From Team 449 Wiki
Chain elevators are systems which use a chain connected to some sprockets to raise and lower a carriage. The chains must be kept taut when in use, and has to be maintained as any other chain would. Chains are very often high maintenance and can get jammed easily if not set up and maintained properly. The chain is also used to vertically move the carriage, via a sprocket or some other part which can interact with a chain. The carriage also has to be mounted in such a way that it can slide up or down the beams on which it is mounted. For example, 80/20 sliders can be used. It is important to make sure the tautness of the chain does not affect other components of the robot; for instance, the two vertical 80/20 bars on Totedile became very bent due to the chain. The advantage of chain is that it is much very good at getting a good grip and not wearing down. It does, however, require a very large amount of maintenance if it is to be used.
Rack and Pinion Elevators
Rack and Pinion elevators use a rack and pinion setup to move the elevator up and down. A rack is a type of metal beam which has grooves in it, somewhat like a timing belt, which line up with the grooves on a pinion, which is essentially a gear. As the pinion is spun using a motor, it can push the rack up or down using simple gear mechanisms. A carriage can then be mounted onto the rack or a pair of racks so it can be moved up and down using this method. Rack and pinion elevators are easier to set up and maintain, but are less structurally stable.
Scissor lifts use several folding X-shaped supports, also known as scissor mechanisms. These can be folded down or extended up by modifying the distance between the "elbows" of the criss-crossing X-pattern bars. There is a negative correlation between the distance between the elbows and the height which the scissor lift extended. Pressure only has to applied to the bottom parts of the scissor lift in order to raise or lower the entire device. Scissor lifts often work either mechanically, pneumatically, or hydraulically. Scissor lifts have the advantage of having lower friction and don't jam as much, but they are not horizontally stable to any degree.