Keys and splines are widely used to transmit torque between a shaft and a hub such as a gear, pulley, sprocket or coupling. Choosing between them is not simply a matter of finding a standard size that fits the shaft.
The connection must transmit the required torque while also satisfying the requirements for assembly, location, accuracy, fatigue life, axial movement, manufacture, maintenance and cost.
This guide explains the main differences between parallel keys, taper keys, Woodruff keys, straight-sided splines and involute splines, and helps identify which type of shaft-to-hub connection is appropriate before moving to the detailed RoyMech dimension and calculation pages.
A conventional keyed connection is simple, well understood and economical. For many gears, pulleys, sprockets and couplings it is entirely appropriate. It should not, however, be treated as the automatic solution whenever a hub has to transmit torque.
Machining a keyway removes material from the shaft and introduces a geometric discontinuity. This reduces the effective shaft section and produces a stress concentration. The effect becomes increasingly important where the shaft is highly stressed, subject to fluctuating or reversing loads, or required to have a long fatigue life.
The key and keyways also introduce clearances, tolerances and contact surfaces. Depending on the fit and loading, relative movement between the shaft, key and hub can lead to fretting, wear or damage to the keyway.
Before specifying a key, consider:
A key fits into matching keyways in the shaft and hub. Torque is transmitted through contact at the sides of the key.
For preliminary design, the two principal key checks are normally:
The shaft must also be checked because cutting the shaft keyway reduces its section and introduces a stress concentration. It is therefore possible for the shaft or keyway region to govern the design even when the key itself has adequate nominal strength.
RoyMech provides a separate worked calculation method covering shaft, key and spline strength:
Key and Spline Strength Calculations
| Connection | Generally used for | Typical reason for choosing it | Main advantages | Points to consider |
|---|---|---|---|---|
| Parallel key | Gears, pulleys, sprockets, flywheels and couplings on general-purpose machinery. | A straightforward fixed shaft-to-hub connection where a conventional keyed joint provides adequate torque capacity. | Simple, standardised, economical and straightforward to manufacture and replace. | Keyway weakens the shaft. Check key shear, bearing stress, fit, fretting and shaft fatigue where relevant. |
| Taper key | Gears, pulleys, flywheels and other hubs where a driven wedging connection is appropriate. Gib-head versions are particularly useful where subsequent removal is required. | A keyed connection where the wedging action of a taper key is beneficial. | Simple construction and a driven fit between the shaft, key and hub. Gib-head versions provide a feature to assist removal. | Wedging action can affect hub position and concentricity. Assembly and removal must be considered. The specified taper is part of the design and should not be altered simply to change the fit. |
| Woodruff key | Small shafts, tapered shafts, machine components and assemblies where alignment of the key with the hub keyway during assembly is useful. | The ability of the key to rock in its curved shaft seat can assist alignment during assembly. | Self-aligning action can simplify assembly and is useful on some tapered-shaft connections. | The relatively deep curved seat removes more shaft material than a shallow parallel-key seat and can be undesirable where shaft strength or fatigue is critical. |
| Straight-sided spline | Gearboxes, transmissions, sliding gears, couplings and other machinery requiring multiple driving faces or axial movement between shaft and hub. | Higher torque, multiple load-carrying faces, repeated assembly or a hub that must slide axially while remaining rotationally coupled. | Torque is distributed over several spline teeth. Can provide positive rotational engagement while permitting controlled axial movement. | More complex to manufacture and inspect than a simple key. Load distribution, fit, alignment, wear and lubrication can become important. |
| Involute spline | Automotive and industrial transmissions, gearboxes, drive shafts, couplings and other production machinery requiring accurately controlled splined connections. | A higher-performance shaft-to-hub connection requiring multiple teeth, controlled fits and good manufacturing repeatability. | Multiple teeth distribute torque around the circumference. Involute geometry is well suited to established generating processes and controlled spline fits. | Design and inspection are more involved than for a keyed joint. Tooth geometry, fit class, alignment, load distribution, fatigue and wear all need appropriate consideration. |
| Alternative / keyless connection | Precision hubs, high-speed rotating components, fatigue-sensitive shafts and applications using interference fits, shrink fits, clamping hubs or shaft-locking devices. | A conventional keyway is undesirable because of fatigue, precision, backlash, assembly or other design requirements. | Can avoid cutting a conventional shaft keyway and, depending on the method, can provide a backlash-free connection and good concentricity. | Assembly force, temperature, surface condition, contact pressure, removal, manufacturing tolerances and failure mode must be considered for the chosen method. |
The parallel key is usually the first keyed connection to consider for a conventional fixed hub. It is widely used because the component geometry is simple and both keys and keyways can be produced using common manufacturing methods.
