Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/655822
22 March - April 2016 | www.machinerylubrication.com The swashplate is moved by two internal cylinders, which are controlled by a sepa- rate valve or manual lever. To drive the hydraulic motor forward (Figure 3), the bottom cylinder extends to angle the swashplate and deliver fluid out the "A" port. Oil flow is then directed to the motor for rotating the shaft. As the shaft rotates, the oil that flows out of the motor will return to the "B" port on the pump. This port will act as the suction port in this direction. To drive the motor in reverse, the top cylinder will extend, allowing the swash- plate to angle in the opposite direction (Figure 4). The "B" port will then serve as the pressure port, and the "A" port will be the suction port. The amount the swash- plate angles in each direction will determine the flow from the pump. Charge Pump A charge pump is mounted on the back end of the main pump. This is sometimes referred to as a replenishing pump. In some cases, the charge pump is located inside the main pump assembly. The charge pump volume is normally 10-15 percent of the main pump volume. When the main pump is in idle mode, the charge pump volume prefills the "A" and "B" ports with fluid. The pressure will continue to build in both ports until the relief valve setting is reached. The charge pump relief is usually set between 200-300 pounds per square inch (PSI). 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 oil then returns to the tank through the case drain line. The purpose of the charge pump is to provide makeup fluid to the system during operation. There are tight tolerances between the pistons and the barrel in the pump and motor. 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. Because of this bypassing, less oil flows out of the motor than what the main pump actually requires. The charge pump will supply makeup oil through the check valve, preventing pump cavitation. The charge pump is also used to supply oil to the spring-loaded cylinders for stroking the main pump. Charge Pump Relief Valve The charge pump relief valve provides a flow path for the excess pump volume to return to the tank in idle mode. The relief HYDRAULICS Figure 2. The main pump in idle mode Figure 3. Driving the motor forward Figure 4. Driving the motor in reverse