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 displacement per shaft 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 roattion is correct.
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Gear Pumps and Motors
1. Measure gear width. W.
2. Measure bore diameter of one of the gear chambers: this is D.
3. Measure distance across both gear chambers: this is L.
GPM @ 1800 RPM =
47 x W x (2D - L) x ( (L - D) / 2)
A speed of 1800 RPM is used in the formula. At other speeds, GPM is proportional to RPM.
C.I.R. Displacement =
6 x W x (2D - L) x ( (L - D) / 2)
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Vane Pumps and Motors
(Balanced type, not variable displ.)
1. Measure width of rotor. This is W.
2. Measure shortest distance across bore; this is D.
3. Measure longest distance across bore: this is L.
GPM @ 1800 RPM =
94 x W x ((L + D) / 4) x ((L - D) / 2)
A speed of 1800 RPM is used in the formula. At other speeds, GPM is proportional to RPM.
C.I.R. Displacement =
12 x W x ((L + D) / 4) x ((L - D) / 2)
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Piston Pumps and Motors
1. Find piston aread from piston diameter; this is A in formula.
2. Measure length of stroke; this is L in formula.
3. Count number of pistons; this is N in formula.
GPM @ 1800 RPM = A x L x N x 1800 / 231
A speed of 1800 RPM is used in formula. At other speeds, GPM is proportional to RPM.
C.I.R. Displacement = A x L x N
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.
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