HYDRAULIC SYSTEM CD3340B/YB4411
4-14
Published 04/07/2015 Control # 569-00
Holding Valve
The holding valve has three functions:
1. Inhibit cavitation of the cylinder,
2. Give full control of the lowering of the boom,
3. Hold the load in event of a hydraulic failure.
Pilot pressure from the rod side of the piston opens the
holding valve. If the cylinder starts to retract faster than the
pump can fill the cylinder, there will be a decrease in the pilot
pressure. The holding valve will close and interrupt the flow
of oil from the cylinder. The holding valve will interrupt the
flow as often as necessary to keep the cylinder filled. Also, if
there is a failure of the pump or a hydraulic line, the holding
valve will hold the boom in position.
Engine speed is important for good lowering of the boom. At
low engine speed, normally there will not be enough oil from
the pump to keep the cylinder filled. As a result, the boom will
move down in a movement that is not regular.
Lift Cylinder Leakage Check
If internal leakage in one or both of the cylinders is
suspected, check for leakage as follows:
1. Remove the suspected cylinder from the machine.
2. Mount the cylinder in a suitable cylinder stand.
3. Connect a portable hydraulic system to the cylinder
base end.
4. Apply pressure to extend the cylinder rod.
5. Check the amount of leakage from the rod port of the
cylinder. If the cylinder leaks more than a few drops of oil
per minute, replacement of the piston seals is
necessary, see page 4-44.
WARNING
Do not adjust the holding valve setting. The valve is
adjusted by the manufacturer.
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