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.

 64ths 32nds 16ths 8ths 4ths Decimal Inches Millimeters 1/64 0.01562 0.397 1/32 0.03125 0.794 3/64 0.04688 1.191 1/16 0.06250 1.588 5/64 0.07812 1.984 3/32 0.09375 2.381 7/64 0.10938 2.788 1/8 0.12500 3.175 9/64 0.14062 3.572 5/32 0.15625 3.969 11/64 0.17188 4.366 3/16 0.18750 4.763 13/64 0.20312 5.159 7/32 0.21875 5.556 15/64 0.23438 5.953 1/4 0.25000 6.350 17/64 0.26562 6.747 9/32 0.28125 7.144 19/64 0.29688 7.541 5/16 0.31250 7.938 21/64 0.32812 8.334 11/32 0.34375 8.731 23/64 0.35938 9.128 3/8 0.37500 9.525 25/64 0.39062 9.922 13/32 0.40625 10.319 27/64 0.42188 10.716 7/16 0.43750 11.113 29/64 0.45312 11.509 15/32 0.46875 11.906 31/64 0.48438 12.303 1/2 0.50000 12.700 33/64 0.51562 13.097 17/32 0.53125 13.494 35/64 0.54688 13.891 9/16 0.56250 14.288 37/64 0.57812 14.684 19/32 0.59375 15.081 39/64 0.60938 15.478 5/8 0.62500 15.875 41/64 0.64062 16.272 21/32 0.65625 16.669 43/64 0.67188 17.066 11/16 0.68750 17.463 45/64 0.70312 17.859 23/52 0.71875 18.256 47/64 0.73438 18.653 3/4 0.75000 19.050 49/64 0.76562 19.447 25/32 0.78125 19.844 51/64 0.79688 20.241 13/16 0.81250 20.638 53/64 0.82812 21.034 27/32 0.84375 21.431 55/64 0.85938 21.828 7/8 0.87500 22.225 57/64 0.89062 22.622 29/32 0.90625 23.019 59/64 0.92188 23.416 15/16 0.93750 23.813 61/64 0.95312 24.209 31/32 0.96875 24.606 63/64 0.98438 25.003

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.

 mm inches mm inches mm inches mm inches 1/4 .00984 18 .70866 38 1.4961 58 2.2835 1/2 .01970 19 .74803 39 1.5354 59 2.3228 3/4 .02953 20 .78740 40 1.5748 60 2.3620 1 .03937 21 .82677 41 1.6142 61 2.4016 2 .07874 22 .86614 42 1.6534 62 2.4409 3 .11811 23 .90551 43 1.6929 63 2.4803 4 .15748 24 .94488 44 1.7323 64 2.5197 5 .19685 25 .98425 45 1.7727 65 2.5591 6 .23622 26 1.0236 46 1.8110 66 2.5984 7 .27559 27 1.0270 47 1.8504 67 2.6378 8 .31496 28 1.1024 48 1.8898 68 2.6772 9 .35433 29 1.1417 49 1.9291 69 2.7165 10 .39370 30 1.1811 50 1.9685 70 2.7559 11 .43307 31 1.2205 51 2.0079 71 2.7953 12 .47244 32 1.2598 52 2.0472 72 2.8346 13 .51181 33 1.2992 53 2.0866 73 2.8740 14 .55118 34 1.3386 54 2.1260 74 2.9134 15 .59055 35 1.3780 55 2.1654 75 2.9528 16 .62992 36 1.4173 56 2.2047 76 2.9911 17 .66929 37 1.4567 57 2.2441 77 3.0315

© 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.