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Hydraulic motors – a user-friendly guide ben lee

If you're reading this then the operation of hydraulic motors is probably no secret to you, but perhaps the people you work with or machine operators struggle to grasp exactly what is happening inside the equipment, and more importantly, why?  We have put together a short, user-friendly guide to hydraulic motors that can serve as an educational tool, for those not in the know and will hopefully reduce the number of repeat questions you have to answer.


In a nutshell, all hydraulic power systems comprise the same four basic elements.  They are:


  • A driving surface area subjected to different pressures
  • A method of timing the flow of hydraulic fluid through the motor
  • An output shaft
  • A mechanical connection between the surface creating the output pressure and the motor itself


The size of these components can affect the speed, pressure, flow, strength and efficiency of the hydraulic motor but the basic concept is the same across the board.  Essentially a hydraulic motor uses varying pressures conducted via hydraulic fluid to increase and magnify force in an energy-efficient and reliable manner. 


The jargon terms used to describe hydraulic motor operation can seem confusing and complex to the lay person, but learning what these words mean and how they relate to the hydraulic equipment is important to fully understand what is happening during normal running, and also what is happening where there is a system failure.


Torque is probably the most important term which refers to hydraulic motors.  It is used to describe the ability of the engine to translate pressure into motion and is measured in Newton Metres (Nm) or inch pounds (lbf).  A hydraulic motor will have a starting torque and a running torque.  The starting torque is the force required to start the motor turning and the running torque refers to the pressure generated to maintain operation, at a certain pace.  Torque ripple refers to the difference between the minimum and maximum torque delivered during a single rotation of the motor.


Motor displacement is an important term to know.  It refers to how much hydraulic fluid is needed to turn the motor through one revolution and is measured in centimetres or inches cubed per revolution.  A motor may be a fixed or variable displacement type, meaning that either torque or speed is the priority.  A fixed displacement motor has torque as the priority, running at a constant pressure.  Speed can be controlled by varying the amount of fluid going into the motor.  In a variable displacement motor both torque and speed can be controlled.


Hydraulic fluid replacement is also something that machine operators should be trained in, if they are expected to top up the reservoir or replace the fluid.  Hydraulic oil comes in a variety of weights, which refers to the viscosity of the fluid.  Different types of hydraulic fluid can withstand different temperature ranges and different chemical make-ups of hydraulic fluid are recommended for different applications.  It is vital that the correct fluid is used, as any mistakes can cause costly damage to the equipment.  When replacing or topping up hydraulic fluid, it is important that it is filtered before entering the system, (we have written more about this topic previously on this blog).  Contaminated hydraulic fluid causes the same problems as using the incorrect product and it is crucial for operators to know how their actions can affect the operation of the machinery and cause problems.



Of course, there is a lot more to hydraulics than we have covered here, but the very basics that we have covered, should help hydraulic machinery operators understand a little more about their equipment, how it works, and most importantly, what can cause it not to work.

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