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?

So what resists flow? For starters, hoses or pipes that are too small resist flow.

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

A far the greater source of resistance to flow occurs as hydraulic fluid tries to flow through heavily loaded actuators.

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.

The exact system pressure that shows on a gauge is determined by the resistance of the entire system.

This includes fittings, hoses, valves, cylinders, motors, filters, etc. In this example, lets assume that the only resistance in the system is from the load to be lifted by the cylinder.

If the pump pushes fluid into a cylinder that is required to lift a 10,000 pound brick, how much pressure will there be?

In order to determine the pressure, we will also need to know the surface area of the piston.

If the piston is 10 square inches, then the system pressure gauge will read 1000 pounds per square inch (psi).

Pressure is developed in the system by resistance to flow, which is easy to calculate when weight and actuator surface areas are known.

Pressure also builds in systems whenever there is a pinch or blockage in the lines. This can be caused from a malfunction or from the setting on a restrictive valve such as a flow control or throttle valve.