Fluid Power Part1

Mechatronics

Swinburne University TAFE division

VBP267 - Set up Fluid Power Controlled Engineering Systems

Read and study the following: 1.0 Fluid Power1.1 IntroductionCommercially, there are three main means of transmitting power from one point to another. Mechanical transmission is through shafts, gears, chains, belts, etc. Electrical is through wires, transformers, etc. Fluid power is through liquids or gas in a pipe. Some applications are best suited to one of these methods, other applications to another method. For example, fluid power is better than mechanical transmission on those applications for transmitting power to moderate distances or to inaccessible and out-of-the-way places, and better than mechanical or electrical, where a fine degree of control, including reversibility and infinite speed variations are an important requirement. Fluid power is the term used that relates to both hydraulic and pneumatically operated machines or equipment. A fluid power system is one that transmits and controls energy through the use of pressurised liquids or gas (It may also use vacuum to transmit power) Hydraulics deals with using liquids under pressure to do work. An example of this would be the braking system of a car. Pneumatics deals with using gases (namely air) under pressure to work. An example of this would be the braking system on the trailer section of a semi-trailer.

1.2 Fluid power: HistoryThe use of gases and liquids as a means of transmitting power can be traced back a long way. It could be consider that the early hunters were using fluid power as a means of transmitting useful energy with the development of the blowgun Using their lungs with capacities of about 1.68 l/s (6000in 3/min), they could develop pressures of 1to 3psi. Mans lungs were a poor compressor for creating a large useful working force. The first recorded use of compressed air was by a Greek named Ktesibios. He developed a compressed air cannon more than 2000 years ago. The term pneuma is derived from the ancient Greek, and means breath of wind. The word hydraulic is also derived from the Greek word hydro and means water. This is because the first hydraulic fluid was mainly water. Traditionally, the use of water or air to produce power depended on the movement of vast quantities of fluid at relatively low pressures. Nature supplied this pressure. However, it is only during the past 200 hundred years that more powerful and efficient means of mechanically compressing gases and pumping fluids have been developed. Fluid power technology began in 1650 with discovery of Pascals law. Simply stated, this law says that pressure in a fluid at rest is transmitted equally in all directions. During the development of fluid power as a means of transmitting energy there were some notable successes and failures, particularly in the development of compressed air.

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VBP267 - Set up Fluid Power Controlled Engineering SystemsDuring the early 1800s an attempt was made to power a mill with compressed generated from a compressor located at a waterfall 900metres away from the plant site. The emerging and not fully understood, technology stumbled here. Clay pipe was used to connect the compressor to the remote plant and while fine for transporting water, is not airtight. The resultant pressure was insufficient to turn the mill.

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By the beginning of the industrial age in Europe, fluid power became prominent as the means of powering presses, cranes, winches and extruding machines. In 1849, the first real application of modern pneumatics was implemented by using pneumatic power tools to cut coal in a coalmine, the major advantage being that air is inherently explosion proof. This success led to compressed air being used on a large scale to dig a tunnel through the French Alps. Using pneumatic rock drills, operating from over 4miles of airlines, the tunnel was successfully completed in 14 years (using manual drilling methods, the 13.7km tunnel would have taken 30 years to complete) Air as an energy transmission came of age when a central plant was built in Paris to supply all of its industries. In 888, Paris installed a 65Hp (48kw) compressor feeding 4 miles of mains and 30 miles of branches (a converted sewer system) delivering 90psi (approximately 6 bar). By 1891, the capacity was increased to 25,00 Hp (18,642Kw). Hydraulics followed a similar development, listed below are some important dates regarding the development of hydraulic fluid power: 1650 1750 1790 1850 1860 1906 1926 Discovery of Pascals law (Pascal was 27 years old) Bernoulli developed a law concerning the conservation of energy in a flowing fluid Joseph Bramah developed the first hydraulic powered press using water as the means for transmitting power Fluid power (water) became prominent in industry (England) Both London and Manchester had central industrial hydraulic distribution systems (water) Oil was replacing water as the hydraulic medium United States developed the self-contained hydraulic power units

Today most factories have their own fluid power systems, often using both pneumatic and hydraulic systems to power various tools and operations.

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1.3 Fluid Power DefinitionsFor a more complete list of definitions for fluid power terms, refer to last page in the notes FLUID - A "fluid" can be any material capable of flowing, but is usually understood to be a liquid or a gas. More specifically, when speaking of fluid power, the "fluids" are compressed air and hydraulic oil. We use the word "fluid" in this book when referring to circuits and principles, which apply to both air and oil. The more specific terms "air", "oil", "pneumatic", "hydraulic", "compressed air" are used when the circuit or principle applies only to one medium. FLUID POWER SYSTEM - This is a system that generates, transmits, and controls the application of power through the use of pressurized and moving fluids within an enclosed circuit. Power is transmitted through pipes as electricity is transmitted through wires. There are many similarities between the action of an-electrical circuit and a fluid circuit. PNEUMATIC FLUID POWER - Normally called an "air" system. Compressed air is used because it is readily available and economical. Occasionally another gas may be used, as for example near a highpressure gas line where the expenditure of gas may be cheaper than the power required to compress air. Most air systems compress and store air in a storage tank for later use or for distribution to many outlets. There may be an occasional exception where the air is compressed at the same rate it is used PRESSURE - Force per unit area, usually expressed in pounds per square inch in the English system.

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Read and study the following: 2.0 SAFETYGeneral Safety Rules for the Operator of Fluid-powered Machines1. 2. 3. 4. 5. 6. 7. Understand the function and operating principle of all the component parts of the machine. Understand the function and operation of the machine's control system. Report any change in operating characteristics, such as abnormal gauge readings, unusual sounds, faulty or erratic performance, and leakage from components. Log any occurrence or observation that may be a clue that preventive maintenance is needed. Contribute information to the safety and well being of others. Attach "Warning" signs to start switch, ignition, etc. Warn others before operating machine

Personal Safety Rules for the Operator of Fluid-powered Machines1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Wear proper clothing that will not catch in rotating or moving parts. Wear safety glasses to protect eyes against flying objects or fluid spray from ruptured lines. Observe the operating rules for the machine, such as safety guards, two-hand control, automatic pull-aways, and other important devices. Know how to shut down the machine and to prepare it for safe inspection or maintenance. Know how to check out controls for restarts after an emergency shut down. Do not make hasty emergency repairs that may endanger life or damage the equipment. Stand clear of any overhead loads Watch for pinch points Do not point air hoses at others Do not clean yourself using air jets

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VBP267 - Set up Fluid Power Controlled Engineering Systems Machine Safety Rules for the Operator of Fluid-powered Machines1. 2. 3. 4. 5. 6. 7. Do not operate controls in a reckless manner that may cause hydraulic shock and damage costly hydraulic components. If the machine is equipped with an accumulator, be sure its pressure energy is released when preparing for shutdowns. Know which valves or controls move the machine parts forward and reverse, in case of an accident. Fit safety bars to prevent loads from dropping Depressurise system before removal of any component Remove fittings slowly Watch out for whipping hoses under pressure

Note: 'No job is so important that you cannot take time to do it safely.'

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Read and study the following: 3.0 Fluid Power TechnologyToday, the machines, equipment and appliances, which rely upon fluid power systems for their functioning, are valued in many billions of dollars, and it is difficult to find a manufactured product which has not been formed, treated or handled by fluid power technology at some stage of its production or distribution. Why, one reason is automation remote control, automatic control of machinery that manipulates and moves parts and products between processes. This is fluid powers strength. A fluid can go anywhere a pipe or tube can go. Valve levers and pushbuttons are all that are required to start, stop, and control a fluid power system. Usually the source of energy for a machine is not at the point where the work is done. For almost all machines, the energy that does the work is mechanical energy. Actuators transform electrical, pneumatic and hydraulic energy into mechanical energy. For this reason, an actuator is required at point of work.

3.1 Comparison of Energy Transfer SystemsThere are three main means of transmitting power from one point to another. Mechanical transmission is through shafts, gears, chains, belts, etc. Electrical is through wires, transformers, etc. Fluid power is through liquids or gas in a pipe. Each method of energy transmission has its own advantages and disadvantages. Therefore a machine may be equipped with a combination of mechanical, electrical, pneumatic and hydraulics systems. To appreciate what type of system or combination of systems should be applied we need to compare the energy transmission systems we have available.

