Lecture 32 - WordPress.com · List the most common causes of hydraulic system breakdown. Explain the importance of cleanliness of hydraulic systems. Explain the problems caused by

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Lecture 32

MAINTENANCE OF FLUID POWER SYSTEMS Learning Objectives

Upon completion of this chapter, the student should be able to:

List the most common causes of hydraulic system breakdown. Explain the importance of cleanliness of hydraulic systems. Explain the problems caused by air in hydraulic systems. List the causes and remedy for excessive noise, incorrect flow, pressure and faulty operations. Describe the various general safety rules for electricity and electronics. List possible faults in solenoid valves. Explain the method for maintaining and disposing of fluids.

1.1 Introduction

The working medium in hydraulic systems is a fluid. Till the early 20th century, water was used as a

fluid. Water as a working fluid had many drawbacks, such as low freezing point, corrosive (rust

formation) nature and poor lubrication characteristics. Gradually, various oil-based fluids that had the

desirable properties were developed for use in hydraulic systems.

In a hydraulic system, a hydraulic fluid has to perform various functions, such as follows:

To transmit power, which is the primary function?

To lubricate various moving parts, so as to avoid metal-to-metal contact, and reduce wear and

noise.

To carry the heat generated in the system due to friction between moving parts and moving fluid, and to dissipate to the environment either through a suitable heat exchanger or through the reservoir.

To perform these functions and make the system work efficiently, a hydraulic fluid must be clean and

should possess certain properties.A hydraulic system is fairly easy to maintain: the fluid provides a

lubricant and protects against overload. But like any other mechanism, it must be operated properly. You

can damage a hydraulic system by too much speed, too much heat, too much pressure or too much

contamination.

The following is a list of most common causes of hydraulic system breakdown:

Clogged or dirty oil filters.

Inadequate supply of oil in the reservoir.

Leaking seals.

Loose inlet lines that cause the pump to take in air.

Incorrect type of oil.

Excessive oil temperature.

Excessive oil pressure.

Most of these and similar kinds of problems can be eliminated if a plant-preventive maintenance

program is undertaken. This starts with the fluid power designer in the selection of high-quality,

properly sized components. It is important for the total system to provide easy access to components

requiring periodic inspection such as filters, strainers, sight gauges, drain and fill plugs, flow meters, and

pressure and temperature gauges.

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Over half of all hydraulic system problems have been traced directly to the oil. The test kit may be used

on the spot to determine whether fluid quality permits continued use. Test that can be performed include

the determination of viscosity, water content and particulate contamination level. Viscosity is measured

using a visage viscosity comparator. Water content is determined by the hot-plate method. Contamination

is eliminated by filtering a measured amount of hydraulic fluid, examining the particles caught on

the filters under microscope and comparing what is seen with the series of photos indicating

contamination levels.

For preventive maintenance techniques to be truly effective, it is necessary to have a good report and

record system. This report should include the following:

The types of symptoms encountered, how they were detected and the date.

The description of the maintenance performed. This should include the

replacement of parts, the amount of downtime and the date.

Records of dates when oil was tested added or changed. Dates of filter changes

should also be recorded.

Proper maintenance reduces hydraulic troubles. By caring for the system using a regular maintenance program, we can eliminate common problems and anticipate special ones. 1.2 The Importance of Cleanliness

Cleanliness is the first requirement when it comes to servicing hydraulic systems. Keep dirt and other contaminants out of the system. Small particles can score valves, seize pumps, clog orifices and cause expensive repair jobs. How to keep the hydraulic system clean? Let us put it this way:

Keep the oil clean.

Keep the system clean.

Keep your work area clean.

Be careful when you change or add oil.

1.3 Importance of Oil and Filter Changes

We cannot get peak performance out of a hydraulic system that is not clean. Despite all the precautions taken when working with the hydraulic system, some contaminants get into the system anyway. Good hydraulic oils hold these contaminants in suspension and filters collect them as the oil passes through. Good hydraulic oil contains many additives that work to keep contaminants from damaging or plugging the system. However, these additives lose their effectiveness after a period of time. Therefore, oil should be changed at the recommended intervals to make sure that the additives do their job. The system filters can absorb only a limited amount of dirt particles and other contaminants from the oil. After that the filters stop working. At this point, the filters should be cleaned or replaced with new ones so that the cleaning process can be maintained.

