Pressure & Flow

Objectives

  • Differentiate between hydraulic pressure and flow
  • List causes for increased and decreased system pressure
  • Explain the effect of an increased flow rate on a hydraulic actuator
  • Define fluid velocity

What Creates Pressure?

Many people think pressure is created by the pump. But lets think about this.

If we take a pump that moves one gallon per minute (1 GPM) and connect a one inch (1") diameter hose that is one foot (1') in length from its outlet, and we install a pressure gauge at the end of the hose, should we expect to see any noticeable pressure reading on the gauge?

1 Foot
1 GPM
Pump
1 Inch
Diameter

Resistance to Flow

No, there is no pressure on the gauge. This is not because of a problem with the pump. If the pump is filling one gallon per minute, then the pump is working fine. There was no pressure because there was no resistance to flow.

1 Foot
1 GPM
Pump
1 Inch
Diameter
There is practically no pressure at the gauge.
Key Concept:
The pump, or more accurately, the action of pumping, does not create nor determine the pressure value in the system.

What Resists Flow?

Pressure is caused by resistance to flow.

So what resists flow? Hoses or pipes that are too small resist flow.

Key Concept:
The pressure value indicated by a gauge in a circuit where fluid is moving, is only a result of downstream resistance.
Pump
Motor
250 PSI
125 PSI
0 PSI
1
GPM
1/4"
100'
Check out the pressure values on the gauges above to see how pressure is affected by downstream resistance.
Key Concept:
The gauge can only indicate back-pressure. This means the highest normal working pressure in a hydraulic system would be at the pump outlet.

Elbows in the pipe will also cause more resistance, creating more pressure near the pump.

Pump
Motor
3 PSI
2 PSI
1 PSI
0 PSI
1"
1"
12"
1 GPM

In hydraulics, resistance to flow mainly occurs as hydraulic fluid tries to flow through heavily loaded actuators.

Motor
A heavily loaded motor.
An example of a cylinder lifting a boom.

A heavily loaded actuator could be a hydraulic motor trying to move a heavy material along a conveyor belt in a mine, or a cylinder trying to lift the boom on a crane.

In very basic terms then, pressure is an indication of force.

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We hope you enjoyed Pressure & Flow

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Atmospheric Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
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