Tensioner

A tensioner is a mechanism that applies force to a belt or chain drive to ensure proper function. Without correct tension, the drive system may fail catastrophically.

Shims

Shimmed Chain Tensioner

design judges HATE him
most structurally sound
most smoothbrain
likely the lightest[citation needed]
-assembly
+weight

-get laughed at by design judges bc they are all biased against it smh my head

Idler

Another sprocket or pulley is used to add tension to the chain or belt. It does not add any torque to the system and freely spins.

Idler Chain Tensioner

Guide

Chain guides use a durable low friction material to squeeze the chain into tension. This is most commonly used for timing chains (kept lubricated inside the engine) and sometimes on bicycles

Guide Chain Tensioner

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Eccentric

big brain time
+rigidity
+infinite adjustability if using friction (may slip)
-design complexity
-limited finite number of tensioning options if using shear pins

Eccentric Chain Tensioner

Turnbuckle

-play
-wear
-packaging
-rigidity[citation needed]
+infinite adjustability
can be done w coarse/fine pitch instead of handedness (i completely forgot what the name is for this kind of turnbuckle, but they have less wear/play [citation needed] but allow a smaller range of motion)

Turnbuckle Chain Tensioner