PUMPS

PUMP Pump is the heart of the system. A hydraulic pump takes oil from the tank and delivers it to

the rest of the hydraulic circuit. In doing so it raises the oil pressure to the required level.

Fig. a) Operation of Pump Fig. b) Pump symbol, arrow shows the direction of flow

CAVITATION When the suction pressure drops below the vapour

pressure, the liquid oil starts changing to vapour state. They began to appear as bubbles. When these bubble collapse, vacuum is created. Due to these liquid from the surrounding region rushes to

the spot and create a momentarily high pressure of very high order (about 105 bar).

This results in damage of the metallic parts of pump. Cavitation occurs on the control edges of the hydraulic

device. High increase in temperature occurs.

BASIC PUMPThe basic operating principle that moves fluid through a

pump is similar in all pumps,

Enlarging the volume of a chamber allows fluid to

enter the pump.

Reducing the chamber volume moves fluid to the

system

Inlet and discharge valves or ports control fluid

movement through the pump.

BASIC PUMPIncreasing the size of the pumping chamber moves fluid into the pump.

BASIC PUMPReducing the size of the pumping chamber forces the fluid into the system.

CLASSIFICATIONPUMPS

POSITIVE DISPLACEMENT

NON-POSITIVE DISPLACEMENT

CENTRIFUGAL PUMPS

AXIAL PUMPS

NON-POSITIVE DISPLACEMENT PUMP Non positive displacement pumps – high clearance between

rotating and stationary parts. Characterized by a high slip that increases as the back pressure

increases, but if the opposing pressure is high then the kinetic energy of the liquid becomes insufficient to overcome the pressure and the liquid cannot move. Due to more clearance gap between impeller and casing, the liquid keeps on rotating inside casing.

They do not develop a high pressure but move a large volume of fluid at low pressure.

The displacement between the inlet and outlet is not positive, therefore the volume of the fluid delivered by the pump depends on the speed at which pump is operated and the resistance at discharge side.

In short, the discharge does not remain constant.

NON-POSITIVE DISPLACEMENT PUMP

ADVANTAGES & DISADVANTAGES

ADVANTAGEa. Non-displacement

pumps have a fewer moving parts

b. Initial and maintenance cost is low

c. They give smooth continuous flow

d. They are suitable for handling all types of flow especially slurries

e. Their operation is simple and reliable.

DISADVANTAGEa. Non-displacement

pumps are not self priming and hence they must be positioned below the fluid level

b. Discharge is a function of output resistance

c. Low volumetric efficiency

POSITIVE DISPLACEMENT PUMP Positive displacement pumps – have a very little slips, are

self priming and pump against a very high pressure but their volumetric capacity is low.

These types of pump eject a fixed amount of fluid into the hydraulic system per revolution of the pump shaft.

Such pumps are capable of overcoming the pressure resulting from mechanical loads on the system as well as the resistance of flow due to friction

These are always protected by relief valves to prevent damage to the pump or system.

A majority of fluid power pumps fall in this category, such as Gear, Piston and Vane pumps.

ADVANTAGES OF PDP OVER NPDP They can operate at very high pressure of up to 800 bar

( used for lifting oils from very deep oil wells) They can achieve a high volumetric efficiency of 98 % They are highly efficient and almost constant throughout

the designed pressure range. They are a compact unit having a high power to weight ratio They can obtain a smooth and precise controlled motion. By proper application and control, they produce only an

amount of flow required to move the load at the desired velocity

They have a great flexibility of performance. They can be made to operate over a wide range of pressures and speeds.

ROTARY PUMPS Rotary pumps are positive displacement pumps

They have two or more rotating components.

The discharge of rotary pumps remains almost same

irrespective of pressure. They can give discharge even at a

high pressure. They are limited to pressure below 140 bar.

The discharge of rotary pumps is smooth, continuous and not

pulsating. A very less vibration and noise is observed. They

are compact with less number of moving component and less

sensitive to contaminations.

CLASSIFICATION-ROTARY PUMPSI. GEAR PUMPS

I. External gear pumpsII. Internal gear pumpsIII. Lobe pumpIV. Ge-rotor pumpV. Screw pump

II. VANE PUMPSI. Unbalanced vane pumpII. Balanced vane pump

III. PISTON PUMPSI. Axial piston pump

I. Straight axis piston pumpII. Bent axis piston pump

II. Radial piston pumpI. Stationary cylinder typeII. Rotating cylinder type

EXTERNAL GEAR PUMPS

EXTERNAL GEAR PUMPS It consist of pump housing in which a pair of precisely

machined meshing gears run with minimal radial and axial clearance.

One of the gears, called as Driver is driven by a prime mover. The Driver drives another gear called a Follower.

As the teeth of the two gears separate, the fluid from the pump inlet gets trapped between the rotating gear cavities and pump housing.

