950G Series II Wheel Loader Caterpillar

Pump Displacement



Non-Positive Displacement Pumps

Non-positive displacement pumps have more clearances between the moving and the stationary parts than positive displacement pumps. The extra clearance allows more oil to be pushed back between the parts as the outlet pressure (resistance to flow) increases. Non-positive displacement pumps are less efficient than positive displacement pumps because the output flow of the pump decreases greatly as the outlet pressure increases. Non-positive displacement pumps are generally either centrifugal impeller-type or axial propeller-type. Non-postive displacement pumps are used in low pressure applications. Some applications are automotive water pumps or charge pumps for piston pumps in high pressure hydraulic systems.

Centrifugal Impeller Pump

Illustration 1g01061997

The centrifugal impeller pump consists of two basic parts: the impeller (2) and the housing (3). The impeller is mounted on an input shaft (4), that has a solid disc back with curved blades (1) that are molded on the input side.

Oil enters the center of the housing (5) near the input shaft and flows into the impeller. The curved impeller blades propel the oil outward against the housing. The housing is shaped in order to direct the oil to the outlet port.

Axial Propeller Pump

Illustration 2g01062101

The axial propeller type pump is shaped like an electric air fan. The axial propeller is mounted in a straight tube. The axial propeller has an open blade propeller. Oil is propelled down the tube by the rotation of the angled blades.

Positive Displacement Pumps

There are three basic types of positive displacement pumps: gear, vane and piston. Positive displacement pumps have much smaller clearances between components. This will reduce leakage and this will provide a much higher efficiency, when displacement pumps are used in a high pressure hydraulic system. The output flow in a positive displacement pump is basically the same for each pump revolution. Positive displacement pumps are classified by both the control of the output flow and the construction of the pump.

Positive displacement pumps are rated two ways. One way is by the maximum system pressure (i.e. 21000 kPa (3000 psi)) at which the pump is designed to operate. The second way is by the specific output that is delivered either per revolution or at a given speed against a specified pressure. The pumps are rated either by liter per minute @ rpm@ kPa or by gpm @ rpm @ psi (i.e. 380 lpm @ 2000 rpm @ 690 kPa or 100 gpm @ 2000 rpm @ 100 psi).

When the flow rate is expressed in output per revolution, the flow rate can be easily converted by multiplying by the speed in rpm (i.e.: 2000 rpm) and dividing by a constant. For example, you will calculate the flow of a pump that rotates 2000 rpm and has a flow of 11.55 inch3/rev or 190 cc/rev.

Table 1
GPM = inch3/rev X rpm/231     LPM = cc/rev X rpm/1000    
GPM = 11.55 X 2000/231     LPM = 190 X 2000/1000    
GPM = 100     LPM = 380    

Volumetric Efficiency

As pressure increases, the close clearances between the parts in a positive displacement pump do not produce the same output flow as input flow. Some oil will be forced back through the clearances between the high pressure chamber and the low pressure chamber. The resultant output flow, when compared to the input flow, is called volumetric efficiency. (Input flow is generally defined as the "output flow @ 100 psi.") Volumetric efficiency changes as pressure changes and must be specified for a given pressure. When a pump that is rated at 100 gpm @ 2000 rpm @ 100 psi is operated against 1000 psi, the output may drop to 97 gpm. This pump would have a "volumetric efficiency" of 97% (97/100) @ 1000 psi.

Volumetric efficiency @ 1000 psi = output flow/input flow

Volumetric efficiency @ 1000 psi = 97/100

Volumetric efficiency @ 1000 psi = .97 or 97% efficient at 1000 psi

When the pressure increases to 2000 psi, the output may drop to 95 gpm. It would then have a volumetric efficiency of .95 or 95% @ 2000 psi. The rpm must remain constant when you are measuring volumetric efficiency.

Fixed Displacement Versus Variable Displacement

Illustration 3g01062103

The output flow of a fixed displacement pump (left side) is only changed by varying the speed of the pump rotation. The pump rotation must be rotated faster in order to increase the flow. Also, the displacement pump must rotate slower in order to decrease the flow. The gear type pump (not shown) is also a fixed displacement pump.

The vane type pump (not shown) and piston type pumps may be fixed or variable. The output flow from a variable displacement pump may be increased or decreased independently of the speed of rotation.

The output flow can be controlled in the following way:

  • Manually controlled

  • Automatically controlled (not shown)

  • Combination of manually and automatically controlled

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