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1.3.1 Draining the System

Periodic draining of the entire hydraulic system is very important. This is the only positive way to

completely remove contaminants, oxidized fluid and other substances from the system. The machine

operator’s manual tells the method to be used, and the frequency, depending on conditions.

1.3.2 Cleaning and Flushing the System

In some hydraulic systems, there might be deposits left in the system. It is advisable to clean and flush the system after draining the oil out. After draining the system, clean any sediment from the reservoir and clean or replace the filter elements. Flush out the old oil remaining in the system after

draining, particularly if the oil is badly contaminated. Drain out the flushing oil and refill the system

with clean hydraulic oil of the recommended type. Be sure to clean or replace the system filters

before refilling the system.

1.3.3 Filling the System

Before filling the system, be sure the area around the filler cap is clean. Fill the reservoir to the

specified level with the recommended hydraulic oil. Use only clean oil and funnels or containers, and

then be sure to replace the filler cap before operating the equipment.

1.3.4 Preventing Leaks

There are internal and external leakages. Internal leakage does not result in actual loss of oil but it

does reduce the efficiency of the system. External leakage does result in direct loss of oil and can

have other undesirable effects as well. A hydraulic system should always be monitored for leaks and

remedial actions should be taken immediately.

1.3.5 Preventing Overheating

Heat causes hydraulic oil to breakdown faster and lose its effectiveness. This is why cooling of the oil

is needed. In many systems, enough heat is dissipated through the lines, the components and the

reservoir to keep the oil fairly cool. But on high-pressure, high-speed circuits, oil coolers are needed to

dissipate the extra heat.

To help prevent overheating, keep the oil at the proper level; clean dirt and mud from lines, reservoirs

and coolers; check for dented and kinks lines; and keep relief valves adjusted properly. Also be

careful to not over speed or overload the system and never hold the control valve in power position

longer than necessary.

1.4 Problems Caused By Gases in Hydraulic Fluids

Gases can be present in a hydraulic fluid (or any other fluid) in three ways: free air, entrained gas and

dissolved air.

1.4.1 Free Air

Air can exist in a free pocket located at some high point of a hydraulic system (such as the highest elevation of a given pipeline). This free air either existed in the system when it was initially filled or

was formed due to air bubbles in the hydraulic fluid rising into the free pocket. Free air can cause the

hydraulic fluid to possess a much lower stiffness (bulk modulus), resulting in a spongy and unstable

operation of hydraulic actuators.

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1.4.2 Entrained Gas

Entrained gas (gas bubbles within the hydraulic fluid) is created in two ways. Air bubbles can be created when the flowing hydraulic fluid sweeps air out of a free pocket and carries it along the fluid

stream. Entrained gas can also be created when the pressure drops below the vapor pressure of the

hydraulic fluid. When this happens, the bubbles of hydraulic fluid are created within the fluid stream.

Entrained gas can cause cavitation problems in pumps and valves. These gases can greatly reduce the

effective bulk modulus of hydraulic fluids, resulting in spongy and unstable operation of hydraulic

actuators.

Vapor pressure is defined as the pressure at which a liquid starts to boil and thus begins changing into a

vapor. The vapor pressure of a hydraulic fluid (or any other liquid) increases with an increase in

temperature. Petroleum-based and fire-resistant phosphate ester fluids have very low vapor pressures.

Cavitation occurs because the vapor bubbles collapse as they are exposed to the high pressure at the

outlet port of the pump, creating extremely high local fluid velocities. This high -velocity fluid

impacts internal metal surfaces of the pump. The resulting high impact forces cause flaking or pit ting the

surfaces of internal components such as gears, vanes, etc. Cavitation also interferes with the

lubrication of mating moving surfaces and thus produces increased wear.

One indication of cavitation is a loud noise emanating from the pump. The rapid collapsing of gas bubbles

produces vibrations of pump components, which are transmitted into pump noise. Cavitation also causes a

decrease in the pump flow rate because the pumping chambers do not completely fill with the hydraulic

fluid. As a result, the system pressure becomes erratic.

