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| Illustration 1 | g02176594 |
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Left travel motor and travel counterbalance valve (1) Left travel motor (2) Travel counterbalance valve (3) Crossover relief valve (forward travel) (4) Port (supply port for reverse travel) (5) Port (supply port for forward travel) (6) Crossover relief valve (reverse travel) | |
Travel counterbalance valve (2) consists of crossover relief valve (3) and crossover relief valve (6). The travel counterbalance valve is bolted to the travel motor. The travel counterbalance valve has the following four functions.
- The travel counterbalance valve prevents a shock load when travel is stopped.
- The travel counterbalance valve prevents overspeed while the machine is traveling down a slope.
- The travel counterbalance valve prevents cavitation.
- The travel counterbalance valve routes a portion of the oil to the travel parking brake in order to release the brake.
Counterbalance Valve Operation During Level Travel
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| Illustration 2 | g02176830 |
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Travel counterbalance valve (level travel) (2) Counterbalance valve (3) Crossover relief valve (forward travel) (4) Port (supply port for reverse travel) (5) Port (supply port for forward travel) (6) Crossover relief valve (reverse travel) (7) Spool (8) Check valve (9) Check valve (10) Spring (11) Spring (12) Passage (oil flow to rotary group during reverse travel) (13) Passage (14) Spring chamber (15) Passage (16) Ball (17) Damper chamber (18) Throttling slots (19) Passage (return oil flow from rotary group during reverse travel) (20) Passage (21) Passage (22) Spring chamber (23) Passage (24) Spring chamber (25) Damper piston (26) Spring (27) Spring chamber (39) Passage (parking brake release oil) | |
Counterbalance valve (2) consists of spool (7), check valve (8), check valve (9), spring (10) and spring (11). During reverse travel, pump oil is supplied to port (4). The oil flows through port (4) and opens check valve (9) in spool (7). The oil also flows through passage (12) to the rotary group of the travel motor.
A portion of the pump oil in port (4) flows through passage (13). The oil enters spring chamber (14). This pump oil flows through passage (15). Ball (16) shifts. This action allows pump oil to flow through the orifice and into damper chamber (17). Oil also flows from spring chamber (14) to damper chamber (17). The pressure oil in spring chamber (14) and the pressure oil in damper chamber (17) shifts spool (7) to the left against the force of spring (10). Throttling slot (18) now opens.
Pressure oil also flows through passage (21), spring chamber (22), and passage (23). The oil now enters spring chamber (24). The pressure oil shifts damper piston (25) to the right against the force of spring (26). The movement of damper piston (25) against spring (26) dampens the force of the sudden increase in pressure at travel crossover relief valve (6).
As spool (7) shifts to the left, throttling slots (18) open. Return oil from the travel motor flows through passage (19), throttling slots (18) and port (5) to the hydraulic tank. The oil in spring chamber (27) and passage (20) will be the same pressure as the return oil.
When the oil flow from port (4) is blocked, the pressure in spring chamber (14) and damper chamber (17) decreases. The force of spring (10) shifts spool (7) to the right. Throttling slots (18) close. Return oil from the travel motor at passage (19) is blocked and the rotation of the travel motor stops.
When the direction of travel is reversed, pump oil flows to spool (7) through port (5). The return oil flows through port (4). During forward travel, the travel counterbalance valve operates in the same manner as the reverse travel operation.
Counterbalance Valve Operation During Slope Travel
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| Illustration 3 | g02176837 |
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Travel counterbalance valve (slope travel) (2) Counterbalance valve (3) Crossover relief valve (forward travel) (4) Port (supply port for reverse travel) (5) Port (supply port for forward travel) (6) Crossover relief valve (reverse travel) (7) Spool (10) Spring (13) Passage (14) Spring chamber (15) Passage (16) Ball (17) Damper chamber (18) Throttling slots (19) Passage | |
When the machine travels down a slope, the travel motors rotate at a higher speed. The higher speed is due to the mass (weight and size) of the machine. When this condition occurs, the pumps cannot maintain the oil supply to the travel motors. The lack of oil supply will cause cavitation in the travel motor. A pressure decrease occurs at port (4). A pressure decrease occurs in spring chamber (14) as well.
