When an excavator is working hard and the cylinder stalls, the hydraulic system hits the main relief valve, which is usually set around 5,000 PSI. But when an operator drops a heavy rock out of the bucket and the boom shocks, the pressure can spike to 8,000 PSI for a millisecond. Why doesn't the cylinder explode? Because of the shock relief valve-a component mechanics frequently mistake for the main relief.
The main relief valve is located in the main control valve bank. It is designed for steady-state pressure limiting, but it is mechanically too slow to react to a shock load (water hammer). By the time the main relief spool cracks open, the shock wave has already destroyed the cylinder seals.
To protect the machine, each cylinder circuit has a "cross-port" or "shock" relief valve mounted directly on the cylinder itself (or right at the boom joint). These are small, pre-loaded poppet valves that react in microseconds. If a shock wave travels down the hose, the cross-port relief instantly dumps oil across the A and B ports before it reaches the piston. If a machine is experiencing blown cylinder seals or bursting hoses under shock loads, do not adjust the main relief. The main relief is fine; the tiny cross-port shock reliefs on the cylinder are stuck shut and must be cleaned or replaced.
