Belts wear out. T-belts (as T5) should not be used since they can slip on the motor pulley for a fraction of the tooth distance. This will negatively impact the printing result. Such effects occur when the direction changes. Strong wear can cause a stepper motor trip, when the belt tooth hits a pulley tooth. GT2 belts are better suited than T-belts.

If changing pulleys it can happen that the diameter changes (as when moving from T5 to GT2) check that the belts still are horizontal, otherwise the printed objects get irregularities (due to Mr Pythagoras).

A belt tensioner spring has two functions; Putting force into the belt and hinder belt oscillations