BOLT 4J0862 - Caterpillar

Illustration 1 g06194553
View of left-side frame
(1) Steering control valve
(2) Priority valveThe steering control valve is mounted on a plate attached to the left-side frame under the cab along with the steering priority valve. The Steering control valve and priority valve work along with the metering pump (HMU) to steer the machine.Left Turn
Illustration 2 g06195147
Schematic of HMU in left turn example
(1) "Gerotor" pump inside the HMU
(2) Rotary spool inside the HMU
(3) Selector spool for steering control valve
(4) Spool dampening orifices
(5) Pilot load check valves
(6) Steering cylinders
(7) Crossover relief valves
(8) make up valves
(9) Directional spool for steering control valve
(10) Steering control valve
(11) Priority Valve
(12) Hand Metering unit (HMU)
(13) Charge pressure oil from accumulatorsShown above is a schematic of the Steering Control Valve (10) and HMU (12) in the STEER LEFT operation.As the operator turns the steering wheel to the left, rotary spool (2) is rotated (moved to the right, above) and "gerotor" pump (1) also rotates. Steering charge oil from Steering Control Valve (10) is metered through rotary spool (2) by the "gerotor" pump (1) through a valve port as signal oil to open priority valve (11), increasing oil flow rate from the accumulators. "Gerotor" pump (1) also sends reduced (pilot) pressure oil through another port to the Steering Control Valve (10).The pilot oil moves selector spool (3) to the right, further sending the pilot oil to move directional spool (9) to the right. As directional spool (9) moves right, steering charge pressure oil from priority valve (11) flows across the spool to the rod end of the left steering cylinder (6) and the head end of the right steering cylinder (6). The left cylinder rod retracts and the right cylinder rod extends, steering the machine to the LEFT, since the cylinder rods connect to the steering arm rearward of the pivot points. Oil returning from the steering cylinders flows back across directional spool (9), directly to tank.When the desired turning radius is reached and the operator ceases rotating the steering wheel, "gerotor" pump (1) stops metering oil and the pilot pressure slowly drains to tank through the spool damping orifices (4) , then through the opposite pilot load check valve (5), allowing directional spool (9) and then selector spool (3) to move slowly toward center. This action stops the cylinders from moving, but holds them in the position last commanded. More movement of the steering wheel results in more movement of the wheels, in the direction of steering wheel rotation.Crossover Relief Valves
During operation of the machine, external forces which try to turn one wheel or both in a direction other than requested, can induce pressure spikes on one side of the cylinder pistons and vacuums on the other sides of the pistons. The crossover relief valves (7) protect the cylinders and other components from pressure spikes.These valves open at approximately 19700 kPa (2857 psi). The makeup valves (8) open to replace oil when the difference in pressure across the valves is approximately 35 kPa (5 psi), preventing cavitation.Priority Control Valve
The priority