Vane Pump
Definition: When sliding vanes are used to increase pressure of oil, it is known as vane pump.
Principle of vane pump:
It is working on principle of:
(a) Changing volume of oil continuously with the help of sliding movement of vanes along with the inner contour of the ring.
(b) Change in volume occurs due to eccentric positioning of the rotor with respect to circular ring.
Important Features of Vane Pump:
● It can work with a maximum pressure of 200 bar.
● It gives pulse free delivery.
● It has low level of noise.
● It can handle moderate viscosity liquids.
● It uses sliding vanes of various types.
● It has 85-90% volumetric efficiency.
Types of Vane Pumps:
Vane pumps are classified as:
(a) Fixed displacement, unbalanced vane pump
(b) Fixed displacement, balanced vane pump
(c) Variable displacement, unbalanced vane pump
(d) Variable displacement pressure compensated vane pump
(a) Unbalanced vane pump (fixed displacement)
Principle:
● It works on the principle of sliding vanes allowed to track along a ring called as cam ring.
● The pressure at inlet is partial vacuum (low pressure) while at the outlet, pressure is high, hence it is called as unbalanced vane pump.
Construction:
This type of vane pump consists of following essential parts:
(1) Driven rotor with slots :
● Rotor is a circular disc which has rectangular slots cut evenly from the centre to the outer face. It accommodates the vanes in the slots.
● The rotor is mounted eccentrically with respect to stationary circular ring.
(2) Sliding vanes :
● The sliding vanes are mounted in rectangular slots of the rotor and slides due to centrifugal force.
● The vanes has various types which includes :
i) Dual vane: It consists of two vanes in each slot of the rotor. This vane is balanced with positive sealing.
(ii) Pin vanes : It has pin shape and pressure is directed underside the pin.
(iii) Intra vane: It consists of smaller vane with in a larger vane with a bevelled edge.
(iv) Spring loaded vane : In this type of vane, a spring is used to force a vane.
(v) Angled vane : The vanes are located at an angle in the rotor.
(3) Stationary circular ring (cam ring):
● Stationary circular ring is provided which helps to create number of chambers with vanes and rotor.
● The vanes are allowed to track inside surface of the circular ring during rotation of rotor.
(4) Inlet and outlet ports:
● The inlet port is provided to suck the oil and outlet port is provided opposite to the inlet port where area is reduced to discharge oil.
(5) Housing :
● It is the outer casing which accommodates all above parts.
Working :
● When the rotor shaft is driven by the motor, it will cause the rotation of the slotted rotor.
● The ones which are free to slide in their slots, moved outward due to centrifugal force.
● The vane makes contact with the circular ring and creates low pressure at the inlet port. At the suction side the chamber volume increases and it is filled with oil from the suction side.
● As the vanes moves forward, the oil is also carried between the moving vanes in the forward direction. At the pressure side near the outlet port. The chamber volume decreases and oil is forced in to the outlet pressure port.
● The amount of eccentricity between rotor axis and circular ring axis determines the displacement of the pump.
(b) Balanced Vane Pump :
Principle : It works on the principle of sliding vanes in which pressure at inlet and outlet is balanced with the help of the two pressure quadrants opposes each other.
Construction:
● It consists of a circular rotor with vane slots are concentrically positioned with the axis of an elliptical cam ring.
● Elliptical cam ring helps to create two inlet and two outlet chambers such that the two pressure quadrants oppose each other and the two inlet chambers are directly opposite to one another.
● The pressure at inlet and outlet of the pump is balanced, therefore the forces acting on the shaft are fully balanced and side loading of the shaft is eliminated completely.
Working :
● Working is similar to unbalanced vane pump. Due to elliptical cam ring, the radius of vanes is not constant and changes volume accordingly.
● During rotation of rotor with slots, the vanes are forced out due to centrifugal force and in contact with the inner surface of the elliptical cam ring.
● The pressure at the inlet chamber is less than atmospheric pressure which helps to suck the oil to the inlet suction ports of the pump.
● The oil from inlet port is moved to forward direction with rotating vanes and carried to the outlet port where its volume decreases.
● These types of pumps are constant volume or fixed displacement pumps.
● They can work up to 175 bar pressure. They are relatively quiet and having simple construction.
(c) Variable Displacement Vane Pump
Definition: When flow rate of the pump can be changed according to the requirement of the hydraulic system, it is called as variable displacement pump.
In variable displacement vane pump, eccentricity between rotor and cam ring can be changed to vary the flow rate of the pump.
Principle :
● The delivery of vane pump is proportional to the eccentricity between axis of rotor and cam ring.
● By changing the geometric position of the ring relative to the rotor centre will change the delivery volume as per need of the system.
Construction :
The main parts are:
(1) Cam ring : It is circular ring whose axis is adjusted by using screw adjustment to vary eccentricity with the rotor.
(2) Rotor : It is same for vane pump with slots to accommodate rectangular vanes.
(3) Vanes : Sliding vanes moves in the slots of the rotor with respect to cam ring.
(4) Screw adjustment : It is important component which consists of screw and nut mechanism.
Screw moves forward to change the eccentricity between axis of cam ring and rotor.
