Not long ago, I was called to a paper mill to diagnose a winder with a cylinder that moved at different speeds whenever the weight of its load changed. The cylinder was operated by a proportional valve, so the speed should have been determined solely by the command signal applied to the valve. Now the whole purpose of a proportional valve is to provide accurate speed control. A standard directional valve is always either fully open or fully closed, but a proportional valve will open by an amount proportional to the electronic command signal applied, hence its name. The more the valve spool opens, the greater flow is delivered to the actuator and the faster it will move. In order to maintain production until the problem was located, the command signal to the proportional valve had to be manually adjusted whenever the load weight on the cylinder changed enough to adversely affect its performance.