Electrical systems

Ideal over long distances Ideal for rotary power output Limited for linear force output Hazardous could cause fire/explosion or electrocution High efficiency converts 95% of energy input to output Automation possible using relays and sensors

Mechanical systems

Positive application of force Positional control wear and tear create problems with backlash High linear and rotary force output Non hazardous Converts 60% of energy input to output, power losses due to friction Automation possible using cams and trips

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VBP267 - Set up Fluid Power Controlled Engineering Systems Hydraulic systems

Applications limited to covering short distances, eg up to 15 metres. Ideal for rotary output infinite control of torque High linear force output but limitations on speed Non hazardous fire hazard eliminated using inert oils Converts 80% of energy input to output. Automation possible using cams, sensors and relays

Pneumatic Systems

Applications limited to covering medium distances, eg up to 300 metres. Must be coupled to hydraulic control for accurate positioning and operating speeds Limited linear force output but high operating speeds Clean and quiet no oils, use of silencers on exhaust ports Non hazardous fire/ explosion hazard eliminated Converts 25% of energy input to output. Automation possible using cams, sensors and relays

3.2 Comparison of Hydraulic and Pneumatic SystemsFluid power systems can be designed to use compressed air, hydraulic fluids or a combination of both as the working medium. In some cases, either pneumatics or hydraulics can be used to perform a function. However, in certain cases the nature of the function will dictate the choice of the power medium At first glance it would appear that the difference between hydraulics and pneumatics are negligible; with actual choice being a matter of the designers own particular preference. Certainly there are a great many similarities. Both use confined fluids to transmit power by means of similar devices such as pumps, directional controls, flow controls, and a variety of actuators. But similarities end at that point, fundamentally due to the behaviour of the fluid (air or oil) that is being used to transmit and control power.

AdvantagesPneumaticValve switching very fast. Safe to use in difficult areas. Portable hand tools. Ease of control in difficult areas. Ease of replacing components. Overload safe. Very high revs developed. No return lines required. Simplification of circuit design Clean

HydraulicsAble to absorb external shock loads. Ideal for modifying output power. Absence of wear self-lubricating. Good power to weight ratio. Low inertia for stop - start reversal. Overload safe can be stalled under load. Infinitely variable speeds and feeds. Wide range of applications. Large force output. Long life

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VBP267 - Set up Fluid Power Controlled Engineering Systems DisadvantagesPneumatic Low torque at low RPM. Smaller force produced. Noisy loud exhaust air Cold to handle portable tools. Running costs expensive Low rigidity gases compress. Hydraulics Noisy pump running all the time. Over heating friction in pipes. Fire hazard. Operates at lower revs than air system. Strong tubes and containers needed. Problems with contaminants in system

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Read and study the following: 4.0 FluidsReference: Chapter 12 Hydraulic Fluids Industrial Hydraulic Control - Rohner

4.1 IntroductionHydraulic Fluid Power - Originally water was used as the "fluid" because it was cheap and readily available. In fact it was the only liquid available in sufficient quantity. It was circulated once and discarded. Most systems today use refined petroleum oil because it prolongs the life of components, while reducing the size of the system and increasing its efficiency by permitting operation at higher pressures. The oil is re-circulated, with a reserve supply maintained in a reservoir. Where there is a fire hazard, synthetic fluids or water with additives is used. Most hydraulic systems generate an oil flow at the rate of use. But there are systems, which pump the oil and store it in accumulators under pressure for later use.

4.2 Functions of a Hydraulic FluidTransmit PowerThe fluid must be able to transmit the applied force from one part of the system to another.

LubricationHydraulic fluids must provide adequate lubrication in bearings and between sliding surfaces in pumps, valves and actuators.

SealingHydraulic oil must be able to maintain an adequate seal with a minimum leakage from high pressure passages to low pressure passages.

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4.3 Properties of Hydraulic Oils4.3.0 IntroductionFor a hydraulic fluid to be able to perform the various functions identified on the previous page, the fluid needs to have some particular properties. It is these properties that we will now look at and they are in brief:

Oil operating range (viscosity) Pour Point Chemical Stability Demulsibility

4.3.1 Oil Operating Range (viscosity)Viscosity is the internal resistance of a fluid to flow. (This is relative to the oil thickness.) It is generally considered as one of the most important physical properties of a hydraulic fluid. Since if affects both the ability to flow and to lubricate moving parts. Petroleum oil is an excellent lubricant for a hydraulic system, but not at all viscosities. If viscosity of an oil were too low, its fluid film would be like water and consequently too thin. This could result in: Excessive internal leakage losses. Excessive wear, due to insufficient lubrication in pumps and motors. Decrease in pump and motor efficiency. Increase in fluid temperature due to internal leakage losses.

If an oil's viscosity were too high, insufficient amounts of fluid would flow into bearings and component clearances. This could result in: High resistance to flow, causing pump cavitation and sluggish actuator movement. Increased power consumption, due to frictional losses. Increased pressure drop through lines and valves.

For this reason, manufacturers of rotating equipment (pumps, motors) which are especially dependent on proper bearing lubrication, specify the viscosity range at which their components are to be operated. When these components are sufficiently lubricated it usually means the rest of the system is lubricated as well.ISO Viscosity Grade | ISO VG 2 ISO VG 3 ISO VG 5 ISO VG 7 ISO VG 10 ISO VG 15 ISO VG 22 ISO VG 32 ISO VG 46 Mid-Point Viscosity cSt at40.0C 2.2 3.2 4.6 6.8 10 15 22 32 46 Kinematic Viscosity Limits cSt at 40.0C ISO Viscosity Grade ISO VG 68 ISO VG 100 ISO VG 150 ISO VG 220 ISO VG 320 ISO VG 460 ISO VG 680 ISO VG 1000 ISO VG 1500 Mid-Point Viscosity cSt at40.0C 68 100 150 220 320 460 680 1000 1500 Kinematic Viscosity Limits cSt at 40.0C

Min.1.98 2.88 4.14 6.12 9.00 13.50 19.80 28.80 41.40

Max.2.42 3.52 5.06 7.48 11.00 16.50 24.20 35.20 50.60

Min.61.20 90.00 135.00 198.00 288.00 414.00 612.00 900.00 1650.00

Max.74.80 110.00 165.00 242.00 352.00 506.00 748.00 1100.00 1650.00

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VBP267 - Set up Fluid Power Controlled Engineering SystemsA few SHELL grades do not conform to the standard ISO classifications. For example the numbers 37, 78 and 800 are SHELL 'ISO type' numbers that have been allocated to meet certain important viscosity requirements that are not met by standard ISO numbers. If components have a required viscosity range, then this information, along with the temperature range of the system, indicates the use of a specific oil. For example, a particular system at its operating temperature requires a minimum/ maximum viscosity of 70-250 SUS (15-54 CST). If the operating temperature range were 80-140F (26.760C), hydraulic fluid Y would be used. If the temperature range were 110-170F (43.3-76.7C), hydraulic fluid Z would be used. Since temperatures can become quite low even in industrial environments, oil can become extremely viscous. To ensure that their pumping mechanisms will fill, pump manufacturers also specify the maximum viscosity allowable at start-up. In general, these viscosities are 1000 SUS (216 CST), and 7500 SUS (1618 CST) for piston, vane and gear equipment, respectively.

4.3.2 Pour PointThe temperature at which fluid congeals is called the pour point. At low temperatures, wax structures begin to form in hydraulic fluids containing any petroleum base crude. These wax formations hinder and may even stop flow. Pour point of a hydraulic fluid is the lowest temperature at which it will pour under a laboratory test. In an actual system, if the maximum viscosity start-up specification is adhered to, the pour point of a fluid is generally not considered. But, when a system has the possibility of operating under extremely low temperature conditions, pour point of the oil should be at least 11C below the lowest expected temperature. This will ensure that the oil will flow and supply the inlet side of the pump.

4.3.3 Chemical Stability Oil Oxidation StabilityOxidation is a process by which material chemically combines with oxygen; this is a common occurrence, If you have ever taken a bite out of an apple, you know that the pulp quickly turns brown as it is exposed to air. Many things on earth, including oil, oxidise in this manner. Oxidation of hydraulic fluid can be pinned down to basically two system locations; reservoir and pump outlet. In both cases, oil reacts with oxygen but in different ways and the oxidation products are not the same. In a reservoir, the free surface of the oil reacts with oxygen in the air. The product of this reaction includes weak acids and soaps. Acids weaken and pit component surfaces; soaps coat surfaces and can plug pressure-sensing orifices and lubrication paths.

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VBP267 - Set up Fluid Power Controlled Engineering SystemsHeat is a major contributor to oil oxidation in a reservoir. As a rule, oil oxidises twice as fast as normal for every 18-20F (10-11C) rise in temperature above an average reservoir temperature of 130F (54.4C). Reservoir oil also oxidises more readily in the presence of iron and copper particles and water droplets. Besides the reservoir, another location where oil oxidation occurs is at pump outlet. If air bubbles are present in a pump suction line as a result of an air leak in the suction line or returning fluid velocity churning up the reservoir, they suddenly collapse upon being exposed to high pressure at pump outlet. This action generates a high temperature, which according to some calculations can rise to 2100F (1149C) when the bubble is compressed from 0-3000 PSI (207 bar). Strong evidence exists that collapsing air bubbles at pump outlet is a major influence in rapid oil degradation. The high temperature fries the oil, forming resinous products, and causes the oil to acquire a characteristic burnt odour. As high-temperature oxidation at pump outlet occurs, resinous materials are formed, but dissolve in the oil. When a hot surface (pump rotor, relief valve spool) is encountered, resins come out of solution forming a varnish or lacquer coating on the hot surface; this causes moving parts to stick. Resinous material can also form sludge, which combines with dirt and floats around the system plugging small openings in valves and filters, and interferes with heat transfer to reservoir walls.