1.4.3 Dissolved Air

Dissolved air is in the solution and thus cannot be seen and does not add to the volume of the

hydraulic fluid. Hydraulic fluids can hold an amazingly large amount of air in the solution. A

hydraulic fluid, as received at atmospheric pressure, typically contains about 6% of dissolved air by

volume that increases to 10% when pumped.

Dissolved air creates no problem in hydraulic systems as long as the air remains dissolved. However,

if the dissolved air comes out of the solution, it forms bubbles in the hydraulic fluid and thus becomes

entrained air.

The following will help control or eliminate pump cavitation by keeping the suction pressure above the

vapor pressure of the fluid:

Keep suction velocities below 1.5 m/s.

Keep pump inlet lines as short as possible.

Minimize the number of fittings in the pump inlet line.

Mount the pump as close as possible to the reservoir.

Use low-pressure drop-pump inlet filters or strainers.

Use a properly designed reservoir that can remove entrained air from the fluid

before it enters the pump inlet line.

Use proper oil as recommended by the manufacturer.

Keep the oil from exceeding the recommended maximum temperature level.

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1.5 Troubleshooting Guides

The following troubleshooting guides are arranged in five main categories. The heading of each is a

symptom that indicates some malfunction in the system. The causes and remedies are given in Tables

1.1–1.5.

1.5.1 Fluid Maintenance Fluid maintenance can be done in the following ways:

Before opening a drum, clean the top and the bung

Use only clean containers and hoses to transfer the hydraulic fluid.

Provide a 200-mesh screen in the reservoir filter pipe.

Store drums indoor or under a roof.

1.5.2 In-Operation Care of Hydraulic Fluid This can be done in the following ways:

Keep the system tight and repair all leaks immediately.

Use proper air and fluid filtration.

Establish fluid change intervals.

Keep the reservoir filled properly to take the advantage of its heat-dissipating characteristics

and prevent moisture from condensing on inside walls.

Table 1.1 Excessive noise

Symptom Cause Remedy Pump noisy Cavitation Any or all of the following:

Replace dirty filters. Wash strainers. Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Change to proper pump drive motor speed. Overhaul or replace the pump. Check fluid temperature.

Air in fluid Any or all of the following: Tighten leaky inlet connections. Fill the reservoir to proper level. Bleed air from the system. Replace the pump shaft seal.

Coupling misaligned

All of the following:

Align unit.

Check the condition of seals, bearings and couplings. Pump worn or damaged

Overhaul or replace defective parts

Motor noisy Coupling misaligned

All of the following:

Align unit.

Check the condition of seals, bearings and couplings.

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Motor or coupling worn or damaged


Relief valve noisy

Setting too low or too close to another valve

setting

Install and adjust pressure gauge

Table 1.2 Excessive heat

Symptom Cause Remedy Pump heated Fluid heated See symptom “fluid heated”

Cavitation Any or all of the following:

Replace dirty filters. Wash strainers. Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Change to proper pump drive motor speed. Overhaul or replace the pump. Check fluid temperature.

Air in fluid Any or all of the following: Tighten leaky inlet connections. Fill the reservoir to proper level. Bleed air from the system. Replace the pump shaft seal.

Excessive load All of the following:

Align unit.

Check the condition of seals, bearings and

couplings. Locate and correct mechanical binding. Check for workload in excess of circuit

design.

Pump worn or damaged


Relief or unloading valve set too high


Motor heated Fluid heated See symptom “fluid heated” Relief or unloading valve set

too high


Excessive loading All of the following:

Align unit.

Locate and correct mechanical binding. seals, bearings and couplings.

Check for workload in excess of circuit design.

Motor or coupling worn or damaged


Relief valve heated

Fluid heated See symptom “fluid heated” Valve setting incorrect


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Worn or damaged valve


Fluid heated System pressure too high

Install and adjust pressure gauge Unloading valve set too high

Install and adjust pressure gauge Fluid dirty or low supply

Change filters.

Check system fluid viscosity, change if necessary.

Fill the reservoir to proper level.

Incorrect fluid viscosity

Change filters.

Check system fluid viscosity, change if necessary.


Faulty fluid cooling system

Clean the cooler and/or strainer.

Replace the cooler control valve. Repair or replace the cooler.

Worn pump, valve, motor, cylinder or other

component


Table 1.3 Incorrect flow

Symptom Cause Remedy No flow Pump not receiving fluid Any or all of the following:

Replace dirty filters.

Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Overhaul or replace the pump.

Pump drive motor not operating Overhaul

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Pump to drive coupling sheared Check for the damaged pump Replace and align coupling.

Pump drive motor turning in the wrong direction

Reverse rotation

Directional control set in the wrong direction

Any or all of the following: Check the position of manually

operated controls. Check the electrical circuit on

solenoid- operated controls. Repair or replace pilot pressure

pump.

Entire flow passing over the relief valve

Adjust part Damaged pump Check for the damaged pump

Replace and align coupling.

Incorrectly assembled pump Overhaul or replace part Excessive flow Flow control set too high Adjust part

Yoke actuating device inoperative (variable displacement pumps)

Overhaul or replace part

Rotation per minute (RPM) of pump drive incorrect

Replace with correct unit

Improper size pump used for replacement

Replace with correct unit

Low flow Flow control set too low Adjust part Relief or unloading valve set too low Adjust part Flow bypassing through the partially open valve

Overhaul or replace part or any or all of the following:

Check the position of manually operated controls

Check the electrical circuit on solenoid- operated controls.

Repair or replace the pilot pressure pump.

External leak in the system Bleed air from the system.

Yoke actuating device inoperative (variable displacement pump)


RPM of pump drive motor incorrect Replace with correct unit

Worn pump, valve motor, cylinder or other


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Table 1.4 Incorrect pressure

Symptom Cause Remedy No pressure No flop See “incorrect flow”, symptom

“no flow” Low pressure Pressure relief path exists See “incorrect flow,” symptom

“no flow” and “low flow” Pressure-reducing valve set too low Adjust part Pressure-reducing valve damaged Overhaul or replace part Damaged pump, motor or cylinder Overhaul or replace part

Erratic pressure Air in fluid Tighten leaky connections.


Bleed air from the system.

Worn relief valve Overhaul or replace part Contamination in fluid Replace dirty filters and system

fluid Accumulator defective or had lost charge Check the gas valve for

leakage. Change to correct

pressure.

Overhaul if defective.

Worn pump, motor or cylinder Overhaul or replace part Excessive pressure

Pressure-reducing, relief or unloading valve misadjusted

Adjust part

Yoke actuating device inoperative (variable displacement pumps)


Pressure-reducing, relief or unloading valve worn or damaged


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or replace part

Table 1.5 Faulty operation

Symptom Cause Remedy No movement No flow or pressure See “incorrect flow”

Limit or sequence device

inoperative or misadjusted Overhaul or replace part

Mechanical bind Locate bind and repair No command signal to the servo amplifier

Repair command console or interconnecting wires

Inoperative or misadjusted servo amplifier

Adjust, repair or replace part

Inoperative servo valve Overhaul or replace part Worn or damaged cylinder or

motor Overhaul or replace part

Slow movement Low flow See “incorrect flow” Fluid viscosity to high Check fluid

temperature. Check system fluid

viscosity, change if

necessary.

Insufficient control pressure for

valves See “incorrect pressure”

No lubrication of machine ways

or linkage Lubricate

Misadjusted or malfunctioning

servo amplifier Adjust, repair or replace part

Sticking servo valve Clean and adjust or replace part.

Check the condition of system fluid and filters.

Worn or damaged cylinder or


Erratic movement Erratic pressure See “incorrect pressure” Air in fluid See “excessive noise” No lubrication of machine ways

or linkage Lubricate

Erratic command signal Repair command console or

interconnecting wires Misadjustment of

malfunctioning servo amplifier Adjust, repair or replace part

Malfunctioning feedback

transducer Overhaul or replace part

Sticking servo valve Clean and adjust or replace part.

Check the condition of system fluid and filters.

Worn or damaged cylinder or


Excessive speed or movement Excessive flow See “incorrect flow” Feedback transducer Overhaul

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1.6 General Safety Rules for Electricity and Electronics

Following are the general safety rules for electricity and electronics:

Use approved tools, equipment and protective devices.

Avoid wearing rings, bracelets and similar meal items when working around

exposed electric circuits.

Never assume that a circuit is OFF. Double check it with an instrument that is

supposed to be surely operational.

Some situations require a “buddy system” to guarantee that power would not be turned ON

Never tamper with or try to override safety devices such as interlock (a type of

switch that automatically removes power when a door is opened or a panel

removes).

