Belt Drive Design Problem

[Chris is me]

Originally Posted by Monochron

Can you explain why this is the case? As I understand it, belts are great if you have perfect C-C distance with no need for tensioning. Chain however, due to their slight stretching, does require tensioning.

We're now entering our 5th season with belts, and our 4th season using exact centers with no tensioners. As long as you pick a strong enough belt / pulley combination, this setup will have no problems at all. There is no noticeable stretch or wear over an FRC robot's life span. We put the drive together Week 4 and never touch it again. Perhaps for other applications you may want a tensioner, but in a drive we have done this year after year and it's just "set it and forget it".

Originally Posted by sdcantrell56

Due to belt tolerances, the only way to get perfect c-c distances would be to lay out and measure the actual belt and pullies. This solves it for that particular belt but what happens when you have to replace it? Last time I checked the belt tolerance is +-.010 on the circumference so the chances of it being perfect with another belt are slim. With a means of tensioning the belt this problem disappears.

I would actually argue (it sounds totally nuts, but hear me out) that for the majority of teams, adding tensioners to a belt system will make it more likely to fail. For the precise, detail oriented teams (e.g. 971, 254, etc) tensioners are probably a performance boost to account for that tolerance.

However, for the average team, adding tensioners makes it very easy to over or under tension a belt. It is surprisingly easy to over-tension a belt as a "perfect" center distance belt has more slack than you would expect. Overtensioning a belt significantly weakens the system. In some specific cases with under-sized belts and pulleys, this can and absolutely has led to drive failure. Another common problem is include differing tension in two belts on the same driveline. Again, teams that pay a lot of attention to detail and design great tensioners can find success, but it's easier to fail a tensioned system than an untensioned system in my experience.

Despite the wider tolerance in belt length, we've just never had a problem doing it this way. It just works. We've done this to at least 16 individual belts in different drivetrains now.

[pfreivald]

Originally Posted by Chris is me

We're now entering our 5th season with belts, and our 4th season using exact centers with no tensioners. As long as you pick a strong enough belt / pulley combination, this setup will have no problems at all. There is no noticeable stretch or wear over an FRC robot's life span.

Agreed. 15mm belts are plenty durable for the abuse of one FRC season--though you should design your drive for easy swap-outs just in case.