4.3.4 Demulsibility Water in Hydraulic OilWe know from experience that water and oil do not mix (except for water-soluble oils). Attempts to mix large amounts of water and oil will result in water settling out at the bottom of a tank. In small quantities, however, water is broken into small droplets, which are carried around by the oil. If an oil contains acidic and resinous products of oxidation, it has an increased tendency to take on water. To counteract this, the oil used should have good demulsibity characteristics ie. the oil resists mixing with water, so that the water will separate out easily and sink to the bottom of the tank.

4.4 Oil Additives4.4.1 Lubrication and antiwearA good quality petroleum base hydraulic fluid is not a good enough lubricant for some systems. As pressures climb, the hydrodynamic fluid wedge between moving parts has more of a tendency to break down. This means lubrication is more dependent on a fluid's inherent lubricity. To aid in lubricity or boundary lubrication at high pressures, hydraulic fluids are equipped with chemical additives.

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VBP267 - Set up Fluid Power Controlled Engineering Systems Antiwear AdditivesAntiwear (AW) or wear resistant (WR) additives can be divided into three types. One type, sometimes called an oiliness or lubricity agent, is a chemical made up of molecules that attach themselves vertically like blades of grass to metal surfaces. This creates a chemical film, which acts as a solid when an attempt is made at penetration. The additive molecules support the load, allowing a moving part to slip by. But this film is not very durable, tending to break down at high temperatures. Another type of antiwear additive chemically combines with a metal surface to form a protective film. This film forms as low frictional heat is generated between contacting points of moving surfaces. They serve to smooth out or polish surfaces so that friction is reduced. Another antiwear agent, known as an extreme pressure (EP) additive, forms a film on a metal surface as high frictional heat is generated. In a high-pressure system, as mechanical interaction between surfaces becomes excessive, heat becomes excessive and the surfaces attempt to weld together. The extreme pressure additive comes out of solution at this point, keeping the surfaces apart. All three types of antiwear additives are not found in the same fluid and are not used in the same applications. When oiliness agents are used, they are generally found in relatively low-pressure systems (below 1000 PSI/68.97 bar). When extreme pressure additives are found in a hydraulic system, the system will probably be operating above 3000PSI (207 bar), or the same fluid that is used to lubricate gears and machine ways is also used in the hydraulic system. A very common antiwear additive is the one, which operates in the medium pressure range (1000-3000 PSI/68.97-207 bar).

4.4.2 FoamingAs oil returns to a reservoir, it should release any entrained air bubbles, which have been acquired in the system. In some systems where leaks are prevalent and/or returning oil is churned up as it enters a reservoir, foaming of the oil occurs. As a result, entrained air is pumped into the system, causing spongy, erratic operation, rapid oil oxidation and noise. In more severe cases, oil foam could bubble out of reservoir creating a housekeeping problem. Probably the best solution for alleviating foaming oil is to fix any system leaks and redesign the return part of the system with baffles or larger return lines which reduce fluid velocity. Sometimes, because of economics, convenience, or a lack of training, chemicals are used to solve the problem.

Anti-Foam AdditivesIn an attempt to discourage oil foaming hydraulic fluids can be equipped with anti-foam additives. In some cases, these additives work by combining small air bubbles into large bubbles, which rise to a fluid surface and burst. In other cases, these additives function by interfering with air release which action reduces foaming, but increases the amount of air bubbles in the system. If an anti-foam chemical is desired in the oil, care should be taken that the agent selected does allow air to escape.

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4.5

Fire Resistant Hydraulic Fluids.

A main disadvantage of petroleum based fluid is its ability to burn. When a hydraulic system is located near high temperature equipment or other sources that could ignite the fluid, the use of fire resistant hydraulic fluids is recommended. There are three basic types of fire-resistant fluid.

Water glycols Water-oil emulsions Synthetic fluids. 4.5.1 System CompatibilityThe various fire resistant fluids, composed of many complex chemical components, display a variety of effects on the materials used in a hydraulic system. A convenient, but generalised list of material behaviour to fire resistant fluids is shown below. Water Glycol Paints: Common Industrial Epoxy & Phenolic Metals: Ferrous Brass, Copper Zinc Aluminium, Unanodised Aluminium, Anodised Seals: Teflon Viton Neoprene Buna N Butyl Rubber E. P. Rubber Silicone Rubber NC C C C NC NC C C C C C C C C Phos. Ester & Oil Synthetic blends NC C C C C C C C C NC NC C C C W/O emulsions NC C C C C C C C C C C NC NC C

Key:

C = Compatible

NC = Non-compatible

4.6

Summary of the Requirements for selecting a Hydraulic Oil1. 2. 3. 4. 5. 6. Maintain correct temperature. Prevent corrosion. Prevent formation of sludge, gum and varnish. Depress foaming. Separate out water. Compatibility with seals.

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Read and study the following: 4.7 AirReference: Chapter 13 Pneumatic Industrial Control - Rohner

The air we breathe is springy, squashy and fluid in substance, we take it for granted that wherever there is space it will be filled with air. Air is composed mainly of nitrogen and oxygen

Air pressureThe atmospheric pressure is caused by the weight of air above us, it gets less as we climb a mountain, more as we descend into a mine. The pressure value is also influenced by changing weather conditions. Pressures will be quoted in kilopascals gauge (kPa) and bar unless otherwise stated An atmospheric pressure of 101.3 kPa absolute has been adopted for the conversion of gauge pressures to absolute pressures for use in calculations shown in the text. The relationship between gauge and absolute pressure is shown in fig. 13-2. Most pressure gauges used on pneumatic equipment are calibrated in gauge pressure. Thus, if a pressure gauge is calibrated for kPa. (A) it is capable of measuring pressures in the vacuum range ("A" stands for absolute pressure calibration). But if it is calibrated just for kPa then the pressure gauge shows pressures above atmospheric pressure only The power of atmospheric pressure is apparent in industry where pick and place suction cups and vacuum forming machines are used, air is removed from one side allowing atmospheric pressure on the other to do the work Pressures are in bar g gauge pressure (the value above atmosphere). Zero gauge pressure is atmospheric pressure Absolute pressures are used for calculations Pa = Pg + atmosphere For quick calculations assume 1 atmosphere is 1000 mbar For standard calculations 1 atmosphere is 1013 mbar There are many units of pressure measurement. Some of these and their equivalents are listed below. 1 bar = 100000 N/m2 1 bar = 100 kPa 1 bar = 14.50 psi 1 bar = 10197 kgf/m2 1 mm Hg = 1.334 mbar approx. 1 mm H2O = 0.0979 mbar approx. 1 Torr = 1mmHg abs (for vacuum)

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VBP267 - Set up Fluid Power Controlled Engineering Systems Free airFree air (or ambient air) is defined as air at atmospheric conditions at any specific location and time. It should be noted, however, that because the altitude, barometric air pressure and air humidity may vary at different geographical localities and times, the term "free air" does not mean air under identical or standard conditions. In view of such variations and in order that stated air flows relating to pneumatic equipment can be calibrated and compared, a standard is essential. There are unfortunately many standards depending on the industry or country where equipment is produced, but the two most common are the European Standard and the Imperial Standard. The European standard for free air is based on a pressure of 101.3 kPa absolute, at a temperature of 0C, whereas the Imperial standard is based on 14.7 p.s.i. absolute at 15.6C. Hence, the term "free air" based on the previously mentioned standards is quite a simple and useful rating to size and compare delivery rates of air compressors. If the compressor, however, is installed in an environment, which has a free air pressure and temperature other than either of the standards above, then its flow rate will alter from the manufacturer's standard specification. Compressors, for example, are always rated for flow output based on the flow rate (ambient air) they take directly from the atmosphere through their air intake. To illustrate: If a compressor had a rated output of 4 m3/min F.A.D. (free air delivery), for each minute the compressor was operating, eight cubic metres of "free air" would be drawn into the intake from the atmosphere. This would be comparable to a volume of air 2 metres long by 2 metres wide by 1 metre deep, being drawn in each minute The output rate from the compressor, however, would be far less, since the air would now be compressed to a much smaller volume, but to a much higher pressure than at the original ambient pressure prevailing at the compressor intake.

Altitude effects on air compressionIn some locations, the effects of altitude on the output of air compressors and air consuming devices must be considered. At altitudes of for example 500 and 1000 metres, there is a corresponding reduction in atmospheric pressure of approx. 5 kPa and 10 kPa respectively. For compressors, these reduced intake pressures cause a decrease in the compressor power requirement. However, the lower atmospheric intake pressure also means that the compression ratio of the compressor is higher. This results in an increased power requirement. The net result for a compressor operating at an altitude above sea level would mean a decrease in the power required to drive the compressor, and a decrease in the free air delivery. Air consuming devices such as air tools and linear actuators, if operated at sea level, would consume more air than if operated above sea level.

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Read and study the following: 5.0 BASIC PRINCIPLES OF FLUID POWER1. Fluids have no shape of their own.(a) (b) (c) Liquids will take up the shape of the container up to the filled level. Gasses will take up and fill the entire shape of the container. Because of this, fluids will flow in any direction and into any passage or cavity of any size or shape.

2.

