 Welcome to Unloading Valves. Unloading valves are often used in the following situations. In applications where the first portion of a cylinder stroke requires high flow and low pressure. For the remainder of the cylinder stroke, which requires low flow and high pressure. A practical example is a metal stamping machine. During the first part of its machine cycle, the cylinder moves quickly downward into position. It requires minimal pressure because the only resistance is the flow resistance and the friction of the cylinder. When contact is made with the sheet metal, the stamping machine slows as a resistance develops when the metal is deforming. Unloading valves are similar to pressure relief valves. Their symbols are shown at the right. Both types of valves send fluid back to a tank when a preset pressure is reached. However, unlike a pressure relief valve, which is activated using pressure from its own line, the unloading valve dumps the fluid when the fluid from an external line reaches a certain pressure. Unloading valves are typically used in systems that have two fixed displacement pumps. There are two functions of the unloading valve. Back the prime mover and save energy. During the initial extension of the cylinder, both pumps provide fluid to the system, causing a rapid movement. When the cylinder reaches a position that causes resistance, pressure increases. When a pre-determined pressure level is reached, the unloading valve opens and directs fluid from the high flow pump to the tank. Once the fluid that flows to the cylinder comes only from the low flow pump, the flow is reduced. However, the required pressure needed to perform the second part of the cycle is achieved. When the cylinder reaches full extension, a back pressure develops. The relief valve opens and directs fluid from the low flow pump to the tank when 3500 pounds per square inch is reached. Here is a diagram of the unloading valve. If the piston first extends, there is minimal resistance at the cylinder, so there is negligible back pressure. The piston extends rapidly because it receives fluid from both pumps. When the piston encounters resistance, a back pressure forms. When the pressure becomes high enough to overcome the spring setting of 1000 pounds per square inch on the unloading valve, the valve opens and directs the flow from pump one to the tank. A check valve prevents the fluid from pump two flowing into the tank. Instead, the fluid is directed to the cylinder. Since the fluid that goes into the cylinder is now being pumped by one pump instead of two, the rod extension slows down. When the piston rod reaches the full extension, the back pressure increases to 3000 pounds per square inch and activates the pressure relief valve so that the fluid from pump two goes to the tank. Typically, the unloading valve is set at 1000 to 1500 pounds per square inch, while the pressure relief valve is set at pressures ranging from 2500 to 3000 pounds per square inch. Congratulations! You have completed the activity unloading valves.