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Shafts and Shaft Components 391
Table 7–6 Shaft Diameter Key Size

Inch Dimensions for Over To (Incl.) w h Keyway Depth
Some Standard Square- 5 7 3 3 3
16 16 32 32 64
and Rectangular-Key
7 9 1 3 3
Applications 16 16 8 32 64
1 1 1
Source: Joseph E. Shigley, 8 8 16
9 7 3 1 1
“Unthreaded Fasteners,”
16 8 16 8 16
Chap. 24 in Joseph E. Shigley,
3 3 3
Charles R. Mischke, and 16 16 32
Thomas H. Brown, Jr. (eds.), 7 1 1 1 3 3
8 4 4 16 32
Standard Handbook of
1 1 1
Machine Design, 3rd ed., 4 4 8
McGraw-Hill, New York, 2004. 1 3 5 1 1
1 1
4 8 16 4 8
5 5 5
16 16 32
3 3 3 1 1
1 1
8 4 8 4 8
3 3 3
8 8 16
3 1 1 3 3
1 2
4 4 2 8 16
1 1 1
2 2 4
1 3 5 7 7
2 2
4 4 8 16 32
5 5 5
8 8 16
3 1 3 1 1
2 3
4 4 4 2 4
3 3 3
4 4 8
For less important applications, a dowel pin or a drive pin can be used. A large vari-
ety of these are listed in manufacturers’ catalogs. 7
The square key, shown in Fig. 7–16a, is also available in rectangular sizes. Standard
sizes of these, together with the range of applicable shaft diameters, are listed in
Table 7–6. The shaft diameter determines standard sizes for width, height, and key depth.
The designer chooses an appropriate key length to carry the torsional load. Failure of the
key can be by direct shear, or by bearing stress. Example 7–6 demonstrates the process to
size the length of a key. The maximum length of a key is limited by the hub length of the
attached element, and should generally not exceed about 1.5 times the shaft diameter to
avoid excessive twisting with the angular deflection of the shaft. Multiple keys may be
used as necessary to carry greater loads, typically oriented at 90° from one another.
Excessive safety factors should be avoided in key design, since it is desirable in an over-
load situation for the key to fail, rather than more costly components.
Stock key material is typically made from low carbon cold-rolled steel, and is
manufactured such that its dimensions never exceed the nominal dimension. This
allows standard cutter sizes to be used for the keyseats. A setscrew is sometimes used
along with a key to hold the hub axially, and to minimize rotational backlash when the
shaft rotates in both directions.

7 See also Joseph E. Shigley, “Unthreaded Fasteners,” Chap. 24. In Joseph E. Shigley, Charles R. Mischke, and
Thomas H. Brown, Jr. (eds.), Standard Handbook of Machine Design, 3rd ed., McGraw-Hill, New York, 2004.