H06- Positive displacement Pump (Hydrostatic pump)

Hydrostatic/ Positive displacement pump:

- In hydrostatic pump the conversion of mechanical energy to hydraulic energy takes place with comparatively small quantity of velocity of liquid.

-In positive displacement pump, the pumping volume changes from maximum to minimum during each pumping cycle i.e the pumping element expands from a small volume to a large volume and then is contracted to a small volume as shown in animation below. During this decrease in volume the pressure of fluid increases.

- These type of the pumps gives a fixed volume of oil per pumping cycle into the hydraulic system.

- These type pumps are capable of overcoming the resistance due to external load as well as resistance to flow friction.

- Since a fixed volume of oil is discharged per pumping cycle the displacement characteristics of the pumping chamber and the volume of oil pumped for a given pump size is dependent on the number of pumping cycle made by the pump OR pumping element per unit time.

- This type of the pump is provides positive internal seal against slippage.

- Closing the outlet of a positive displacement pump causes an instantaneous increase of pressure. This increase of pressure can causes break down of component.

Advantages of the positive displacement pump:

a. Capable of generating high pressure

b. Small and compact

c. Very high power to weight ratio

d. Very small change in efficiency through out operation

e. Very high volumetric ratio

f. Greater flexibility of performance

Dis-advantages of the positive displacement pump:

a. The pressure generated may cause failure of the pump

b. Initial and maintenance cost

Classification of positive displacement pump

Positive displacement pumps are classified based on

i. The type of motion of the pumping element

ii. The displacement characteristics

iii. The type of pumping element

Based on the type of the motion of the pumping element we have-

A. Reciprocating type - e.g. Piston pump

B. Rotary type- e.g. Gear pump, vane pump, screw pump etc.

Base on the displacement characteristics we have-

A. Fixed displacement pump- Pumps whose displacement cannot be varied without replacing the certain element is called as fixed displacement pump. Example- Gear pump

B. Variable displacement pump- Pumps whose displacement varies by varying the size of the pumping chamber are called variable displacement pump. Example- Vane pump

Based on the pumping element we have-

A. Gear pump

B. Vane pump

C. Piston pump

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