The parallel key does not normally provide the primary means of axial retention of the hub. The designer should separately consider how the hub is located along the shaft using shoulders, nuts, circlips, retaining plates, spacers or another suitable arrangement.
The hub-to-shaft fit and the key fit are separate design decisions. A key transmits torque; it should not be expected to compensate for an inappropriate shaft-to-hub fit.
Use the RoyMech dimensional reference for metric parallel keys:
BS 4235-1 Parallel Key and Keyway Dimensions
A taper key has a longitudinal taper and is driven into position so that wedging action develops between the shaft, key and hub.
This can provide a secure connection, but the wedging action is also an important difference from a parallel key. Driving the key further is not a general-purpose method of adjusting an arbitrary fit: the taper and keyway geometry are specified parts of the connection.
A gib-head taper key incorporates a projecting head that provides a feature for extracting the key. This can be valuable where dismantling is expected, provided there is adequate access around the assembly.
The wedging action should be considered where accurate hub concentricity or position is important. The effect of assembly forces on the hub should also be considered, particularly with relatively thin hub sections.
Metric taper-key dimensions are provided here:
BS 4235-1 Taper Key and Keyway Dimensions
A Woodruff key is a segment-shaped key fitted into a circular recess in the shaft. Its geometry allows it to rock slightly in the shaft seat, which can help the key align with the hub keyway during assembly.
This makes Woodruff keys useful in particular assembly arrangements and on some tapered shafts. They are commonly associated with relatively small shaft-to-hub connections.
The benefit comes with a design trade-off: the circular seat extends more deeply into the shaft than many conventional parallel-key keyways. Where the shaft is highly stressed or fatigue is important, the reduction in shaft section and the local stress concentration need careful consideration.
Woodruff-key dimensions are provided here:
BS 4235-2 Woodruff Key and Keyway Dimensions
A spline can be considered when a single key is no longer the most suitable way to transmit torque or control the relationship between the shaft and hub. Instead of transmitting load through one key, a spline provides multiple engaging teeth around the shaft circumference.
Reasons for considering a spline include:
A spline should not automatically be regarded as a stronger key. Its actual capacity depends on the shaft diameter, spline geometry, tooth engagement, material, fit, alignment, load distribution and operating conditions.
Straight-sided splines use parallel-sided teeth on the shaft and corresponding grooves in the hub. They provide multiple load-carrying surfaces and can be used for both fixed and sliding connections.
Where axial movement is required, the spline fit, alignment, lubrication and wear behaviour become particularly important. A connection that can slide when assembled is not necessarily suitable for continuous sliding under substantial transmitted torque without further design consideration.
RoyMech provides dimensions and fit information for straight-sided metric splines here:
ISO 14 Straight-Sided Spline Dimensions and Fits
An involute spline uses tooth profiles based on involute geometry rather than parallel-sided spline teeth. This geometry is well suited to established gear-type manufacturing methods and provides a systematic basis for tooth dimensions, fits and inspection.
Involute splines are commonly considered where the application justifies a more sophisticated shaft-to-hub connection and where control of fit, manufacture and inspection is important.
The choice of an involute spline is not simply a torque decision. The designer must also consider the required fit, centring, tooth loading, alignment, fatigue, wear, lubrication where applicable, manufacturing capability and inspection method.
RoyMech provides the detailed involute-spline design information here:
Involute Spline Design and Engineering Data
One of the most useful early questions is whether the hub is intended to remain in one axial position or move along the shaft.
For a fixed gear, pulley or sprocket, a parallel key may provide a simple solution provided the torque, shaft strength, fit and assembly requirements are satisfactory.
