I designed this chain tensioner for use with FTC Tetrix beams. There usually isn't an easy way to tension chains being used with Tetrix beams, because they use different units (metric versus imperial). This lead-screw method tensioner can be attached to the end of a Tetrix beam and can extend the beam to the desired length to tension your chain, with a long screw that fits in the hole, and a nut that fits in the slot.
I attached my engineering notebook entries for the whole Relic Arm - it includes assembly instructions for a chain tensioner.
Null Robotics (8103) used this with their Relic Arm for the Relic Recovery (2017-18) FTC season.
Here is Null Robotics's relic claw: https://www.thingiverse.com/thing:2808240
The inner piece MUST BE STRONG! Consider printing it with >1mm walls and ceiling, and between 60-100% infill.
Print the inside part on the SIDE and use supports! (see pictures in engineering notebook.) If you print it upright on an end, it will snap along the layer-grain because of the torque being applied on the chain side.