#John deere 750 dozer brakes manual#
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With these systems, checking the case flow would not provide an accurate indication of bypassing. On systems with hot oil shuttle valves, the tank port of the shuttle relief valve is sometimes ported into the hydraulic motor case drain line. The motor case drain flow should be checked and recorded for future troubleshooting purposes. Maximum pressure to the motor is controlled by the crossport relief valve settings. The speed and direction of the motor is determined by the variable displacement hydraulic pump. On these systems, the pressure override is set below the crossport relief valve settings. The pump will only deliver enough oil to maintain the pressure override setting. When the pressure override setting is reached, the pump volume is reduced to an output of nearly zero GPM. Some drives may have a maximum pressure override, which operates similarly to a pump compensator.

The valves typically are preset to 200 to 400 PSI above the maximum operating pressure. Depending on the system, the valves may be located on the pump, mounted in a separate block or on the hydraulic motor. To best absorb the pressure spikes, the valves are generally mounted as close to the motor as possible. The valves also absorb shock spikes in the system. If the motor should mechanically stall, the relief valve on the high-pressure side would open and dump fluid back to the low-pressure side of the loop, protecting the motor from overpressurizing. The check valves are normally accessed by removing the charge pump.Ĭrossport relief valves limit the maximum pressure in the system. At the same time, oil in the high-pressure side is blocked to the low-pressure side by the opposite check valve.

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Makeup check valves permit free flow from the charge pump to the low-pressure side of the loop. On systems that utilize a hot oil shuttle valve, a shuttle relief valve determines the pressure on the low side of the loop when driving the motor. In the system shown in Figure 2, the relief valve setting determines the pressure in the system when in idle mode. The outlet flow of this relief valve is usually ported into the pump case where it returns to the tank through the main pump’s case drain line. The relief valve is normally mounted on or near the charge pump. The charge pump relief valve provides a flow path for the excess pump volume to return to the tank in idle mode. The charge pump is also used to supply oil to the spring-loaded cylinders for stroking the main pump. The charge pump will supply makeup oil through the check valve, preventing pump cavitation. This means that some of the oil inside the pump and motor will bypass the pistons and flow back to the tank through the case drain lines.īecause of this bypassing, less oil flows out of the motor than what the main pump actually requires. There are tight tolerances between the pistons and the barrel in the pump and motor. The purpose of the charge pump is to provide makeup fluid to the system during operation. The oil then returns to the tank through the case drain line. Once the valve’s spring setting is reached, the charge pump volume will flow through the charge pump relief and into the pump case. The charge pump relief is usually set between 200-300 pounds per square inch (PSI). The pressure will continue to build in both ports until the relief valve setting is reached. When the main pump is in idle mode, the charge pump volume prefills the “A” and “B” ports with fluid. The charge pump volume is normally 10-15 percent of the main pump volume. In some cases, the charge pump is located inside the main pump assembly. This is sometimes referred to as a replenishing pump. The amount the swashplate angles in each direction will determine the flow from the pump.Ī charge pump is mounted on the back end of the main pump.

The “B” port will then serve as the pressure port, and the “A” port will be the suction port. To drive the motor in reverse, the top cylinder will extend, allowing the swashplate to angle in the opposite direction (Figure 4). This port will act as the suction port in this direction. As the shaft rotates, the oil that flows out of the motor will return to the “B” port on the pump. Oil flow is then directed to the motor for rotating the shaft. To drive the hydraulic motor forward (Figure 3), the bottom cylinder extends to angle the swashplate and deliver fluid out the “A” port.

#John deere 750 dozer brakes manual#

The swashplate is moved by two internal cylinders, which are controlled by a separate valve or manual lever. In Figure 2, the pump swashplate is in the vertical position, which means the pump output is zero gallons per minute (GPM). The pump volume can range from zero to the maximum amount. A piston-type pump is always used in a hydrostatic system.