Buna -N, NBR or Nitrile Rubber is a commonly used material for a spider insert in soft jaw couplings. It is soft for a number of reasons

1. Absorption of equipment vibration, protecting the vibration from a motor or drivetrain to the sensitive equipment on the other side of the jaw coupling.
2. Allows angular misalignment – The whole premise of the jaw coupling is the “spider insert” has a degree of flexibility so the coupling can be flexible. That flexibility for a Buna-N insert is only 1 degree.

The Buna-N spider has its downsides as well.

1. The more it is used at an angle the quicker it wears the “legs” of the spider and the quicker it needs to be replaced.
2. General use means the elastomeric insert is compressed between the two jaw hubs and this leads to the insert absorbing less and less vibration until it needs replacement.

Buna-N or any jaw coupling will always have the disadvantage of only being flexible to 1 degree or less. The replacement of spider inserts while cheap but constantly adds to the cost of using jaw couplings, the labor cost alone is triple the cost of the insert plus the equipment downtime.

Twin Spring Couplings are flexible to 25 degrees, have no insert that needs replacing, the dual spring design absorbs vibration much like the insert does and the built in shear pins make them much like the safety coupling that jaw couplings are known for.