Fluids Transmit applied pressure in all directions.Pascal's law states that pressure applied to a confined fluid acts equally in all directions and at right angles to all containing surfaces. This not the case with solids as force can only be applied in one direction.

3.

Fluids are able to apply a great increase in applied force.Force multiplication with a fluid power system is in some ways similar to a mechanical lever. In a fluid power system the same principles may be achieved by applying a small force to a small diameter piston that in turn applies pressure to a larger piston thereby increasing the force output.

4.

Liquids are practically incompressible.In most hydraulic fluid power systems, the hydraulic oil is considered to be incompressible. Therefore if a piston moves a certain distance the oil will move an equal distance and what ever the oil is pushing on will also move the same distance.

5.

Gasses are readily compressed.The fact that gasses are able to be compressed makes possible the use of compressed air for use in pneumatic systems. When air is compressed it develops potential energy similar to a spring and will react in a similar way as a spring when applied to a movable member

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5.1 Basic Calculations for Fluid Power Systems5.1.1 Force TransmissionPneumatic and hydraulic systems are fluid power systems. They use a fluid as the medium of energy transmission. Pneumatic systems use a highly compressible, gaseous fluid. Hydraulic systems use a relatively incompressible, liquid fluid. Just as the other transmission systems (mechanical, electrical), fluid power systems are capable of transmitting a static force (potential energy) as well as kinetic energy. When a static force is transmitted through a fluid, it happens in a special way. To illustrate, we will compare how a force is transmitted through a solid with force transmission through a confined fluid.

Force Transmitted Through a SolidA force transmitted through a solid is transmitted basically in one direction only. If we pushed on a solid block, the force would be transmitted in the direction of the applied force, to the opposite side only.

Force Transmitted Through a FluidUnlike a solid, a force applied to a confined fluid (gas or liquid) is transmitted equally in all directions throughout the fluid in the form of fluid pressure is known as Pascal's law in honour of Blaise Pascal who first defined the principle. If we pushed on a container filled with fluid, the pressure of the applied force would be transmitted equally throughout the fluid. In the case of a gaseous fluid, the applied force would push the piston down compressing the gas. Piston movement would continue until the intensity of the applied force was equalled by gas pressure. A gas absorbs the intensity of an applied force. A confined gas or liquid will transmit pressure in a similar manner regardless of how it is generated. As far as a fluid is concerned, an applied force results in pressure whether the application of force comes from a hammer, by hand, weight, fixed or adjustable spring, or any combination of forces. Fluids take the shape of their container. Consequently, pressure will be transmitted in all directions regardless of container shape.

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5.1.2 Hydrostatic PressureUnder static conditions and with no other internal pressure, the pressure at any point within a liquid system is proportional to the height of the liquid column above that point. Torricelli called this pressure at the bottom of the fluid column (tank) the "head pressure". The head pressure is equal to the height of the liquid column, multiplied by the density (mass per unit volume) of the liquid in the tank, multiplied by the gravitational constant. Note: Density for water is 1000 kg/m3. Density for hydraulic oil is 860 kg/m3. Gravitational constant is 9.81 N/kg.

Effect of Hydraulic PressureThe effective area on which hydraulic pressure acts to produce a force is either the projection of the ball-seat contact area (check valve) projection of the pressure exposed piston area (actuator) calculated effective piston area (unloading valve)

5.1.3 Fluid Pressure to Mechanical ForceIn transmitting pressure through a confined fluid, some sort of movable member has been used to apply the pressure. In the examples used so far, the movable member has been a piston. To determine the intensity of a force, or pressure, being applied to a system, the force is divided by the area of the movable member. For example, if an applied force of 100N (approx 10kgs) were applied to a piston area of 1m2, the resulting pressure would be 100 N/m2 (Pascal). Applying a force to a fluid and transmitting the resulting pressure throughout the fluid in various shaped containers does very little good for its own sake. Fluid pressure must be converted into mechanical force before work can be done. This is the function of a fluid power actuatorto accept fluid pressure and convert it into a mechanical force. One very common type of actuator is the fluid power cylinder.

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VBP267 - Set up Fluid Power Controlled Engineering Systems Mechanical Force MultiplicationMechanical forces can be multiplied using fluid power. The determining factor for force multiplication is the area on which pressure is applied. Since pressure is transmitted equally in all directions throughout a confined fluid, if a cylinder piston has more area than the movable member developing the pressure, output force will be greater than input force. In our example, assume that the resisting object is stationary and will not move. A 10N force on the 1m2 area piston results in a pressure of 10N/m2 (Pascals Pa) throughout the system. The 10Pa acts on the cylinder piston with a 2m2 area resulting a mechanical force of 20N

in

Force (N) = pressure N/m2 x area (m2) Movement SacrificedIt has been illustrated that a cylinder can be used to multiply a force by the action of fluid pressure acting on a piston area. When multiplying a force with fluid pressure, it may have appeared that something was received for nothing. It appeared that a smaller force could generate a larger force under the right circumstances, and nothing was sacrificed. This is relatively true in a static system. But if the force were to be multiplied and moved at the same time, something would be sacrificedmovement. Each cylinder has a stroke and volume. The stroke of a cylinder is the distance through which a piston and piston rod travel. A cylinder volume is the piston's displacement. It is calculated by multiplying its stroke in metres by piston area in square metres. This will give a volume in cubic metres

cylinder volume = piston area x stroke (m3) (m2) (m)In the illustration, the system is filled with hydraulic fluid. The small piston must move through a distance of 10mm. to make the large cylinder piston move 5mm. In both cases the work done is the same.

When forces are multiplied with fluid pressure, movement is sacrificed .

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Fluid Pressure MultiplicationIntensifierAn intensifier multiplies fluid pressure.

What Intensifiers Consist ofAn intensifier basically consists of a housing with inlet and outlet ports, and a large area piston connected by a rod to a small area piston. The volume between the two pistons is vented.

How Intensifiers WorkThe inlet of an intensifier is connected to a source of fluid pressure either air or hydraulic. Intensifier outlet is connected to part of the system containing hydraulic fluid. An intensifier multiplies, or intensifiers, an existing fluid pressure by accepting an air or hydraulic pressure at the large area piston and applying the resultant force to the small area piston. Fluid pressure is therefore intensified or multiplied at the actuator. In our example, assume that the object is to be clamped. An input pressure of 3Mpa at intensifier inlet ultimately results in a high output clamping pressure of 12MPa.

12MPa

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5.2 Pressure and Force CalculationsPressure it equal to the amount of force exerted over a given area.

FORCE Pressure is equal to AREA

F P=A

The unit for pressure is PASCAL(Pa)

EXAMPLE1) FORCE = 100 NEWTONS AREA = 1M X1M = 1M2

EXAMPLE2) FORCE = 100NEWTONS AREA = 2M X2M = 4M2

FORCE Pressure = AREAPRESSURE = 100 PRESSURE = 100 PASCALS

FORCE Pressure = AREAPRESSURE = 25 PRESSURE = 25 PASCALS

CalculationsAll calculations should be done in base units. Base units: Force = N Area = m2 Pressure = Pa Volume = m3 Power = W Distance = m Work = J Velocity = m/S

Derived UnitsDerived units are used as a more convenient method of expressing values. BASIC UNIT Newton (N) Pascal (Pa) Meter (m) X 1000 or 103 Kilo Newton KN Kilo Pascal KPa Kilo Meter Km X 1,000,000 or 106 Mega Newton MN Mega Pascal MPa Mega Meter Mm

NOTE: Pressure is often related to atmosphere which is measured in bar an 1 bar is equal to 100,000Pa

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Review Questions: 5.2.1 Force &Pressure CalculationsExercise 1 Change the following pressures into derived units: kPa kPa kPa kPa kPa MPa MPa MPa MPa MPa bar bar bar bar bar

10000 Pa 950 Pa 1000000 Pa 800000 Pa 1000 Pa

Exercise 21 m2 0.01 m2 0 3 m2

Change the following areas into derived units:cm2 cm2 cm2 cm2 cm2 mm2 mm2 mm2 mm2 mm2

0.004 m2 0.0007m 2

Exercise 3

Which answer is correct?

The pressure p=24 N/m2. The force acting on 10000 cm2 is therefore: 2.4N 24N 240 Ng Impossible to calculate

Exercise 4

Which answer is correct?

If P= A and F=200 N and A=0.08 m2; then p is: p = 4 Pa p = 25 N/m2 p=0.25N/cm2 p=160Pa

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VBP267 - Set up Fluid Power Controlled Engineering Systems Exercise 5 Which answer is correct?

A force of 250 N is acting on an area of 2 m2. The pressure is therefore: 125Pa 250 Pa 500 Pa Impossible to calculate

Exercise 6

Which answer(s) are correct?

There is a force of 315 N acting on a flat surface. The pressure must not exceed 45 N/cm2. The area on which the force acts must therefore be at least: 7cm2 45 cm2 35 cm2 14625cm2

Exercise 7

Which answers are correct?

