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Minimum opening pressures and pressure drops ... what's the difference?

Hydraulic motors are not easy. Everything is interdependent, interconnected. And yet, each one has its own role, despite being intertwined with the others. Hydraulic valves can give you trouble and cause problems, that's for sure. Several cross-cutting concepts can also interfere with each other. For example, the notions of minimum opening pressure and pressure drop can be confused. What about each? We'll tell you all about it here!

Hydraulic motors in general and in brief

A hydraulic motor has a simple definition. It is a motor whose purpose is to create mechanical energy by channelling and directing a hydraulic source, often a special industrial liquid. Usually oil. The conversion of pressurised liquid into mechanical force is known as hydrostatic transmission. This is the basis of the engine we are talking about today.

This way of generating energy uses fluid pressure. It is the combination of pumps, valves and other reservoirs that enable these fluids to be put under pressure. These fluids are then channelled towards the engine to produce mechanical energy and keep it running. The movement thus created is sent to a load. The mechanical energy generated is then transformed into the rotational movement of the output shaft. So our motor can turn and do its job!

In this vast and complex system, what are the concepts of minimum opening pressure and pressure drop?

Minimum opening pressure

These two concepts can be confused, because they have a consequence that can be identical: a reduction in the total volume of liquid delivered. However, apart from that, they play a completely different role in our engine.

Let's start with minimum opening pressure.

The minimum opening pressure corresponds to the pressure that will be applied to the inlet valve to force the spring to let the input liquid through. The aim is therefore to allow liquid to enter the motor at a guaranteed minimum level.

Below this minimum pressure, the liquid cannot pass. The pressure is therefore calculated to deliver the minimum quantity required for optimum engine operation. In fact, while it guarantees the union minimum delivery, it cannot influence the maximum quantity delivered. That's the job of other valves.

Now for the other concept we need to talk about today: pressure drop. What is it?

Pressure loss in the engine

Although it can be caused by insufficient liquid entering our engine, which is in line with the notion of minimum opening pressure, the concept of pressure drop is more global.

It is in fact everything that is directly or indirectly affected by the reduction in engine load that is concerned. In other words, any interruption or reduction in the supply of operating fluid.

In addition to a defect in the minimum required supply, i.e. the concept of minimum opening pressure, there are also various concepts and problems linked to the state of the fluids or the pipes and valves that can hinder the proper circulation of the fluid in the engine.

These various technical problems can be as follows:

  • Oil viscosity
  • Presence of air
  • Sticking of certain spool valves
  • Oscillations

We can see that, although the concept of minimum opening pressure is sometimes similar to that of pressure drop, the latter can also refer to other issues and is therefore much broader.

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