I'm working on a small project where an armature is actuated by a servo via a couple feet of tendon. The servo+tendon needs to be able to pull about 5-10 lbs. There will be springs at the endpoints to allow some flexibility and shock absorption, so I don't need the tendon to incorporate much flexibility of its own. What's the appropriate type of wire to use for the tendon? Should I use Nylon? Or stranded steel? I see a lot of projects using tendons, but I don't see much description of what's the best material to use to avoid snapping and breakage.
I have successfully used a non-stretch polymer known by various names such as Dyneema, GelSpun or Spectra for a similar purpose. It is easily purchased from most fishing supply stores.
The material is extremely strong and non-elastic - some care is needed with termination since it is slippery. Crimps, wire loops and model aircraft clevises are all good terminations.