Adding a VFD to a conveyor

A Gearbox trades torque for speed; i.e. lower speed, higher torque and vice versa. A VFD MAINTAINS torque at any speed. An incline conveyor needs MORE torque as weight is increased on the conveyor, so the issue is going to depend on the nature of your load application. In other words, not enough information given.

For example is this a box conveyor and when you put on bigger boxes are they the same spacing, or is the spacing different? In other words are you speeding it up so that there is MORE space between the bigger boxes, therefore the same weight on the belt, or is there LESS total weight on the belt because you are slowing down the in-feed rate at the same time you slow down the conveyor? Or is it the other way around?

Or, is this a continuous flow conveyor, like a crushed product or a powder or grain where if you slow it down against the same feed rate, MORE product gets deposited on the belt?

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I would shoot for a maximum conveyer speed a little higher than what you expect to require for the maximum speed. Then, size the motor so you have enough power at all operating speeds.

Given the choices in the original post, gearing down and speeding up the motor with a VFD is the clear choice. A VFD can increase base speed of a motor but it cannot add torque. The other option would overload the motor.

The product is cucumbers. The belt will generally have the same weight on it at all times. When we are running a smaller cucumber, the belt pulls alot at one time and that is when I would slow down the conveyor so it dumps less cucumbers onto the take-away conveyor so we can inspect the product better without it being all bunched up. Then on the other hand, when we are running larger cucumbers, the incline conveyor dumps the same weight but less product due to the size so we can inspect that product faster so I would speed up the conveyor. My next question is, how much can I speed up a slow gearbox before doing any damage. Thanks again.

"How much can I speed up a slow gearbox before doing any damage?" That is a question which can only be answered by the gear manufacturer. 10 % more than the rated speed should not be a problem and I can only imagine overheating of the gear as a possible problem at high speed but to be safe you must ask the gear manufacturer,