Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/778560
www.machinerylubrication.com | January - February 2017 | 13 found all of these to be good. So why was the actuator slowing down under heavier loads and speeding up under lighter loads? The problem turned out to be a bad hydro- stat module. The module had not been replaced because the valve was assumed to be a pressure-reducing valve based on its hydraulic symbol. Since the pressure down- stream of the valve stack seemed normal, they thought the pressure-reducing valve was functioning properly. Understanding Hydrostats Hydrostats are often misunderstood, and mistaking them for pressure-reducing valves is not uncommon. The schematic symbol for a hydrostat is indeed the same as that for a pressure-reducing valve, and in fact that is exactly what the hydrostat is. It's easy to miss the way the hydrostat is connected, particu- larly on the hydraulic installation schematics made by the equipment manufacturer. In Figure 1, a typical pressure-reducing valve installation is depicted. Figure 2 shows a common hydrostat configuration. Notice that the drain line of the pressure- reducing valve goes directly to the tank, whereas the drain line of the hydrostat is connected in the line to the full piston side of the cylinder. Whatever pressure is exerted to extend the cylinder will also be applied to move the hydrostat toward its closed position. Thus, the heavier the load on the cylinder, the more pressure is applied to close the hydrostat. The result will be that the pressure drop across the proportional valve will be equal to the spring setting of the hydrostat. For example, if the weight of the load requires 500 pounds per square inch (PSI) to move (as in Figure 3), the hydrostat setting is 200 PSI and the pump compen- sator is set to 1,500 PSI, the pressure upstream of the proportional valve will be regulated to 700 PSI. If the load weight increases to 1,100 PSI (Figure 4), the hydro- stat will regulate the pressure upstream of the proportional valve to 1,300 PSI. Using a Hydrostat Module So what does all of this have to do with the speed of the actuator? Isn't speed deter- mined by volume, not pressure? To comprehend this fully, you must first under- stand how the pressure drop across an orifice affects flow. Essentially, the higher the pressure drop across an orifice, the more flow will be delivered through it. A propor- tional valve functions exactly the same as a variable orifice flow control. The greater the command signal, the more the proportional valve will open, the more flow is delivered and the faster the actuator moves. Just as with any other orifice, the greater the pres - sure drop across a proportional valve, the more flow will pass. Using a hydrostat module effectively converts the proportional valve to a pressure-compensating flow control so that the flow through the valve will not be affected by changes in pressure. Back to the paper mill, when we tried to adjust the hydrostat, we found that it would not adjust. The internal spool was stuck. The hydrostat was replaced, and the cylinder on the winder moved at a constant speed, regardless of the weight of the load. In conclusion, don't overlook the importance of your hydrostats. These modules are essential for regulating pres- sure when the weight of the load changes. Be sure to keep them adjusted for the minimum acceptable pressure drop so you can ensure your hydraulic systems run smoothly . About the Author Jack Weeks is a hydraulic instructor and consultant for GPM Hydraulic Consulting. Since 1997 he has trained thousands of electricians and mechanics in hydraulic troubleshooting methods. Jack has also taught radio-wave propagation for the U.S. Air Force and telecom - munications equipment operation and repair for the Central Intelligence Agency at American embassies overseas. Figure 3. With the weight of the load requir- ing 500 PSI to move, the hydrostat setting at 200 PSI and the pump compensator at 1,500 PSI, the pressure upstream of the proportional valve is regulated to 700 PSI. Figure 4. When the load weight increases to 1,100 PSI, the hydrostat regulates the pressure upstream of the proportional valve to 1,300 PSI.