pic: Concept 8wd Drivetrain

[B.Johnston]

It can be...

But here it was done by reducing the number of parts.

The second driven gear was added (per side).

And 2 sprockets and a chain were eliminated (per side).

The 8 wheels design inherently lent itself to being efficiently divided up into 4 identical modules.

[sdcantrell56]

This is one of the reasons why I have gravitated toward 8wd is the redundancy of the chain. Unless 2 chains break on each side of my drivetrain, there will always be at least one center wheel plus its outer wheel powered. Also once again this drivetrain was designed for a game with climbing obstacles. I did not design it to drastically increase traction over a 6wd although I believe the traction would be increased by a very small amount given the use of roughtop. Also I like the use of 8wd because there will be less rock than a 6wd. Finally, treadwear will be reduced. These advantages in my mind outweigh the disadvantage of slightly more complexity and about a 2.5lb weight gain.

If the game is a perfectly flat field than chances are very good that we will go with a 6wd version of this wooden drivetrain.

[Cory]

Originally Posted by sdcantrell56

This is one of the reasons why I have gravitated toward 8wd is the redundancy of the chain. Unless 2 chains break on each side of my drivetrain, there will always be at least one center wheel plus its outer wheel powered.

This is also true of any direct drive to the center wheel 6wd robot.

I'm always curious why so many people cite the desire to have a robot that can continue driving through multiple chain/sprocket/etc failures. In the hundreds of matches my teams have competed through, as well as our collaborative partners, I have only ever witnessed two chains break-in both cases it was a sprocket failure.

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

[sdcantrell56]

Originally Posted by Cory

This is also true of any direct drive to the center wheel 6wd robot.

I'm always curious why so many people cite the desire to have a robot that can continue driving through multiple chain/sprocket/etc failures. In the hundreds of matches my teams have competed through, as well as our collaborative partners, I have only ever witnessed two chains break-in both cases it was a sprocket failure.

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

I do agree with you and in fact with all of the teams I have been a part of, I have never had a chain break. The first year we had multiple chains come off of sprockets, but ever actually had one break. I guess I am always worrisome and because of this like to design redundancy into the system. I do agree though it is probably unnecessary as long as the chain is properly aligned and tensioned there is really a very minimal chance of breaking, even with #25 chain.

[Triple B]

Originally Posted by Cory

This is also true of any direct drive to the center wheel 6wd robot.

I'm always curious why so many people cite the desire to have a robot that can continue driving through multiple chain/sprocket/etc failures. In the hundreds of matches my teams have competed through, as well as our collaborative partners, I have only ever witnessed two chains break-in both cases it was a sprocket failure.

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

"just say no" to chain tensioners.
i think maybe a 7wd base, lighter than 8wd and 1 wheel cooler than 6wd.
and the "coolness factor" is one of thee most important design features.
mike d

[vivek16]

Originally Posted by Triple B

"just say no" to chain tensioners.
i think maybe a 7wd base, lighter than 8wd and 1 wheel cooler than 6wd.
and the "coolness factor" is one of thee most important design features.
mike d

Yes, I bet our 5wd last year could give you guys a run for your money.

-Vivek

p.s. no kidding, PM me if you want more details about the 5wd

[sdcantrell56]

Mike please tell me how you used #25 chain without tensioners? I think your drivetrains are my absolute favorite in all of FIRST. I love the use of triangular bearing blocks to allow for easy wheel removal.

[B.Johnston]

One way to avoid tensioners is design.

Not that you can see the chain in the pic I attached.

We:

1. used sprockets with an even number of teeth.
2. spaced them apart a multiple of the chain pitch (#25 = 1/4")
   our axle CL to CL = 10"
3. changed the chain loop when it stretched to sag more than 3/8"

And to answer Cory:

Originally Posted by Cory

I'm always curious why so many people cite the desire to have a robot that can continue driving through multiple chain/sprocket/etc failures. In the hundreds of matches my teams have competed through, as well as our collaborative partners, I have only ever witnessed two chains break-in both cases it was a sprocket failure.

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

Here's why I like redundancy and fault tolerance.

2006

2nd match of the Greater Toronto Regional (Finals)

We're(1680) up on 1114 by 1 match

30 seconds in our driven transfer sprocket shatters kinking the chain.

We spin in circles for the rest of the match.

now even 1 -1 against our sister triplets (this was an amazing tank system)

Change the broken sprocket with a replacement (from 1114 BTW)

Third match in autonomous we throw the chain...

The rest is history.

With redundancy and fault tolerance you shouldn't be susceptible to these sort of events.

Those who fail to learn from history will repeat it.

[AdamHeard]

Originally Posted by B.Johnston

One way to avoid tensioners is design.

Not that you can see the chain in the pic I attached.

We:

1. used sprockets with an even number of teeth.
2. spaced them apart a multiple of the chain pitch (#25 = 1/4")
   our axle CL to CL = 10"
3. changed the chain loop when it stretched to sag more than 3/8"

And to answer Cory:


Here's why I like redundancy and fault tolerance.

2006

2nd match of the Greater Toronto Regional (Finals)

We're(1680) up on 1114 by 1 match

30 seconds in our driven transfer sprocket shatters kinking the chain.

We spin in circles for the rest of the match.

now even 1 -1 against our sister triplets (this was an amazing tank system)

Change the broken sprocket with a replacement (from 1114 BTW)

Third match in autonomous we throw the chain...

