Plastic modular belt are basically driven like a bicycle chain. Well, it is rather powered by an electrical motor than by muscles. But in most cases it has just one or more catenary sags after the drive sprockets each containing of an unsupported belt distance between 0.8 m and 1 m length. This belt sag creates a small tension by its own weight back to the drive sprockets which is called back-tension that is an important player to ensure a proper belt to sprocket engagement.
In my role as Application Engineer I often see installations with poor sprocket engagement. In most cases it is due to lack of back-tension that allows the belt to easily jump over the sprockets teeth and to lose its engagement.
For an optimal drive configuration use a sprocket size to minimize the polygon effect (see earlier article). Medium-size sprockets with teeth between 12 and 28 teeth are generally preferred. Larger sprockets >36 teeth are not recommended because accumulation of the belt pitch and sprocket tolerances may cause engagement issues.
To keep the belt properly engaged the belt shall be wrapped with an angle of minimum 180° to approximately 200° around the sprockets. The increased belt wrap can be easily reached by adding a snub roller or installing the belt back-bending rollers close to the sprockets.
In particularly for lower head and center drive configuration the distance between sprocket and snub roller(s) must be as short as possible. Take care, that the belt does not get squeezed in-between. The worst configuration is if the drive sprockets position is very low and the catenary sag small. In this particular case the problem is that the weight of the catenary sag is below the weight of the belt section around the sprocket. As result the catenary sag will establish around the sprockets and cause disengagement.