View Single Post
  #23   Spotlight this post!  
Unread 30-01-2008, 18:18
JHSmentor's Avatar
JHSmentor JHSmentor is offline
Registered User
AKA: Bryan Molloseau
FRC #3210 (Falcons)
Team Role: Mentor
 
Join Date: Jan 2008
Rookie Year: 2008
Location: Oregon
Posts: 84
JHSmentor is a name known to allJHSmentor is a name known to allJHSmentor is a name known to allJHSmentor is a name known to allJHSmentor is a name known to allJHSmentor is a name known to all
Re: Position idlers to get better chain wrap -- a question about good practices.

Quote:
Originally Posted by M. Krass View Post
If you can vary the gear ratio provided in the Kit of Parts by replacing either of the sprockets. To go faster, use a smaller sprocket on your wheel and leave that on the Toughbox output unchanged -- or similarly, use a larger output sprocket on the Toughbox.

Keep in mind when sourcing sprockets that the Toughbox output is a 1/2" diameter, keyed shaft. It's very difficult to find COTS sprockets that have a 1/2" bore and keyway. You may have better luck buying #35 sprockets from www.andymark.biz or www.ifirobotics.com, as those contain a bolt pattern that will mate directly with the Kit of Parts wheels.

Idlers can be made in a variety of ways. Search these forums for "idlers" or "chain idlers" or similar and you'll find lots of information.
Another option that we considered was to buy the wheel hub from AndyMark and a larger sprocket (I think the KOP wheel sprockets are 22 tooth - you could use a larger one on the output shaftO)- use this as the output gear of the toughbox instead of actually attaching it to the wheel.

for the chain tensioner question, we designed a custom spring loaded idler with bearings pressed into the 22 tooth sprockets. we are not concerned about the chain wrap issue, however - just tensioning.

I agree that a little serpentine alignment of the idler and output sprocket will work fine like the ones used in cycling (for a bike, the sprockets are directly above each other and spaced about 2" apart). they are usually 11-12 tooth sprockets so the serpentine path engages ~1/2 of the teeth. they are spring loaded derailures, however, so a stagnant idler like your design will take a lot of force. Your arrangement will not likely slip (which is why you have it in the first place) but will cause a lot of stress on the frame, idler, and gearbox. to get around this much stress just make sure that the chain is not too tight - you want to just barely take up the slack. On a fixed gear bike (no derailure and no coasting) the chain can move about 1/4" when the wheel is locked in place - this relieves all of the drag on the chain.

I hope this helps.