Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.

Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system other components could also be at fault, but this would require the services of an experienced technician.

By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, then if necessary, each component in that area can be tested or replaced.


Cylinder Testing
Run the piston to one end of its stroke and leave it stalled in this position under pressure. Crack the fitting on the same end of the cylinder to check for fluid leakage.

After checking, tighten the fitting and run the piston to the opposite end of the barrel and repeat the test. Occasionally a cylinder will leak at one point in its stroke due to a scratch or dent in the barrel. Check suspected positions in mid-stroke by installing a positive stop at the suspected position and run the piston rod against it for testing. Once in a great while a piston seal make le ak intermittently. This is usually caused by a soft packing or O-ring moving slightly or rolling into different positions on the piston, and is more likely to happen on cylinders of large bore.

When making this test on hydraulic cylinders, the line should be completely removed from a cylinder port during the test. The open line from the valve should be plugged or capped since a slight back pressure in the tank return line would spill oil from the line if not plugged. Pistons with metal ring seals can be expected to have a small amount of leakage across the rings, and even "leak-tight" soft seals may have a small bypass during new seal break-in or after the seals are well worn.



4-Way Valve Testing
For testing 4-way valves, either air or hydraulic, it is necessary to obtain access to the exhaust or tank return ports so that the amount of leakage can be observed. To make the test, disconnect both cylinder lines and plug these ports on the valve. Start up the system and shift the valve to one working position. Any flow out the exhaust or tank return line while the valve is under pressure is the amount of leakage. Repeat the test in all other working conditions of the valve.

Replaced a pump or motor 

To select a replacement for a broken or worn out hydraulic pump or motor which has no nameplate or has no rating marked on its case, use the formulas below after making internal physical measurements.

When replacing a pump, catalog ratings will usually be shown in GPM at a specified shaft speed. On a motor, catalog ratings will usually be in C.I.R. (cubic inches per revolution). Formulas are given for calculating either GPM at 1800 RPM or calculating C.I.R. Use the formula which is appropriate. Make all measurements in inches, as accurately as possible. Convert fractional dimensions into decimal equivalents for use in the formulas.

Make sure the catalog pressure rating is adequate for your application, and in the case of a pump, be sure direction of shaft rotation is correct.

If a pump of higher GPM has to be used, it will require more HP at the same pressure and cylinders in the system will move faster. If one with lower GPM is used, the system will have plenty of power but cylinders will move more slowly than originally.

If a motor with greater displacement is used, it will deliver more torque at a reduced RPM, but will require no more fluid HP from the pump. If it has less displacement it will rotate faster with less torque.


  Gear Pump

Vane Pump
(Balanced Type Only)