Piston Pumps
Definition : When pumping of hydraulic oil is carried out with the help of reciprocating piston in a cylinder, it is called as piston pump.
Principle :
● It works on the principle of reciprocating movement of piston or plunger inside finely machined cylinder bore.
● The piston while moving outside sucks the oil from the tank and pushes it from the cylinder bore when moving inside. In this manner oil is supplied with increased pressure through the outlet.
Types of Piston Pumps
(a) In-line crank shaft driven pump
(b) Axial piston pump
(1) Bent axis piston pumps
(2) In-line axial piston pump
(a) Swash plate axial piston pump
(b) Wobble plate axial piston pump
(c) Radial piston pump
(a) In-Line Crank Shaft Driven Pump:
Definition : When crank shaft is used for pumping action of fluid using piston and cylinder mechanism, it is called as in line crank shaft driven pump
Construction and Working:
● It consists of crank shaft with parallel arrangement of cylinders along with pistons. It requires more space.
● When crank shaft rotates it will cause reciprocating movement of pistons in the respective cylinders and causes suction and delivery of hydraulic oil, alternately.
● The pump are suitable for applications where one or two cylinder piston pumps are used.
● It is used in mobile applications like small forklift, simple loaders, etc.
● With 24 cylinders (12 cylinders in two opposite rows) crank driven can generate very high pressure up to 400 to 450 bar with flow rate of 40 Liter/min.
(b) Axial Piston Pump :
Definition: When pistons of pump are arranged axially parallel to the driving shaft of the pump, it is known as axial piston pump.
Principle :
● It works on the principle of reciprocating motion of pistons with the help of swash plate or wobble plate for increasing pressure of oil to be supplied to the hydraulic system.
Types of axial piston pump :
Axial Piston pumps are classified according to:
Axis of pump:
(1) In-line axial piston pump
(2) Bent axis piston pumps
(1) In-line axial piston pump:
In-line axial piston pumps has the pistons are arranged inside the piston block parallel to each other i.e. in-line hence they are called as in-line axial piston pumps.
In-line axial piston pumps are classified as:
(a) Swash plate axial piston pump
(b) Wobble axial piston pump
(a) Swash plate axial piston pump :
Definition : When swash plate or tilting plate is used to obtain to and fro motion of pistons for pumping of an oil, it is known as swash plate axial piston pump.
Principle :
● The angular surface of swash plate helps to produce reciprocating motion when rotates along with the piston rods of the pistons and increases pressure of oil.
Construction:
It consists of following main plates :
a) Drive shaft with cylinder block : It is rotating element of the pump in which cylinder block is mounted with shaft.
b) Pistons : Pistons are mounted in the cylinder block with piston rods.
c) Shoe and shoe plate : The piston rod ends are covered with shoe and fitted on the shoe plate.
d) Swash plate : The cam plate or swash plate being kept fixed and positioned at an angle a with the axis of the cylinder block. The shoe plate is attached to the angular surface of the swash plate.
e) Suction and discharge ports : They are provided for suction and delivery of oil.
Working :
● As the drive shaft rotates, it will cause rotary movement of cylinder barrel. The piston shoes follows the surface of the swash plate.
● Since the swash plate has angular surface, the pistons reciprocates within the cylinder bore and thus oil is sucked during one half of the circle of rotation and during other half of rotation, the oil is forced to the outlet port.
● In another design of swash plate axial piston pump, the location of stationary swash plate and the cylinder block are reversed and piston arrangement remains same.
● For fixed capacity axial piston pumps, the angle of swash plate is mostly 30°. The flow rate of pump depends upon the number of cylinders (n), speed of the pump (N), the bore diameter and stroke length of piston (e) and angle of the swash plate.
Function of swash plate:
● Swash plate is an angular surface plate used in swash plate axial piston pump. It is provided to obtain to and fro motion of the pistons in the bore of cylinder.
● The stroke length of a piston is determined by the swash plate angle. With a large swash plate angle, the pistons have a larger stroke whereas smaller swash plate angle gives shorter stroke length.
● When the angle of swash plate becomes zero, no displacement of pistons takes place.
● The angle of swash plate can be changed to get variable piston displacement and volume flow rate. The maximum angle is generally limited to 17.5° from various design considerations. By varying the angle, pump output and flow can be changed from zero to maximum.
(b) Wobble plate axial piston pump :
Definition: When the swash plate rotates while cylinder box is kept fixed, for obtaining reciprocating action of pistons, it is called as wobble plate axial piston pump.
