Metric Parallel Keys and Keyways
Parallel key and keyway sizes for shafts from 6 to 130 mm (ISO 773).
A key is a small block that fits into slots (keyways) in a shaft and a hub to carry torque between them. The standard parallel key is chosen from the shaft diameter: this table gives the key width and height and the depth of the keyway in the shaft (t1) and in the hub (t2).
The table
| Shaft diameter (mm) | Key width b (mm) | Key height h (mm) | Keyway depth in shaft t1 (mm) | Keyway depth in hub t2 (mm) | Key cross-section b x h (mm2) |
|---|---|---|---|---|---|
| Over 6 to 8 | 2 | 2 | 1.2 | 1 | 4 |
| Over 8 to 10 | 3 | 3 | 1.8 | 1.4 | 9 |
| Over 10 to 12 | 4 | 4 | 2.5 | 1.8 | 16 |
| Over 12 to 17 | 5 | 5 | 3 | 2.3 | 25 |
| Over 17 to 22 | 6 | 6 | 3.5 | 2.8 | 36 |
| Over 22 to 30 | 8 | 7 | 4 | 3.3 | 56 |
| Over 30 to 38 | 10 | 8 | 5 | 3.3 | 80 |
| Over 38 to 44 | 12 | 8 | 5 | 3.3 | 96 |
| Over 44 to 50 | 14 | 9 | 5.5 | 3.8 | 126 |
| Over 50 to 58 | 16 | 10 | 6 | 4.3 | 160 |
| Over 58 to 65 | 18 | 11 | 7 | 4.4 | 198 |
| Over 65 to 75 | 20 | 12 | 7.5 | 4.9 | 240 |
| Over 75 to 85 | 22 | 14 | 9 | 5.4 | 308 |
| Over 85 to 95 | 25 | 14 | 9 | 5.4 | 350 |
| Over 95 to 110 | 28 | 16 | 10 | 6.4 | 448 |
| Over 110 to 130 | 32 | 18 | 11 | 7.4 | 576 |
Source and notes: ISO 773 and DIN 6885 sheet 1 (parallel keys, form A, normal fit). The key length is chosen from the standard length series to carry the torque: see the shaft and key sizing calculator.
How to use this table
- Find the shaft diameter range (for example “Over 30 to 38” for a 35 mm shaft).
- Read the key width × height and the two keyway depths.
- Choose the key length from the torque to be carried and the standard length series (the shaft and key calculator does this).
Worked example: a 40 mm shaft with a 50 mm long steel key
- A 40 mm shaft is in the range “Over 38 to 44”: key 12 × 8 mm, shaft keyway depth 5.0 mm, hub keyway depth 3.3 mm.
- Torque limited by shear of the key (allowable 80 MPa): T = τ × b × L × d/2 = 960 N·m.
- Torque limited by crushing of the keyway side (allowable 140 MPa on half the key height): T = 560 N·m, so crushing governs at about 560 N·m.
Notes and cautions
- A shaft with a keyway is weaker in fatigue than a plain shaft: the sharp keyway corners concentrate stress. Use a fillet at the keyway ends and check the shaft for torsion and bending.
- The table is for the normal fit (the key is a tight fit in the shaft keyway and slides in the hub). A tight fit for reversing loads uses different tolerances.
- Keys on shafts above 130 mm and special keys (Woodruff, tapered, gib head) have their own tables.
Related tools and guides
- Shaft and Key Sizing Calculator
- Shafts, Keys and Couplings (guide)
- Inch Square Keys and Keyseats
- All reference tables
FAQ
What size key for a 25 mm shaft?
8 × 7 mm (the shaft range is “Over 22 to 30”), with a keyway depth of 4.0 mm in the shaft and 3.3 mm in the hub.
How long should a key be?
Long enough to carry the torque without crushing or shearing: a common rule is 1 to 1.5 times the shaft diameter. The key must not be longer than the hub.
Sources
- ISO 773 (rectangular or square parallel keys and keyways) and DIN 6885 sheet 1 (parallel keys, form A).
- ISO 773 and DIN 6885 sheet 1 (parallel keys, form A, normal fit). The key length is chosen from the standard length series to carry the torque: see the shaft and key sizing calculator.