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Hydraulic Charge Pump 101: The Basics You Should Know

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There is a wide range of closed-loop hydraulic applications used in a variety of industries. From large tractors and harvesters to small self-propelled lawn mowers, the one thing these things often have in common is an HST (hydrostatic transmission) with a hydraulic charge pump.

It is essential that you know the basics of hydraulic charge pumps to properly perform maintenance and repairs when they are needed.

What Is a Hydraulic Charge Pump?

A hydraulic charge pump is usually a fixed displacement pump that is integrated into the hydraulic system. However, some system configurations will use a variable displacement pump.

Additionally, most systems use gear pumps as their charge pump. A charge pump is separate from the main hydraulic pump in the system.

Instead, it functions to charge or feed fluid through the HST system loop. It can either be integrated directly into the HST system or be a separate stand-alone pump that is attached to the HST system.

What Do Charge Pumps Do?

Charge pumps provide several key functions for the HST system:

Replace/Replenish Fluid

When a charge pump is used to replace and replenish fluid in closed-loop systems, it provides fluid to the main pump. Once a charge pump reaches a preset PSI, it pumps the fluid through the charge pump relief valve and into the primary pump.

Then the hydraulic oil is returned to the tank. As a result, the oil returning to the pump from the motor(s) will ensure they remain topped up so as to prevent pump cavitation. This is the primary function of the charge pump.

Maintain Back Pressure

HST systems require inlet pressure to be maintained constantly in order for the system to function correctly. Charge pumps do this by using a charge pump relief valve, which can be customised to fit specific inlet pressure PSI levels as needed for your HST system.

When the hydraulic system is idling, the pressure and fluid flow often need to be maintained. This is the secondary function of the charge pump.

It will continue to draw fluid from the fluid reservoir and feed that to inlet ports on the main pump. Any excess fluid is released through the charge pump relief valve, where it flows back into the fluid reservoir.

Operate Auxiliary Controls or Brakes

Charge pumps can occasionally be used for auxiliary functions to provide sufficient power to perform other functions like operating remote hydraulic servo controls or hydraulic brakes. They help provide the necessary back pressure to release hydraulic brakes and prevent freewheeling.


Many HST pumps will have a built-in filter on the charge pump inlet manifold. The charge pump pushes the oil through the filter prior to it reaching the main pump. This way all the drain oil is filtered and, over time, all the oil in the circuit is cleaned.

Common Charge Pump Issues

Factory Engineer Operating Hydraulic Tube Bender

Another essential hydraulic charge pump basic you need to know is being able to diagnose and troubleshoot common charge pump issues. Identifying these problems can help prevent complete system failure and extended downtime, including the following:

  • Noticeable Pressure Drop – When a charge pump is wearing out, a pressure drop is common. You may notice the system is no longer maintaining proper pressure. However, a pressure drop could also be caused by external and internal leakages from fluid loss.
  • System Overheating – When the system is not getting sufficient fluid from the charge pump, it can operate at higher temperatures and overheat.
  • Screeching or Squealing Sounds – When the main system pump and motor is not receiving sufficient fluid from the charge pump, the lack of fluid can cause these sounds.
  • Control Resistance – If you notice that system controls are resistant to operating as intended, this could signify the charge pump is getting ready to fail.
  • Movement Stops – When a charge pump fails, the hydraulics will no longer move. By this time, the extent of damage often requires rebuilding the system with new hydraulic parts and components.

If you suspect the charge pump is failing, you need to check the charge pump pressure. This will give you a clear indication if the problem is with the charge pump or another hydraulic component.

Charge Pump System Design, Repairs, and Replacements

If you are designing a new HST system or need repairs or replacement charge pumps or other hydraulic parts, pumps, motors, and components, please feel free to contact White House Products, Ltd. at +44 (0) 1475 742500 today!

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