Quality
Transmission
Components
Quality Transmission Components
125 Railroad Avenue, Garden City Park, NY 11040-5016
Phone: (516) 437-6700 • Fax: (516) 328-3343
e-mail: support@qtcgears.com, support@econobelt.com
R-0
Selecting a Preferred Size
The idea for developing metric standards worldwide
comes from a preferred numbering system. Its first known
application was in the 1870's by Charles Renard, a French
army captain who reduced the different diameters of rope for
military balloons from 425 to 17.
Nominal metric sizes are identical where the metric
systems have been in use for several years. These reflect
preferred sizes for components such as threaded fasteners,
steel plates, sheets, and bars used throughout the world.
The accompanying table shows how the general system
works.
For example, if a designer was choosing a hydraulic
cylinder, bolt, or plate thickness, the sizes in the First-choice
column would be preferred. Second- and Third-choice
columns are self-explanatory. The table extends to smaller
and larger sizes.* For instance, 60 mm sizes would be a
preferred choice as would 2.5 mm devices.
The three columns to the far right are the originating
Renard numbers. In the First- choice column, each succeed-
ing number is 1.6 times the previous, with some rounding.
These three columns provide the basis for the values on the
left side of the table. The inch values show close correspond-
ing English units.
The form of the first table carries through to other
tables in the standard. The number series shown are
recommended to reduce the number of standard sizes for
items such as screw threads, steel plates, steel sheets,
round steel bars, lifting capacities, and hydraulic cylinder
diameters.
4.5
5.6
7.1
9
11.2
14
18
22.4
28
35.5
5
8
12.5
20
31.5
4
6.3
10
16
25
40
0.156
0.172
0.188
0.219
0.25
0.281
0.313
0.344
0.375
0.438
0.5
0.563
0.625
0.688
0.75
0.875
—
1.125
1.188
1.375
1.563
5/32
11/64
3/16
7/32
1/4
9/32
5/16
11/32
3/8
7/16
1/2
9/16
5/8
11/16
3/4
7/8
1
1-1/8
1-3/16
1-3/8
1-9/16
3.97
4.37
4.76
5.56
6.35
7.14
7.94
8.73
9.53
11.11
12.7
14.29
15.88
17.46
19.05
22.23
25.4
28.58
30.16
34.93
39.69
4.5
5.5
7
9
11
14
18
22
28
35
5
8
12
20
30
4
6
10
16
25
40
* The values in the first three columns of the table may be extended to cover smaller or larger sizes by multiplying or
dividing sizes by 10.
Reprinted from Kverneland, K.O., "How ISO Standards Cut Manufacturing Costs," Machine Design,
pp 126-130, November 5, 1998. E-mail: kok@kok.com Website: http//www.kok.com
Preferred Sizes (mm)
Customary Sizes
Preferred Numbers
First
Choice
Second
Choice
mm
inch
Fractions
inch
Decimals
First
Choice
Second
Choice
Third
Choice
Third
Choice
