W345B Series II Material Handler Hydraulic System Travel Counterbalance Valve Caterpillar


Travel Counterbalance Valve
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1.1. Construction of Counterbalance Valve
2.1. Travel Operations
3.2. Level Travel
4.2. Slope Travel
5.1. Operation of Crossover Relief Valves
6.1. Parking Brake
7.1. Makeup Oil



Illustration 1g00819714

Travel motor and counterbalance valve (side view)

(1) Swivel

(2) Travel motor

(3) Counterbalance valve

The counterbalance valve is bolted to the travel motor. The counterbalance valve is part of the travel brake valve. The travel brake valve has the following functions:

  • prevent the occurrence of a shock load during a travel stop.

  • prevent the development of cavitation (a vacuum condition).

  • prevent overrun during slope travel.

  • send oil to the parking brake in order to release the brake.




Illustration 2g00818696

Travel motors and travel brake valves

(1) Travel brake valve

(2) Travel counterbalance valve

(3) Crossover relief valve

(4) Crossover relief valve

(5) Left travel motor

(6) Right travel motor

Each travel brake valve (1) contains two counterbalance valves (2) and two crossover relief valves (3).

Construction of Counterbalance Valve




Illustration 3g00818720

Travel brake valve

(1) Crossover relief valve

(2) Passage

(3) Spring

(4) Valve

(5) Port

(6) Passage

(7) Passage

(8) Passage

(9) Valve

(10) Spring

(11) Passage

(12) Port

(13) Crossover relief valve

(14) Ball

(15) Valve

(16) Chamber (guide)

(17) Orifice

(18) Spring chamber

(19) Spring

(20) Passage

(21) Passage

(22) Check valve

(23) Port

(24) Passage

(25) Passage

(26) Passage

(27) Port

(28) Check valve

(29) Passage

(30) Spring

(31) Orifice

(32) Brake pilot valve

(33) Travel brake valve

(34) Counterbalance valve

Counterbalance valve (34) has the following components:

  • valve (15)

  • spring (19)

  • check valve (22)

  • check valve (28)

  • spring (30)

Travel Operations

The following sections describe the actions of the counterbalance valve during travel operations.

Note: Refer to Illustration 3 as you read the following description.

Level Travel

During forward travel, pump oil flows through port (23) and into passage (24). The oil flows through passage (24) and into valve (15). Check valve (22) opens, and the oil flows through passage (2) and passage (5). The oil then flows into the piston of the travel motor, and the oil powers the travel motor.

A portion of the pump oil in port (23) flows through passage (21) and into spring chamber (18). The oil then flows through passage (20) and into chamber (16). The oil in spring chamber (18) and chamber (16) shifts valve (15). Valve (15) shifts to the right against the force of spring (30), and passage (8) opens.

The motor return oil flows through port (12), passage (11), passage (8), passage (26), and out port (27). The oil flows back to the tank.

When oil flow from port (23) is blocked, there is a pressure decrease in chamber (18) and chamber (16). Spring (30) forces valve (15) to the left. This closes passage (8). Return oil flow from the motor is blocked and the motor rotation stops.

If the travel direction is reversed, pump oil flows into valve (15) and through port (27). The oil then flows out of port (23). The operation is similar to the operation that is described above.

Slope Travel

When the machine moves down a slope, the travel motors rotate at a higher speed due to the mass of the machine. The pumps cannot maintain the oil supply to the motors, and the lack of oil supply will cause cavitation in the travel motor. The pressure decreases at port (23), and the oil pressure in spring chamber (18) also decreases. Spring (30) forces valve (15) to the left, and passage (8) closes. Oil is trapped between passages (11) and (27). The return oil flow to the hydraulic tank is restricted, and the oil flow to the suction port of the travel motor is also restricted. The rotation of the travel motor will slow down.

The pump oil pressure at port (23) increases. Part of the oil flows through passage (21). Valve (15) moves to the right, and passage (8) opens.

Modulation of valve (15) maintains the proper opening of passage (8) when the machine moves down a slope. The motor rotates according to the output of the pump. This prevents the motor from cavitating.

