938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Braking System General Information (Braking) Caterpillar


General Information (Braking)
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938G Series II Wheel Loader and IT38G Series II <BR>Integrated Toolcarrier Braking System [RENR6074]
BRAKING SYSTEM
938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Braking System General Information (Braking)



Illustration 1g00275087

Hydraulic Schematic for the Brake System

(1) Rear service brakes. (2) Service brake control valve. (3) Left pedal assembly. (4) Right pedal assembly. (5) Inverse shuttle valve. (6) Accumulator charging valve. (7) Service brake oil pressure switch. (8) Spool for cut-in and cutout. (9) Flow control valve. (10) Front service brakes. (11) Accumulator for the rear service brakes. (12) Accumulator for the front service brakes. (13) Steering pump. (14) Implement pump. (15) Brake pump. (16) Relief valve. (AA) Pressure tap for the rear service brakes. (BB) Pressure tap to check the service brake accumulator pressure. (CC) Pressure tap for the front service brakes.

The hydraulic pump group is a three-section pump that consists of steering pump section (13), implement pump section (14), and brake pump section (15). These components are mounted in a tandem style. Brake pump (15) is a straight vane pump which provides the hydraulic oil that is required in order to operate the brake system.

Oil flows from brake pump (15) to accumulator charging valve (6). Accumulator charging valve (6) contains inverse shuttle valve (5) and pressure relief valve (16) .

The flow of oil from brake pump (15) to the accumulators is controlled by accumulator charging valve (6). Pressure relief valve (16) is located in accumulator charging valve (6). The pressure relief valve limits the oil pressure that flows from brake pump (15) to the brake circuit.

A check valve in accumulator charging valve (6) prevents oil from flowing through the accumulator charging valve, and back to brake pump (15). Flow control valve (9) is located in accumulator charging valve (6). The flow control valve regulates the oil flow in inverse shuttle valve (5). This keeps accumulators (11) and (12) charged. Any excess oil is dumped back to the hydraulic oil tank.

Oil flows through inverse shuttle valve (5) to accumulators (11) and (12) during oil charge. However, the oil flow between the two accumulators is kept separate. Inverse shuttle valve (5) is designed to charge each accumulator when the pressure reaches the cut-in pressure. A third port in the inverse shuttle valve is used to connect service brake oil pressure switch (7) .

Inverse shuttle valve (5) is located in accumulator charging valve (6). Pressure oil from inverse shuttle valve (5) flows to the oil end of the two accumulators. Both accumulator (11) and accumulator (12) contain a precharge of dry nitrogen.

When oil flows into an accumulator, the pressure oil moves the piston inside the accumulator. The piston moves against the charge of dry nitrogen gas. This puts compression on the dry nitrogen until the oil pressure reaches the cutout pressure. The oil also flows to service brake control valve (2) .

Service brake control valve (2) is in the circuit from the accumulators to the service brakes. When a brake pedal is pushed, oil from each accumulator flows to the service brakes. The service brakes are then applied.

When service brake control valve (2) is released, the lines from the accumulators are blocked. The brake lines are connected to the drain port. This allows the service brakes to release. The supply oil is then dumped back into the hydraulic oil tank.

Each brake application removes some oil from the accumulators. The piston in each accumulator moves toward the oil end until the oil pressure decreases to the cut-in pressure. The accumulator charging valve then permits oil from brake pump (15) to flow to accumulators (11) and (12), until the pressure increases to the cutout pressure.

The hydraulic circuits for the front service brakes and for the rear service brakes are completely separate. The right pedal assembly actuates service brake control valve (2). Applying force to a brake pedal causes the tandem spool in service brake control valve (2) to move. This allows oil from both accumulator (11) and accumulator (12) to flow to the disc brakes in each axle housing. The pressure of the oil causes the wet disc brakes that are enclosed in each axle to engage. The heat from the friction of the brakes is removed by the oil in the axle housings.

Note: Some machines may have only one service brake pedal. Machines that have two service brake pedals function in the same manner as machines that have only one service brake pedal. This is because the brake pedals are mechanically coupled together.

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