CONTROL GP-TRANSMISSION HYD 6P5139 - Caterpillar

Illustration 1 g01249029
Lift circuit relief valveThe lift circuit relief valve is located under the cover on the front frame. When the lift spool in the main control valve is in the HOLD position, the lift circuit relief valve for the head end of the lift cylinders limits the maximum pressure in the lift circuit.
Illustration 2 g01041965
Section view of the lift circuit relief valve (1) Spring (2) Valve (3) Passage (4) Passage (5) Valve (6) Inlet (7) ChamberAny external force on the lift cylinders increases the oil pressure at inlet (6). Oil flows through a orifice in the centre of valve (5) and into chamber (7). The oil pressure in chamber (7) acts against valve (2). If oil pressure exceeds the force of spring (1), valve (2) will open and allow the oil in chamber (7) to return to the hydraulic tank through passage (3). Oil pressure in chamber (7) decreases, the oil pressure at inlet (6) causes valve (5) to open. Oil can now flow through passage (4), and return to the hydraulic tank. The pressure in inlet (6) is reduced in order to prevent damage to the components of the lift cylinder circuit.The lift circuit has a makeup valve in the main control valve. When the lift circuit relief valve opens, the lift cylinders retract. If the lift cylinders retract too fast, the lift circuit can not supply enough oil to the rod end of the lift cylinders creating a pressure decrease. The pressure decrease causes the makeup valve to open allowing makeup oil to flow to the rod end of the lift cylinders.