Inch Square Keys and Keyseats
Square key sizes by shaft diameter for inch shafts from 5/16 to 4-1/2 inches (ANSI B17.1).
In inch practice a shaft up to about 6 1/2 inches takes a square key whose width is about one quarter of the shaft diameter. This table gives the standard square key for each shaft diameter range, with the keyway depth in the shaft, which is half the key height; the hub keyway takes the other half.
The table
| Shaft diameter | Square key size (in) | Square key size (decimal in) | Square key size (mm) | Keyway depth in shaft, half the key (in) | Keyway depth in shaft (mm) |
|---|---|---|---|---|---|
| Over 5/16 to 7/16 in | 3/32 | 0.0938 | 2.38 | 0.0469 | 1.19 |
| Over 7/16 to 9/16 in | 1/8 | 0.125 | 3.18 | 0.0625 | 1.59 |
| Over 9/16 to 7/8 in | 3/16 | 0.1875 | 4.76 | 0.0938 | 2.38 |
| Over 7/8 to 1-1/4 in | 1/4 | 0.25 | 6.35 | 0.125 | 3.18 |
| Over 1-1/4 to 1-3/8 in | 5/16 | 0.3125 | 7.94 | 0.1563 | 3.97 |
| Over 1-3/8 to 1-3/4 in | 3/8 | 0.375 | 9.53 | 0.1875 | 4.76 |
| Over 1-3/4 to 2-1/4 in | 1/2 | 0.5 | 12.7 | 0.25 | 6.35 |
| Over 2-1/4 to 2-3/4 in | 5/8 | 0.625 | 15.88 | 0.3125 | 7.94 |
| Over 2-3/4 to 3-1/4 in | 3/4 | 0.75 | 19.05 | 0.375 | 9.53 |
| Over 3-1/4 to 3-3/4 in | 7/8 | 0.875 | 22.23 | 0.4375 | 11.11 |
| Over 3-3/4 to 4-1/2 in | 1 | 1 | 25.4 | 0.5 | 12.7 |
Source and notes: ANSI B17.1 (keys and keyseats), square keys: the key width equals the height, the shaft keyway is half the key height deep, and the hub keyway takes the other half. Rectangular keys are used where a square key would be too tall for the shaft.
How to use this table
- Find the shaft diameter range.
- Read the key width (the key is as high as it is wide).
- Cut the shaft keyway half the key height deep, and the hub keyway to match.
Worked example: a 1-1/2 inch shaft
- The 1-1/2 inch shaft falls in “Over 1-3/8 to 1-3/4 in”: a 3/8 inch square key (9.53 mm).
- The keyway in the shaft is half the key height deep: 0.1875 inch (4.76 mm).
- Check by the rule of thumb: key width ≈ shaft diameter ÷ 4 = 1.5 ÷ 4 = 0.375 inch = 3/8 inch.
Notes and cautions
- For large shafts or where a square key would be too tall for the shaft, a flat (rectangular) key is used.
- Setscrews and keys can be combined, but the key carries the torque.
- The same cautions apply as for metric keys: a keyway weakens the shaft in fatigue.
Related tools and guides
- Shaft and Key Sizing Calculator
- Shafts, Keys and Couplings (guide)
- Metric Parallel Keys and Keyways
- All reference tables
FAQ
What size key for a 1 inch shaft?
A 1/4 inch square key (the range is “Over 7/8 to 1-1/4 in”), with a keyway 1/8 inch deep in the shaft.
Why is the keyway half the key height?
So the key sits half in the shaft and half in the hub, and each side carries the load on a bearing surface of the same height.
Sources
- ANSI B17.1, Keys and Keyseats (square parallel keys); ASME B17.1.
- ANSI B17.1 (keys and keyseats), square keys: the key width equals the height, the shaft keyway is half the key height deep, and the hub keyway takes the other half. Rectangular keys are used where a square key would be too tall for the shaft.