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

Tensioning Systems

There are a few ways to create tension in a belt or chain drive system. You can increase the distance between the drive and driven sprocket such as in the shimmed or eccentric systems. Another method is to lengthen the path of the chain as in the idler method.

Shims

Shims are one of the simplest methods of tensioning the chain. A shimmed system only requires a differential mount that has a vertical split that can be bolted together. By inserting shims between the front and rear halves of the differential mount(s), the differential can be moved further away. The shims can be made of any stiff material. Anecdotally these are frowned upon by design judges, but your mileage may vary.

If correctly designed, shims can be the stiffest option for tensioning the chain as the entire system will be held in compression, there are no threads, and no moving parts. The shimmed design also presents an incredibly cheap option.

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 bicycle^[1].

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. A common implementation is the secondary sprocket on a bicycle derailleur.

Eccentric

An eccentric chain tensioner holds the driven sprocket with a circular mount. The driven sprocket is held off of the neutral axis such that when the mount is turned, the driven sprocket is moved closer or farther from the drive sprocket. The circular mount may be retained by friction around the circumference or by shear pins to the larger structure. A frictional retention method allows for infinite adjustability. Shear pins mean the mount can only be tensioned in discrete steps, but the use of a vernier scale style hole pattern allows incredibly fine control.

Eccentric mounts tend to be larger than other mounting schemes and thus may be inherently stiffer. By necessity, they require multiple moving parts which can drive additional complexity and design time.

Turnbuckle

Often implemented on a swing arm type design. -play
-wear
-packaging
-rigidity[citation needed]
+infinite adjustability
can be done as a differential screw ^[2]^[3]