Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/778560
12 | January - February 2017 | www.machinerylubrication.com HY DR AULICS Know About Hydrostats By Jack Weeks, GPM Hydraulic consultinG 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 stan- dard 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 manu- ally adjusted whenever the load weight on the cylinder changed enough to adversely affect its performance. The first action the technician took was to replace the (very expensive) valve, assuming that the speed variation must have been caused by incorrect shifting of the valve spool. He was quite disappointed to find that this did not correct the problem. Actually, this unnecessary component replacement could have easily been avoided by checking the valve with a test box. This particular proportional valve was equipped with a linear variable displacement trans - ducer (LVDT), which electronically feeds back the actual position of the valve spool. The test box could have been connected to drive the valve either by the programmable logic controller (PLC) or an external signal generated by the box and measuring the LVDT feedback. If the valve behaves the same when driven by the test box and the LVDT feedback corresponds correctly to the command signal, the proportional valve is very unlikely to be bad. When I arrived, we tested the valve, its amplifier and the PLC signal to the valve. We What You Should Figure 1. A typical pressure-reducing valve installation Figure 2. A common hydrostat configuration