The rest is history.

With redundancy and fault tolerance you shouldn't be susceptible to these sort of events.

Those who fail to learn from history will repeat it.

Yes, redundancy and fault tolerance are good, no one is disputing that.

What people are disputing is putting too much emphasis on it rather than what the problem truly is.

If you are throwing chains or breaking sprockets in a drivetrain, the issue isn't that you don't have enough redundancy built in; the issue is something is causing you to throw chains and/or break sprockets. Sure, adding a lot of fault tolerance is a way to reduce the negative effects of the problem, but I would rather have a system that didn't break as much period.

Also, when it comes to choosing numbers of wheels in a drive, fault tolerance should not be a factor. Fault tolerance is something that results from good design.

[sdcantrell56]

I completely agree that if it is a design flaw that is causing the failure, then creating redundancy to lessen the impact is the absolute wrong way to do things. A team should build a drivetrain, and the entire robot so that all that needs to be done once it is assembled is tightening the chains. This should also be a simple task.

[B.Johnston]

This is going to be a long post so please bear with me.

I agree with you and adam above that there is no substitute for good design.

I believe that good design includes the analysis of the functional requirements of the task you are performing.

This includes appropriately matching component properties in a system.

I think this is where my emphasis on fault tolerance and redundancy is being misunderstood.

For me the meaning of fault tolerance is the ability to recover (perhaps not fully) from an unanticipated occurrance.

For example if you have a tank track system and your tread is stripped off your robot because opponents have discovered that when you are pushed sideways they can be broken.

A fault tolerant design would, perhaps allow for you to drive with no track on(rather than being immobilized because your drive is sitting on a skid plate or bogies).

An example of redundancy does not neccessarily include the use of a second system which fully duplicates the function of the primary system either.

An arm mechanism driven by 2 #25 chains operating in parallel to support the functionality needed rather than a single heavier #35 chain would provide a redundant system capable of withstanding the failure of one chain and sprocket system (accidental entanglement with another robot) if done properly.

Here full function may not be possible but some function would remain.

Neither would be an example of poor design, they are both designed to mitigate the outcome of failures that while not wholly unexpected are somewhat probable.

To design a system so "strong" that it never breaks is not inherently good design.

If we apply enough force to a wheel we can break traction and start it spinning.

This force needed would be our design target (plus a moderate margin) for the strength of the components in the system.

To use components which far exceed this constraint is not good design.

You would carry a combination of penalties in weight, cost, bulk, parasitic losses etc that while nothing broke, something was affected along the way.

A function that couldn't be included, a speed that couldn't be reached, an incline that couldn't be climbed etc.

While I don't advocate either redundacy or fault tolerance as a substitute for good design they can be used as part of a strategy which maximizes the outcome despite the circumstances.

B.T.W. The 2006 sprocket that fractured was a 19 tooth #35 martin sprocket capable of absorbing about twice the power that our double 2.5 cim/andymark 2 speed drive could deliver. It fractured at an inclusion near the keyway due to an internal flaw in its structure. The other sister drive trains were good enough to win three regionals that year.

[AdamHeard]

Bruce, I realize I got too wrapped up in my point, and was really ambiguous as to what I said applies to.

I mainly was referring to the teams that think they need all this redundancy because they break 5 sprockets a regional, or a chain snaps every other match.

You're right, fault tolerance is something that really should be designed in (and with good design, can be done pretty elegantly as many robots prove) and not ignored.

No hard feelings, this is all just some good technical discussion

[Aren_Hill]

Originally Posted by Cory

If your drivetrain is well designed and the chains are properly tensioned, the odds of losing a chain, let alone more than one chain are nearly negligible.

We had 7 runs of #25 chain on the drivetrain alone this year (swerve)
and not once did any of those 7 come off.

I agree completely with Cory on the fact chains aren't a common failure point unless you make them by mistreating them.

[sdcantrell56]

Yeah I must agree I have never actually witnessed a robot I was involved with breaking chain. It can definitely happen but for the most part as long as it is tensioned and aligned there's just not enough force to break the actual chain.

I'm currently working on a new version that is 6wd with a plywood frame and the center wheel is directly driven by the transmission shaft. Hopefully I'lll have it up within a week or so.

[cbale2000]

I should check the search page for posts about 703 more often, but alas at least I'm only a few days late.

I heard someone was looking for info on powerful drive trains? I'd be happy to answer some questions if there are still any about our drive systems (I noticed a few a page or so up)

Originally Posted by JVN

Does anyone have any quantitative measurements of their performance, or are we basing everything off anecdotal evidence (as usual)?

Yep, 703 was great at pushing in 2007. They had an incredible defensive machine. Yet... I bet that NO ONE made any quantitative measurement of it's performance. Here is what I would looooooooove to see:

• Step 1 - Measure it's linear pulling force while on carpet (use a fish-scale).
• Step 2 - Remove all but the 4 outer wheels.
• Step 3 - Throw the extra wheels on top of the frame, so the overall robot weight doesn't change.
• Step 4 - Measure it's linear pulling force again, while on carpet.

I speculate that these numbers will be extremely similar. Within 5%.
Maybe I'm wrong. I would love to be wrong. I would LOVE for someone to prove me wrong with some solid engineering analysis beyond the usual "703 pushed really hard, they pushed against truck town and everyone knows that nobody pushes truck town..."

That could be very easily arranged if you're still interested, we've still got that bot laying around and (for the most part) working.