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To select a replacement for a broken or worn out hydraulic pump or motor which has no nameplate or has no rating marked on its case, use the formulas below after making internal physical measurements.

When replacing a pump, catalog ratings will usually be shown in GPM at a specified shaft speed. On a motor, catalog ratings will usually be in C.I.R. (cubic inches per revolution). Formulas are given for calculating either GPM at 1800 RPM or calculating C.I.R. Use the formula which is appropriate. Make all measurements in inches, as accurately as possible. Convert fractional dimensions into decimal equivalents for use in the formulas.

Make sure the catalog pressure rating is adequate for your application, and in the case of a pump, be sure direction of shaft rotation is correct.

   
  REFERENCE
>> Troubleshooting
> Hydraulic Troubleshootin g
>
Cylinder & Valve Testing
> Replacing a Pump or Motor
>> Formulas
>> Conversions
 
 
 
Gear Pump

Vane Pump
(Balanced Type Only)
 
Gear Pumps & Motors

  Piston Pumps & Motors

  Vane Pumps & Motors

 
  1. Measure gear width, (W)
2. Measure bore chamber of one of the gear chambers, (D)
3. Measure distance across both gear chambers, (L)

  1. Find piston area from piston diameter, (A)
2. Measure length of stroke, (L)
3. Count number of pistons, (N)
  1. Measure width of rotor, (W)
2. Measure shortest distance across bore, (D)
3. Measure longest distance across this bore, (L)
 
GPM @ 1800 RPM =   GPM @ 1800 RPM =   GPM @ 1800 RPM =  
47 x W x (2D - L) x (L - D)
2
  A x L x N x 1800 ÷ 231  
94 x W x (L + D) x (L - D)
4 2
 
  A speed of 1800 RPM is used in the formula. At other speeds, GPM is proportional to RPM.   A speed of 1800 RPM is used in the formula. At other speeds, GPM is proportional to RPM.   A speed of 1800 RPM is used in the formula. At other speeds, GPM is proportional to RPM.  
C.I.R. Displacement =   C.I.R. Displacement =   C.I.R. Displacement =  
6 x W x (2D - L) x (L - D)
2
  A x L x N  
12 x W x (L + D) x (L - D)
4 2
 

If a pump of higher GPM has to be used, it will require more HP at the same pressure and cylinders in the system will move faster. If one with lower GPM is used, the system will have plenty of power but cylinders will move more slowly than originally.

If a motor with greater displacement is used, it will deliver more torque at a reduced RPM, but will require no more fluid HP from the pump. If it has less displacement it will rotate faster with less torque.

 
 

Reference > Replacing a Pump or Motor


 
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