Posted by Owen Aaland on June 04, 2008 at 05:36:12 from (65.165.246.71):
In Reply to: 560 hydraulic problem posted by Wayne-FDL on June 04, 2008 at 04:55:04:
This is usually caused by a plugged or restricted orifice screen assembly or worn spool valves.
The hydraulic system on the 560 is a combination of closed center and open center hydraulics. They use an open center type pump and closed center valves. The unloader valve is designed to open whenever the valves are closed or the system exceeds maximum pressure. The flow through the orifice is used to control the unloader valve. It is also ported to the spool valves. When the valves are closed there is no flow past them and the pressure builds up to open the unloader valve. Normally this results in about 60 PSI of pressure in the hydraulic system. When a valve is moved the flows from the orifice is directed to the load side of the valve and this causes a drop in pressure which will allow the unloader valve to seat. When the pressure builds to the maximum for the system the unloader valve is again unseated to control maximum pressure.
If contamination collects on the orifice screen enough to restrict the flow to less than what leaks past the spool valves there will not be enough pressure built up to open the unloader valve and the system will go on high pressure.
In your case, since it is OK when the oil is cold, I would expect to find too much wear in the spool valves so that there is too much leakage to unseat the unloader valve when the oil warms up.
Check the orifice screen assembly first. If everything looks OK there you can compensate for the worn spool valve by enlarging the orifice hole. If you drill out the hole, only go a few thousands of an inch at a time. Go just far enough th correct the problem with the hydraulic oil hot. You will need a set of number drill bits to get a small enough increment in hole size.
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