Pilot pressure is the pressure that is used to control or operate another valve in a hydraulic system. Pilot valves are small valves that regulate the flow of a low-pressure control feed to a larger valve that handles a high-pressure or high-flow supply. Pilot valves are useful because they allow a small and easy-to-control feed to control a much larger pressure or flow feed that would otherwise require a much greater force to operate. Pilot valves can also be used to remotely control other valves, pumps, cylinders, and motors in a hydraulic circuit.
Types of Pilot Valves
There are different types of pilot valves depending on their design and function. Some common types are:
- Pilot-operated directional control valve (DCV): This is a valve that controls the direction of fluid flow in a hydraulic system. It consists of a main spool valve and a pilot valve that shifts the spool position. The pilot valve can be operated by a solenoid, a lever, a pedal, or another pressure source. The pilot valve applies line pressure to one end of the spool, while the other end is connected to a spring or another pressure source. The spool shifts when the pilot pressure exceeds the spring or the opposing pressure, allowing fluid to flow through the desired ports. A pilot-operated DCV can provide large shifting forces without the impact or wear that a manually operated valve would experience.
- Pilot check valve: This is a valve that allows free flow in one direction and blocks flow in the opposite direction until a pilot pressure is applied. It consists of a poppet valve and a pilot valve that opens and closes the poppet. The poppet valve has a spring that keeps it closed, while the pilot valve has a piston that pushes the poppet open when pilot pressure is applied. A pilot check valve can be used to lock a load in place or to prevent backflow in a hydraulic system.
- Pilot-operated relief valve: This is a valve that limits the maximum pressure in a hydraulic system by releasing excess fluid to a tank or a low-pressure line. It consists of a main relief valve and a pilot valve that controls the opening and closing of the main valve. The main relief valve has a spring that keeps it closed, while the pilot valve has a pressure sensor that detects the system pressure and opens the main valve when the pressure exceeds a set value. A pilot-operated relief valve can provide more accurate and stable pressure regulation than a direct-acting relief valve.
Pilot Ratio and Pilot Pressure
The pilot ratio is the ratio of the pilot area to the relief area of a pilot valve. The pilot area is the area of the piston or the spool that is exposed to the pilot pressure, while the relief area is the area of the poppet or the spool that is exposed to the system pressure. The pilot ratio determines how much pilot pressure is required to open or close the main valve. A higher pilot ratio means a lower pilot pressure, and vice versa.
The pilot pressure is the pressure that is applied to the pilot valve to operate the main valve. The pilot pressure can be derived from the system pressure, from an external pressure source, or from a solenoid. The pilot pressure must be greater than or equal to the product of the system pressure and the pilot ratio, plus the spring force, to open the main valve. The pilot pressure must be lower than or equal to the product of the system pressure and the pilot ratio, minus the spring force, to close the main valve.
The following table summarizes the formulas for calculating the pilot pressure for different types of pilot valves:
| Type of Pilot Valve | Formula for Opening Pilot Pressure | Formula for Closing Pilot Pressure |
|---|---|---|
| Pilot-operated DCV |
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| Pilot check valve |
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| Pilot-operated relief valve |
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