Hirth joint
Encyclopedia
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.
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 angle
s 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.
crankshaft
s 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 Coupling
s.
Hellmuth Hirth
Hellmuth Hirth was a German engineer who founded the Mahle GmbH and Hirth companies, manufacturing engine components and complete aircraft engines respectively.Hirth was born in Heilbronn, the son of an engineer and tool-maker...
. 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 milledMilling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
or ground
Grinding machine
A grinding machine, often shortened to grinder, is a machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool...
(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 angle
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 . This definition is applicable to every type of gear for which a pitch surface can be defined...
s 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.
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 turbineGas turbineA gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
s). - If there is some frettingFrettingFretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
wear resulting in looseness, tightening the axial thread can regain tightness.
Disadvantages
- The manufacturing process is complex and time consuming and consequently expensive.
Uses
Hirth joints were first used in gasoline enginePetrol engine
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....
crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
s 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
Campagnolo
Campagnolo is an Italian manufacturer of high-end bicycle components with headquarters in Vicenza, Italy. The components are organised as groupsets and are a near-complete collection of a bicycle's mechanical parts. Campagnolo's flagship components are the Super Record, Record, and Chorus...
's "Ultra-Torque" bicycle crankset
Crankset
The crankset or chainset , 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, which in turn drives the rear wheel...
, and in Bicycle Torque Coupling
Bicycle Torque Coupling
An S and S Coupling also known as a Bicycle Torque Coupling or BTC is a coupling which enables bicycle frames to be separated into smaller pieces, usually to facilitate packing and transporting. They can be built into the frame by the frame manufacturer when the frame is originally assembled or...
s.