TUBE 3617535 - Caterpillar

Left Turn
When the steering wheel is turned to the left, the same components move that did for a right turn, only in the opposite direction. The oil flow through the SCU is also in the opposite direction to that of a right turn. Oil still comes in at inlet port (1) and passage (6) but then goes to rotor (11) through passage (5). From rotor (11), the oil flows through passage (13) and goes through the sleeve and spool to left turn port (4). From port (4), the oil flows to the steering cylinders.The oil volume supplied to the cylinders is proportional to the angular rotation of the steering wheel.SCU Valves
SCU Schematic
(24) Inlet check valve. (25) Relief valve. (26) Suction valve. (27) Passage. (28) Anti-cavitation valves. (29) Shock valves. (30) Passage.The SCU contains suction valve (26) and a valve block that consists of inlet check valve (24), relief valve (25), anti-cavitation valves (28) and shock valves (29). All of the valves except for suction valve (26) are in the valve block.If there is an operating fault before the SCU, such as a broken oil line, pump or engine failure, suction valve (26) allows the SCU to automatically convert to a manual emergency pump.When the steering wheel is turned for a right or left hand turn, the SCU will draw oil from the return line through outlet port (3). This will cause the suction valve ball to move and allow oil to pass into passage (6). Oil is then sent through the SCU and to the steering cylinders.SCU Valve Block
SCU Valve BlockThe valve block is mounted directly to the SCU. The valve block contains two shock valves (29) with anti-cavitation valves (28), pressure relief valve (25) and inlet check valve (24).Shock valves (29) are set to open at 20 000 kPa (2900 psi). They cushion sudden high pressure increases caused by shocks on the steered drum. They protect the working lines and prevent damage to the steering system. The valves, which are direct acting, react very quickly. This allows oil which has been submitted to a sudden shock in the right and left turn working lines to pass through shock valves (29). The oil then flows into the SCU and out through outlet port (3).Two anti-cavitation valves (28) make sure there is oil suction to the chambers of the steering cylinders. Without these valves, cavitation could occur during poor steering conditions.These valves work in conjunction with the shock valves. Whenever one working line is submitted to a sudden shock, the corresponding anti-cavitation valve on the same line will be closed. At the same time, the anti-cavitation valve on the opposed working line will open due to the displacement of the hydraulic cylinder piston. Suction created by the movement of the cylinder piston will cause the anti-cavitation valve ball to move. Oil will be drawn into the SCU from the return line where it flows through passage (30) and into the working line through passage (27). This prevents cavitation.Pressure relief