A Hirth coupling is a self-centring face serration: two parts are joined by radial teeth on their end faces and clamped together axially, so the teeth interlock positively (a form fit) and the parts centre themselves. It sits where torque has to be transmitted and a position has to be repeated exactly, for example in rotary tables of machine tools and in turbomachinery. The teeth are straight and triangular with a 60° form. This page explains the geometry and function, compares Hirth with Curvic serrations, describes milling, grinding and forming, and says plainly which machines make them. It closes with a short note for owners of such machines. The related connection for shafts is covered in serrations explained, the overview in how gears are made.
What is a Hirth coupling?
A Hirth coupling is an axially acting face serration used as a form-fit coupling for rotating elements such as shafts. Form fit means the torque is carried by the interlocking teeth, not by friction alone. The teeth are not on the circumference of a cylinder but radially on the end face of the part. “Face serration” here means teeth on the end face, not the cylindrical teeth of a spur gear.
Geometrically it is a 60° face serration with straight, triangular teeth. The teeth are radial and tapered. When clamped they lie flat against each other and centre the parts themselves. According to Gleason, the coupling was invented in 1928 by Carl Albert Hirth.
How a Hirth coupling works
In a Hirth coupling the tooth height increases from the inside to the outside: the tooth tip rises radially outward and the tooth root falls accordingly. The angle between tooth tip and the axis of rotation depends only on the number of teeth. According to Gleason, the duplex taper design makes it possible to mill each tooth space in a single stroke with a disc cutter. Both coupling halves are built identically.
Unlike a dog clutch, a Hirth coupling needs axial clamping for its force fit, and how strong that clamping must be depends on the torque and on friction of the flanks. Because of the 30° flank inclination, transmitting torque creates an axial force. To transmit it, the preload must be at least Fa = Fu · tan 30°, where Fu is the mean circumferential force at the tooth. For design, one manufacturer gives a safety factor of 1.8 to 3.0. Clamping is by disc springs, hydraulic cylinders or, in special cases, bolts. If the elements to be coupled can move slightly axially, the coupling is built in two parts; for rotary tables or tool holders that must not lift off, there is also a three-part design with outer, inner and bridge ring.
How many teeth a Hirth coupling gets depends on the diameter. A calculation handbook gives these guide values:
| Diameter | Recommended number of teeth |
|---|---|
| up to 30 mm | 12 |
| 30 to 60 mm | 24 or 36 |
| 60 to 120 mm | 36 or 48 |
| over 120 mm | 72 or 96 |
Permissible surface pressures are calculated from the material properties, analogous to DIN 6892 for parallel key connections.
Where Hirth couplings are used
- Rotary tables and indexing devices: rotary indexing tables in machine tools use Hirth serrations because, after release, the table returns exactly to the same position. In indexing applications, pitch (indexing) accuracies down to one arc second, 1/3600 of a degree, are reached.
- Turbomachinery: according to Wikipedia, the serration sits between the rotor discs of compressors in turbine jet engines and in stationary turbo compressors and gas turbines.
- Shafts and drivetrain: Hirth serrations join shaft ends to gears, discs and crankshafts.
- Vehicle gearboxes: the serration transmits high and alternating torque without backlash and quietly, which is why it is also named in gearboxes of electric vehicles.
Manufacturers of Hirth rings also name energy technology and wind power, aerospace and heavy industry as sectors.
Advantages and limits
Manufacturers name a form-fit and force-fit, self-centring connection, low axial and radial runout, small space requirement, high torque capacity, very accurate repeatability of positioning, and wear resistance and long life. In indexing technology a Hirth coupling is therefore the preferred solution when static stiffness and position repeatability count.
There are two limits. First, the transmittable torque is limited by the necessary axial clamping: the clamping elements must take the axial force that comes from the flank inclination. Second, manufacture is regarded as complex, time-consuming and therefore expensive. Hirth serrations are found where positioning accuracy and stiffness justify the effort.
Hirth and Curvic serrations compared
The Curvic serration is closely related but has curved instead of straight teeth. One part carries barrel-shaped teeth, the mating part hourglass-shaped teeth, and every tooth in mesh contributes to self-centring. According to Gleason, the Curvic coupling was introduced in 1942. The curved tooth form results because the parts are milled with a cutter head or ground with a cup wheel.
| Feature | Hirth coupling | Curvic coupling |
|---|---|---|
| Tooth form | straight, triangular teeth | curved teeth, barrel and hourglass shaped |
| Manufacture | space by space with a disc cutter; for highest accuracy pre-mill, harden, grind | cutter head (milling) or cup grinding wheel; in grinding, the serration is finished in one workpiece revolution |
| Machines | milling and grinding machines | machines similar to those for bevel gears; common Curvic grinding machines are the Gleason No. 887 and 888 |
| Typical use | rotary tables, indexing devices, turbomachinery, shaft couplings | rotor discs in jet engines and power plant turbines, also precision indexing devices on machine tools |
The Curvic serration grew out of the manufacture of Zerol and spiral bevel gears and is made on similar machines. For Curvic serrations the family of bevel gear cutting machines is therefore the closest relative. According to Gleason, most Hirth couplings are unground and therefore suitable only for moderate speeds, while rotors in jet engines and power plant turbines are stacked exclusively with Curvic couplings. For high-accuracy rotary tables and indexing devices, manufacturers on the other hand offer ground Hirth rings. The terms are not always kept apart: where Wikipedia speaks of Hirth serrations between turbine discs, Gleason assigns the same job to Curvic couplings.
