Allen Engineering Corporation HD 550, Operations Manual And Parts Book

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1A
OPERATIONS
1A-17
1.20 Hydraulic Troubleshooting
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 pres-
sure. Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated sys-
tem other components could 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.
STEP 1 - Pump Suction Strainer ...
Probably the field trouble encountered most often is cavitation of the hydraulic pump inlet caused by restriction
due to a dirt build-up on the suction strainer. This can happen on a new as well as am older system. It produces the
symptoms described above: increased pump noise, loss of high pressure and/or speed.
If the strainer is not located in the pump suction line it will be found immersed below the oil level in the reservoir.
Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these
conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the
whole system and damage to the pump.
The suction strainer should be removed for inspection and should be cleaned before re-installation. Wire mesh
strainers can best be cleaned with an air hose, blowing from inside out. They can also be washed in a solvent which is
compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum base hydraulic oil. Do not
use gasoline or other explosive or flammable solvents. The stainer should be cleaned even though it may not appear to
be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is
mechanical damage, the strainer should be replaced. When reinstalling the strainer, inspect all joints for possible air leaks,
particularly at union joints. There must be no air leaks in the suction line. Check the reservoir oil level, with all cylinders
extended. If it does not cover to this depth there is danger of a vortex forming which may allow air to enter the system
when the pump is running.
STEP 2 - Pump and Relief Valve ...
If cleaning the pump suction strainer does not correct the trouble, isolate the pump and relief valve from the rest
of the circuit by disconnecting the hydraulic lines so that only the pump, relief valve, and pressure gauge remain in the
pump circuit. Cap or plug both ends of the plumbing which was disconnected. The pump is now deadheaded into the
relief valve. Start the pump and watch for pressure build-up on the gauge while tightening the adjustment on the relief
valve. If full pressure can be developed, obviously the pump and relief valve are operating correctly, and the trouble is to
be found further down the line. If full pressure cannot be developed in this test, continue to STEP 3.
STEP 3 - Pump and Relief Valve ...
If high pressure cannot be obtained in STEP 2 by running the pump against the relief valve, further testing must
be conducted to see whether the fault lies in the pump or in the relief valve. Proceed as follows:
If a flow meter is available, the flow can be measured and compared with the pump catalog rating. If a flow meter
is not available, the rate of flow on small pumps can be measured by discharging the hose into a bucket while timing with
a watch. For example, if a volume of 10 gallons is collected in 15 seconds, the pumping rate is 40 GPM, etc.
If the gauge pressure does not rise above a low value, say 100 PSI, and if the volume of flow does not substan-
tially decrease as the relief valve adjustment is as instructed in STEP 5. If the oil substantially decreased as the relief
valve adjustment is tightened, and if only a low or moderate pressure can be developed, this indicates trouble in the pump.
Proceed to STEP 4.