Hirth joint

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.

Construction

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 is rapidly decreasing at 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 a number of kinds of matching serrations can be made on the end faces of shafts. 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 number of grooves, the outer diameter of the cylindrical feature, the bottom angle of the grooves (to the axis of the cylindrical feature), and the depth of grooves.

Finally, an axially oriented bolt holds the two parts together.

Advantages

• Very high loads can be transferred in a small enclosure of only a few parts (two serrated faces and a bolt fixing them together).
• There is no lag in the joint.
• The joint is self centering (because of this the Hirth coupling is used in very high RPM gas turbines).
• If there is some fretting wear resulting in looseness, tightening the axial thread can regain tightness.

Disadvantages

• The manufacturing process is complex and time consuming -- i.e. expensive.

Uses

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.

References