Hirth joint

A Hirth joint or Hirth coupling is a type of mechanical connection named after its developer Albert Hirth. It is used to connect two pieces of a shaft together and is characterized by tapered teeth that mesh together on the end faces of each half shaft.

Hirth joints consist of radial teeth formed by grooves milled or ground into the end face of a cylindrical shaft.

The teeth mesh around a ring, as the torque capacity of teeth increases with their diameter. This ring is arranged to be at the maximal possible diameter for the space available. The centre of a shaft is not toothed, as this would add little torque capacity to the coupling and the increasingly narrow teeth would become impractical to cut.

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.

Tapered, symmetrical serrations are used. Profile angles of 60 and 90 degrees are used. A key feature of the Hirth joint is that, unlike simple splines, the load-bearing faces of the joint are tapered. This allows the joint to be tightened so that there is no backlash by simply applying an axial load. This is done by either bolting the shafts together or by applying spring pressure from an external housing. This lack of backlash also reduces wear due to fretting.

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. Hirth joints are designed as mating pairs and, unlike splines, there is no standardised off-the-shelf sizing system for them.

Hirth's joints were first used in aircraft engine crankshafts. A large number of cylinders and often the need for reduction gears and supercharger drives made it impractical to manufacture a one-piece crankshaft, so Hirth joints were used to couple the sections. They have used in complex crankshafts for many years afterward, particularly where the output power was taken from a central drive gear. As a large gear could neither be formed-in-place nor passed over the crank webs, it could be machined separately and placed between two half-crankshafts with Hirth's joints. They are also used in gas turbine shafts, in accessories for surgical operating tables, in agricultural machines for fixing tools etc., and in bike parts and frames, such as Campagnolo's "Ultra-Torque" bicycle crankset, and in Bicycle Torque Couplings.

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