How Hirth couplings are made: milling, grinding, forming
Milling
A Hirth coupling is milled with a disc cutter. Thanks to the duplex taper, each tooth space is made in a single stroke. According to contract manufacturers, unhardened materials up to a strength of 1,100 N/mm² are milled, and milled rings reach about 1,000 mm in diameter. The load-bearing contact area of milled serrations is up to about 70 percent.
Grinding
For high-accuracy connections, the teeth are pre-milled, hardened and ground in the hardened state, respecting the required case depth. Ground Hirth serrations then show almost no change in pitch and centring accuracy even after many indexing cycles. The load-bearing contact area, the share of the tooth flank that carries in mesh, rises to up to 80 percent. That lowers surface pressure and raises transmittable torque. Diameter limits for ground rings are 400, 500 (800 on request) or 950 mm, depending on the supplier; one supplier gives a load-bearing contact area of up to 97 percent. The figures clearly depend on the machine and process of the individual supplier. Grinding is used especially where standard solutions reach their limits, for example in hard-to-machine materials such as hardened tool steels, quenched and tempered steels, stainless steels and duplex and super duplex steels.
The machines used are jig grinding centres, profile grinding machines and vertical multi-purpose grinding machines. For Hirth serrations, an angular accuracy of the workpiece spindle of one arc second is one of the main requirements.
Forming and other processes
As a chipless alternative, a Hirth serration can be cold rolled. A rolling machine designed for parts of 80 to 120 mm diameter handles tooth counts of 20 to 110, tooth heights of 0.3 to 4.5 mm and space angles of 40° to 100°. The advantage is that the serration can be integrated as the last operation in a process chain without changing the body of the part. The literature also names die forging, flow forming, sintering and injection moulding for applications with lower requirements.
| Process | Starting condition | Machine type | Data |
|---|---|---|---|
| Milling | unhardened, up to 1,100 N/mm² | milling machine with disc cutter | load-bearing contact up to about 70 %, rings up to about 1,000 mm diameter |
| Profile grinding | pre-milled and hardened | jig or profile grinding machine | load-bearing contact up to 80 % (up to 97 % at another supplier), repeatability at most 0.001 mm |
| Cold rolling | forming without chip removal, body of the part unchanged | rolling machine | example: 80 to 120 mm diameter, 20 to 110 teeth, tooth height 0.3 to 4.5 mm |
Accuracy, tolerances and inspection
With Hirth serrations, pitch accuracy comes first. One manufacturer aims for ±2 arc seconds on ground rings and guarantees ±3 arc seconds, with a repeatability of at most 0.001 mm and a hardness of 54 ± 2 HRC. For rotary tables, what matters is that the serration hits the same position after every release and clamping. A contract manufacturer documents inspection with 3D measuring technology to VDI/VDE 2614.
Hirth rings: standard or custom-made?
If you do not make Hirth serrations yourself, you can buy rings or finished parts. Suppliers offer standard sizes available at short notice, custom ring sets and the machining of a Hirth serration on parts you supply. Before you ask for a quote, clarify:
- the diameter of the serration and the number of teeth,
- the required pitch accuracy,
- whether the teeth should be milled or ground, unhardened or hardened,
- whether a two-part or three-part design is needed.
Milled, unhardened versions are possible up to a material strength of 1,100 N/mm²; one manufacturer’s hardened rings have a hardness of 54 ± 2 HRC and are then ground.
Which machines make Hirth couplings?
We could not document a machine that makes only Hirth serrations in the manufacturer sources reviewed. The serration is made on milling machines and, for highest accuracy, on grinding machines. Profile grinding is the same basic principle used in gear grinding: a profiled grinding wheel works space by space. See selling gear grinding machines. Curvic serrations, by contrast, have their own Curvic grinding machines. Because no machine family of its own for Hirth serrations could be documented, we do not run a separate machine-type page for it; the machines belong to the wider range of gear cutting machines.
What this means for used machines
Anyone making Hirth serrations needs above all accuracy in pitch: the condition of the rotary table and workpiece spindle, the control and an accuracy record such as an inspection report are among the factors that determine the value of a grinding machine. Workpiece diameter and length also count, because they set the range of use. We ask for these details with every valuation.
If you own a machine that made Hirth serrations
We buy your grinding or gear machine. Sale through our buyer network: commission stated upfront; you are paid before pickup. The offer usually follows 24–48 hours on working days after we have all the details once we have all information. Direct purchase: bank transfer before pickup; no machine leaves your site without confirmed payment. We organise dismantling and transport, and whether costs are included is stated in the offer upfront. The whole process usually takes 7 to 15 working days on a direct purchase.
For the type pages, see gear grinding machines and the overview at selling gear cutting machines. What your gear machine is worth explains the value factors, and how it works describes the process.