Position idlers to get better chain wrap -- a question about good practices.

[wilsonmw04]

http://www.chiefdelphi.com/media/photos/27587

Here's a picture of our drive last year. I think it's similar to what you are looking at doing. We had zero problems with this and it worked very well. So well in fact, that we are using a version of it this year. Hope this helps.

[Scott Ritchie]

We used the little things that you take off of the gear end of the big CIMs that come in the kit. They are sort of a spring loaded idler. We have used lots of other hand-made items and also moved the wheels in and out in years past. Last year we had no problem with tension. They kept the chain tight even in extreme situations of flex due to the constant pressure of the spring load. I am sure they can even be used here to get your desired serpentine effect you want which would mimic the serpentine belt system on the alternator on your car. I am not with Cyber Blue (234) anymore but if you contacted one of them I bet they would send you a picture.

[ConKbot of Doom]

I'm a fan of having 2 separate chain loops, one for front one for rear, it gives a lot of engagement on all the sprockets, and provides redundancy in the event of losing a chain. We had another robot run on top of us in 2005, and bend our front axles enough that one of our chains came off. But as long as we were flat on the ground (and not tipping forward like it liked to so so much) control was not hindered much at all.

If you have a metal idler sprocket, then you still have 4 sprockets for each side, so any weight savings from having the single loop should be minimum. We used sliding axle mounting blocks for tensioning, each one with at least 6 socket head cap screws to hold it in place in the frame rail

[meaubry]

Madison,
We've built serpentine #35 - 6 wheel drive systems that incorporated idler / chain tensioners mechanisms in the past, along with many other types of chain drive systems.

The distance apart and the amount of chain wrap is directly related in a design like the one you are showing. We've had them as close as "the teeth almost touching", to a few inches apart with 3 teeth engaged.

So, it can be done - but, there are other issues to also consider when routing the chain this way.

The chain wrap and chain tensioning will impact the drive motor output and temperature.
Too much tension is not good and too little tension is not good. Too much wrap is not good and too little wrap is not good.

The chain tensioning can also be achieved by placing shims or blocks beneath the idler or motor mount. Thus lifting one or the other upward, taking up the chain tension and increasing the chain engagement. This can be done either in conjunction with, or seperate from, sliding fore and aft on the rail.

Also watch that the longest unengaged side of the chain (in this case it is on the bottom side) is protected. Any object that can push against the side of the loosest side will act like a derailer.

I may not have provided a specific answer to your question, but I hope this helps,

Mike Aubry

[deejay021]

ok so i had a quick question... with te tough boxes we recieved a sprocket that fits perfectly ont the box... however the output sprocket is at least 2x the size of the input... so... for every rotation in the tough box we get .5 on the actual wheel...for a 1st year team we are trying to get a drivable robot thatd be a decent accomplishment in out eyes... and we are working towards that but a faster driving robot would be nice...

and my second question is actually relevant to this forum... where do you get idlers... and any ideas on mounting them?

[Madison]

Originally Posted by deejay021

ok so i had a quick question... with te tough boxes we recieved a sprocket that fits perfectly ont the box... however the output sprocket is at least 2x the size of the input... so... for every rotation in the tough box we get .5 on the actual wheel...for a 1st year team we are trying to get a drivable robot thatd be a decent accomplishment in out eyes... and we are working towards that but a faster driving robot would be nice...

and my second question is actually relevant to this forum... where do you get idlers... and any ideas on mounting them?

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.

[MrForbes]

Another way to get a faster speed out of the kit transmissions is to take out the second reduction stage. You have to move some parts around, add a washer to space the gear on the output shaft, and add more holes to the aluminum plate to clear the ends of the motor shafts. Also if you want to use the gear tooth sensors, you'll have to relocate them (after you get them repaired). Then you can use about 3:1 chain reduction (with 6" wheels) to give you a reasonable "high gear" robot speed. The Andymark 12 tooth transmission sprocket, as well as 36 tooth wheel sprockets from Andymark or IFI, will do this.

[JHSmentor]

Originally Posted by M. Krass

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.

[Dave Campbell]

Originally Posted by M. Krass

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.

As another source: McMster-Carr http://www.mcmaster.com
"Finished Bore" is keyed - just as Madison posted, be sure to check bore size and key size....page 1012, 9 tooth to 25 tooth are available in 1/2" bore for #35 Chain

We're also experimenting with new idler designs for chain drive system tensioning.
Chris H - A white paper would be great! The "Milwrights and Mechanics Guide" is tough for our kids to follow...