Roymech engineering encyclopedia

Keyways & Splines



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Metric and Imperial Keys and Splines

Brief Description of Keys / Keyways and Splines

Keys and splines are mechanical components used in engineering to transmit torque and motion between rotating components, such as shafts and hubs. They are used to provide a positive drive and prevent slippage between mating parts, ensuring efficient power transmission and precise angular positioning. 

Keys are rectangular or square metal bars or strips that are inserted into a keyway, which is a straight-sided slot or groove cut into a shaft or hub. The keyway and key are designed to have a precise fit, with the key preventing relative rotation between the shaft and hub. Several keys and keyways are available covered below; parallel keys, taper keys and woodruff keys in metric and imperial.

Splines are parallel ridges or teeth on a shaft or hub that interlock with corresponding grooves or spaces on the mating component. Splines come in various shapes, including involute, straight-sided, and serrated, and can be internal (on the inside of a bore) or external (on the outside of a shaft). Splines can transmit torque and motion without the need for a separate key, as the ridges and grooves provide the necessary positive drive. Splines are often used in applications where axial movement is required, such as in gearboxes, transmissions, and other mechanical systems.

Metric keys and Splines

Parallel Keys

Parallel metric keys and keyways for engineering


Parallel keys are machine key used to connect a rotating shaft to a hub or gear. These keys are called "parallel" because they have the same width along their entire length. Parallel keys are commonly used in mechanical applications where torque transmission between a shaft and a hub is required. They are typically made of steel, and can be either rectangular or square in shape.

Parallel Keys

Taper Keys

Taper metric keys and keyways for engineering


Taper keys (also known as gib-head keys) are a type of machine key that are used to connect a rotating shaft to a hub or gear. They are called "taper" keys because they have a wedge shape, with one end narrower than the other. Taper keys are typically used in applications where high torque is required, such as in heavy machinery, automotive transmissions, and industrial equipment.

Taper keys

Woodruff Keys

Woodruff metric keys and keyways for engineering


Woodruff keys are semi circular, fitting partially in a circular cut in the shaft, achieved by plunge cutting with a woodruff key cutter. The woodruff key can be easily removed by the blow of a hammer.  Common applications include machine tools, automotive applications and marine propellors. 

Woodruff Keys

Straight Sided ISO Splines

Straight Sided ISO Splines for engineering


Straight Sided ISO Splines are a type of mechanical connection used to transmit torque between two rotating shafts. They consist of a series of equally spaced grooves or teeth cut into the surface of a shaft or hub, which interlock with a mating component to create a strong and secure connection.



Straight Sided ISO Splines

Involute Splines

Involute Splines for engineering


Unlike straight-sided splines, involute splines have a curved tooth profile that follows the shape of an involute curve. This allows the teeth to engage smoothly and gradually, minimizing stress and wear on the spline and mating components. Involute splines are commonly used in applications where high torque is required, such as in gearboxes, couplings, and other power transmission systems. They are also used in various types of machinery, such as pumps, compressors, and machine tools.

Involute Splines

Key and Spline Strength

Key and  Spline strength for engineering


Key and spline strength calculations based on operating conditions, resulting in the stress based on torque applied. Operating conditions include design factor, application factor, load distribution factor, fatigue life factor and wear life factor (primary related to splines).




Key and Spline Strength

British Standards for Metric keys and Splines

  • BS 4235-1:1972: Specification for metric keys and keyways - Parallel and taper keys
  • BS 4235-2:1977, ISO 3912-1977: Specification for metric keys and keyways - Woodruff keys and keyways
  • BS 5686:1986, ISO 14-1982: Specification for straight-sided splines for cylindrical shafts: metric nominal dimensions, tolerances and gauging requirements
  • BS ISO 4156:2005_1: Straight cylindrical involute splines. Metric module, side fit. Generalities
  • BS ISO 4156:2005_2: Straight cylindrical involute splines. Metric module, side fit. Dimensions
  • BS ISO 4156:2005_3: Straight cylindrical involute splines. Metric module, side fit. Inspection

Imperial Keys and Splines

BS46 Parallel Rectangular Keys

BS46 Parallel Rectangular Keys for engineering


BS46 keys are standardised according to the British Standard BS 46: Part 1, which specifies the dimensions, tolerances, and other parameters for different key sizes. BS46 keys are commonly used in various industrial and mechanical applications where torque transmission between a shaft and a hub is required.


BS46 Parallel Rectangular Keys

BS46 Square Keys

BS46 Square Keys for engineering


BS46 square keys are standardised according to the British Standard BS 46: Part 1, which specifies the dimensions, tolerances, and other parameters for different key sizes. BS46 square keys are commonly used in various industrial and mechanical applications where torque transmission between a shaft and a hub is required.


BS46 Square Keys

BS46 Rectangular Taper Keys

BS46 Rectangular Taper Keys for engineering


BS46 rectangular taper keys are standardised according to the British Standard BS 46: Part 1, which specifies the dimensions, tolerances, and other parameters for different key sizes. They are commonly used in various industrial and mechanical applications where torque transmission between a shaft and a hub is required, and where the angle of the taper can be varied to adjust the fit between the key and the keyway.          

BS46 Rectangular Taper Keys

BS46 Square Taper Keys

BS46 Square Taper Keys for engineering


BS46 square taper keys are standardized according to the British Standard BS 46: Part 1, which specifies the dimensions, tolerances, and other parameters for different key sizes. They are commonly used in various industrial and mechanical applications where torque transmission between a shaft and a hub is required, and where the angle of the taper can be varied to adjust the fit between the key and the keyway.

BS46 Square Taper Keys