Technical Details (Catalogues)
Standard Displacement Range Selector
The Application and Selection of Hydraulic Motors.
The application and selection of the correct Hydraulic Motor(s) for any given application is a specialised process. Seemingly simple, but only able to be done properly, if all aspects of the application are considered.
There are various types available on the market today and it is very much a "horses for courses" situation.
As an initial evaluation consideration, it is very wrong to look at the COST of a Hydraulic Motor. Only once all aspects of the application have been fully considered should any decision be made as to the actual type of Motor to be used.
For example, THEORETICALLY, Displacement for Displacement, a Gerotor Motor can cost a fraction of that of a Radial Piston Motor. THEORETICALLY, they produce the same Torque but in PRACTICE they are poles apart.
On the other hand however, if on any given application it is known that a Gerotor Motor performs to an acceptable standard, it would most unlikely to be cost effective to consider a Radial Piston Motor.
TYPES of Hydraulic Motors.
GEAR - Internal or External - Very poor efficiency, Poor Low Speed running, High Leak rate. Only available as FIXED Displacement. - otherwise known as GEAR, GEROTOR or GEROLLER. Generally, only to be considered for low efficiency drives, which start off load and run at about 200 RPM.
VANE -
AXIAL PISTON - Poor Low Speed performance. High Speed Performance good. Efficiency improves with speed. Axial Motors are also available with Variable Displacement.
RADIAL PISTON - High Starting Efficiency results in Excellent Slow Speed Performance. The SAI range of Radial Piston Hydraulic motors are exceptional in that what little internal leakage there is very consistent. Low running speeds depend on this as it means that the support flow required is equally consistent without need for any sophisticated feedback system. Most Radial Piston Motors are considered "LOW SPEED" but the SAI Range can run at speeds of up to 2500 RPM in certain sizes meaning they can be considered as at least MEDIUM or even HIGH Speed units. SAI Radial Piston Motors are available as Dual Displacement or Variable Displacement in certain sizes making the SAI Range THE MOST VARIED of all Radial Piston Motors.
Tabulating the pros and cons of the different basic Motor designs gives the basis to make an initial appraisal
| Type | Max Speed | Min Speed | Starting Efficiency | Running Efficiency | Variable | Duty | Cost |
| Gear | 1500 | 500 | Rubbish | Poor | No | Light | Low |
| Gerotor or Geroller | 350 | 25 | Poor | Not brilliant | No | Light | Low |
| Vane | 500 | 200 | Poor | Average | Not Usual | Light | Medium |
| Axial | 3000 | 200 | Poor | Excellent | Available | Heavy | Medium/High |
| Radial (SAI) | 2000 | 0 | Excellent | Excellent | Available | Heavy | Medium |
| Radial - Others | 250 | 0 | Good/Excellent | Excellent | Not Usual | Heavy | Medium/High |
SPEED Range - refer to the above General Tabulation with special regard to Starting Efficiency. These 2 factors alone can immediately eliminate certain Types.
TORQUE Requirement - To ascertain the basic THEORETICAL displacement we need to establish the Torque requirement of the Motor. (Call SAI (GB) Limited if you do not know how to arrive at the TORQUE requirement)
PRESSURE Availability - is there a Hydraulic Power source existing?
FIRST CALCULATION - TORQUE (Nm) divided by desired running Pressure (Bar) multiplied by 63 - will give an APPROXIMATION of the THEORETICAL displacement required in cc/rev.
DISPLACEMENT - Now you have an IDEA of the displacement - how does this relate to the maximum speed required when you consider the flow you have available? cc/rev multiplied by speed (RPM) divided by 1000 will give you the flow in Litres per Minute
SPEED of Application, Flow availability with relation to Flow requirement. Only if it seems that you have plenty of flow available you can consider Gear or Gerotor/Geroller Motors (unless heat is an additional consideration as the high leakage of these motors can cause high operating temperatures)
STARTING - Is there a need to START under load. If this is an important consideration, DISCOUNT anything other than Radial Piston Motors.
CONSISTENCY of SPEED with respect to VARYING Load. If this is important DISCOUNT anything other than Radial or Axial Piston Motors.
QUICK SELECTION for a very BASIC selection of a Motor from the SAI range download our FREE QuickSelect calculator (this is NOT a replacement for a fully detailed selection and is offered for GUIDANCE ONLY). CLICK THIS LINK
We hope the above has enabled you to at least begin to understand the vagaries connected with Hydraulic Motor selection.
SAI (GB) Limited have EXPERTS on hand to immediately assess your Hydraulic Motor requirements. One call will get you STRAIGHT THROUGH (NO automated switchboard!) to a qualified Engineer who will give an unbiased opinion of which Motor is most suited to your application.
SAI (GB) Limited GUARANTEES
that you will get the BEST FREE ADVICE
available when it comes to selecting a Hydraulic Motor by calling us on
01268 272030.