Not all rollers are visible; some can be hidden from view inside a centre drive module for example. If you know all of the diameters, then you need to refer back to the belt manufacturer's brochure again because there will be a minimum roller diameter given for each belt material. The reason for this dimension and the reason why it's so important is that the more rigid the belt is, the less flexible it is and the less likely it is to wrap around a small roller. If this happens you will lose traction at the drive end and the belt will rise up at each end of the conveyor, not good. On a centre drive conveyor the belt will not want to grip the powered roller and your belt could well remain stationary.

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