Keep tools and test equipment clean and in good working condition. Replace

insulated probes and leads at the first sign of deterioration.

Some devices, such as capacitors, can store a lethal charge. They may store this

charge for long periods of time. It must be certain that these devices are

discharged before working around them.

Do not remove grounds and do not use adapters that defeat the equipment ground.

Use protective clothing and safety glasses when handling high vacuum

devices such as picture tubes and cathode ray tubes.

Do not work on equipment before knowing proper procedures and having

awareness of any potential safety hazards.

1.6.1 Solenoid Valves Pneumatics will continue to dominate the power section and retain its positions in the control section. On

the other hand, it is impossible to deny the advances made by electronic controls, systems and

components due to the following advantages:

Low power consumption.

Short switching times.

Higher contact ratings.

Long service life.

Maintenance-free operation.

1.6.1.1 Possible Faults in Solenoid Valves Following are the possible faults:

Directed short (at power supply, electrical bus and load): A direct short is when too much

current is sent back to the power supply overloading it, generally blowing a fuse.

Cross short: A cross short is created by one or more wires (cables) bypassing the load causing a

direct short to occur.

High resistance connections (too many connections at the terminal eye).

Low voltage or over voltage at the solenoid.

Corrosion.

Partially or fully blocked hoses.

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Wire connections are open internally

Lack of source pressure (at the compressor or on the service unit).

Sticking spool.

Diaphragm not working.

Exhaust ports blocked.

Gaskets mounted incorrectly.

Faults caused by wear or external influences.

Caution: Short circuiting of the power supply is not recommended without the installation of a

“circuit breaker” to protect the equipment and the user.

Causes and remedy for troubleshooting for direct shorts and faults in relay coil is given in Tables 1.6

and 1.7.

Table 1.6 Troubleshooting for direct shorts

Fault Cause Remedy When the push button is pressed, solenoid Y1 is not

activated and the circuit breaker must be reset.

Relay coil K1 has been shorted

directly to the ground bypassing the load (relay coil K1)

Pull the ground cable connected to solenoid Y1, retry the circuit.

If the power supply continues to short, the location of the direct short is at relay coil K1. If not,

the direct short is at solenoid

Y1.

Solenoid Y1 has been shorted directly to ground bypassing the

load (solenoid coil). This causes the relay coil to buzz (noise like

a bee) and the circuit breaker

must be reset. When proximity limit switch a1

and pressure switch a2 are

activated, solenoid Y3 is not

activated and the circuit breaker must be reset.

Relay coil K2 has been shorted

directly to ground, bypassing the load (relay load).

Pull the ground cable connected to solenoid Y3. Retry the

circuit. If power supply continues to short, the location

of the direct short is at relay coil K2. If not, the direct short is at

solenoid Y3.

Solenoid Y3 has been shorted

directly to ground, bypassing

the load (solenoid). The circuit

breaker must be reset.

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Table 1.7 Faults in relay coils

Fault Cause Remedy When powered, the relay coil does not function.

One or more of the wires (leads) have an infinite resistance.

Use the voltmeter to check the

potential difference across each

cable and the push button when switch is open, then closed. If

the problem is with any of these

components

Open (infinite resistance) in the coil. Low voltage: Below specification.

Relay coil is not energized by

electrical limit switch (or

proximity limit switch).

The electrical limit switch is not being activated.

Visually examine the electrical limit switch to make sure that

the roller is fully activated. If

not, reposition the sensor so that

physical contact is achieved (or

see LED).

Use the voltmeter to test the

difference in potential across

the limit switch (24 V when open, 0 V when closed); replace

if not functioning or remove the

limit switch from the circuit and

use the ohmmeter to measure

the resistance of the limit switch

(infinite pressure when open,

approximately 0 when closed). One of the cables (wires) is

either not connected or there is

infinite resistance in the cable

(wire).

Use the multimeter to check the difference in potential across

each cable (wire) in the

corresponding ring. Then

remove the suspect cable(s) and

use the ohmmeter to confirm

your findings, replace the cables

if required. The relay coil itself is

malfunctioning, that is, low

voltage, an open (infinite

resistance) circuit, loose

connection, high-resistance

connection.

