A Hirth joint or Hirth coupling is a type of mechanical connection named after its developer Hellmuth Hirth. It is used to connect two pieces of a shaft together and is characterized by teeth that mesh together on the end faces of each half shaft.
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Hirth joints consist of radial grooves milled or ground (or both) into the end face of a cylindrical feature of a part. Grooves are made one by one into the part tilted by the bottom angle of the grooves, and rotated from groove to groove until the serration is complete.
Usually the grooves mesh together within a ring, because the load-bearing capacity of teeth decreases rapidly with smaller diameters. For instance a shaft of 60 mm diameter can be toothed in a 12 mm wide ring only (inner diameter is 36 mm) without jeopardizing the load-bearing capacity of the shaft.
Theoretically, any matching serrations types can be made on the shaft end faces. Only symmetric serrations are used in practice: the profile of a tooth is a symmetric triangle, and the tooth's head and bottom angle is the same too. Even the profile angles are not arbitrary: angles of 60 and 90 degrees are used.
The coupling is defined by the groove count, the outer diameter of the cylindrical feature, the bottom angle of the grooves (to the axis of the cylindrical feature), and their depth.
Finally, an axially-oriented bolt holds the two parts together.
Hirth joints were first used in gasoline engine crankshafts[1] and are now used in jet engine shafts, in accessories for surgical operating tables, in agricultural machines for fixing tools etc., and in bikes parts/frames such as Campagnolo's "Ultra-Torque" bicycle crankset, and in Bicycle Torque Couplings.