A relief valve limits the highest pressure in a hydraulic system. It stays closed while the pressure is below its setting and opens when the pressure reaches it, sending oil back to the tank so the pump, hoses and cylinders are never loaded beyond their rating. On an injection molding machine the main relief valve sits on the valve manifold near the pump supply and is often called the safety valve. On the drawing, it is the valve mounted on the right side of the manifold block, with its spring housing and adjusting screw outside.
How a relief valve works
Direct-acting relief valve
A spring holds a poppet or ball on a seat. When the pressure under the poppet multiplied by the seat area exceeds the spring force, the poppet lifts and oil flows to the tank. An adjusting screw sets the spring force, and a lock nut holds the setting. Direct-acting valves respond fast and are simple, but the pressure rises noticeably as more flow passes through them, so they are used for smaller flows and as the pilot stage of larger valves.
Pilot-operated relief valve
A main poppet or spool carries the full flow. System pressure acts under it and, through a small damping orifice, also in the chamber above it, where a light spring holds it closed. A small direct-acting pilot valve on top limits the pressure in that chamber. When the system reaches the pilot setting, the pilot opens, oil flows through the orifice, and the pressure difference across the main stage lifts it so it can pass a large flow with little further rise in pressure.
The pilot chamber is usually brought out to a vent port. Connecting the vent to the tank through a small solenoid valve unloads the pump at low pressure; connecting it to a remote or proportional pilot relief valve lets the pressure be set from elsewhere.
The relief valve / safety valve we supply is pilot-operated, with a pressure range of 3–25 MPa and a maximum rating of 31.5 MPa.
The relief valve and pressure control
- On fixed pump machines, the proportional valve sets the working pressure for each step, and a mechanical safety relief is set above the highest working pressure as a backstop.
- On variable pump and servo pump machines, the pump control or the servo drive sets the pressure, and the relief valve stays as protection.
- Other relief valves on the machine include the back-pressure valve used during screw recovery, the relief valve in the accumulator safety block and, on some machines, cross-port relief valves on hydraulic motors.
Signs of a faulty relief valve
- The system cannot reach its set pressure, or the maximum pressure is lower than it used to be.
- No pressure at all: the main stage stays open, or the vent line is open to the tank.
- Pressure higher than the setting, or pressure peaks: the valve is stuck closed, which is dangerous.
- A squeal, whistle or chatter from the valve.
- Hot oil: the relief is passing flow continuously, for example because it is set too close to the working pressure.
Why relief valves fail
- Contamination blocks the damping orifice or holds the poppet off its seat.
- Erosion of the seat and poppet by oil passing at high pressure.
- Spring fatigue, which lowers the opening pressure.
- Wrong adjustment, by someone trying to cure a pressure problem elsewhere in the circuit.
Testing and setting a relief valve
The relief valve protects people and equipment. Only a trained technician should adjust it, using the values and the procedure in the machine manual, and never above the maximum the manual allows or the rating of the components.
- Fit a calibrated pressure gauge at the test point on the pump outlet.
- Follow the manual's order for setting. On a fixed pump machine, the safety relief must end up above the highest pressure the proportional valve is set to give, or the two valves fight each other.
- In manual mode, build pressure with a movement held against its end stop, as the manual describes.
- Read the pressure, adjust slowly, tighten the lock nut and record the setting.
- If the pressure cannot be raised, will not stay steady or the valve is noisy, clean or replace the valve rather than turning the adjuster further.
Our guide to troubleshooting hydraulic pressure loss explains how a gauge at the pump outlet separates supply problems from leaks further down the circuit.
What to send for a replacement quote
The model code from the valve, its size (interface or cartridge size), the mounting (subplate, manifold, inline or cartridge), the pressure range, whether a vent line is connected, and photos of the valve and its nameplate. Add the machine model and serial number, then request a quote. For the block it is mounted on, see the valve manifold. The hydraulic system guide shows the other parts it protects.
Updated by the IMSPARES technical team.