The force of spring (10) moves spool (7) to the right. Throttling slots (18) begin to close. This blocks oil flow between passage (19) and port (5). The return oil from the travel motor and the oil flow to the suction port of the travel motor are restricted. The rotation of the travel motor slows down.
The pressure of the pump oil at port (4) increases. Part of the oil flows through passage (13). Spool (7) shifts to the left. Throttling slots (18) open. Return oil from the travel motor flows through port (5). The modulation of spool (7) maintains the proper opening of throttling slots (18) when the machine travels down a slope. The travel motor begins to rotate in accordance with the amount of pump oil supply. This action prevents cavitation in the travel motors.
When the machine is traveling down a slope, or the machine is suddenly stopped, spool (7) suddenly closes throttling slots (18). This action causes a hydraulic pressure spike to occur. A damper is provided at both ends of spool (7) in order to prevent hydraulic pressure spikes. As spool (7) shifts to the right, the oil in damper chamber (17) is pressurized. Ball (16) moves to the left. Passage (15) closes. The oil in damper chamber (17) is forced through the orifice into spring chamber (14). Spool (7) slowly moves to the right. Throttling slots (18) slowly close. The size of the orifice and the position of the orifice maintains the proper shock damper.
Operation Of Travel Crossover Relief Valves During Machine Stop
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| Illustration 4 | g02176831 |
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Travel counterbalance valve (travel stop) (2) Counterbalance valve (3) Crossover relief valve (forward travel) (4) Port (supply port for reverse travel) (5) Port (supply port for forward travel) (6) Crossover relief valve (reverse travel) (7) Spool (10) Spring (12) Passage (18) Throttling slots (19) Passage (21) Passage (25) Damper piston (27) Spring chamber (28) Passage (29) Passage (30) Spring chamber (31) Poppet (32) Passage (33) Orifice (34) Spring (35) Plunger (36) Passage | |
If the travel control levers are returned to the NEUTRAL position during machine movement, the pump oil is blocked from the travel motors. The pressure at port (4) of the travel counterbalance valve decreases. The force of spring (10) moves spool (7) to the right to the neutral position. The mass (weight and size) of the machine causes the travel motor to continue to rotate. Throttling slots (18) close and the flow of return oil from passage (19) is blocked. A sudden pressure increase occurs in passage (28). The return oil in passage (28) flows to crossover relief valve (3) by flowing through passage (29). The oil flows into spring chamber (30) and the oil acts on poppet (31). At the same time, the oil flows through passage (32), orifice (33) and spring chamber (27). The oil now acts on piston (25). The oil pressure rises to the set pressure of the force of spring (34) of piston (25). Piston (25) moves to the left and the piston compresses spring (34). When piston (25) reaches the end of the stroke, the oil pressure opens poppet (31). The oil flow through the open poppet seat causes a pressure decrease in spring chamber (30). The pressure decrease in spring chamber (30) allows the high oil pressure in passage (29) to move plunger (35) to the right. Oil in passage (29) flows past the open plunger (35) into suction passage (12) of the travel motor.
Crossover relief valves (3) and (6) protect the travel motor by releasing the high-pressure oil.
Crossover relief valves (3) and (6) also provide makeup oil from the outlet side of the travel motor to the inlet side of the travel motor. This makeup oil prevents a vacuum condition in the travel motor.
During forward travel of the left travel motor, oil flow opens crossover relief valve (6) when the machine is stopping. During reverse travel of the left travel motor, crossover relief valve (3) is activated when the machine is stopping.
In order to adjust the pressure setting of the crossover relief valves, refer to the crossover relief valves by the functions of the travel control levers. Refer to crossover relief valve (3) as left travel (forward). Refer to crossover relief valve (6) as left travel (reverse).