Metric Parallel Keys and Keyway Dimensions

Metric parallel keys and keyways provide a positive connection for transmitting torque between a shaft and a hub. The dimensional data on this page is based on BS 4235-1:1972 – Specification for metric keys and keyways – Parallel and taper keys.

The tables provide standard key sizes according to shaft diameter, together with keyway widths, fit tolerances, shaft and hub keyway depths, corner radii, key dimensions and standard key lengths.

Parallel Keys and Keyways

A parallel key is fitted into matching keyways in the shaft and hub. Torque is transmitted principally through contact between the side faces of the key and the corresponding faces of the keyways.

Standard key width and height are selected according to shaft diameter. The required key length must also be sufficient for the transmitted torque and should be checked as part of the complete shaft-to-hub connection.

Machining a keyway removes material from the shaft and introduces a stress concentration. This can be important where the shaft is highly loaded, subjected to fatigue or reversing loads, or where the keyway is positioned close to a shoulder or another change in shaft section.

BS 4235-1 Keyway Dimensions

The table below relates the nominal shaft diameter d to the standard parallel key size b × h and the corresponding shaft and hub keyway dimensions.

• d – nominal shaft diameter
• b – key and keyway width
• h – key height or thickness
• t₁ – shaft keyway depth
• t₂ – hub keyway depth
• r – keyway corner radius

Keyway Fits and Tolerances

The dimensional table includes the tolerance arrangements for free, normal and close/interference keyways. The required arrangement depends on how the shaft and hub are intended to assemble and operate.

A connection that must permit axial movement has different fit requirements from a fixed hub where relative movement between the shaft, key and hub is to be restricted. The tolerances should therefore be selected according to the function of the assembly rather than simply choosing the nominal key size.

Metric Parallel Key and Keyway Dimensions

Parallel Key Dimensions and Standard Lengths

The following table gives dimensional tolerances for the parallel keys themselves, together with chamfer dimensions and the range of standard key lengths associated with each key size.

Standard Metric Key Lengths

Standard key lengths are selected from the preferred series below. The required length should take account of the available hub length and the strength required to transmit the design torque.

6 - 8 - 10 - 12 - 14 - 16 - 18 - 20 - 22 - 25 - 28 - 32 - 36 - 40 - 45 - 50 - 56 - 63 - 70 - 80 - 90 - 100 - 110 - 125 - 140 - 160 - 180 - 200 - 220 - 250 - 280 - 320 - 360 - 400

Increasing key length increases the available contact area for transmitting torque, but the connection should still be considered as a complete design. Hub length, shaft strength, load distribution, manufacturing and assembly can all influence the appropriate key arrangement.

Parallel Key Design Considerations

A keyed connection should be considered as a system comprising the shaft, key and hub. Checking the key alone is not sufficient where the connection is significantly loaded.

The principal strength considerations include shear of the key, bearing or crushing stress on the contacting surfaces, shaft strength and the strength of the hub surrounding the keyway. The keyway itself also reduces the shaft section and introduces a geometric stress concentration.

Where loading is cyclic or reversing, the effect of the shaft keyway can become particularly important. The keyway position, its end geometry, nearby shaft shoulders and changes in shaft diameter should therefore be considered as part of the fatigue assessment.

The key is intended to transmit torque through its side faces. Key and keyway tolerances should therefore be selected to provide the required assembly and operating condition rather than relying on inappropriate contact at the top of the key.

Typical Parallel Key Sizes

Examples from the dimensional table include:

• 6–8 mm shaft – 2 × 2 mm key
• 10–12 mm shaft – 4 × 4 mm key
• 22–30 mm shaft – 8 × 7 mm key
• 50–58 mm shaft – 16 × 10 mm key
• 75–85 mm shaft – 22 × 14 mm key

These examples are useful for quick identification, but the complete table should be used when selecting dimensions and tolerances.

Engineering Applications

Parallel keyed connections are widely used with shaft-mounted components such as gears, pulleys, sprockets, couplings and flywheels. They provide a relatively simple positive-drive connection that can normally be manufactured and assembled using conventional machining methods.

Where greater torque capacity, repeated axial movement, accurate angular location or improved distribution of torque around the shaft is required, a splined or alternative shaft-to-hub connection may be more appropriate.

Standard Reference

BS 4235-1:1972 – Specification for metric keys and keyways – Parallel and taper keys.

The dimensional tables on this RoyMech page are presented as engineering reference information based on BS 4235-1.

Related Key and Spline Data

• Metric Taper Keys and Keyways
• Woodruff Keys and Keyways
• Straight-Sided Splines
• Involute Splines
• Key and Spline Strength Calculations
• BS 46 Parallel Rectangular Keys
• BS 46 Square Keys
• BS 46 Rectangular Taper Keys
• BS 46 Square Taper Keys