Matching the Size of a Broken Pump or Motor
FLUID POWER  Design Data Sheet 2
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 
Download a PDF of Fluid Power Design Data Sheet 2  Matching the Size of a Broken Pump or Motor.
© 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.
