HYDRAULICS 1.49 OPERATION Do not adjust the factory set relief valves in the crowd spool. General NOTE: An identification number in bold type, after an item, refers to the number of that item in the main hydraulic schematic illustration. The crowd spool contains two relief valves, one for the crowd forward or dump (RV2) (35) set at 175 bar (2538 psi) and one for the crowd back (RV3) (39) set at 265 bar (3843 psi). Hydraulic fluid reaches the control valve inlet section (PVSP) (36) from the load-sensing pump. Spools are closed centre, dead headed into a pressure adjustment spool.
When a spool is actuated pressure is fed through the shuttle valve circuit to the spring chamber behind the pressure adjustment spool. This action restricts fluid return to tank and increases pump pressure to operate the selected circuit through the spool valve slice. Hydraulic pressure is now directed to the service with return flow diverted to tank. Relief valves RV2 and RV3 are factory set and cannot be adjusted. The pressure setting is stamped on the valve, the spring colours do not relate to the pressure rating of the valve. If the control/spool is not released when the spool reaches the end of its travel, the fluid builds up pressure.
This pressure is also sensed in the LS circuit, when this pressure reaches maximum of 250 bar (3625 psi) the pump de-swashes. It is important that the relief valves are not allowed to remain open for more than a few seconds, otherwise extreme fluid temperatures will result and cause damage to the hydraulic seals and reduce the life of the hydraulic fluid. The speed of operation of the services is related to the amount of spool movement and therefore fluid flow. If external pressure is generated by accidental contact with buildings or road surfaces, etc, the System is protected by installation of relief valves.
Relief valves Relief valves are fitted to the control valve to control system pressure and the carriage/crowd service pressure. The boom extend/retract, lift/lower and all auxiliary pressures are protected by the main system relief valve (RV1) (32) in the outlet section (PVP) (25) of the control valve (25). The relief valve is set at 290 bar (4205 psi). It is unlikely that this is opened, as in normal circumstances the pump de-swashes when 250 bar (3625 psi) is sensed in the LS line.