All hydraulic systems leak to some degree and any suspended load held on a ram will eventually creep down due to gravity. However there are occasions when a ram will creep out when the ram is mounted horizontally or even pointing upwards.
Whenever I am asked to investigate this fault I always look for the following:
1) Pressure is applied to the valve all the time (usually pressure compensated pump).
2) The control valve has a closed centre position (all ports blocked).
If we analyse this arrangement we must consider what is happening to the control valve. Pressure is being applied to the closed port (P). The A&B ports are situated either side of the pressure port (P) and the tank ports (T) are furthest away. When the valve is brand new the leakage is minimal but as time passes spool wear increases together with leakage from port P (pressure) to A&B. The same pressure applied to both sides of a ram will cause it to extend due to the difference In the area (full bore and annular).
There are two ways you can resolve this problem.
1) Replace the control valve with same.
2) Replace the valve with centre position P blocked with A&B to T and add a double PO check valve.
In this set up any leakage across P will flow directly to tank and should not create enough pressure to force the ram out. The PO check valves will lock the ram in position against external loads.
Hope you find this helpful
Bob Jackson (The Hydraulicman)
More of this can be found at www.hydraulicbrain.com
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