Contributed by Robert Sheaf, Founder of CFC Industrial Training
I was asked to travel to California to troubleshoot a problem with a hydraulic system. This system stroked a cylinder which pushed product into a furnace. The main directional valve was changed due to a porosity hole that started leaking oil only when the valve was shifted. After this valve change-out, it stopped cycling.
The customer was under considerable pressure to get it up and running and felt he had done all that he could. He also did not fully understand the circuit. He claimed the only thing he did was change the valve with a spare they had in stock that looked just like the original. I told him how to check the pilot’s orifice inside the valve to make sure the pilot was getting a signal from the main system supply. He found that blowing shop air into the “X” port resulted in air coming out of the “P” port. This showed it was not blocked with a solid pilot pressure plug.
What would you do next? Can you identify what was the problem? We welcome responses of all kinds directly to Robert Sheaf at [email protected] or to [email protected]. The solution will be provided in conjunction with next edition of Fluid Power World and will be published online next month.
Solution
The valve that was replaced also had a pilot choke to control how fast the main spool shifted. Many people don’t understand that there are two common ways flow controls operate. Turning the adjustment clockwise on some valves “opens” the flow controls and others are set up to close the valve if turned clockwise. See the illustration, which shows how one type restricts how far the bypass check closes to reduce the flow and others use a common needle valve. The customer assured me that the pilot chocks were fully open when they were fully closed, thus blocking the pilot flow to the main stage spool.
Filed Under: Components, Cylinders & Actuators
