What is a unloading valve?

Unloading valves are pressure-control devices that are used to dump excess fluid to tank at little or no pressure. A common application is in hi-lo pump circuits where two pumps move an actuator at high speed and low pressure, the circuit then shifts to a single pump providing high pressure to perform work.

What is the main function of unloading valve?

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

What is the function of unloading valve and sequential valve?

Pressure valves with a switching task are referred to as pressure valves with fixed throttle: sequence valves and unloading valves. Sequence valves are used to direct the flow to a secondary circuit. When the primary hydraulic circuit reaches a preset pressure, flow is permitted to the secondary circuit.

What are relief valves used for?

A pressure relief valve, at times called a relief valve, is one that is used to control or put a limit on the pressure within any given system to prevent it from building up and causing damage to equipment or causing more serious hazards.

When does an unloading valve start to unload?

The valve begins unloading the accumulator system when the accumulator-system pressure drops to the lower limit of the set pressure range. The valve unloads and bypasses the fluid output of the pump back to the reservoir when the accumulator pressure reaches the upper limit of the set pressure range.

How are unloading valves similar to pilot operated relief valves?

Unloading valves are very similar to pilot operated, balanced relief valves. The only difference is where the pilot pressure comes from. In unloading valves when external pressure is applied to Port X that is greater than the pilot setting, the pilot poppet will crack open, allowing the main poppet to open.

What kind of pressure regulator is an unloading valve?

A special type of pressure regulator installed as part of an accumulator system.

Why are relief and unloading pressure controls important?

Some bypass fluid at low or no pressure when activated. This chapter only covers relief valves and unloading valves because they are closely associated with hydraulic pumps. The other pressure-control valves are part of the control circuit and will be dealt with after directional control valves. Why relief valves?