A liquid exerts a force of 180 N over an area of 0.002 m2. The pressure p is: 1/9N/cm2 90 kPa 90000 Pa 36 N/cm2 0.36 N/m2 1.111 Pa 90000 N/m2 0.0036 N/m2 0.9 bar

Exercise 8GIVEN P= 3MPa F =15kN F = 6MN P = 477 bar F = 8.3kN d = 42mm FIND d= d= P m; mm; MPa; mm m bar

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VBP267 - Set up Fluid Power Controlled Engineering Systems Exercise 9GIVEN d =800 mm F =12 MN d = 130 mm p =1200kPa FIND p= F= bar; kN; MPa M

Exercise 10GIVEN AP = 0.049 m2 AR = 0.005 m2 F = 750kN F = 0.12 MN p = 40 MPa FIND p= p= AA = m= MPa bar m2 kg

Exercise 11GIVEN DP = 125 mm dR = 100 mm p = 16 MPa dP =450 mm dR =280 mm F =2144 kN FIND AA = F = m = AA = p = p = m2 kN kg m2 MPa bar

Exercise 12 Work out each stroke separately and assume no back pressure.GIVEN dp = 10 cm dR = 8 cm p1 = 0.6 MPa p2 = 1.683 MPa Fret = kN FIND AA = Fext. = m2 kN

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VBP267 - Set up Fluid Power Controlled Engineering Systems Exercise 12 Refer to fig. 6-4.2

The surface area of the lifting piston in a fork-lift truck 0.006 m2. The maximum load on the platform is 1200 kg. The lifting mechanism, which also has to be raised, weighs 6000 N The minimum pressure on the piston must therefore be: p= kPa MPa bar

Exercise 13 Refer to fig. 5-3.1A1 = 0.005 m2 A2 = 0.150 m2 F1 = 2250 N Calculate: m d2= p= p= F2= F2= m kPa bar kN MN

Exercise 14 Circle the correct answer. The ratio of forces (F1:F2) in fig. 5.3.2 is:3:8 1:30 17:8 8:3 30:1

Exercise 15 The following data apply to the fork-lift truck in fig; 5.3.3.Piston diameter d= 100 mm Weight of lifting mechanism W1 = 1 kN Load to be lifted W2=7 kN Calculate the minimum required system pressure in: bar kPa MPa N/m2

Exercise 25 A boiler must be pressure tested to 1.2 MPa. The internal diameter is 2.4m. What force in MN acts onto each of the domed end caps of the vessel?

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Read and study the following: 5.3 Fluid FlowIf work is to be done in a fluid power system there must be movement (W = F x D) In order for movement to occur an unbalancing of forces needs to occur. When movement occurs the fluid will be forced to flow. The tendency to cause flow may be supplied by a mechanical pump or may be caused by the weight of the fluid. Torricelli proved that if a hole is made in the bottom of a tank of water, the water, will run out faster if the tank is full and the flow rate decreases water level decreases. Measuring Flow: 1) There are two ways to measure the flow of a fluid.

Velocity (V)

Velocity is the speed at which a fluid travels past a given point. It is measured in metres per second. .:V= M/S 2) FlowRate(Q)

Flow rate is the volume of a fluid that passes a given point. It is therefore a combination of volume and speed. It is measured in cubic metres per second. Q V A = = = Flow Rate(m3/s) Speed (m/s) Area (m2)

Q =V AWe shall now further consider flow under the following headings: (a) Flow and pressure drop (b) Flow through an orifice (c) Laminar and turbulent flow.

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VBP267 - Set up Fluid Power Controlled Engineering Systems Flow and Pressure Drop A basic rule of hydraulics is that wherever there is flow, there must be a pressure difference or pressure drop. Conversely, where there is a difference in pressure, there must be either flow or at least a difference in the level of the liquid. Whenever a gas is flowing, a condition of unbalanced force must prevail to cause fluid motion. Hence, when a gas (and compressed air is a gas) flows through a pipe with a constant diameter, the pressure will always be lower downstream than at any point upstream (fig. 5.3.1).

fig. 5.3.1

The pressure difference when a liquid is flowing is used to overcome friction and to lift the fluid where necessary. When a liquid is flowing, the pressure is always highest upstream and lowest downstream. The pressure differential or pressure drop is caused by the friction of the gas or liquid molecules amongst each other and friction of gas/liquid molecules on the walls of the pipe. Thus the inside surface condition does influence the pressure drop in the piping system. This friction creates heat, and so the pressure drop (or loss) is a permanent loss, transferred into heat energy, which cannot be regained.

fig. 5.3.2

As soon as the flow stopped Pascals Law must be applied for the now static condition, and pressures in all parts of the piping system (fig. 5.3.2).

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VBP267 - Set up Fluid Power Controlled Engineering Systems Flow through an Orifice Pressure drop occurs to a greater degree when the flow is restricted. An orifice is a restriction often placed in a line deliberately to create a pressure difference. There is always a pressure drop across an orifice so long as there is flow (see diagram below). However, if we block the flow beyond the orifice, Pascal's Law takes over and pressure equalizes on both sides. An orifice is a hole with less cross-sectional area than the pipes or cavities to which it is fitted. The orifice is generally used to control flow (speed control of actuators) or to create a pressure differential (pressure reducing valve). Pressure drop also takes place when passing fluid through a valve or line. The smaller the valve passage or line the greater the pressure drop. In effect, the restrictive area acts as an orifice.

Flow through an orifice to Atmosphere When compressed air is discharged through an orifice to atmosphere, the speed at which it flows through the discharge orifice, either sonic( the speed of sound) or subsonic (lower than the speed of sound) The speed depends on two factors: The shape or type of orifice; The pressure differential across the orifice

The pressure upstream of the orifice is 140 kPa (1.4 bar) higher than the downstream pressure then it can be surmised that the speed at which the compressed air flows is sonic. No matter how much the pressure upstream is increased now, the flow will remain sonic. This phenomenon is an important characteristic of compressed air on which the effective speed control of double acting linear actuators depends, and for that reason, any pneumatic actuator or pneumatic motor (Fig. 5.3.3.)

Fig 5.3.3 Flow through an orifice to atmosphere

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VBP267 - Set up Fluid Power Controlled Engineering Systems Laminar and Turbulent Flow Laminar flow (sometimes referred to as streamline flow) occurs when the liquid particles flow smoothly in even layers, and frictional losses are at a minimum. The sketch (Fig. 5.3.4) illustrates laminar flow and highlights the greater restriction next to the walls of the tube.

Fig 5.3.4 laminar flow patterns

Generally it can be said that the flow is laminar if the liquid flows slowly enough and remains laminar at greater velocities if the diameter of the pipe is small. If the velocity of flow or size of pipe increases sufficiently, the flow becomes turbulent. Turbulent flow occurs when the liquid particles flow in a random or erratic pattern as is illustrated in Fig. 5.3.5.

Fig 5.3.5 turbulent flow patterns

Flow Law If fluid flows through a pipe with different diameters the speed of the flow changes, (ie) faster speed in the smaller diameter. Example : Fig. 5.3.6 shows that an equal volume of fluid will take the same time to flow through a pipe, this means that the speed of the fluid flows will be faster in the small diameter pipe.

T 1 T 2

T 3

V 1

V 2

V 3

T 1 x V1

=

T 2 x V2 =

T 3 x V3

Fig 5.3.6 fluid flow through pipes of different diameters

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Review Questions: 5.5.1 Flow - Calculations1. Express the following volumes in derived volumes 6 litres = 117mL = 220cm3 = mL cm3 mL cm3 L m3 m3 m3 L

2.

GIVEN d = 80mm Q = 4L/min V = 15m/min d = 60mm Q = 0.5L/sec V = 12mm/sec

FIND V= V= Q= Q= A= d= m/min mm/s L/min L/sec m2 mm

3.

Calculate the flow rate of a pump in (l/min) if it is driven with a speed of 1400rpm and its geometrical displacement is 255cm3. Assume a volumetric efficiency of 91%.

4.

Calculate the required RPM for a piston pump with a geometrical displacement of 107mL and a volumetric efficiency of 97% The pump must produce a flow rate of 3.46L/s.

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5.

VBP267 - Set up Fluid Power Controlled Engineering Systems Calculate the required geometrical displacement in litres if the pump is driven at 1400 RPM and must deliver a flow rate of 72L/min. The stated volumetric efficiency (v) is 95%

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Read and study the following: 6-0 Actuators IntroductionReference: Chapter 3 - Linear Actuators Pneumatic Control for Industrial Automation - Rohner

Actuators are used to convert the stored (static) energy of compressed air or liquid flow (kinematic energy) into mechanical force or motion. Although the actuator itself produces motion, a variety of mechanical linkages and devices may be attached to it to produce a final output force which is rotary, semi-rotary or a combination of linear and rotary. Levers and linkages may also be attached to achieve force multiplication or force reduction as well as an increase or reduction of motion speed (fig. 6.0.1).

Fig 6.0.1 simple actuator mechanisms The generation of thrust force with an actuator is very simple and direct. The compressed air or liquid when delivered to one end of the actuator, acts against the piston area and produces a force against the piston (force = pressure x area). The piston with the attached piston rod starts to move in linear direction as long as the reacting force is smaller. The developed force is used to move a load, which may be attached either to the protruding piston rod or to the actuator housing (fig.6.0.1). The distance through which the piston travels is known as the stroke.

