PISTON 9J1776 - Caterpillar

When signal oil pressure combines with the force of springs (15), flow compensator spool (12) moves downward. Then, the flow compensator spool blocks the flow of pump oil through passage (35).While the flow compensator spool (12) is lowered, the oil in the chamber of destroke piston (29) can flow into passage (30). The oil flows past the cavity of spring (24), past flow compensator spool (12), and through return oil line (27). The oil then flows back to hydraulic tank (26).Oil from the steering pump flows through passage to upstroke piston (41) and into spring chamber (34). The oil in the chamber of destroke piston (29) is now vented. The combined force of spring (7) and of the pump oil in chamber (34) causes upstroke piston (8) to move swashplate (3) toward the maximum angle.When the force of pressure that is acting on flow compensator spool (12) becomes greater than the force of springs (15) and of the force of signal oil in the chamber of spring (15), the flow compensator spool moves upward.When the flow compensator spool moves all the way to the top, the pump oil in passage (35) can flow past the spool, through passage (31), and then to the chamber for destroke piston (29).Destroke piston (17) is larger than upstroke piston (8). Because of this difference in size, the force of oil pressure that is acting against the destroke piston exerts a greater amount of force than the combined forces that are acting against the upstroke piston.The oil pressure that is acting against the destroke piston (12) overcomes the force of spring (18), and the combined force of the oil and of the spring in chamber (34). This causes destroke piston (17) to move downward.As destroke piston (17) moves downward, swashplate (3) moves toward the minimum angle. This causes destroking of the steering pump. As the angle of the swashplate moves toward the minimum angle, the pump output flow decreases.When the pump pressure decreases, the signal pressure oil in line (32) combines with the force of springs (15). This moves flow compensator spool (12) downward. This allows pump oil pressure in the chamber of the destroke piston to vent to the hydraulic tank. This causes the steering pump to upstroke again.The continuous rise of the flow compensator spool and fall of the flow compensator spool will maintain the pump pressure in passage (36). The pressure in passage (36) should equal 2100 105 kPa (305 15 psi). This pressure is greater than the signal pressure in the chamber of springs (15). The force of springs (15) is equal to 2100 105 kPa (305 15 psi). This difference is called the margin pressure.Destroking
Illustration 4 g01066072
Operation of steering pump and pump control valve (destroking)
(2) Pump
(3) Swashplate
(7) Spring
(8) Upstroke piston
(9) Pump control valve
(12) Flow compensator spool
(15) Springs
(17) Destroke piston
(18) Spring
(19) Outlet for the pump discharge pressure
(21) Pressure cutoff spool
(24) Spring
(26) Hydraulic tank for the steering and brake systems
(27) Return oil line
(28) Drain line to the