SCREEN 9T5003 - Caterpillar

Steering Control Unit (SCU)
STEERING CONTROL UNIT 1. Body.2. Left turn port.3. Outlet to tank.4. Right turn port.5. Inlet from pump.6. Passage.7. Passage.8. Gerotor.9. Drive.10. Sleeve.11. Passage.12. Right turn passage.13. Left turn passage.14. Pin.15. Spool.16. Centering springs.A. Control valve section.B. Pump section.The steering control unit (SCU) has two main sections; control section A and pump section B. These two sections work together to send oil to the steering cylinders. The SCU is located just under the front of the cab.The direction and the speed of a turn are controlled by the SCU. As the steering wheel is turned faster, there is an increase in the flow of oil to the steering cylinders. This increased flow causes the cylinders to move farther and faster, which turns the machine faster.When the steering wheel is not being turned and the steering control unit is in the NEUTRAL position, oil from the flow control and relief valve group enters the SCU through inlet (5) and passage (6). Since the SCU is open-centered, a passage is open through sleeve (10) and spool (15) that lets the flow go through to outlet (3) and back to tank.When the steering wheel is turned for a right turn, pump oil flows into the SCU through inlet (5) and passage (6). From passage (6) it goes into the gerotor through passage (11). With the steering wheel turning, gerotor (8) is turning and pumping oil. The oil flow comes out through passage (7), where it is directed to the steering cylinders through the right turn port (4). When the steering wheel started to turn; shaft spool (15), pin (14) and drive (9) also started to turn. Sleeve (10) did not start to turn at the same time as the spool because the diameter of the holes in the sleeve are slightly larger than the diameter of pin (14). This allows spool (15) to turn inside of sleeve (10) enough to put the small holes in sleeve (10) in line with the grooves in spool (15). The alignment of these holes and grooves provides the path for oil flow to gerotor (8) and to the steering cylinders. Centering springs (16) are compressed when the spool moves in relation to the sleeve. When the steering wheel is no longer turning, the springs will bring the spool and sleeve back to a NEUTRAL position. This means that the wheels will stay in the position they were in when the steering effort stopped. The steering wheel must be turned in the opposite direction to bring the wheels back to straight ahead, or to make a left turn. As the oil goes out to the cylinder through port (4) for a right turn, return oil from the cylinder is coming back into the SCU through outlet (3). This oil is sent to the tank.
SPOOL AND SLEEVE
10. Sleeve. 15. Spool. 17. Slots for springs. 18. Holes for pin. 19. Small holes for oil flow. 20. Grooves for oil flow.When the steering wheel is turned to