Hey,
Team 694 was considering building a drive train over the summer, and using belts as opposed to chain to cut down on weight. I have two questions relating to this. First, is it a bad idea to use the nylon pulleys? Are they too weak or will the teeth wear? Second I was wondering how much belt stretches. I know that chain can stretch quite a bit and was wondering if belt does the same. Any teams have experience with this?
Thanks a lot.
Nylon pullys will wear faster than aluminum ones.
Is it a bad idea to use them? For that answer you’ll need to calculate the sheer force that your drivetrain will produce and look at the sheer strength of the material. From there you should have a good idea if you have enough safety margin (I usually look for a safety margin of 5x or better due to shock loads)
Belts do stretch a little. You can get belts that have been pre-stretched and ones that have fiber mesh reinforcement (both will limit stretching … but it will still stretch). I would recomment an idler so that you can adjust the tension as the belt stretchs.
Here is one team’s experience with a belt drivetrain: http://www.chiefdelphi.com/forums/showthread.php?t=61512
look at 125’s robot too
1023 has used cogged belts instead of chains the last 2 years.
Our 2007 robot had aluminium pulleys which we had no problems with. The belts stretched during build season and we retightened the belts (once). The drivetrain did not have to be worked on through 3 competitions it just worked.
The 2008 robot also used belt drive but had some problems. Our original ratio was not right and the belts would slip as we turned; we did not use an idler pulley. Another issue for 2008 was when we used plastic and metal pulleys. They came without a keyway and we had them keyed. We later learned they weren’t ment for the stresses we put them under, the plastic seperated from the metal leaving us spinning around the field.
So I would recommend belts as long as the pulleys are metal and the belt is tensioned.
Two things. The Gates belting supplied in the kit is really good stuff. As long as it is tensioned its tough stuff. You can find it on McMaster Carr or look on their site for a local dealer. Local dealers can often get custom sizes for belting and a much larger selection of ratios. The second is that there are steel lined belts on the market. I don’t know of a specific brand that makes them (Goodyear does I think). They have really high tensile strengths and quite adaptable. I’ve used them in other applications and they worked very well.
This year we used timing belt in our lifting mechanism. We used polycarbonate pulleys with aluminum hubs. We ended up having to make new, larger hubs after they stripped, but we could have prevented that with some screws through the pulley and into the hub (they were also weakened by machining.)
As far as material, urethane and rubber belts are both very strong and can be purchased with kevlar or fiberglass belting from McMaster-Carr. I’ve never heard of one breaking in a FIRST robot (not to say it can’t happen).
If you’re using pulleys/belts, a few things to keep in mind:
If it is a prototype drivetrain, go for it. You can always go back and switch to chain if you do not like it.
We used belts this year and it worked out great. They are MUCH lighter than chain and sprocket (1/6th of the weight for us) and you can get the belts in the exact lengths you need although you will still need to tension them.
We used Kevlar reinforced belts and found that they did NOT stretch noticeably in any fashion. Our tensioner was a piece of delrin turned down to the correct size, no springs, no adjustments, just the same amount of tension the entire season and we never needed to adjust them.
Belts CAN be done, do the calculations, make sure its right for you, and go for it, like i said, worst case is you go back to chain and sprocket and learn your lesson.
Brandon,
Can you tell us what belt and pulley combinations worked well for you? We’d like to take a shot at incorporating them into our drive train.