Hi
The drive roll OD (normally the HP=head pulley) is normally determined by several factors. Superficially, any combination of the following factors may play a role: 1. The OD and gearbox ratio is often combined to determine the conveyor speed (where a specific speed is required). 2. When it is required to wrap the belt around the HP at a greater angle than 180° (usually 270°), a larger pulley is used to provide space for the snub pulley (offset, behind the HP). 3. The weight of the belt and it’s contents are drawn foreword by the HP; this is even more significant on inclined belts, where much of the weight of the belt is also carried by the HP. This means the pulley shaft (by which this weight is driven) is often quite large. This shaft is connected to the pulley-shell by ring-fedders, which may require substantial space (diameter-wise) where large forces are concerned. This limits the minimal size of the pulley OD. 4. Often the HP only looks larger than the tail pulley due to the 10mm thick rubber lagging on the HP (which makes a pulley look much larger). 5. The tail pulley size is determined only by the radius at which the belt can be bent without damaging the belt carcass ; this would then be the minimum allowable size of pulleys for a specific belt-application. The HP size is determined mostly by the forces exerted on it, which has much more significance than the belt-requirements only. 6. Often one would attempt to standardize on component sizes as far as possible to minimize on spares; one would then would the have a fixed size HP for a certain range of applications, which means you use a slightly larger HP than actually required in some applications.
These are some of the factors that play a role. In reality however there exists formulas and theory by which conveyor components are calculated for each application. Now days this is done with a software package (eg “Helix”) which would then give you all the conveyor design specifications taking all factors into consideration/calculation. Conveyor suppliers such as Dunlop and Goodyear will give you all the theory involved on these calculations if you approach them for help.
Regards
George
