What are closed hydraulic systems used for?

It all depends on the pressure in the closed circuit

If a function does not run with the hoped-for performance when commissioning a hydrostatic, the call for more pressure can be heard from hydraulic fitters. Which pressure valve should then be adjusted? And does it achieve the desired effect?

During the closed circuit training, various pressure setting options are explained. Image: IHA

Nov 30, 2016 | 07:00 am | by Robert Becker, IHA

In the closed circuit there can be a multitude of pressure setting options, depending on the version. Depending on the machine and application, the valves for maximum pressure limitation or pressure cut-off must also be selected and set appropriately. The maximum pressure depends on the maximum pressure of the components used and should not be reached in normal operation.

If the shock valve responds due to an overload, oil flows from the high to the low pressure side. This can generate a lot of power loss, which increases the oil temperature. Using this valve too often can create a temperature problem.

Use pressure cut-off

The more efficient way to limit the maximum pressure is to use a pressure cut-off. The pressure cut-off ensures that the variable pump swivels back to the minimum swivel angle when the pressure cut-off setting is reached. At this operating point, the pump maintains the maximum pressure and compensates for leaks.

If the pressure cut-off is set too low, the hydraulic motor delivers a lower drive torque. If the pressure cut-off is set too high, this can lead to temperature problems. Overloading of the pump's drive motor is also possible - regardless of whether it is an electric or diesel motor.

Set pressure relief valves correctly

The correct setting of the two pressure relief valves for the feed and flushing circuit is of essential importance for the function and operational safety of the hydrostat. The closed circuit is filled with the feed circuit and supplied with fresh oil. In addition, oil from the supply circuit can also be used to adjust the pump. A connection between the low pressure and the flushing circuit is established for cooling and filtration.

However, so that the low pressure does not run dry, a pressure relief valve is provided in the flushing circuit. Incorrect setting of these valves can lead to permanent damage to the pump and motor due to overheating or wear. If the feed pressure is too low, it is still possible that the full pump output cannot be obtained.

During commissioning of a hydrostat, the settings of all pressure valves should be checked and adjusted if necessary. This can increase the functional and operational safety.

Workshop "Mobile hydraulics - closed circuit / hydrostatic travel drive"

  • Theoretical basics of hydrostatics, hydrodynamics, symbols
  • Difference and structure of an open and closed circuit
  • Basic structure of a hydrostatic transmission
  • Comparison of different variants with regard to requirements, structure and function
  • Structure and function of additional components such as feed circuit, filtration, cooling
  • Practical exercises, settings and measurements on the test bench
  • Installation / replacement of components
  • Behavior of individual pump controllers
  • Simulation and measurement of driving scenarios on the test bench
  • Practical error analysis on the real system

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