Construction and working:
● In this pump, the swash plate rotates with the drive shaft while the cylinder block is fixed. The shoe plate and other components are similar to swash plate piston pump.
● The swash plate rotates on the surface of the shoe plate to produce a motion on it and causes to and fro movement of pistons.
● This movement helps for pumping of oil to the required pressure.
(II) Bent axis piston pumps :
In bent axis piston pump, the axis of cylinder block is made to form an angle with the axis of the drive shaft for making reciprocating movement of pistons.
Bent axis axial piston pump:
Definition : If the axis of the cylinder block is made to form an angle with the axis of the drive shaft to obtain reciprocating movement of pistons for pumping of oil. It is called as bent axis axial piston pump.
Principle :
When cylinder block is inclined at angle a with respect to drive shaft, it will produce to and fro motion required for pumping of oil. The cylinder block angle can be charged to vary the stroke and flow rate of the pump.
Construction and working :
● Bent axis pump consists of a cylinder block, pistons, universal link which keys block to drive shaft, shaft bearings and drive shaft.
● The cylinder block is mounted at an angle a with the drive shaft with inlet and outlet ports.
● The drive shaft is connected to the electric motor with the help of coupling.
● As the drive shaft rotates it causes rotation of the cylinder block. Thus pistons inside the box also get to and from motion.
● The rotating cylinder block bores are open to inlet and outlet slots of the valve plate which is fixed.
● During rotation of cylinder block, the pistons are pulled to suck the oil through inlet and get pushed during next rotation for pumping of oil through discharge port.
● The angle of cylinder block may be fixed or variable. It is maintained with the help of yoke and actuating control mechanism. The maximum angle is generally kept upto 30°
● To reduce function between cylinder block and valve plate, an oil film is needed.
● Bent axis pump can generate pressure upto 700 bar with flow rate maximum upto 800 liters/min.
(c) Radial Piston Pump:
Definition : When pistons are arranged along a radius of the cylinder block similar to vanes in the vane pump, it is called as radial piston pump.
Principle :
The reciprocating motion of the pistons mounted radially is obtained with the help of rotating cylinder block mounted eccentrically with the pump housing.
Construction :
It consists of:
(a) Cylinder block : The pistons are arranged radially (4 pistons) in a slots of cylinder block. They move perpendicular to the shaft centre line.
(b) Radial pistons : The radial pistons are mounted in the slots or bores of cylinder blocks usually 4 to 6. They are provided with the spring to maintain contact with the cam ring.
(C) Cam ring : It is the outer ring which is positioned eccentrically with the cylinder block axis. It helps to obtain to and fro movement of pistons. The eccentricity can be varied by using adjusting screw.
(d) Inlet and outlet ports : The inlet and outlet ports are provided at the centre of the block and separated with the help of pintle.
Working :
● When driving shaft is rotated, it will cause rotation of the cylinder block (rotor). During rotation of block, the pistons are thrown out by centrifugal force and pushed back due to eccentricity between cam ring and cylinder block.
● When movement of rotor is observed step by step it shows that during movement of rotor from piston (1) to the piston (2), the radius of rotation increases and piston is forced out towards cam ring. The piston is pulled out and causes suction of oil.
● During next part of rotation, the piston moves from point (2) to point (3). The radius of piston movement is almost same and oil is carried out by the slots of the rotor.
● When rotor moves from (3) to (4), the radius of rotation gradually decreases and piston moves towards the centre of the pump. As piston get pushes towards centre, it will cause pumping of oil to the discharge port.
● From point pine wall is in contact with the piston no suction and delivery occurs.
● These pumps are quite robust and work at a high rpm with very high flow rate about 2000 lit/min.
● It can generate pressure in the range of 450 - 500 bar.
Advantages and Disadvantages of Piston Pumps
◆ Advantages :
1. High range of pressure:
Piston pumps are able to produce high pressure in the range of 450-500 bar. Maximum pressure is about 1000 bar.
2. High volumetric efficiency:
It has high volumetric efficiency greater than 95%.
3. Variable delivery:
By changing eccentricity between cam ring and rotor flow rate can be varied.
4. Viscosity range high :
It can handle oils of wide range of viscosity.
5. Noiseless operation :
It has low level of noise hence silent operation.
◆ Disadvantages :
1. Cost is high :
Due to complicated construction, it is costly than other pumps like gear pump.
2. Cleaning difficult :
It is difficult to clean closely fitted parts located inside the pump.
