Contributed by Robert Sheaf • Founder of CFC Industrial Training
I was recently called by a customer who attended one of our advanced hydraulic training sessions. He was having a problem with an old rolling mill.
The servo-controlled cylinder was oscillating that held position on one of the rolls. This cylinder helped maintain the thickness of the sheet steel they produce. He said it seemed like it was having problems finding and maintaining its position.
He sent us the attached circuit drawing made by a local hydraulic shop that he deals with for repairs and service. It was a hand-drawn sketch that we redrew for this article.
He had replaced the Moog servovalve, replaced the filter elements and the electronic position feedback sensor. We asked him if there was any work done before this problem showed up. He informed us that they replaced a leaking steel tube and its fitting on the cap end of the cylinder with a new hose.
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 our next edition of Fluid Power World.
Solution
Servo systems are every sensitive to the compressibility of the oil and stretch of the mechanical system. Replacing a steel line with a hose caused the problem. Hoses expand when pressurized and contract when the pressure drops. Harmonics of the system was changed when replacing a steel tube with a hose. Replacing the hose with a steel tube solved the problem.
Filed Under: Components, Cylinders & Actuators, News
