[patrickrd]

Timing belts and pulleys are typically advertised as a replacement to chains and sprockets. Chains have a couple major problems: a low lifetime (which FIRST teams generally don't need to worry about), and a tendency to stretch. Companies like Mectrol and Gates make belts that they claim do not stretch, and have an extremely long life.

The downside is that they are a pain in the neck. Unlike chain, you can't simply remove or add a link. So you've got to design your system right the first time. You cannot allow any distance change in between your two (or more) pulleys. If one pulley moves towards the other, for example, your belt is going to strip teeth or skip teeth. Finally, you have to pretension your belt to a known pretension that you can calculated based on the belt geometry. If your pretension is to small, it will cause excessive belt wear or the teeth will skip. If too high, your belt may snap under high loads. It's very hard to come up with a simple way to pretension the belts. There are two ways I've considered when designing a belt transmission. First, you can place spring-loaded idlers in the system, which add the tension you want. It sounds simple, but it makes design complicated. Further, for power transmission applications like FIRST, you need to move in both directions. So you've got to have an idler on both sides of the belt (slack side and powered side). The other option is what I prefer, but still is not simple. You need to set up your system so that when you install the belt, you have a means of increasing the center distance between the two pulleys until the belt is properly tensioned. Then, you need a way to lock the system in that location so that it never moves.

The bottom line is that belts are great for some industrial applications because they can have little to no maintenance. On the other hand, FIRST robots have a very short lifetime and you don't have much time to spend on design... So chains make a lot of sense to me.

Patrick