Usually the rollers or pulleys are curved (See Roller design for conveyor belts)
But this is not enough in some cases: side loading, short and wide belts, need for high precision of straightness.
In this case, you can add a guide:
The guide uses the roll groove to guide the conveyor belt.
Grooves can also be machined:
In this case the rollers are provided with guides.
In the case of moulded belts, it is possible to form grooves or guides in order to ensure perfect guidance on the rollers:
It is also possible to produce Poly-V power transmission belts for low forces: