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.
Elephant Fluid Power supplies Rexroth, Sauer, Komatsu, Kawasaki and other brand piston pumps to provide charge pumps.
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.
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