May, 21 2010

Hydraulic

The Unloading Valve - Part I

In this edition of Newsletters that Teach we will examine the unloading valve and find out how it minimizes energy waste.

Mechanical Engineering Symbol

This symbol indicates a mechanical point of mount or friction. It is used in the schematic below to indicate that the spring is mounted to a fixed point.

The unloading valve is a device that is typically used in a high-low circuit. The circuit is sometimes referred to as a fast approach, slow-feed circuit. The valve is put to good use in a system where a large flow volume is needed at a lower pressure, and then later, a very low flow volume is required with a higher pressure.

Typically, two pumps, plumbed in parallel, are used in such a circuit. One fixed displacement pump provides volume for the lower pressure, high flow mode, while a small pump provides the low flow at the higher pressure. Both pumps provide a flow volume in the low pressure, high-flow mode. The purpose of using the two pumps is to save energy draw at the prime movers during the long periods when the only function of the hydraulic system is to maintain an even maximum pressure with very little or no flow.

When the system cylinders are extending quickly at low pressure (any pressure below the setting of the unloading valve), both pumps send their full volume into the cylinder.

When the cylinders encounter enough mechanical resistance to bring the system pressure up to the setting of the unloading valve, the high flow volume pump is directed to tank through the piloted unloading valve. Unlike a relief valve, the restriction of the operated unloading valve on the flow passing through it is minimal. Therefore, the unloading valve does not pose a heating problem. The volume of the smaller pump cannot escape to tank through the unloading valve due the check valve in the system. The presence of the check valve makes the schematic symbol for the unloading valve distinct from a typical relief valve.

As the cylinder continues to move against resistance (system pressure) that is higher than the unloading valve setting, the motion is slower as only the volume from the small pump is available. In many cases the cylinders come to a halt as the machine is actively clamping an assembly or machine apparatus of some sort. At this point, the main relief valve will open and limit the maximum system (or clamping) pressure. If a relief valve is not used to limit maximum system pressure, then the small pump will be of the variable displacement/pressure compensated variety.

In many systems, the large pump is referred to as the volume pump. The small pump is often referred to as the pressure pump since it is often just used to support a system pressurizing function when the cylinders are used in a clamp mode. The small pump will only supply enough volume to the cylinders to make up for any internal leakages in the system.

In The Unloading Valve - Part II, we'll look at a plant application and a mobile machine circuit where unloading valves are utilized.