Load-holding Valve: Securing Working Machines and Lifting Platforms

The load-holding valve is a small, yet very important component, primarily used in hydraulically operated working machines. Its function is exactly what its name implies: to hold a load in the defined position and prevent it from dropping. Technically, the load-holding valve has a relatively simple design. However, its correct installation is a challenge.

Load-holding Valve: Load-side Pressure in Hydraulic Systems

When a hydraulic cylinder lifts a load against gravity, the weight always exerts counter-pressure to the hydraulic pressure. As long as the pump maintains sufficiently high pressure against it, the forces are balanced, and the load remains in place. However, this requires the hydraulic pump to be in continuous operation. This is not only uneconomical but also puts significant strain on the entire hydraulic system. As soon as a malfunction occurs, such as a seal failing or a hose bursting, the lifted load will drop. This can lead to dangerous situations.

The load-holding valve ensures that the pre-tensioned pressure in the cylinder is maintained even when the pump is switched off or a leakage occurs. It is therefore a central component for machine safety in demanding working machines.

Controlled Lowering with Load-holding Valves

In addition to its primary function of maintaining pressure in the cylinder, the load-holding valve can be used for another purpose. By finely metering the opening of the valve, the lowering speed of a loaded cylinder can be precisely adjusted. The load-holding valve thus not only fulfills safety tasks but also significantly contributes to the better handling of the hydraulic system.

Structure of a Load-holding Valve

Simply put, a load-holding valve is a simple shut-off valve equipped with a coil spring. The coil spring builds up the counter-pressure to the load on the hydraulic cylinder. As long as the spring tension is greater than the pressure in the fluid, the load-holding valve blocks the flow. If the spring is equipped with an adjusting screw, the spring tension can be varied. The more the spring is relaxed, the more hydraulic pressure can escape past the load-holding valve. This makes dosing the lowering speed very convenient.

Two Principles for Load-holding Valves

The described load-holding valve, consisting of a valve cone, spring, and adjusting screw, is also known as a “direct-acting load-holding valve.” In addition to these standard valves, there are also “differential area valves.”

Direct-acting load-holding valves are simple in design and inexpensive. However, they have a disadvantage: if the load forces the hydraulic cylinder to retract, the hydraulic oil is pushed back into the tank. This causes the pressure in the hydraulic system to drop, and the load-holding valve closes again. To counteract this, the spring tension would have to be constantly adjusted. Otherwise, swaying and jerking would occur during the lowering process.

Load-holding valves with differential area technology provide a constant flow of hydraulic oil. The lowering movement is thus much smoother and more controlled. These valves achieve this performance through a specially shaped cone geometry inside them.

Application of Load-holding Valves

Load-holding valves are used wherever a load must be held. They are standard components in dump trucks, lifting platforms, excavators, and material handling machines. In addition to their main function of keeping the lowering movement of the load as controllable as possible, they also relieve weaker components of a hydraulic system. This significantly contributes to extending the lifespan of hoses and seals.