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VBP267 - Set up Fluid Power Controlled Engineering Systems Linear Actuators (cylinders) A cylinder is a term used in fluid power for a device for translating the energy contained in a system into an equivalent amount of mechanical energy. Cylinders can give either straight movement or rotary movement. Commonly those actuators that are referred to as cylinders give straight-line movement. Those that are rotary actuators are termed motors. Cylinders can be either single or double acting. Single Acting Cylinders The single acting cylinder has a power stroke on extension by means of introducing compressed air into the cylinder behind the piston. The return stroke is achieved by springs or weights on the piston rod when the air is exhausted from the piston side of the cylinder. When using a single acting actuator the force available from the air operated stroke is reduced by the opposing spring force. It must also be borne in mind, that the force due to the spring compression is progressively decreasing along its stroke Single acting actuators are usually built with a stroke length up to 100 mm. Some manufacturers of pneumatic actuators make a special type of single acting actuator called a short stroke clamping cylinder or diaphragm actuator. These actuators have extremely short strokes ranging from 1 to 10 mm and piston areas ranging from 100 to 3000 mm2, whereby the large piston (diaphragm) actuators normally have the extremely short strokes of 1 to 2 mm. Such clamping actuators are normally retracted by either an inbuilt spring or by the pretensioned diaphragm A very special type of single acting actuator is the air bag. The air bag actuator consists of s flexible "bellow" (or bellows) made from plies of nylon-reinforced cord, encased by neoprene rubber. The cord is meant to provide a restraining effect as pressure is applied and helps to maintain the effective area of the internal air column, which provides the thrust when filled with compressed air. The bellows may be made in single, double or triple convolutions. "Girdle hoops" of metal or metal wire covered with rubber, provide additional restraint on multiple bellow types Metal end pieces are attached to the top and bottom of the bellow(s) and the air enters and exhausts through a port in the end piece. Manufacturers of air bags provide performance charts that contain such information as: useable stroke length, load-carrying capacity related to stroke and air pressure, air volume at various stages of inflation, and mounting methods. The main applications are heavy duty lifting, logging industry, air presses, isolation of vibration, cause of vibration. shock absorption and clamping.

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VBP267 - Set up Fluid Power Controlled Engineering Systems Standard Double Acting. Power stroke is in both directions and is used in the majority of applications. The double acting cylinder is retracted when pressure air is applied to the rod side of the cylinder and the air from the piston side is exhausted to atmosphere. Double-Rod Cylinders. Used when equal displacement is needed on both sides of the piston, or when it is mechanically advantageous to couple a load to each end. The extra end can be used to mount cams for operating limit switches, etc. Ram Type, Single-Acting Cylinders. Containing only one fluid chamber, this type of cylinder is usually mounted vertically. The weight of the load retracts the cylinder. They are sometimes known as "displacement cylinders", and are practical for long strokes. Telescoping Cylinders. Available with up to 4 or 5 sleeves; collapsed length is shorter than standard cylinders. Available either single or double-acting, they are relatively expensive compared to standard cylinders. Tandem Cylinders. A tandem cylinder is made up of two cylinders mounted in line with pistons connected by a common piston rod and rod seals installed between the cylinders to permit double acting operation of each. Tandem cylinders allow increased output force when mounting width or height is restricted. Duplex Cylinders. A duplex cylinder is made up of two cylinders mounted in line with pistons not connected and with rod seals installed between the cylinders to permit double acting operation of each. Cylinders may be mounted with piston rod to piston (as shown) or back to back and are generally used to three position operation. Rotary actuators Motors are used where a semi-rotation or full rotation is needed. They are found on air tools such as grinders where the motor is a vane type. Air winches employ rotary actuators, which require great force but not as high a speed as the air grinders. Air winches utilise piston motors to generate the high torque required.

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6.1 Cylinder ConstructionMajor Parts of Cylinders The major components of a cylinder are the head caps, tube, tie rods, piston, piston rod, rod bearing and seals. Cylinder Heads and Caps are usually made from rolled steel or cast iron. Some are also from aluminium or bronze. Cylinder Tubes are usually brass, steel or aluminium. The inside, and sometimes the outside, is plated or anodised to improve wear characteristics and reduce corrosion. In some applications, cylinder tubes can also be made Pistons vary in design and materials used. Most are made of cast iron or steel. Several methods of attaching the piston to the rod are used. Cushions, are an available option on most cylinders and most often, can be added with no change in envelope dimensions. Piston Rods are generally high strength steel, case-hardened and ground, polished and hard- chrome plated for wear and corrosion resistance. Corrosive atmosphere conditions usually require rods of stainless steel, which may be chrome plated for wear resistance. Rod Glands or Bearings are used on the head end of most industrial cylinders to support the piston rod as it travels back and forth. The gland also acts as a retainer for the rod packing or seals. Most are made of ductile iron or bronze and usually are removable without disassembling the entire cylinder. The gland usually contains a piston rod wiper or scraper on the outboard side to remove dirt and contamination from the rod, and prevent foreign material from being drawn into the packings. A primary seal is used to seal the cylinder pressure. Seals are generally made from Nitrile or fluoro carbon elastomers, polyurethane, leather or Teflon'9. Commonly used seal shapes are shown in illustration b-8. The Lipseal'9 shape is commonly used for both piston and piston rod seals. Generally, O-Rings are used for static applications such as head to tube; piston to rod; and head to gland. Cup or V-packings are used for sealing piston and piston rod. Piston rings are usually cast iron. Tie-Rods are usually high tensile steel with either cut nr rolled threads, prestressed during assembly. Prestressing with proper torque prevents separation of parts when subjected to pressure and reduces the need for locknuts, although locknuts are sometimes used.

Fig 6.1.1 cylinder construction

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Cylinder MountingCylinders can be mounted in a variety of ways, some of which are shown below. The type of mounting is determined by the manner of fitting the cylinder to fixtures and machines. The cylinder can be designed with a permanent type of mounting, alternatively it can be converted to another type of mounting at a later date by using suitable accessories. Modular construction of cylinders allows the use of basic cylinders types in many different applications.

Fig 6.1.2 cylinder mounting styles Short non-centerline mounted cylinders Relatively short, fixed non-centerline mounted cylinders can subject mounting bolts to large tension forces which, in combination with shear forces, overstress the bolts. In applications involving large forces, cylinders with non-centerline type mountings tend to sway under load. The use of non-centerline type mountings may require strengthening of machine members to resist bending under load

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VBP267 - Set up Fluid Power Controlled Engineering Systems Long Cylinder Mountings Relatively long cylinders with fixed mountings often require additional supports to prevent excessive sag or vibration. The following arrangements can applied: Use of an extra mounting block located midway along cylinder body. A spreader type tie rods are used to increase rigidity in center portion of cylinder. Where one end of a cylinder must be overhung, an additional supporting member can be provided. Cylinders using Dowel Pin Location Cylinders that are pinned in place to help secure alignment and resist shock loads should be pinned at either end, choice of end depends upon direction of major shock load. If dowel pins are used across corners, cylinder may be warped by operating temperatures and pressures or shock loads. Shear Key Mounting Shear keys are often used to absorb shear forces developed at cylinder mounting surfaces. Proper placement of shear keys depends upon direction of major load, (a). Shear keys should never be mounted at both ends of a cylinder, (b). Otherwise, shock-absorbing capabilities of cylinder elasticity can be lost, and changes in cylinder length due to temperature and pressure effects can cause trouble. .Manufacturing tolerances for similar components could also make replacement difficult with such arrangements. Trunnion Mounting Trunnions for pivot mounting of cylinders are generally designed to resist shear loads only. The use of self-aligning bearings that have small bearing areas acting at a distance from the Trunnion and cylinder head introduce forces that can overstress the trunnions.

Cylinder MisalignmentStandard type actuators are not designed to absorb piston rod side loading. Thus actuators must be mounted with care and accuracy, to ensure that the load moves precisely parallel and in alignment with the actuator centreline. In many cases the cylinder must have a clevis or trunnion mount to allow it to swing as the direction of the load changes. Use guides on the load mechanism, if necessary, to assure that no side load is transmitted to the cylinder rod. A self-aligning piston-rod coupling can also be used to compensate for both angular and radial misalignment, but the angular misalignment must not exceed 4C in either direction and the radial alignment must not be more than 1 mm out (fig. 6.1.3).

Fig 6.1.3 self - aligning coupling

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VBP267 - Set up Fluid Power Controlled Engineering Systems Misalignment of fixed-mounted cylinders with work slides can be of two types. The cylinder can tolerate slight misalignment that increases with stroke. It cannot operate properly with constant misalignment,

Sometimes, a relatively long-stroke cylinders can be made somewhat self-aligning by allowing the rod end head to Float. Holes in the side lugs at the rod cylinder head permit some movement of the front of cylinder with respect to dowel pins. Cylinder body flexes slightly about fixed mounting.

