Positive Displacement Pumps

Positive Displacement Pumps

This type of pump ejects a fixed quantity of fluid perrevolution of the pump shaft. As a result the pump discharge isfairly constant and not dependent on system pressure. Thismakes them particularly suitable for fluid power systems.

Positive displacement pumps must be protected againstoverpressure if the resistance to flow becomes very large. Thiscan happen if a al e is completel closed and there is nocan happen if a valve is completely closed and there is nophysical place for the fluid to go. A pressure relief valve isused to protect the pump against overpressure.

There are various types of positive displacement pumps usedin the industry. Among them Gear pumps, Vane pumps andin the industry. Among them Gear pumps, Vane pumps andPiston pumps are most widely used.

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Gear pumpsN d i f h t lNumerous designs of gear pumps such as external gearpump, internal gear pump, screw pump, lobe pump, etc.are commercially available.

The above figure shows the operation of an external gearpump, which develops flow by carrying fluid between theteeth of two meshing gears. One of the gears is connected to ateeth of two meshing gears. One of the gears is connected to adrive shaft. The second gear is driven as it meshes with thedriver gear. 13

Oil chambers are formed between the gear teeth, the pumphousing and the side wear plates. The suction side is whereteeth come o t of mesh There the ol me e pands bringingteeth come out of mesh. There the volume expands, bringingout a reduction in pressure to below atmospheric pressure.Fluid is pushed into this void by atmospheric pressure.

The discharge side is where teeth go into mesh, causing adecrease in the volume (and rise in pressure) between matingdecrease in the volume (and rise in pressure) between matingteeth. It results in discharge positively against higher systempressure. 14

The volumetric displacement of a gear pump can be found byThe volumetric displacement of a gear pump can be found bycalculating the volume of a hollow cylinder of outsidediameter Do, inside diameter Di and length L. There areact all t o s ch c linder ol mes (beca se there are t oactually two such cylinder volumes (because there are twogears) where oil could fill the inside of the pump if there wereno teeth. However, one half of these two volumes are taken bythe gear teeth of both gears. Thus the volumetric displacementcan be written as

Thus the theoretical flow rate is15

Example: A gear pump has 75 mm outside diameter, 50 mmi id di t d 25 l th If th l t iinside diameter, and 25 mm length. If the volumetricefficiency is 90% at rated pressure, what is the correspondingactual flow rate? The pump speed is 1000 rpm.

Solution:The volumetric displacement can be written asThe volumetric displacement can be written as

= 0.0000614 m3/rev = 0.0614 litre.

The theoretical flow rate is

= 61.4 lpm

Actual flow rate, QA = ηQT = 0.9x61.4 = 55.3 lpm.Actual flow rate, QA ηQT 0.9x61.4 55.3 lpm.

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Vane pumps

The above figure shows the operation of a vane pump. Theeccentric rotor which contains radial slots, is attached to thedrive shaft and rotates inside a cam ring. Each slot contains avane designed to mate with the surface of the cam ring as thevane designed to mate with the surface of the cam ring as therotor turns. Centrifugal force keeps the vanes out against thesurface of the cam ring. 17

During one-half revolution of rotor rotation, the volumeDuring one half revolution of rotor rotation, the volumeincreases between the rotor and the cam ring. The resultingvolume expansion causes a reduction of pressure. This is thes ction process hich ca ses fl ids to flo thro gh the inletsuction process, which causes fluids to flow through the inletport and fill the void.

As the rotor rotates through the second-half revolution, thesurface of the cam ring pushes the vanes back into their slots,and the trapped volume is reduced. This positively dischargesand the trapped volume is reduced. This positively dischargesthe trapped fluid through the discharge port.

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There is an eccentricity between the centre of the rotor and theThere is an eccentricity between the centre of the rotor and thecenter of the cam ring. If the eccentricity is zero there will beno net flow. If the cam ring diameter is DC and the rotordiameter is D then the maximum eccentricity isdiameter is DR, then the maximum eccentricity is,

The maximum eccentricity produces the maximum volumetricdisplacement:

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Substituting the expression for the maximum eccentricityigives,

Thus the theoretical volumetric displacement at an eccentricitye is:

Some vane pumps have provisions for mechanically varyingthe eccentricity. Such a design is called a variabledisplacement pumpdisplacement pump.

Example: A vane pump has a 50 mm diameter rotor, a 75 mmdiameter cam ring, and 50 mm wide vanes. If the eccentricityis 8 mm, determine the volumetric displacement.Ans:Ans:

(0.050+0.075)(0.008)(0.050) =0.0000785 m3

= 0.0785 litre 20

Piston pumpsA i t k th i i l th t i tiA piston pump works on the principle that a reciprocatingpiston can draw in fluid when it retracts and discharges itwhen it extends.

There are two basic types: axial piston pump and radialpiston pump. In axial design pistons are parallel to the axis ofpiston pump. In axial design pistons are parallel to the axis ofthe cylinder block. In radial design pistons are arrangedradially in a cylinder block. Axial piston pumps again can beeither of the bent a is config ration or of the s ash plateeither of the bent axis configuration or of the swash platedesign.

Axial piston pump (Bent axis design)An axial piston pump contains a cylinder block rotating withthe drive shaft. However, the axis of the cylinder block is setthe drive shaft. However, the axis of the cylinder block is setat an offset angle relative to the axis of the drive shaft.

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The cylinder blockcontains a number ofpistons arranged alongp g ga circle. Piston rods areconnected to the driveshaft flange by ballshaft flange by ball-and-socket joints.

h i f dThe pistons are forcedin and out of theirbores as the distancebetween the drive shaftflange and cylinderblock changesblock changes.

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The volumetric displacement of the pump varies with the offsetangle. No flow is produced when the cylinder block axis isg p yaligned to the drive shaft axis.

Or23

The total displacement volume equals the number of pistonsmultiplied by the displacement volume per piston:p y p p p

Substituting, we have

The theoretical flow rate can then be obtained b multiplying thedisplacement volume with the pump speed in rpm.

In-line piston pump (Swash plate design)

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Radial piston pump

The operation and construction of a radial piston pump isillustrated in Figure 5-29. This design consists of a pintle todi fl id i d f h li d li d b l i hdirect fluid in and out of the cylinders, a cylinder barrel withpistons, and a rotor containing a reaction ring.

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The pistons remain in constant contact with the reaction ringd if l f d b k h i Fdue to centrifugal force and back pressure on the pistons. Forpumping action, the reaction ring is moved eccentrically withrespect to the pintle or shaft axis. As the cylinder barrel rotates,p p y ,the pistons on one side travel outward. This draws in fluid aseach cylinder passes the suction ports of the pintle. When apiston passes the point of maximum eccentricity it is forcedpiston passes the point of maximum eccentricity, it is forcedinward by the reaction ring. This forces the fluid to enter thedischarge port of the pintle. 26