651E and 657E Wheel Tractor-Scrapers Hydraulic System Cushion-Hitch Circuit Caterpillar


Cushion-Hitch Circuit
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1.1. The Cushion Ride Is Turned Off and the Engine Is Running.
2.1. The Cushion Ride Is Turned On and the Engine Is Running.
3.1. The Cushion Ride Is Turned On and the Engine Was Just Stopped.

The Cushion Ride Is Turned Off and the Engine Is Running.




Illustration 1g01000493

The Cushion Ride Is Turned Off and the Engine Is Running.

(1) Drain passage for the accumulators

(2) Accumulator valve stem

(3) Line to the bail valve for push-pull machines only

(4) Passage for pilot oil

(5) Passage for pilot oil supply

(6) Check valve (cushion-hitch)

(7) Manifold

(8) Solenoid valve (cushion-hitch)

(9) Leveling valve

(10) Passage to the head end of the load cylinder

(11) Piston pump (cushion-hitch)

(12) Passage to the accumulators

(13) Outlet to the hydraulic oil tank

(14) Inlet passage from the piston pump (cushion-hitch)

(15) Rod

(16) Directional spool

(17) Detent ball

(18) Passage to the rod end of the load cylinder

(19) Springs

(20) Load cylinder

(21) Leveling stem

(22) Accumulators (cushion-hitch)

(23) Leaf spring

(24) Hydraulic oil tank

(AA) Pump oil

(JJ) Nitrogen

(LL) Return oil

When the engine is started, oil from hydraulic tank (24) flows to piston pump (11). When the system pressure is lower than the setting of the pressure compensator valve, the pump is at maximum output.

When the engine is first started, leaf spring (23) has positioned leveling stem (21) in order to keep the cushion ride off. Passage (5) receives oil from the pump and valve stem (2) is shifted to the left. Pump oil also flows around the leveling stem and through notches in the lower end of directional spool (16) to the rod end of load cylinder (20). The weight of the wheel scraper and the flow of oil from the pump retracts the cylinder rod fully into the load cylinder. The notches in the upper end of the directional spool also opens the head end of the load cylinder to hydraulic tank (24) .

The oil from leveling stem (21) also flows to accumulators (22). At this time, the pressure compensator valve causes the swashplate of piston pump (11) to move toward the minimum angle position. The piston pump is at the minimum displacement that is needed to maintain system pressure and internal pump lubrication. Maximum pressure is now in the system.

Note: When the engine is stopped, the flow of oil to accumulator valve stem (2) is stopped and the spring force pushes the valve stem to the right. The oil under pressure in the accumulators can then be relieved through drain (1) .

The Cushion Ride Is Turned On and the Engine Is Running.




Illustration 2g01000495

The Cushion Ride Is Turned On and the Engine Is Running.

(1) Drain passage for the accumulators

(2) Accumulator valve stem

(3) Line to the bail valve for push-pull machines only

(4) Passage for pilot oil

(5) Passage for pilot oil supply

(6) Check valve (cushion-hitch)

(7) Manifold

(8) Solenoid valve (cushion-hitch)

(9) Leveling valve

(10) Passage to the head end of the load cylinder

(11) Piston pump (cushion-hitch)

(12) Passage to the accumulators

(13) Outlet to the hydraulic oil tank

(14) Inlet passage from the piston pump (cushion-hitch)

(15) Rod

(16) Directional spool

(17) Detent ball

(18) Passage to the rod end of the load cylinder

(19) Springs

(20) Load cylinder

(21) Leveling stem

(22) Accumulators (cushion-hitch)

(23) Leaf spring

(24) Hydraulic oil tank

(AA) Pump oil

(HH) Trapped oil

(JJ) Nitrogen

(LL) Return oil

(PP) Pilot oil

When the cushion ride is turned on, solenoid valve (8) is energized. The solenoid valve allows the pump oil in passage (5) to flow into passage (4) and to directional spool (16). When the oil pressure overcomes the force of springs (19), the directional spool moves downward. Oil from the pump and oil from the accumulators at leveling stem (21) flow around the directional spool and into the head end of load cylinder (20). As the oil pressure increases, the rod of the load cylinder extends.

