A hydraulic charge pump works by supplying make-up fluid to the main pump in a closed-loop hydraulic system, maintaining positive pressure and replenishing fluid losses.
Role in Closed-Loop Systems
In many hydraulic systems, particularly those using variable displacement pumps and motors in a closed loop (where fluid flows directly between the pump and motor with minimal return to the reservoir), some internal leakage is inevitable. This leakage, along with thermal expansion and contraction and fluid used for control circuits, can lead to a loss of volume in the main loop.
This is where a charge pump becomes essential. When a charge pump is used to replace and replenish fluid in closed-loop systems, it provides fluid to the main pump. Its primary function is to ensure the main high-pressure pump always has a positive supply of fluid available at its inlet, preventing cavitation (formation of vapor bubbles) and ensuring efficient operation.
Step-by-Step Operation
The operation of a charge pump involves drawing fluid from the system's reservoir and introducing it into the low-pressure side of the main closed loop.
Here’s a breakdown of the typical process, incorporating the reference information:
- Fluid Supply: The charge pump draws relatively cool hydraulic fluid from the system's reservoir (tank).
- Pressurization: The charge pump, which is usually a small, positive displacement pump (like a gear pump or vane pump), increases the pressure of this fluid.
- Pressure Regulation: Once a charge pump reaches a preset PSI (pounds per square inch), typically a low pressure compared to the main loop pressure but sufficient to prevent cavitation, it continues to pump.
- Flow Through Relief Valve: Excess fluid that is not immediately needed for make-up or cooling is directed. It pumps the fluid through the charge pump relief valve. This relief valve is critical as it maintains the desired charge pressure in the system and diverts surplus flow.
- Entry into Main Pump: The pressurized fluid that passes through or is regulated by the relief valve flows into the primary pump (the main variable displacement pump) on its low-pressure inlet side. This ensures the main pump is 'charged' with sufficient fluid.
- Return to Tank: The fluid that goes through the charge pump relief valve (the excess flow) or is used for cooling or lubrication within the main pump or motor circuits is returned to the tank. This continuous circulation helps cool the fluid and filters out contaminants.
Why Charge Pumps Are Essential
Charge pumps provide several vital functions in closed-loop systems:
- Make-Up Fluid: Compensate for internal leakage within the main pump, motor, and control components.
- Prevent Cavitation: Ensure a positive pressure at the inlet of the main pump, preventing damaging cavitation.
- Cooling: Circulate cooler fluid from the reservoir through the main pump and motor, helping to dissipate heat generated during operation.
- Lubrication: Provide pressurized fluid for lubricating internal components of the main pump and motor.
- Control Fluid: Sometimes provide fluid for system control circuits.
Key Components
A typical charge circuit includes:
- Charge Pump: The pump itself, drawing fluid.
- Charge Filter: Filters fluid before or after the charge pump.
- Charge Relief Valve: Regulates charge pressure and directs excess flow.
- Check Valves: Ensure fluid flows into the correct side of the main loop (the low-pressure side) and prevent high-pressure fluid from entering the charge circuit.
Practical Considerations
Properly functioning charge pumps are crucial for the longevity and efficiency of closed-loop hydraulic transmissions found in equipment like hydrostatic drive vehicles (skid steers, excavators, combines). Issues with the charge pump or its relief valve can lead to overheating, cavitation damage, poor performance, and ultimately, failure of the main hydraulic components.
