Explains why we could easily flatten a couple of teeth on a failed unit using taps from a small hammer.
Suprises me, as being used to compensate for angular mis-alignment which must result in almost point contact, I was expecting them to be hardened to some degree.
Many high performance toothed couplings certainly are hardened. I have a text called the "Maag Gear Book". In connection with MAAG toothed couplings, I quote from this text :
"All external teeth are surface hardened and ground with slight crowning ..... the internal teeth are shaped into the through hardened components. In order to avoid distortion, no further heat treatment is normally applied. If necessary, the internally toothed parts can be nitrided, in which case care must be taken to minimize distortion."
However, this is top of the line Swiss stuff, not El Cheapo. You get what you pay for.
However, they also say "One exception is that of the couplings for high torque at low speed, such as are used in the cement industry. Here the external teeth are not surface hardened."
Gear couplings bear the torque they transmit over a large number of teeth. The teeth in the outer are usually straight cut and are crowned & barrelled on the inner hub to allow flexing. The combination of multiaxial curves and minor elastic deformation of the teeth means that the load is actually carried by a huge area. Also the teeth are near the extremity of the coupling so the forces are lower than near the shaft - torque is force times distance.
Gear couplings transmit more torque per unit diameter than any other coupling type, the limiting factors for gear selection are usually shaft or keyway capability/performance.
