Matching the Size of a Broken Pump or Motor

When replacing a pump, catalog ratings will usually be shown in GPM at a specified shaft speed. On motors, catalog ratings will usually be shown as C.I.R. (cubic inches displacement per shaft revolution). Formula are shown for calculating either GPM at 1800 RPM or calculating C.I.R. Use the formula which is appropriate.

Make all measurements in inches, and as accurately as possible. Convert fractional dimensions into decimal equivalents for use in formula. See conversions on back side of this sheet.

When selecting a new pump or motor, be sure the catalog pressure rating is adequate for your application, and in the case of a pump, be sure the direction of shaft rotation is the same as for the old pump.

GEAR PUMPS AND MOTORS 1. Measure gear width. This is W in the formula. 2. Measure bore diameter of one of the gear chambers. This is D in the formula. 3. Measure distance across both gear chambers. This is L in the formula. 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 will be directly proportional to RPM. C.I.R. Displ. = 6 x W x (2D - L) x [(L - D) ÷ 2]

VANE PUMPS AND MOTORS (Balanced type, not variable displacement) 1. Measure width of rotor. This is W in the formula. 2. Measure shortest distance across bore. This is D. 3. Measure longest distance across bore. This is L. GPM @ 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 will be proportional to speed. C.I.R. Displ. = 12 x W x (L + D) ÷ 4 x [(L - D) ÷ 2]

PISTON PUMPS AND MOTORS 1. Find piston area from piston diameter. This is A in the formula. 2. Measure length of piston stroke. This is L in formula. 3. Count the 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 the formula. At other speeds, GPM will be proportional to RPM. C.I.R. Displ. A x L x N

Notes: After finding GPM or C.I.R. rating of the old pump or motor, choose a replacement with as near as possible the same rating at zero pressure.

If necessary to choose a pump with higher GPM rating, it will require more input HP to produce the same pressure, and cylinders or motors in the system will move faster. If necessary to choose a pump with lower GPM rating, it will require less HP input to produce the same pressure but cylinders and motors will move more slowly.

If necessary to choose a motor with greater displacement, it will produce higher torque at the same pressure than the original motor but will rotate more slowly. The system will require the same HP as before but will deliver a different ratio of torque and speed. If the new motor has less displacement, it will have less torque at the same pressure but will run faster. The system will run at the same HP but with a different ratio of torque and speed.

EQUIVALENTS

Fractions to Decimals & Millimeters

The table below converts inch fractions to inch decimals or millimeters. Since 1 inch = 25.4 mm, millimeter equivalents = inch decimal equivalents multiplied by 25.4.

Millimeters to Decimal Inches

Figures in the table below convert millimeters into decimal inches and are calculated from: 1 inch = 25.4 mm, or 1 mm = .03937 inches.

© 1988 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.