The trapped fluid is then carried around the periphery of the pump casing and delivered to the outlet port.

The teeth of the precisely meshed gears provide almost a perfect seal between a pump inlet and pump outlet.

The inlet is at the point of separation and the outlet is at the point of mesh.

EXTERNAL GEAR PUMPS

ADVANTAGE1. They are self-priming.2. They give constant

delivery for a given speed

3. They are compact and light in weight

4. Volumetric efficiency is high

DISADVANTAGE1. The liquid to be

pumped must be clean, otherwise it will damage the pump.

2. Variable speed drives are required to change the drive.

3. If they run dry, parts can be damaged because the fluid to be pumped is used as lubricant.

LOBE PUMP

LOBE PUMP

LOBE PUMPThe lobe pump is a close relative of the external gear pump.

― Two three-lobed, gear shaped units are often used to form

the pumping element

― Output flow is larger than a gear pump of comparable

physical size because of pumping chamber geometry

― Lower pressure rating than the gear pumps, but higher

discharge is achieved

― Tend to have a pulsating output flow.

INTERNAL GEAR PUMPS

INTERNAL GEAR PUMPS— Another form of gear pump is Internal Gear pump.

— They consist of two gears: An External and an Internal Gear

— The crescent placed in between acts as a seal between the suction and discharge.

— When the pump operates, the external gear drives the internal gear and both rotate in same direction.

— After the un-meshing of the gears at the inlet, the fluid fills the cavities formed by rotating teeth and crescent.

— Both the gears transport the fluid through the pump. The crescent seals the low pressure pump inlet from the high pressure pump outlet.

GE-ROTOR PUMP

GE-ROTOR PUMP

GE-ROTOR PUMP The ge-rotor is a common internal gear pump The inner gear rotor is called as the gerotor element. The element is driven by prime mover and during the

operation drives outer gear rotor around as they mesh together.

The gerotor has one tooth less than the outer idler gear Each tooth of gerotor is always in sliding contact with

surface of outer element, the teeth of two elements engage at just one place to seal pumping chambers

The pocket of increasing size are suction pockets and those of the discharge side are of discharge.

SCREW PUMP Screw pump has pumping elements that consist of one,

two or three rotating screws. As the screw rotates, fluid is trapped and carried along to

the discharge of the pump The design screw allows them to operate at a very low

noise level. A single screw version is called as “Progressive Cavity

Pumping”

SCREW PUMP

VANE PUMPVane pumps are positive-displacement, fixed or variable delivery units.Design is commonly used in Industrial applications.Vane pumps consist of slotted rotor, fitted with moveable vanes, that rotates within a cam ring in the pump housing- Rotor is off centre in the ring, which creates pumping

chambers that vary in volume as the pump rotates- As chamber volume increases, pressure decreases,

bringing fluid into the pump- As volume decreases, fluid is forced out into the system.

VANE PUMP

VANE PUMPVane pump may be pressure unbalanced or pressure

balanced

- Unbalanced has only one inlet and one discharge which

places a side load on the shaft

- Balanced has two inlets and two discharge opposite each

other, creating a pressure balance and therefore no load

on the shaft

PISTON PUMP

PISTON PUMPA basic piston consist of a housing that supports a pumping

mechanism and a motion converting mechanism

- Pumping mechanism is a block containing cylinders fitted

with pistons and valves

- Motion converter changes rotary to reciprocating motion

via cams, eccentric ring, swash plate or bent axis design.

- Rotating the pump shaft can cause piston movement that

pumps the fluid.

FUNCTION OF SWASH PLATE Swash plate is used to change the discharge of the pump

By changing the angle of swash plate, we can change the

stroke length and hence the discharge.

When swash plate is perpendicular to the pump axis, the

discharge is zero

Discharge increases with the increase in swash plate

angle from this perpendicular to pump axis.

BENT AXIS PISTON PUMP

BENT AXIS PISTON PUMP It consist of cylinder block with axial bores Pistons are inserted inside these bores. The other end of

the plate are connected to the shoe plate with shoe joints. In bent axis piston pump, the shoe plate is fixed to a

flange; the flange is keyed to drive shaft. When the shaft rotates, it causes the cylinder block to

rotate, the shoe plate will also rotate with it, causing the piston in the bores to reciprocate

Half rotation of the cylinder block causes suction of the oil into the bore and the next rotation causes the discharge.

COMPARISON OF HYDRAULIC PUMPS

FACTORS AFFECTING THE SELECTION OF TYPE OF PUMP1. Maximum operating pressure2. Maximum delivery3. Type of control4. Pump drive speed5. Type of fluid6. Pump contamination tolerance7. Pump noise8. Size and weight of pump9. Pump efficiency10. Availability and interchangeability11. Maintenance and spares12. cost