Actuator failureRod Bearing Failure Rod bearing failures usually occur when the cylinder is at maximum extension, when side load is not detected early enough or cannot be avoided. Failure occurs most often on hinge or trunnion mount cylinders, in which the rear support point is located considerably behind the bearing.

rod

The following actions should be taken to avoid / minimise rod-bearing failure Where space permits, order cylinders with longer stroke than actually needed. Do not permit the piston to approach close to the front end under load. Use a double - ended piston rod type actuator,

Use an actuator with oversize piston rod, which has less flexing than a standard size piston rod

Rod Buckling Column failure, or the buckling of the rod, may occur if the cylinder stroke too long in relation to the rod length to rod diameter. Tension and Compression Failures Standard cylinders are designed with sufficiently large piston rods. They will never fail either in compression or tension, if the cylinder is operated within the pressure range specified by the manufacturer. 39 is

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6.2 Actuator (Cylinder) SizingThe main criteria on which the size of a linear pneumatic actuator is based are: Force output for extension and retraction; Piston speed for extension and retraction; Impact cushioning at the end of the piston stoke Mechanical stability of the piston rod. Piston Force Calculations The piston force exerted by a working element is dependent on the air pressure, the cylinder diameter, and the frictional resistance of the sealing components. Theoretical Piston Force The theoretical piston force can be calculated using the following formula:

Fth = A x PWhere : Fth A P = = = theoretical piston force useful piston area operating pressure (N) (m2) (Pa)

Effective Piston Force In practice however, the effective piston force is significant. In calculating the effective piston force, the frictional resistance must be taken into account. Under normal operating conditions (pressure range 4 - 8 bar), the frictional forces may be assumed to be between 3-20% of developed force. The effective piston force can be calculated using the following formula: Single acting cylinders :

Fn = A x p -FR -FF Fn = A x p -FR Fn = A' x p -FR(N) (N) (3-20% of Fth) (N) (m2) (m2) (Pa)

Double acting cylinders :

Double acting cylinders (return stroke) : Where : Fn FR FF A A' P = = = = = = effective piston force Frictional force force of return spring useful piston area useful piston ring area operating pressure

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VBP267 - Set up Fluid Power Controlled Engineering Systems Sample Calculation A pneumatic double acting cylinder is used to clamp work pieces in a machine tool. The actuator available has a piston diameter of 50mm. The rod diameter is 12mm.The operating system pressure available is 6 bar. Calculate the clamping force available. Assume a friction loss of 5% (near static thrust) SOLUTION Piston area: A = D2

4

= 0.0502 Fth Fth

3.142 4

= 0.00196345m2

Theoretical piston force :

=p x A = 600, 000 x 0.00196 = 1178.1N = 1.1781 kN

Frictional resistance at 5% is 58.9N Effective piston force : Fn = p x A FR = 600,00Pa x 0.001963m2 58.9N = 1178.1 58.9 = 1119.2N = 1.12 kN Effect of Undersizing an Acutator If an actuator is undersized, it may either not cope with given load force or it may just cope, but the response and acceleration times become extremely long, such that back pressure falls below the level required to maintain sonic speed. Thus the necessary back pressure is no longer there and the speed becomes erratic. In addition, the pneumatic cushioning is totally ineffective since initial back pressure used for cushioning is too low Effective Force TablecylinderDia RodDia piston piston area A(cm2) 21 29 53 82 129 212 333 517 824 1328 2072 3239 5309 8295 12960 Effective Force (N) at p (Bar) 3 17 22 46 69 108 182 280 436 739 1199 1943 3028 4976 7962 12442 28 39 70 109 172 282 444 690 1098 1771 2763 4319 7079 11060 17280 4 21 30 61 92 144 243 373 581 986 1598 2591 4037 6635 10616 16590 5 6 35 45 91 137 216 364 560 871 1478 2397 3886 6056 9953 15924 24885 49 68 123 191 302 493 776 1207 1923 3098 4836 7558 12388 19355 30239 7 41 52 107 160 253 425 653 1016 1725 2797 4534 7066 11612 18579 29032 56 77 141 218 345 563 887 1380 2196 3541 5526 8638 14157 22120 34559 8 46 60 122 183 289 486 746 1162 1971 3196 5181 8075 13270 21233 33180 63 87 158 246 388 634 998 1552 2471 3984 6217 9718 15927 24885 38879 9 52 67 137 206 325 546 840 1307 2218 3596 5829 9084 14929 23887 37327

10 12 16 20 25 32 40 50 63 80 100 125 160 200 250

4 6 6 8 10 12 16 20 20 25 25 32 40 40 50

O.8 0.66 1.1 0.8 2.0 1.73 3.1 2.6 4.9 4.1 8.0 6.9 12.6 10.6 19.6 16.5 31.1 28.0 50.0 45.3 78.5 73.61 122.7 114.6 201.0 188.4 314.1 301.4 490.6 471,0

35 29 42 48 37 58 88 76 106 136 114 164 216 180 259 352 304 422 554 466 665 862 726 1035 1373 1232 1647 2213 1998 2656 3454 3238 4145 5399 5047 6479 8848 8294 10618 13825 13270 16590 21600 20737 25920 FULL PISTON AREA ANNULAR AREA

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Review Questions 6.3 Actuator - Calculations

1.

Calculate the static force output for a clamping actuator with the following measurements:

PISTON DIAMETER = 140mm PRESSURE = 650 kPa

2.

Calculate the force output for a slowly oscillating actuator with a piston diameter of 250mm; a rod diameter of 180mm and a system pressure of 600kpa. Assume a friction loss of 8% and no back pressure. Calculate extension as well as retraction forces.

3.

Calculate the required lifting force and piston diameter for the given application. Assume no back pressure and no losses

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VBP267 - Set up Fluid Power Controlled Engineering Systems 4. A linear actuator required to produce a theoretical total extension force of 7000 Newton with a line pressure of 600 kPa, fails to produce that force, due to a back pressure of 300 kPa resulting from meter-out speed control. The piston rod diameter is 60mm (custom made actuator). It is assumed that a pressure increase will produce the necessary extension force of 7000 Newton (with 700 kPa pressure) if necessary, use a new standard size piston diameter of 160mm. But maintain a piston rod of 60mm. Calculate the following parameters:PISTON AREA ROD AREA NEGATIVE FORCE (AGAINST EXTENSION) ANNULAR AREA AVAILABLE FORCE (EXTENSION) NEW FORCE (INCREASED) NEW PRESSURE (INCREASED) FINAL FORCE (NEW ACTUATOR) AP =D2 x 0.7854 AR =D2 d2 x 0.7854 F=pxA AP A R FEXT FNEG FEXT + FNEG F Ap FEXT FNEG.

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Read and study the following: 7.0 Rotary Actuators (Motors)Additional Reference: Chapter 7 Rotary Actuators Industrial Hydraulic Control Rohner

Motor is the name usually given to a rotary actuator. Rotary actuators (motors) convert hydraulic or pneumatic energy into rotary power. Motors very closely resemble pumps in construction. Instead of pushing on the fluid as the pump does, as output members in the hydraulic system, they are pushed by the fluid and develop torque and continuous rotating motion. Since both the inlet and outlet ports may at times be pressurised, most hydraulic motors are externally drained. The maximum performance of a motor in terms of pressure, flow, torque output, speed, efficiency, expected life and physical configuration is determined by the: Ability of the pressure surfaces to withstand hydraulic force Leakage characteristics Efficiency of the means used to connect the pressure surface to the output shaft. Motor Ratings Motors are rated according to displacement (size), torque capacity, speed, and maximum pressure limitations. Motor Displacement: Displacement is the amount of fluid required to turn the motor output shaft one revolution. Motor displacement (flow output) is expressed in m3 (or cm3or mL) per revolution. Hydraulic motors are built with fixed or variable displacement. Fixed displacement motors provide constant torque and variable speed. Speed varies with the amount of input flow into the motor. Variable displacement motors provide variable torque and variable speed. With constant input flow and constant operating pressure, the ratio between torque and speed can be infinitely varied to meet load requirements. Torque Torque may be defined as a twisting or turning moment and is expressed in Newton metres (Nm). Motor torque figures are usually given for a specific pressure differential, or pressure drop across the motor. Theoretical figures indicate torque available at the motor shaft Torque is a function of system pressure and leverage, whereby the leverage is measured from the centre of the drive shaft to the centre of the pressured exposed area.

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VBP267 - Set up Fluid Power Controlled Engineering Systems The figure shown below illustrates the typical torque requirements for raising a load with a pulley. Note: Torque is always present at the drive shaft, but is equal to the load multiplied by the radius. A given load will impose less torque on the shaft if the radius is decreased. However, the larger radius will move the load faster for a given shaft speed.

