Parallel keys transmit torque between a shaft and hub — coupling, pulley, sprocket, or gear. The keyway cut into both the shaft and the bore must match in width and depth, and the key itself must be the correct cross-section and length. Incorrect key sizing is a leading cause of coupling slippage, groove damage, and emergency shutdowns in UAE industrial plants running mixed European, Asian, and American equipment. This guide covers standard metric keyways, selection rules, and ordering specifications.
How keys and keyways work
A parallel key sits in a rectangular groove (keyway) machined into both the shaft and the hub bore. Torque transfers through the side faces of the key — not the bottom of the keyway. The key prevents relative rotation between shaft and hub while allowing axial sliding for taper lock removal.
Keys are standardized to shaft diameter ranges. Using an undersized key or mismatched keyway width causes fretting, wallowing of the keyway, and eventual torque transmission failure.
Standard metric keyway dimensions (DIN 6885 / ISO)
For metric shafts, keyway width and depth are determined by shaft diameter. The table below lists the most common pairings found on motors, pumps, and taper lock bushes in UAE industrial supply.
| Shaft diameter (mm) | Key width (mm) | Key height (mm) | Keyway depth on shaft (mm) |
|---|---|---|---|
| 6–8 | 2 | 2 | 1.0–1.1 |
| 8–10 | 3 | 3 | 1.4–1.8 |
| 10–12 | 4 | 4 | 1.8–2.5 |
| 12–17 | 5 | 5 | 2.3–2.9 |
| 17–22 | 6 | 6 | 2.8–3.5 |
| 22–30 | 8 | 7 | 3.3–4.0 |
| 30–38 | 10 | 8 | 3.3–4.5 |
| 38–44 | 12 | 8 | 3.3–4.5 |
| 44–50 | 14 | 9 | 3.8–5.0 |
| 50–58 | 16 | 10 | 4.3–5.5 |
| 58–65 | 18 | 11 | 4.4–5.5 |
| 65–75 | 20 | 12 | 4.9–6.0 |
Measuring existing keyways
When replacing a coupling, pulley, or key, measure the keyway width with calipers or a gauge at the shaft opening. Measure keyway depth from the shaft surface to the key bottom. Compare against the table above to confirm shaft diameter and key size.
Also measure the hub keyway on the coupling or pulley bore — both keyways must match. Taper lock bushes come with a pre-cut keyway for a specific key width; confirm the bush bore matches your shaft diameter and keyway.
- Measure keyway width at both shaft and hub
- Confirm keyway depth matches standard for shaft diameter
- Check key length — should span most of hub width without excessive protrusion
- Inspect for wallowing, fretting, or cracked keyways before reusing shaft
Key length selection
Key length is determined by the hub width (the narrower of the two keyways). The key should extend nearly the full width of the hub keyway for maximum torque capacity. A key that is too short concentrates stress and may shear under load.
Standard key lengths follow DIN 6885 preferred lengths: 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 70, 80, 90, 100 mm. Select the longest standard length that fits within the hub keyway.
| Hub keyway width (mm) | Minimum key length (mm) | Preferred key length (mm) |
|---|---|---|
| 10–15 | 8–10 | 14–18 |
| 15–25 | 14–18 | 22–28 |
| 25–40 | 22–28 | 32–45 |
| 40–60 | 32–40 | 50–63 |
| 60+ | 45–56 | 70–90 |
Key material and finish
Standard keys are plain carbon steel (C45 or equivalent), suitable for most industrial drives indoors. Stainless steel keys (AISI 304 or 316) are used in food processing, marine, and corrosive environments common in UAE desalination and F&B plants.
Keys may be supplied with chamfered ends for easier insertion, or with gib head for easier removal. For high-torque or reversing applications, consider keys with tighter tolerance (h9 width tolerance).
- Plain carbon steel — general industrial, indoor
- Stainless steel 304/316 — corrosive or washdown environments
- Chamfered ends — easier assembly, standard for MRO
- Gib-head keys — used where frequent removal is required
Woodruff keys and special cases
Woodruff keys (semicircular) are used on some small shafts, flywheels, and older equipment. They are not interchangeable with parallel keys. Identify by the semicircular keyway cut into the shaft.
Imperial (inch) keyways are common on US-imported motors and gearboxes. Do not assume metric keys fit — measure in inches or use a metric/imperial conversion. Mixed plants in JAFZA, KIZAD, and Sharjah Industrial often have both systems.
- Woodruff keys — semicircular, used on small diameter shafts
- Imperial keys — measure in inches, common on NEMA motors
- Spline shafts — not keyed, require matched spline hubs
- Set screw-only mounting — no keyway, verify torque capacity is adequate
Ordering keys and keyway confirmation
When ordering keys, specify width × height × length, material, and whether chamfered ends are required. For couplings and pulleys with taper lock bushes, confirm the bush bore size includes the correct keyway for your shaft — bush size charts list bore and keyway combinations.
If the shaft keyway is damaged (wallowed or oversized), the shaft may need remachining or replacement before a new key and hub can be fitted reliably.
- Specify: width × height × length (mm)
- State material: plain steel or stainless
- Note chamfered ends if required
- For hubs/bushes: confirm bore diameter and keyway width together
Common mistakes to avoid
- Using a key one size smaller because it 'fits loosely' — causes fretting and failure
- Measuring key length on the shaft instead of the hub keyway width
- Assuming metric keyways on all equipment — many US-imported drives use inch keys
- Reusing a sheared or bent key after a failure — inspect and replace
- Ignoring damaged keyways on the shaft — wallowed grooves will not hold torque
- Mixing keyway depth standards between shaft and hub — width may match but depth may not
- Forgetting to order a key when buying a finished-bore coupling or pulley
RFQ checklist
- Shaft diameter (mm) measured with calipers or micrometer
- Keyway width and depth on shaft
- Keyway width and depth on hub/bore
- Hub width for key length selection
- Material requirement (carbon steel or stainless)
- Quantity and whether chamfered ends needed
- Photo of existing key and keyway if designation unknown