Working:
● The rotor containing the vanes is positioned eccentric with respect to cam ring by means of screw adjustment.
● When rotor is rotated, there is increasing and decreasing volume similar to vane pump discussed earlier.
● If the screw is adjusted slightly so that eccentricity of the rotor to the cam ring is not enough, the flow rate will be less.
● When eccentricity is increased, then delivery volume will be increased with the cam ring completely concentric with the rotor axis, eccentricity is zero and these will be no flow rate as volume remains constant during rotation of vanes.
● Variable vane pumps are used for pressure range of 75-125 bar with 85-90% efficiency.
(d) Pressure Compensated Vane Pump:
Definition: When it is desired that when the predetermined system pressure is reached, the pump should stop pumping further oil to the system, this type of vane pump is called as pressure compensated vane pump.
Principle :
It works on the principle of variable displacement vane pump in which spring is provided to offset the cam ring to the zero eccentricity position when pressure of the system is reached to set value.
Construction and working:
● The pressure compensated variable vane pump consists of an additional spring which is adjusted to offset the cam ring.
● As the pressure acting on the inner contour of the ring is more than the pressure exerted by the spring, the cam ring becomes concentric to the rotor.
● Now the pumping action stops due to zero eccentricity. No flow is permitted to the system.
● In such a pump the pumping mechanism has to act very fast and any build of fluid inside the housing is undesirable. The leakage is generally hot.
● The leakage is drained through external drain called as case drain.
Advantages and Disadvantages of Vane Pump:
Advantages of vane pump:
(1) Low internal leakage : The leakage of oil can be reduced by using spring loaded vanes fitted in the rotor.
(2) Compact design: The pump is compact in size and shape, hence can be used where space limitation is important.
(3) Low noise : These pumps are comparatively quiet in operation and gives smooth flow.
(4) High efficiency: As leakage is less, it has high efficiency about 85 -9 0%.
Disadvantages :
(1) Low pressure : It can efficiently work up to pressure range of 125 - 175 bar which is low compared to gear and piston pump.
(2) Low operating speed : Due to centrifugal force of vanes, high speed may create excessive wear of vanes, hence cannot be used for higher speed.
Definition : When pair of meshing screws is used increase pressure of oil, it is known as screw pump.
Principle :
It utilizes the movement of screw which displace the oil axially and meshing of thread flanks forces the oil in the space between rotor and housing wall and gradually increases pressure of oil.
Construction :
● The screw pumps are available in design with one, two screws and three screws.
● It consists of following main parts:
(a) Screw pair : The meshing pair of screws is mounted with the help of spindle mounted in bearing. One of the screw is connected to the driving shaft. The screws are either left hand or right hand.
(b) Timing gears : Timing gears are provided to maintain a running clearance between two meshing screws.
(c) Suction and discharge ports : Inlet suction port is provided for suction of oil and discharge port is provided to deliver high pressure oil.
(d) Housing : It is body of the pump which accommodates screw pair, timing gears and suction and discharge ports.
Working:
● When pump is started by prime mover, the driving screw rotates and drives the other screw and creates suction of oil.
● The oil entered through inlet suction ports is carried out linearly (axially) due to helical recesses between screw pair. The oil does not rotate but moves linearly.
● Similar to gear pump, the meshing of the thread flanks forces oil to flow in the space between the rotor and housing wall.
● The volume of the oil moves forward uniformly with the rotation of screw along the axis and oil pressure is gradually increased
● The oil from both the ends comes towards the centre of the pump where discharge port is located.
● Now the oil is discharged through discharge port to the hydraulic system. The flow rate of the oil can be varied by varying the drive motor speed.
Three Spindle Screw Pump:
● In this design, the drive motor is connected to the middle screw and on its both sides two idler screws are provided.
● The idler screws are driven by the pressure of the liquid and therefore no timing gears are required.
● The manufacturing of a three spindle screw pump is costly and therefore it is used for special applications where,
(a) Low flow rate is required.
(b) No pressure pulsation.
(c) Maximum noise elimination.
● They are used for hydraulic systems of lifts, elevator, hydraulic press, ship winch, crane, wood working machine, machine tools, etc.
● They work in the pressure range of 250 bar or as high as 400 bar.
Advantages :
(1) Reliable performance : Screw pumps are most reliable hydraulic pump.
(2) Operate at very high speeds: They can operate at very high speed up to 3500 rpm and sometimes even higher speeds.
(3) Continuous discharge : The oil supply is pulsation free and continuous.
(4) Silent operation : These pumps are very quiet because of rolling action of the spindle rotor. This eliminates pulsation and vibration, noise, etc.
(5) Better hydraulic control : There is no oil churning, pump turbulence, etc.
Disadvantages:
(1) Screw manufacturing difficult : Because of specific profile of screw, its manufacturing is difficult when close tolerance is considered.
(2) Unsuitable for high viscosity oil : Pumping of higher viscosity oil may derate the pump characteristics. Pressure rating of the screw is viscosity dependant.
(3) Low efficiency : Overall volumetric and mechanical efficiency is relatively low.
(4) Decrease in efficiency with increase in viscosity of oil.
