Hytrel insert for jaw couplings are a hard elastomeric insert for larger jaw couplings. The maximum amount of flexibility is only 1/2 degrees, which makes it an alternative to a universal joint but without the flexibility .

Jaw couplings are typically chosen for a number of reasons

1. High speed – the application needs to run at high speed, usually higher than what a universal joint is designed for.
2. Maintenance – A universal joint regardless of type (Block and pin/Friction bearing or needle bearing) need to be greased constantly, so the bearings do not fail due to the heat created during use. Aside from the greasing these same bearing will wear over time and need to be replaced, either from day to day use but also the angle of use, the bigger the angle of misalignment the quicker they will wear out.
3. Small angle of use needed – Jaw couplings are used in applications like pumps and motors because the misalignment angle is only very small (1 degree or less) Adding to that the environment (water, dust) they are used in make them a better coupling than a universal joint, but still they do wear the elastomeric spider insert (regardless of type) out and due to compress as well as just plain old wear the insert needs to be replaced.

Hytel like Bronze is a hard wearing insert but its diminished angle of use makes it not very practical

Twin Spring coupling can work in hostile environments, flexible to 25 degrees, and can run at 7000 rpm. To top it off it has no spider inserts that need replacing.