If the hub must move axially while remaining rotationally connected to the shaft, a spline is usually a more natural arrangement. Examples include sliding gears and other mechanisms where axial movement forms part of the machine function.
For a sliding spline, do not consider torque capacity in isolation. Sliding surfaces introduce additional questions of fit, alignment, lubrication, surface finish and wear.
It is tempting to select a connection by torque capacity alone. In practice, several connections may be capable of transmitting the nominal torque while behaving very differently in service.
For example, consider whether the application has:
These conditions can change which connection is most appropriate even when the transmitted power and nominal torque remain unchanged.
A common design mistake is to calculate the key strength but give insufficient attention to the shaft.
The shaft keyway removes material at a location that may already be carrying combined torsional and bending stresses. The keyway also creates a local stress concentration. Consequently, the fatigue effect of the keyway can be more important than the simple reduction in shaft cross-sectional area.
This becomes particularly important for highly loaded rotating shafts because bending stress at a point on the shaft surface can cycle as the shaft rotates, even when the external transverse load itself is steady.
Where fatigue is critical, consider whether the keyway should be avoided altogether or whether another shaft-to-hub arrangement would provide a better design.
The key is only one part of the complete joint. The fit between the shaft and hub controls how the two components locate relative to each other, while the key or spline provides the required rotational engagement.
Do not assume that a tight key should be used to correct a loose or poorly specified shaft-to-hub fit. This can produce poor contact, eccentricity, difficult assembly and unintended loading of the key.
Similarly, where accurate concentricity is important, identify which surfaces are intended to provide the centring function and specify their fits accordingly.
Torque transmission and axial retention are different functions.
A conventional parallel key prevents relative rotation but should not automatically be assumed to locate the hub axially. If the gear, pulley or coupling must be held in a defined axial position, provide an appropriate retention method.
Possible arrangements include shaft shoulders, nuts, circlips, retaining plates, spacers and other positive locating features. The appropriate method depends on the axial load, assembly arrangement and service requirements.
A technically capable connection is not necessarily the best connection to manufacture.
A parallel key and keyway can often be produced with relatively simple machining operations. Splines generally require more specialised cutting, forming or generating processes and additional inspection.
Production quantity matters. A manufacturing method that is difficult to justify for a single replacement shaft may be entirely reasonable for a production component where suitable tooling and inspection are already available.
Assembly access should also be considered at the design stage. A gib-head key, retaining nut or other removable feature is of little practical value if the surrounding machine prevents access to it.
For a conventional shaft-to-hub design, the following sequence provides a useful starting point.
| Design requirement | Connection to consider first | Reason |
|---|---|---|
| General fixed gear, pulley, sprocket or coupling | Parallel key | Simple, economical and widely applicable. |
| Driven wedging action is specifically required | Taper key | The taper develops the required driven fit. |
| Taper key with improved access for extraction | Gib-head taper key | The head provides a feature for removal. |
| Small or tapered shaft where self-alignment during assembly is useful | Woodruff key | The key can rock in its curved shaft seat during assembly. |
| Hub must slide axially while remaining rotationally engaged | Spline | Multiple teeth can maintain rotational engagement during axial movement. |
| Multiple load-carrying faces required | Straight-sided or involute spline | Load can be distributed over several spline teeth. |
| Controlled involute tooth geometry and production manufacture required | Involute spline | Established involute geometry supports systematic manufacture, fits and inspection. |
| Highly fatigue-sensitive shaft | Consider a keyless connection | Avoiding a conventional shaft keyway may improve the shaft design. |
| Existing imperial machinery | Identify the existing standard first | Do not replace an imperial key with a nominally similar metric key without checking the keyways. |
Once the type and approximate size of connection have been selected, its strength should be checked against the actual loading.
RoyMech provides a worked method covering shaft shear, key bearing stress, straight-sided spline loading and involute spline loading, together with application, fatigue, wear and load-distribution factors:
The following withdrawn BS 46 pages are retained for maintenance of existing machinery, historic drawings and replacement components. They should not be treated as the normal starting point for a new metric design.