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#090 Motion Mechanism Design - 7 : Shaft-boss Fastening Methods

Category : Drive mechanism design
October14, 2011

There are various shaft fastening methods available, including ones used to couple two shafts or to fasten a shaft to a boss for power or torque transmission. This section provides a brief comparative summary of shaft-boss fastening methods for power or torque transmission.

(1) Shaft-boss fastening methods

A mechanical element fastened to a shaft is called a boss. Typical shaft and boss fastening methods are shown below:

-Fastening with keys (See Fig. 1.)

[Fig. 1] Fastening with keys

-Fastening with setscrews (See Fig. 2.)

[Fig. 2] Fastening with setscrews

-Coupling with pins (See Fig. 3.)

[Fig. 3] Fastening with pins

-Frictional locking (See Sections 92 and 93.)

(2) Comparison of fastening methods

Shaft-boss fastening method Principle and features Torque transmission capacity
Fastening with keys
Flat key
Saddle key
Sunk key
Round key
  • Keys (that may be shaped rectangular, round, conical, etc.) inserted into keyways in the shaft and boss to fasten them.
  • JIS and DIN standardized
  • Available in various shapes and highly reliable.
Flat key: Moderate
Saddle key: Low
Sunk key: High
Round key: Low
Fastening with setscrews
  • Setscrews inserted through a boss to fasten the shaft.
  • Poor compatibility with overload torque.
Used to secure a small (low-power) motor output shaft to a coupling.
Fastening with pins
  • Pins inserted into thru holes in the shaft and boss to couple them together.
  • Suitable for coupling linkage mechanisms.
Frictional locking
  • Clamping screws apply radial force to the shaft and boss to frictionally fasten them.
  • User-friendly, allowing removal and reattachment and easy shaft alignment.
Low to high
Torque transmission capacity variable depending on frictional force available from Mecha-lock selected.
  • Shaft and boss welded and firmly fastened together.
  • Inconvenient because disassembly impossible after welding.
  • Accuracy variations unavoidable due to weld strain, etc.
Moderate to high