A Hirth joint or Hirth coupling is a type of mechanical connection named after its develope
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.

Construction

Hirth joints consist of radial teeth formed by grooves milled or

ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...

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 angle The profile angle of a gear is the angle at a specified pitch point between a line tangent to a tooth surface and the line normal to the pitch surface (which is a radial line of a pitch circle). This definition is applicable to every type of gea ...

s 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 Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequent ...

. 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.

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 turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...

s). * If there is some

wear resulting in looseness, tightening the fixing bolt can restore firmness.

Disadvantages

* The manufacturing process is complex, time-consuming and consequently expensive.

Uses

Hirth's joints were first used in aircraft engine

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...

s. 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 been 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

shafts, in accessories for surgical operating tables, in agricultural machines for fixing tools etc., and in bike parts and frames, such as

Campagnolo Campagnolo is an Italian manufacturer of high-end bicycle components with headquarters in Vicenza, Italy. The components are organised as groupsets (gruppi), and are a near-complete collection of a bicycle's mechanical parts. Campagnolo's flags ...

's "Ultra-Torque" bicycle

crankset The crankset (in the US) or chainset (in the UK), is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain or belt, which in turn drives the rear wheel ...

, and in Bicycle Torque Couplings. Specialized's "S-Works" top-level racing mountain bike cranksets use a Hirth joint with a center hex-head threaded rod to secure the crank halves together. The crank design and technology was licensed from Lightning Cycle Dynamics, which utilized and patented a two-piece bicycle crank using the Hirth coupling in 1995. Large-diameter Hirth couplings can effect extremely accurate and repeatable rotational positioning. For this reason, they are used in

indexing head An indexing head, also known as a dividing head or spiral head, is a specialized tool that allows a workpiece to be circularly indexed; that is, easily and precisely rotated to preset angles or circular divisions. Indexing heads are usually use ...

s and rotary tables for precision machining and inspection tasks. Commercial products such as the Ultradex achieve sub-arc-second accuracy. Hirth-like joints are often found on camera support equipment, where they are called rosettes.

Curvic coupling

Curvic couplings are similar in concept and appearance to Hirth couplings, except that their teeth are curved instead of straight. Like Hirth couplings, they lock the components together in an unlockable/removable way and provide indexability.

References

External links

{{Commons category, Hirth joints
Elastic Averaging
Rotating shaft couplings