When the machine moves down a slope, or stops, valve (15) suddenly closes passage (8). A hydraulic pressure spike can occur. To prevent pressure spikes, there is a damper at both ends of valve (15). Valve (15) returns to the left from the full open position. The oil in chamber (16) is under pressure. Ball (14) moves to the right, which closes passage (20). The oil in chamber (16) flows through orifice (17) and into the spring chamber. The movement of valve (15) slows down, and valve (15) slowly closes passage (8) .

The damper is maintained by the size and the position of orifice (17) .

Operation of Crossover Relief Valves

Note: Refer to Illustration 3 as you read the following description.

When the machine is in motion and the travel pedals are moved to the NEUTRAL position, the motion of the machine stops. The supply of pump oil to the travel motors and travel brake valves stops. There is a decrease in the oil pressure at port (23) of the brake valve. Spring (30) returns valve (15) to the NEUTRAL position. The travel motor is still rotating because the machine is still in motion. Passage (8) is closed. Return oil is blocked. There is a sudden increase in oil pressure in passage (11). High pressure return oil in passage (11) flows through passage (6). Valve (4) of crossover relief valve (1) opens. The oil flows through valve (4) and into suction passage (2) of the travel motor.

Crossover relief valves (1) and (13) protect the travel motor against damage. Valves (1) and (13) allow the high pressure oil to escape. Valves (1) and (13) also maintain the oil flow from the return side to the inlet side. This oil helps to prevent a vacuum condition in the travel motor.

Crossover relief valve (1) opens just before forward left travel stops. Crossover relief valve (13) opens just before reverse left travel stops. During an adjustment, the tracks must be secured from movement. After the tracks are secured, the left travel pedal is moved to the FORWARD position. Oil flow from port (23) through passage (2) is blocked. The oil pressure in passage (2) increases. This opens valve (9). Oil flows from passage (2) to passage (11). Crossover relief valve (13) opens and crossover relief valve (1) closes.

When the left travel pedal is moved to the REVERSE position, crossover relief valve (1) is open. Crossover relief valve (13) is closed, as described above.

Valves (1) and (13) are designated by the movement of the travel pedal that is used during pressure adjustment.

Parking Brake

When pump oil is supplied to port (23) to start the travel motor, part of the oil in passage (24) flows through passage (25). The oil then flows through brake pilot valve (32) and into the parking brake of the travel motor. This releases the parking brake. Since valve (15) is supplied with pump oil after passage (25), the travel motor does not operate before the brake is released.

To stop the travel motor, the supply of oil to port (23) is blocked. No pump oil is supplied to passage (25) and valve (15). The oil from the piston chamber of the brake in the travel motor is restricted at orifice (31). This action delays the application of the brake, and the action allows the machine to stop moving before the parking brake is activated.

The above operation releases the parking brake just before the travel motor starts rotation. The parking brake is not applied while the travel motor is rotating. The operation applies the brake only after the travel motor has stopped.

Makeup Oil




Illustration 4g00375793

Oil makeup operation

(1) Motor rotary group

(2) Left travel motor

(3) Passage

(4) Check valve

(5) Line

(6) Swivel

(7) Line

(8) Passage

(9) Left travel control valve

(10) Passage

(11) Return passage

Hydraulic tank oil from the travel control valve is used as makeup oil to prevent cavitation (a vacuum condition) in the travel motor when operation is stopped.

The oil makeup operation is described with respect to left travel. The oil makeup operation is identical for right travel.

If the left travel control lever is returned to the NEUTRAL position in order to stop left travel, the supply of pump oil to left travel motor (2) is blocked at passage (10). The left travel motor continues to rotate because the machine is still in motion. A negative oil pressure at passage (3) of motor rotary group (1) opens check valve (4) .

With left travel control valve (9) in the NEUTRAL position, passage (11) is connected to line (7) by passage (8). Oil from the hydraulic tank flows through passage (11), passage (8), and line (7) into swivel (6). The oil then flows through line (5), check valve (4), and passage (3) into motor rotary group (1). This makeup oil eliminates the possibility of cavitation in the travel motor.

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