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Unread 05-07-2014, 22:54
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Re: pic: FrameDrive 6wd Concept

Quote:
Originally Posted by asid61 View Post
I would still put in some tensioners. You may not think you need them, but an initial tensioning of the belt would be beneficial. You don't need to retension with belts, but having cams and some small slots in there would be a good idea.
Just my opinion.
In my experience, they're really just not necessary for a drivetrain. We've used 15mm HTD or GT2 belts at exact center distances, no offset, for 3 seasons without any failures. The calculated center distance works essentially forever in terms of FRC robot run times (a few hundred hours at most). While belts may eventually stretch, it takes thousands of hours under load for it to become noticeable.

Quote:
Originally Posted by EricH View Post
Ah, but that is a bit of a stretch. Belts and chains both stretch over time, especially for the first little bit right after they're put on as they wear in. Having no provision for dealing with that is asking for trouble. If you design for the final distance, the initial belt tension is going to be a bit tight, running the potential risk of a catastrophic stretch.
I've never seen a properly tensioned HTD or GT2 timing belt stretch in an FRC robot. We've used 9mm belts at exact centers for mechanisms and 15mm belts at exact centers for drivetrains and they have stayed the same tension throughout a season and beyond.

Quote:
Originally Posted by Andrew Lawrence View Post
A 15mm HTD belt put in C-C distance should not stretch during an FRC lifetime to the point where it loses tension. IIRC 2791 runs their belts C-C in a WCD-style setup with two 15mm belts, and hasn't had tensioning problems yet, though if Chris could weigh in on this I'm sure he can describe their experiences better than I can.
Basically, we put in the belts week 3 or 4 of build season and we're done. We never need to adjust them through the competition season, they just work maintenance free. When we've disassembled old robots, the belts show very little visible wear. As long as you properly size the belts for the application (use 15mm and it's a lot harder to go wrong) they're a very robust and simple solution for power transmission.

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As for the gearbox in tube, I think the difficulty of assembly and maintenance for the gearbox would offset the packaging advantage. We used to run "one plate" gearboxes integrated with one side of the drive tube, but it made assembly harder and repairs a nightmare. Inside the tube would probably be worse. Belts in the tube I do recommend as there are a lot fewer parts to align and belt failures are quite rare with proper sizing.
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