Breakaway torque is the torque required to get a non-moving load turning. More torque is required to start a load moving than to keep it moving. Running torque can refer to a motors load or to the motor. When used in reference to a load, it indicates the torque required to keep the load turning. When it refers to the motor, running torque indicates the actual torque, which a motor can develop to keep a load turning. Mechanical efficiency is the ratio of actual torque delivered to theoretical torque. Motor Speed Motor speed is a function of motor displacement and the volume of fluid delivered to the motor. Speed of a motor is expressed in revolutions per minute (RPM). Maximum motor speed is the speed at a specific inlet pressure, which the motor can sustain for a limited time without damage. Minimum motor speed is the slowest, continuous, smooth rotational speed of the motor output shaft. Slippage is the leakage across the motor, or the fluid moves through the motor without doing any work. ChangeINCREASE PRESSURE SETTING DECREASE PRESSURE SETTING INCREASE L/MIN DECREASE L/MIN INCREASE DISPLACEMENT (SIZE) DECREASE DISPLACEMENT (SIZE)

SpeedNO EFFECT NO EFFECT INCREASES DECREASES DECREASES INCREASES

Effect on operating pressureNO EFFECT NO EFFECT NO EFFECT NO EFFECT DECREASES INCREASES

Torque availableINCREASES DECREASES NO EFFECT NO EFFECT INCREASES DECREASES

Above table assumes a constant load

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7.1 Motor SizingSome factors for sizing of a rotary actuator (motor) are: Maximum torque required (Nm) Maximum r.p.m. output (r.p.m.) Maximum operating pressure (Pa) Displacement volume per revolution (m3/rev) Most efficient r.pm.

A motor operating below its maximum rated capacity will provide a service life gain more than proportional to the loss in operating capacity. Motor performance is separated into volumetric efficiency, mechanical efficiency, and overall efficiency. The efficiency is expressed as a percentage. Formulae for Motor Application. Listed below are the formulae used for applying hydraulic motors and determining flow and pressure requirements. Note: All of the following formulae are for theoretical torque. To find the torque required for a job use the following formula: torque load = Force(N) radial distance(m) To find working pressure for a given size motor and load: Operating Pressure (p) = torque load (M) Motor Torque Rate

To find torque when pressure and displacement are know Torque (M) = Pressure (Pa) Displacement (V) 2 (6.28)

To find L/min requirements for a given drive speed Flowrate (m3 /min) (Q) = Displacement (V) Revolutions ( rpm ) Efficiency( v ) 100

To find drive speed when displacement and GPM are known RPM = flow rate (Q) efficiency( v ) Displacement (V) 100

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Review Questions: 7.1 Rotary Actuator Calculations1. GIVEN V = 90cm3 Q = 140L/min P = 20Mpa FIND v= n= % RPM

2.

GIVEN M= 100Nm V = 0.032L n = 1500RPM

FIND p= MPA

3.

GIVEN n = 1500RPM Q = 80.5L/min v= 95% p = 20Mpa

FIND M= A= P= Nm m2 Kw

4. Calculate the required motor torque to drive a cable drum with the following specifications: Load mass = 2000kg Cable drum Dia. =1.4m

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VBP267 - Set up Fluid Power Controlled Engineering Systems 5. The cable drum crane described in the previous question is driven by a hydraulic motor with a direct drive (no gearbox). Calculate the required RPM of the motor and the flow requirements in L/min. the load is to be lifted a distance of 82 metres in 22 seconds. The motor has a displacement volume V of 0.60 Litres and operates at a pressure of 12Mpa. Efficiencies are: volumetric v= 95%

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Read and study the following: 7.2 Types of Rotary Actuators (Motors)There are a variety of motors used in industrial applications. The type of motor that is used depends on the demands of each individual application.

Vane motors In vane motors, torque is delivered by pressurised fluid acting onto rectangular vanes, which slide in and out of their slots in a rotor splined to a drive shaft. As the motor turns, the vanes follow the contour of the cam ring, thus forming sealed cavities which transport the hydraulic fluid from the pressurised inlet side to the non-pressurised outlet side. Since the motor vanes must maintain cam ring contact at all times, and centrifugal force is absent during motor start, these vanes are usually fitted with springs to enable rotation to commence. Other designs use coil springs, or feed pressure to the under side of the vanes to force them firmly against the cam. Gear motors There are two basic types of gear motor available External and Internal, application is usually hydraulic. External gear motors Some gear pumps can also be used as motors. The hydraulic fluid enters the gear chamber on the side where gears mesh, and forces the gears to rotate. The fluid exits at low pressure on the opposite side to the inlet. Non-reversible motors have the case drain connected internally to the low-pressure side of the motor. Reversible gear motors must have an external case drain, which must be connected, to the reservoir. Internal Gear Motors Orbit motors, direct drive gerotor motors, and crescent gear motors work in much the same way as internal gear pumps. The orbit motor and direct drive-drive gerotor motors are designed and built for high torque and lows peed applications. Most internal gear motors are flow reversible and thus require a case drain. All internal gear motors are fixed displacement machines, which provide fixed torque but variable speed can be achieved provided input flow is variable. 49 the

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VBP267 - Set up Fluid Power Controlled Engineering Systems Piston motors This type of motor is very compact, provides extremely high torque and high acceleration. They also have an excellent life expectancy. Axial piston motors are capable of operating at speeds from 0.5 rpm to as high as 6000rpm with stable torque output for fixed or variable displacement. Radial piston motors are up to 95% overall efficient, can operate at power outputs of up to 3000Kw, and reach peak speeds of 14,000 rpm. Some larger motors may require a motor input flow rate of 1600L/min.

HSLT Motors In many applications, the motor operates continuously at relatively high RPM. Examples are fan drives, generator drives, and compressor drives. While the speed is high and reasonably constant, the load may be either steady as in fan drives, or quite variable, as in compressors or generators. HSLT motors are excellent for these kinds of applications. The four primary Types of HSLT motors are; LSHT Motors In some applications, the motor must move relatively heavy loads at lower speeds and fairly constant torque. A motor for a crane is one such application. Some LSHT motors operate smoothly down to one or two rpm. LSHT motors are simple in design with a minimum of working parts and are quite reliable and generally less expensive than higher speed motors employing speed reducing devices. Ideally, an LSHT motor should have high starting and stall torque efficiencies. They should start smoothly under full load and provide full torque over their entire speed range. Basic LSHT motors include Internal gear Vane Rolling Vane radial piston axial piston axial ball piston In-line piston Bent axis piston vane gear

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VBP267 - Set up Fluid Power Controlled Engineering Systems Free wheeling control In some applications of hydraulic motors. It is necessary for the motor to freewheel during part of the operating cycle. In some, freewheeling is achieved by means of suitable external valving. To permit flow circulation from port to port. In piston motors freewheeling is sometimes achieved by connecting both inlet and outlet ports to tank, simultaneously ensuring a positive pressure of approx. 50 100kPa. This pressure moves the pistons clear of the cam ring. In this way, freewheeling for speeds of up to 700rpm may be achieved Deceleration control (braking) Regulating the pressure on the motor outlet port controls the motor deceleration. In some applications inlet pressure is maintained while outlet pressure is gradually increased until the motor stops. This method gives accurate deceleration control. In other applications the directional control valve is shifted to neutral, while at the same time a brake valve imposes a restriction on the motor outlet flow. The pressure setting on the brake valve controls the rate of deceleration.

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Read and study the following: 7.4 Actuator ApplicationsCylinders Moving Horizontal Loads Rolling Loads A cylinder thrust of l/10 the load weight will move loads which operate on low-friction needle, roller, or ball bearings. An air cylinder with meter-out flow controls can be used on some applications, even at slow feed rates. Attention should be given to deceleration at the end of cylinder stroke to prevent momentum of the load from damagthe cylinder and/or the machine Sliding Loads Either air or hydraulic cylinders can be used for moving high friction sliding loads. On applications that require rapid indexing from one positive stop to another, an air cylinder will give more rapid action than hydraulics if the load is within its capacity. Air cylinders should not be used for slow speed or controlled feeding of a sliding load with a large area of surface friction, as a chattering motion will result. Hydraulic cylinders with a meter-out flow control should be used in these applications. In some instances an air/oil system will also give acceptable performance. The force needed to push a sliding load varies with surface material, lubrication, unit loading and other factors. For lightly lubricated machined slides, the cylinder thrust should be equal to 1/2 to 3/4 of the load weight to get the load started. A thrust of 1/5 to 1/6 load weight will keep it moving. To operate an air cylinder at high speeds, the cylinder should be sized to develop twice the thrust needed to balance the load.

ing

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VBP267 - Set up Fluid Power Controlled Engineering Systems Cylinders for Lifting Differential Lift Where overhead clearance is not sufficient for direct lift, a differential lift is sometimes an ideal solution. A shorter length, larger diameter cylinder is usually best in this application. The arrangement illustrated will give a 2:1 mechanical reduction. Size the cylinder with twice the piston area and half the stroke needfor a straight lift of the same load. Possible side thrust from the cylinder is prevented by running the pulley attached to the cylinder rod in horizontal guides. In addition to needing less head room, this arrangement allows the cylinder to work on full piston area and the rod packings are not subjected to high pressure. Vertical Lifting Air cylinders used to lift a load must always be sized to exert a force greater than the weight of the load. An air cylinder, which exerts a 2.5kN force can support a 225Kg load, but cannot move it. For normal applications, an air cylinder should develop 25% more thrust than needed to support the load. Twice the force needed to support the load is required for fast operation. A hydraulic or ail/oil system must be used if the cylinder is to be stopped at an intermediate point a

ed

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Read and study the following:8.2

Directional Control ElementsChapter 2 Directional Control Valves Pneumatic Control For Industrial Automation Rohner

Reference:

Fluid power working element (actuator) needs a positive method to allow it to move in and out. The movement of

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