A simple conveyor belt will be stretched between two rollers, one driven roll which powers it and one idle roll which is free to spin as the belt moves. The driven roll will almost always be at the front of the conveyor — that is, the direction of material travel — because as we all know, you can't push a rope. Only in situations with very tight belts and low power can the drive roll be in the back without causing the top of the belt to become loose and the bottom to stay taut.
With very long conveyor belts driving a lot of weight, it may be necessary to increase the number of rolls. Additional rolls under the top of the belt can prevent the conveyor from sagging, although this can also be done by driving the belt on top of a flat surface between the rolls if friction is not a big problem. If the belt is long enough, stretching may be an issue, causing the belt to go slack as it heats up or ages. In this case, a take-up roll can be added, which uses weights, springs, or air cylinders to keep the belt taut by stretching it, usually in the free space underneath the conveyor. The most common set up is to use three rollers, with the take-up roll pulling the belt up between two others.
Another difficulty is keeping the belt centered on the roller, especially in high speed applications. Unfortunately, a tight and slightly elastic belt stretched between two rollers will tend to climb upward, so if the rolls are off parallel, even by just a little bit, the belt will eventually walk itself to one side, whichever side is more taut. This can cause all kinds of problems. The belt might fall off the side of the roll, or it might sustain damage to one side by rubbing against the frame, or it might not deliver the material straight ahead. There are several ways to keep the belt centered, but not all are applicable to all situations.
One method is using a "crowned" roll for the drive roll. A crowned roll is tapered such that it is thicker in the middle than on the ends, which ensures the belt always climbs to the center of the roll. However if the drive roll must be flat this is not an option.
Another method is to groove the rolls or add guides to the sides of the belt, which keeps the belt in place by giving it vertical walls to the sides. However this can damage the side of the belt in some cases, and if an object gets pulled under the belt, it can push the belt up over the groove or guide if it is short enough or the belt has enough elasticity.
A third method is to make the rollers adjustable so that they can be shifted out of parallel on purpose. If the belt is wandering one direction, adjusting the rollers so that they are more taut in the other direction will cause it to wander the other direction.
Because of these difficulties, conveyor belts are only used when they are really needed, for example when moving material that requires a continuous surface or when the conveying process must be very gentle. Other options are screw conveyors, which run a screw through a half-cylinder to move granular material, roller conveyors, which use a series of cylindrical rollers with gaps between them, disc conveyors, which are similar to roller conveyors but use narrow discs on a shaft rather than continuous cylinders, and chain conveyors, which use chains driven by a sprocket. There is also a model called the belt conveyor (as opposed to the conveyor belt). The difference is the belt conveyor uses two or more narrow belts or cables next to each other, running along pulleys driven by a common shaft. These are much easier to keep from wandering as the groove in the pulley keeps the belt in place.