Design Types Basic Operation Performance
1. Calculate the theoretical pump flow rate based on shaft speed,
pump displacement, and volumetric efficiency.
2. Describe the effect of pressure on pump flow rate.
3. Measure the volumetric efficiency of a pump.
4. Describe the effect of oil temperature on volumetric efficiency.
5. Explain basic pump operating principles.
Pump Operating Principles
The maximum amount of flow that runs through a hydraulic
system is determined by the size and speed of the pump. In a
pneumatic system, maximum possible flow is determined by the
size of the compressor. All pumps or compressors operate by
development of a pressure differential across their ports. When
the internal rotating group begins to move, a vacuum is generated
at the inlet. Atmospheric pressure causes fluid to rush into the
inlet because fluids, like electricity, always seek the path of least
resistance. When in the pump or compressor, the fluid is forced to
leave via the outlet and into the system.
There are three basic designs of pumps which are gear, vane, and
piston. All pumps operate by the same principles. A vacuum
develops at the inlet and pressure at the outlet causing fluid to be
forced into the system. Another way to explain this action is by
saying that the pump generates a pressure differential across it’s
ports or that it has an increasing and decreasing volume. However it
is stated, all pumps develop flow and in the same way.
Determining Pump Flow
There are two primary factors that control pump and
compressor flow, which are, their physical size and the
speed at which they are ran. The following formula
illustrates the relationship between speed, size, and
flow rate. This formula is used to calculate theoretical
Flow Rate = Displacement (cubic inches/revolution)
x Shaft Speed ( revolutions/ minute) divided by 231
Volumetric efficiency has to do with the ability of any given pump to
maintain the same rate of displacement while under pressure. Most
hydraulic pumps have efficiencies that are in the range of 80 to 90%.
While we may think of a positive displacement pump as producing
the same amount of flow with each revolution, they in fact loose
some oil flow to internal leakage past moving parts.
Overall efficiency takes into account a pump’s volumetric
efficiency as well as it’s mechanical efficiency and combines
them. This is determined by dividing the hydraulic
horsepower delivered to the system by the input horsepower
of the prime mover.
Pressure Rating and Power
When the term “pressure rating” is used in reference to a
pump, it can sometimes be confused with the function of the
pump. The pressure rating of a pump refers to the pressure
of the environment in which the pump is to operate and not to
mean that the pumps produce pressure when in fact, they do
not. Pump life is in part determined by the pressure of its
operating environment and ratings are designed to give the
pump the greatest possible amount of service life. The
amount of power required by a given system will depend
largely on the pressure and flow requirements. Both internal
leakage and friction have to be taken into account when
calculating the power needed because they affect the pump’s
Calculating the amount of power needed in a system will
ultimately lead to calculating horsepower of the power unit. Many
factors are considered but the primary ones are the work load and
the speed at which we need to perform the work. There are two
formulas which can assist in this process; theoretical horsepower,
and, actual horsepower. Theoretical horsepower does not take
into account volumetric or mechanical deficiencies of a pump and
therefore is for “ballpark” figures.
Theoretical Pump Actual Pump Horsepower:
Flow Rate X Pressure
Flow Rate X Pressure
1714 X Overall Efficiency
1. What action causes oil to flow from the reservoir to the pump’s
2. Name the three types of hydraulic pumps commonly used.
3.What is the main advantage of a gear type pump?
4. Which type of pump would you use if you needed a high efficiency
5. Calculate theoretical pump flow given the following information:
Displacement = 3.0 cubic inches
Speed = 1725
Pressure = 1000 psi
Pump Type = piston
6. Using information found in the volumetric efficiency chart of this
presentation and in question 5, calculate the actual flow rate of the
7. Explain how pressure can affect the flow rate of a pump.
8. Using the data from the experiment, plot the flow rate
vs. pressure data on the graph below. Label your graph.
9. Based on the curve, or lack there of, what happens to
volumetric efficiency as discharge pressure increases?
10. Calculate the power needed to run a unit providing 7.5 gpm
at 1500 psi. The pump’s volumetric and mechanical efficiencies
are 90 and 95% respectively.