Re: Re: HYDRAULIC TUTORIAL

Posted by TomG on August 11, 2003 at 03:47:59 from (206.51.253.39):

In Reply to: Re: HYDRAULIC TUTORIAL posted by Wayne on August 10, 2003 at 19:35:35:

Pretty good trouble-shooting and safety comments here. Sounds like the problem is on its way to being found. Sure does sound like a cylinder side obstruction that's acting as a check valve on the exhaust side. Too bad switching lines without quick-connects is such a pain. If the lift and bucket lines were swapped and the problem moved from the lift to bucket the diagnosis would be almost definite and also suggest that the valve isn't involved except.

Packing or line deterioration sure could do it as was mentioned originally. Disconnecting a quick-connect would simulate a hard cylinder obstruction and could also have been the problem, which is why I used the illustration. It sounds like there's an obstruction that isn't quite as hard and and passes a bit of fluid each time the valve is operated, which is why the bucket can be lowered eventually. Such a condition definitely would open a relief valve.

The basic idea of an open centered system is that the pump circulates oil continuously and there always has to be a return path to the reservoir. With no control valve operated, oil flows through the valve from the inlet to outlet port to the reservoir--often through a manifold/diverter and on to the 3ph. Operating a control valve blocks the return line and creates a high-pressure and a low pressure side of the system at that point. The valve also opens one cylinder line to the high pressure side and the other side to the low pressure side. Pump flow is diverted to the high pressure side of the cylinder. Exhaust oil in the other side is displaced to the return line.

There's a bunch of stuff omitted from this basic idea. I hope it illustrates that a obstruction anywhere on the exhaust oil side can prevent movement. The oil has to have a place to go. Otherwise it acts as an infinite load and a pressure relief valve provides a return path to the reservoir. In testing, it's a good idea to know where relief valves in a system are and ensure that a pump is never dead-ended. A common result is a broken key on the pump drive shaft. Dang! I think I've written too much to take more space to say what I'd actually do to find the problem.

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