As rod (15) of load cylinder (20) extends, the wheel scraper raises in order to provide a cushion ride. Leaf spring (23) pulls leveling stem (21) to the correct position in order to turn on the cushion ride. When the cushion ride is turned on, the leveling stem stops the flow of oil to accumulators (22) and the head end of the load cylinder. The leveling stem also traps the oil between the head end of the load cylinder and the accumulators. The oil pressure in the accumulators is equal to the nitrogen pressure in the accumulators.

When rough ground causes the wheel scraper to move up and down, the oil in the head end of load cylinder (20) flows back and forth to accumulators (22). The pistons in the accumulators move upward when the load cylinder retracts. The upward movement of the pistons compresses the nitrogen gas at the top of the accumulators. This compression increases the pressure in the top of the accumulators. The pistons of the accumulators move downward, when the pressure in the top of the accumulators is greater than the pressure in the bottom of the accumulators. The oil is then forced back into the load cylinder in order to extend the rod. The cushion ride is a result of the compression and the expansion of the nitrogen gas in the accumulators.

When a very large shock occurs, and the rod of the load cylinder extends too much, leveling stem (21) moves down. The leveling stem moves to the lower position in order to relieve the shock load. Detent ball (17) rides on the bevel of the leveling stem. As the leveling stem is pulled out of leveling valve (9) the oil from the head end of load cylinder (20) drains to hydraulic tank (24). The oil drains through notches in the upper end of the leveling stem. When the shock load is relieved, the leveling stem is pushed back into the leveling valve. The leveling stem turns on the cushion ride again. Pump oil flows to the head end of load cylinder (20) and to the accumulators in order to turn on the cushion ride again.

When the oil from the accumulators moves rod (15) upward, the oil in the rod end of load cylinder (20) flows around directional spool (16) and flows back to hydraulic tank (24). The weight of the wheel scraper may cause the rod in the load cylinder to move down. This movement draws oil from the hydraulic tank to the rod end of the load cylinder.

The Cushion Ride Is Turned On and the Engine Was Just Stopped.




Illustration 3g01000496

The Cushion Ride Is Turned On and the Engine Was Just Stopped. The Cushion-Hitch Hydraulic System Is Starting To Release Hydraulic Pressure.

(1) Drain passage for the accumulators

(2) Accumulator valve stem

(3) Line to the bail valve for push-pull machines only

(4) Passage for pilot oil

(5) Passage for pilot oil supply

(6) Check valve (cushion-hitch)

(7) Manifold

(8) Solenoid valve (cushion-hitch)

(9) Leveling valve

(10) Passage to the head end of the load cylinder

(11) Piston pump (cushion-hitch)

(12) Passage to the accumulators

(13) Outlet to the hydraulic oil tank

(14) Inlet passage from the piston pump (cushion-hitch)

(15) Rod

(16) Directional spool

(17) Detent ball

(18) Passage to the rod end of the load cylinder

(19) Springs

(20) Load cylinder

(21) Leveling stem

(22) Accumulators (cushion-hitch)

(23) Leaf spring

(24) Hydraulic oil tank

(HH) Trapped oil

(JJ) Nitrogen

(LL) Return oil

Immediately after the engine is stopped and the cushion ride is still engaged, the pressure is released from the cushion-hitch system.

When the cushion ride is engaged and oil flow to the cushion-hitch system is lost for any reason, the cushion-hitch defaults to the correct position in order to turn off the cushion ride. Accumulator valve stem (2) moves to the right from spring force and drain (1) allows oil pressure in the accumulators to be relieved. Pilot oil pressure in passage (4) is vented to the hydraulic tank. This allows directional spool (16) to move upward because of the force of springs (19) .

The weight of the wheel scraper pushes the oil from the head end of load cylinder (20). The oil returns to directional spool (16) and into hydraulic tank (24) .

Note: Before you start the engine, make sure that the cushion ride is turned off.

Note: Before you park the machine, make sure that the cushion ride is turned off.

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