The hydraulic system of a machine is typically grouped into three classifications: active components, passive components, and transmission lines. Some of the components found on this specific circuit include pumps, pipes, tubes, hoses, motors, and cylinders. These components can help the machine gain the necessary flow and pressure of the hydraulic fluid, which is the key component in providing energy, lubrication, heat transfer, and contamination control throughout the hydraulic machine.
Defining Closed-Loop Hydraulic Systems
One type of hydraulic system that is being used by different hydraulic machines is the closed-loop hydraulic system. In this type of system, the hydraulic fluid flows continuously between the pump and the actuator. Unlike an open-closed hydraulic system, the fluid does not have to enter the reservoir anymore, making it possible for the whole system to operate in either direction easily.
A constant displacement pump, however, is added to make the whole system work. This pump is utilised in generating and managing the circuit fluid. The movement of the actuator, on the other hand, is operated by the variable displacement pump.
The turning of the pump determines the flow within this type of system. However, it only allows enough fluid flow just to cover the needed lubrication of the pump and obtain the basic standby pressure at the directional control valve. The flow will enter a passage once a spool is stroked, while the direction control valve will send a pressure signal to the pump. The said signal informs the pump to produce the needed flow for the completion of the hydraulic flow.
Movement and Behaviour of Fluid Flow
The speed and direction of the fluid flow in a closed-loop hydraulic system are regulated by a displacement controller or a control lever that is located to the swashplate of the piston pump.
Once the control lever is moved forward, the swashplate of the pump will tilt to effectively send fluid to the motor, which subsequently makes the motor turn. Pushing the lever further will then increase the tilt of the swashplate, allowing more fluid displacement and faster turning of the motor. Pulling the lever back to the neutral position will stop the system flow. Displacement of fluid will be reversed once the lever is pulled back in the opposite direction, which also turns the motor in reverse.
Overcoming Heat and Pressure Problems
The absence of a reservoir in closed-loop hydraulic systems is already enough to make them generate overheating issues. As a solution, several systems just mount a hot oil purge valve that can effectively discharge a certain amount of hot oil from the loop for them to be filtered and cooled. As for other systems, this oil is utilised to flush some of their working components.
The charge pump, on the other hand, draws cooled, filtered fluid from a reservoir and sends it into the loop. The purpose of this specific function is to replace any fluid losses due to internal leakage caused by case drains or the diversion of fluid by the hot oil purge valve.
The continuous workload of closed-loop hydraulic systems can also cause excessive pressures. To overcome this problem, these systems maximise the presence of a cross port relief valve. Once the pressure exceeds the limit of the motor, this specific valve will open and short circuits the loops, circumventing the motor and diverting the flow back to the pump. This valve can easily prevent any damages that may occur during operations. Closed-loop hydraulic systems are typically used because of their precise response to actuation, their ability to flow in reverse easily, and their capability of running at higher pressures with less fluid flow. If you have some questions about this type of hydraulic system, feel free to contact us at Mobile Hydraulics.
Factory 89, 38-40 Popes Road
Keysborough, Victoria, 3173
Phone: (03) 9798-6511
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