1. Closed-Loop System Structure
Before we answer the question, let's go over the basics of a closed-loop system. A closed-loop system, also called a hydrostatic drive, has a variable pump and a fixed or variable motor that work together in a sealed circuit. In this system, the oil from the motor doesn't go back to the tank; it goes directly to the pump's intake. The oil flow from the pump can be adjusted, and it can even change direction by moving a part inside the pump. This lets the pump control both the direction and speed of the motor, so there's no need for extra valves like in an open-loop system.
Figure 1 Schematic diagram of the basic principle of a closed system
2. Make-Up Oil Pump
In a closed-loop hydraulic system, the pump and motor can leak oil inside. This causes oil to escape from the system and return to the tank. To keep the system filled with oil during normal operation, a make-up oil pump is used to add oil back into the system. The make-up oil pump is usually installed behind the closed-loop pump, but it can also be set up separately. Typically, the make-up pump's flow is about 20% of the pump's output flow, or 10% of the combined pump and motor flow. If there is more leakage or the system runs at a higher temperature, the flow rate may be slightly higher.
In most cases, the pressure setting for the make-up pump is between 18 and 36 bar, depending on the pump manufacturer and what the customer needs. The basic make-up oil system includes the make-up pump, a relief valve, and two check valves. These parts work together to add oil to the closed-loop system. When the pressure in the system reaches the setting of the relief valve, the make-up pump will send oil through the valve and back through the pump or motor, or both, before returning to the tank.
