Travel Parking Brake

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Travel Parking Brake

The travel parking brake is located in the travel motor. The travel parking brake consists of the following components: the brake spring, the brake piston, the separator plates, and the friction plates. The friction plates are splined to the cylinder barrel. The separator plates are splined to the motor housing.

When the travel pedals or levers are moved from the NEUTRALposition, supply oil from the pump flows to the inlet port of the travel motor from the travel control valve in the main control valve group.

Aportion of the supply oil enters the motor and unseats the brake pilot check valve. The oil then flows to the brake piston. As pressure builds in the brake piston, the piston moves to the left against the brake spring.

The separator plates and friction plates are no longer held together and the motor is able to rotate.

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When the travel pedals or levers are returned to the NEUTRALposition, the supply oil to the motor is blocked by the travel control valves. The counterbalance valves shift back to NEUTRAL.

The brake pilot check valve moves to the right The orifice in the brake pilot check valve allows the oil behind the brake piston to slowly flow to the motor case drain.

The brake spring moves the brake piston to the left to press the separator plates and friction plates together to apply the travel parking brake.

The travel park brake engages approximately four seconds after the travel control valves return to NEUTRAL.

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The travel brake valve consists of the counterbalance valve, two check valves, and two crossover relief valves. The orifice check valves are internal to the counterbalance spool.

During normal travel, supply oil from the travel control valve enters the travel brake valve and flows to the counterbalance valve.

Some of the supply oil flows through the orifice check valve to the right end to shift the counterbalance valve to the left. As the counterbalance initially shifts to the left, some of the supply oil is used to release the parking brake.

As the counterbalance continues to move to the left, a return passage from the motor is opened.

The supply pressure unseats one of the check valves and flows past the check valve to the motor. Return oil from the motor flows around the counterbalance valve, to the travel control valve, and back to the tank.

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When traveling downhill the weight of the machine will cause the machine to try to turn the travel motor faster than the supply oil from the pumps.

When this occurs, the pressure drops in the supply passage to the motor. When the supply pressure drops, the counterbalance valve will shift to restrict the return oil from going back to the tank. This movement by the counterbalance valve slows the speed of the motor and reduces motor cavitation.

One of the crossover reliefs valve may open to send the high pressure return oil into the supply side to prevent motor cavitation.

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When the travel pedals/levers are returned to NEUTRAL, the supply oil is blocked by the travel control valve.

The pressure drops in the spring chamber on one end of the counterbalance valve. The counterbalance shifts to restrict the return oil.

The crossover relief valve opens to dampen the shock when the motor stops and sends some of the high pressure oil in the return passage to the supply passage to prevent motor cavitation.

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The travel crossover relief valves provide a cushioning effect when they open, similar to the swing relief valves. The valves initially open at a lower pressure to handle pressure spikes of short duration and to reduce jerkiness at the start of travel. When travel is stalled, the circuit pressure is at maximum.

NEUTRAL: In NEUTRALthe pistons are moved against the stops by the large springs. The large springs also seat the unloading valves. At this time the relief valves are at their minimum spring setting.

Start of Travel: When travel is started, the high pressure oil in the supply side is sensed at the relief valve on the left. The unloading valve moves left against the large spring to allow some of the oil in the supply side to flow into the return passage to dampen the pressure spike. The relief valve opens at the lower pressure setting.

Since the pressure spike is short in duration, the oil sensed through the stem on the left end of the left relief valve does not increase sufficiently to move the piston to the right. Since the piston does not move, the spring is not compressed and the relief valve pressure setting is not increased.

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Travel Stall: The crossover relief valves are also able to handle high pressure increases.

When travel is stalled, the high pressure in the supply side is sensed at the left relief valve. Not only does the unloading valve move to the left, but oil sensed through the stem, allows for pressure to increase on the left end.

As the pressure increases on the left end, the piston gradually moves to the right to compress the large spring, which causes a modulated increase in the maximum relief valve setting.

Travel Stop: When travel is suddenly stopped the return oil is blocked by the counterbalance valve. Pressure in the return side is very high, while pressure in the supply side is very low.

The high pressure is sensed at the right relief valve. The oil sensed through the stem on the right end of the relief valve may move the piston to the left to partially compress the spring due to higher pressure in the system as compared to a travel start. At the same time the right unloading moves to the right and opens to allow the high pressure oil in the return loop to flow to the low pressure side.