(Reference Standards: ISO 3117:1977, IS 2291:1990/ DIN 268,271)

The tangential or tangent key arrangement is an uncommon, but nevertheless an interesting mean of securing the hub and the shaft to prevent relative movement between a power transmitting shaft and an attached component. This blog posts provides a brief explanation of this design and its applications.

Key attributes of the design:

• Used for large shafts in heavily loaded applications.
• A tangent key transmits torsion in one direction only, by means of compressive force only. This allows to increase the load transmission capability of the assembly.
• A tangential key consists of two tapered keys mated together.

Shortcomings of the design:

• When a single tangential key assembly is chosen, the torque can be transmitted in one direction only. Therefore, two tangential keys, placed 90° to 180° apart, are required for bi-directional rotation applications.
• Assembly is more complex when compared to standard sunk key designs.

Machining:

• Shaft and hub are typically machined to a transition fit tolerance.
• Typically, the hub or coupling is machined by keyway slotting or wire erosion.
• Tapered keys are machined with 1:100 slope and Ra finish of 32 or better.

Assembly:

• Correct fitting of the keys is critical to the integrity and long-term reliability.
• The keys are typically hand fit by verifying contact with transfer blue.
• After correct fit has been established, both sets of keys are hammered simultaneously, or equally into position.
• The keys are then locked into place with a pin or other method such as set screw or tack weld.

Typical drive applications where it is used:

• Large mixers, steel rolling mills.
• Grinding mill drives, mining hoists.

The Bottom Line

Although not common, the tangential key arrangement remains a viable option for large drive applications involving high torque/torsion loads.