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| Illustration 1 | g06602599 |
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Travel brake valve (1) Crossover relief valve (4) Piston (7) Spool (10) Crossover relief valve (11) Check valve (12) Passage (13) Check valve (14) Left travel motor (16) Passage (17) Spring chamber (18) Counterbalance valve (19) Spring chamber (20) Spring (21) Passage (22) Check valve (23) Passage (24) Passage (26) Port (27) Passage (28) Passage (29) Passage (30) Port (31) Passage (32) Passage (33) Check valve (34) Spring | |
The travel counterbalance valve consists of counterbalance valve (18), crossover relief valve (1), and crossover relief valve (10). The travel counterbalance valve is bolted to travel motor (14). 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 to release the brake.
Counterbalance Valve Operation During Level Travel
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| Illustration 2 | g06607433 |
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Left travel motor and travel brake valve (1) Crossover relief valve for reverse travel (10) Crossover relief valve for forward travel (14) Left travel motor (18) Counterbalance valve (26) Port (30) Port | |
Counterbalance valve (18) consists of crossover relief valve (1), crossover relief valve (10), check valve (11), check valve (13), spring (20), and spring (34). All of these components are located in counterbalance valve (18). During forward travel on level ground, pump oil is supplied to port (26). The oil flows from port (26) through passage (23) to check valve (11) in spool (7). The pressure oil forces check valve (11) to open. Oil flows through passage (24) to motor (14) to drive the motor.
A portion of the oil delivery from the left pump in port (23) flows past check valve (22) through passage (21) and passage (12). The oil proceeds to flow into spring chamber (17). The oil in spring chamber (17) pushes spool (7) to the right against the force of spring (34). This action connects passage (31) and passage (32).
As spool (7) shifts to the right, return oil from the travel motor flows through passage (31), passage (32), passage (29), and port (30) to the hydraulic tank.
When the oil flow from port (26) is blocked, the pressure in spring chamber (17) decreases. The force of spring (20) shifts spool (7) to the right. Return oil from the travel motor 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 (30). Spool (7) shifts to the left. The return oil flows through port (26). During reverse travel, the travel counterbalance valve operates in the same manner as the forward travel operation.
Counterbalance Valve Operation During Slope Travel
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 (26). A pressure decrease occurs in spring chamber (17) as well.
Spool (7) in counterbalance valve (18) is forced to the left by the force of spring (20). This blocks oil flow between passage (31) and passage (32). 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 oil delivery from the left pump at port (26) increases. Part of the oil flows to spring chamber (17). Spool (7) shifts to the right. Return oil from the travel motor flows to port (30). The modulation of spool (7) maintains the proper flow of return oil when the machine travels down a slope. The travel motor begins to rotate in accordance with the amount of pump oil supply, thus preventing cavitation in the travel motors.
Operation of Travel Crossover Relief Valve During Travel Stop
If the travel /pedals are returned to the NEUTRAL position during machine movement, the oil supply from the pumps to the travel motor will be blocked. Once the oil supply to port (26) is blocked, the pressure at port (26) decreases and the spool (7) of the counterbalance valve (18) returns to the neutral position by spring (34). Then the counterbalance valve blocks port (30) and inertia rotation of the travel motor will cause pressure increase at passage (31).
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| Illustration 3 | g06268347 |
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Travel crossover relief valve (1) Crossover relief valve (7) Spool (10) Crossover relief valve (14) Travel motor (18) Counterbalance valve (24) Passage (26) Port (30) Port (31) Passage (37) Valve (38) Orifice (41) Orifice (42) Spring (43) Body (44) Chamber (45) Piston (47) Orifice | |
Pressurized oil in passage (31) flows into chamber (44) of the crossover relief valve (1) through orifice (47), (38), and (41). The oil in chamber (44) moves the piston (45) to the right. The pressure in chamber (44) does not change until the piston contacts body (43) at the right stroke end. Once piston (44) reaches the right stroke end, pressurize oil in passage (31) opens valve (37) and the oil flows from passage (31) to passage (24). The relief action of the relief valve (1) controls the pressure in passage (31) and prevents cavitation in passage (24) by providing makeup oil. When piston (45) reaches the left stroke end by the pressure of passage (24), the pressure between chamber (44), orifice (47), (38), and (41) increases and close valve (37).
If the machine is stopped while traveling in reverse direction, relief valve (10) will be activated.
Reference: For more information concerning the pressure settings of the travel crossover relief valves, refer to Testing and Adjusting , "Relief Valve (Crossover) - Test and Adjust".
Travel Parking Brake Operation
When the oil delivery from the left pump is supplied at port (26) or (30), spool (7) shifts. Pump oil flows to passage (28). A portion of the oil delivery in passage (27) releases the travel parking brake. Since passage (28) opens before return oil flows past spool (7), the rotation of the motor rotary group does not start until the travel parking brake is released.
When the oil delivery to port (26) or (30) is blocked to stop the rotation of the travel motor, spool (7) returns to the NEUTRAL position slowly. Check valves (22) and (33) slow the shifting of spool (7). Return oil flow is blocked before passage (28) is closed. These actions allow the movement of the machine to stop before the travel parking brake is engaged.
Reference: For more information concerning the operation of the travel parking brake, refer to Systems Operation , "Travel Parking Brake".