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OPERATIONS

1.20 Hydraulic Troubleshooting

(Cont’d)

STEP 4 - Pump ...

If a full stream of oil is not obtained in STEP 3, or if the stream diminishes as the relief valve adjustment is tight-

ened, the pump is probably at fault. Assuming that the suction strainer has already been cleaned and the inlet plumbing
has been examined for air leaks, as in STEP 1, the oil is slipping across the pumping elements inside the pump. This can
mean a worn-out pump, or too high an oil temperature. High slippage in the pump will cause the pump to run consider-
ably hotter than the oil reservoir temperature. In normal operation, with a good pump, the pump case will run about 20ºF
above the reservoir temperature. If greater than this, excess slippage, caused by wear, may be the cause.

Check also for slipping belts, sheared shaft pin or key, broken shaft, broken coupling, or loosened set screw.

STEP 5 - Relief Valve ...

If the test in STEP 3 has indicated the trouble to be in the relief valve, the quickest remedy is to replace the valve

with one known to be good. The faulty valve may later be disassembled for inspection and cleaning. Pilot-operated relief
valves have small orifices which may be blocked with accumulations or dirt. Blow out all passages with an air hose and
run a small wire through orifices. Check also for free movement of the spool. In a relief valve with pipe thread connec-
tions in the body, the spool may bind if pipe fittings are over-tightened. If possible, test the spool for bind before unscrew-
ing threaded connections from the body, or screw in fittings tightly during inspection of the valve.

STEP 6 - Cylinder ...

If the pump will deliver full pressure when operating across the relief valve in STEP 2, both pump and relief valve

can be considered good, and the trouble is further downstream. The cylinder should be tested first for worn-out or defec-
tive packings by the method described below.

Cylinder Testing - Run the piston to one end of its stroke and leave it stalled in the 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 position and run the piston rod against it for testing. Once in a great while
a piston seal may leak intermittently. This is usually caused by a soft packing or O-ring moving slightly or rolling into dif-
ferent 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 of 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.

STEP 7 - Directional Control Valve ...

If the cylinder has been tested (STEP 6) and found to have a reasonably tight piston seals, the 4-way valve should

be checked next. Although it does not often happen, an excessively worn valve spool can slip enough oil to prevent build-
up of maximum pressure. Symptoms of this condition are a loss of cylinder speed together with difficulty in building up to
full pressure even with the relief valve adjusted to a high setting. This condition would be more likely to occur with high
pressure pumps of low volume output, and would develop gradually over a long period of time. Four-way valves may be
tested by the method described below.

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
exhausts or tank return line while the valve is under pressure is the amount of leakage. Repeat the test in all other work-
ing positions of the valve.

Other Components ...

Check other components such as bypass flow controls, hydraulic motors, etc. Solenoid 4-way valves of the pilot-

operated type with tandem or open center spools may not have sufficient pilot pressure to shift the spool.

1A-18