If the voltmeter identifies low

voltage, then check the power

supply and original source.

Replace or modify the source as

required. If an “open” infinite resistance,

loose or high connection is

suspected, use the ohmmeter to

determine the exact location of

the problem and repair/replace

as required.

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1.7 Maintaining and Disposing of Fluids

Controlling pollution and conserving natural resources are important goals to achieve for the benefit of

society. Thus, it is important to minimize the generation of waste hydraulic fluids and to dispose them

in an environmentally friendly manner. The following are some recommendations that should be

adhered to strictly for properly maintaining and disposing hydraulic fluids:

Select the optimum fluid for the application involved. This includes the consideration of the

system operating pressures and temperatures as well as the desired fluid properties.

Utilize a well-designed filtration system to reduce contamination and increase the useful life

of the fluid. Filtration should be continued and filters should be changed at regular intervals.

3. Follow a proper storage procedure for the unused fluid supply. For example, outdoor

storage is not recommended, especially if the fluid is stored in drums as it is affected by increment weather and resulting weakening of drum seams may occur and cause leakage and contamination.

Fluids from the storage containers to the hydraulic systems should be transported carefully as

the chances of contamination depend to a large extent on handling. The transfer containers

should be covered when not in use.

Operating fluids should be checked regularly for viscosity, acidity, bulk modulus, specific

gravity, water content, additive levels and particle contamination.

The entire hydraulic system, including pumps, piping, filters, actuators and reservoir, should

be maintained according to the manufacturer’s specifications.

Corrective action should be taken to reduce or eliminate leakage from operating hydraulic

systems. Typically leakage occurs due to worn seals or loose fittings. A preventive maintenance program should be implemented to ensure ideal operating conditions and reduce

contamination, leakage, etc.

Fluids must be disposed properly because a hydraulic fluid is considered to be a waste material when it has deteriorated to the point where it is no longer suitable for use in

hydraulic systems. The various environmental government agencies also suggest against mixing hazardous wastes with waste hydraulic fluids being disposed. It is also not allowed to

burn these waste fluids in non-industrial boilers.

Pollution control and conservation of natural resources are critical environmental issues for

society. Properly maintaining and disposing of fluids not only protects the environment but

also conserves our natural resources.

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Objective-Type Questions

Fill in the Blanks

1. The primary function of a hydraulic fluid is to transmit .

2. Over half of all hydraulic system problems have been traced directly to the .

3. Entrained gas can also occur when the pressure drops below the of the hydraulic fluid.

4. Cavitation occurs because the vapor bubbles collapse as they are exposed to the

pressure at the outlet port of the pump, creating extremely high local fluid velocities. 5. Oxidation is caused by the chemical reaction of oxygen from the air with particles of .

State True or False

1. Dissolved air creates no problem in hydraulic systems as long as the air remains dissolved.

2. Most of fire-resistant fluids are compatible with most natural or synthetic rubber seals. 3. The neutralization number is a measure of the relative acidity.

4. A 10 µm filter is one capable of removing contaminants as small as 10 µm in size. 5. Free air can cause the hydraulic fluid to possess a much lower stiffness.

Review Questions

1. Name five reasons for the overheating of the fluid in a hydraulic system.

2. Name four causes of low or erratic pressure. 3. What three devices are commonly used in the troubleshooting of hydraulic circuits?

4. Name five of the most common causes of hydraulic system breakdown.

5.List eight recommendations that should be followed for properly maintaining

and disposing hydraulic fluid.

6.Name two items that should be included in reports dealing with a maintenance procedure.

7. Name the three ways in which a hydraulic fluid becomes contaminated.

8. Name five things that can cause a noisy pump.

9.Name four causes of low or erratic pressure. 10.Name four causes of no pressure.

11.If an actuator fails to move, name five possible causes.

12.If an actuator has slow or erratic motion, name five possible causes. 13. Why is loss of pressure in a hydraulic system not a symptom of pump malfunction?

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Answers

Fill in the Blanks

1. Power

2. Oil 3. Vapor pressure

4. High 5. Oil

State True or False

1. True

2. False

3. True 4. True

5. True

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Lecture 32 - WordPress.com · List the most common causes of hydraulic system breakdown. Explain the importance of cleanliness of hydraulic systems. Explain the problems caused by