pic: 2012 frc971 transmission

[MichaelBick]

I'm wondering if you think that you would run this drivetrain again considering the following:

• weight gain for the extra stage rectified by the less weight of belts
• efficiency loss for the extra stage mildly rectified the the greater efficiency of belts
• more prone to over turns mildly rectified by better software
• more prone to over turns mildly rectified by more space on the inside of the chassis
• 1 more point of failure mildly rectified by less maintenance in general
• harder to implement encoders mildly rectified by less machining required
• harder to replace belts mildly rectified by easier to replace gearboxes
• more expensive(extra gears for extra stage and for encoders) rectified by less machining

Out of all of these things there are a few pretty nice benefits(that still outweigh, at least imo, any negatives associated with them): less machining and less maintenance.

However you lost some one of the most important benefits of a classic WCD to me: the prototype with old drivetrain parts without keeping a large stock of parts(ie belts).

Did you think the trade off was worth it? Do you think for next years you would go with a more conventional belt drive, or maybe even a classic WCD?

[AdamHeard]

Originally Posted by MICHAELABICK

I'm wondering if you think that you would run this drivetrain again considering the following:

• weight gain for the extra stage rectified by the less weight of belts
• efficiency loss for the extra stage mildly rectified the the greater efficiency of belts
• more prone to over turns mildly rectified by better software
• more prone to over turns mildly rectified by more space on the inside of the chassis
• 1 more point of failure mildly rectified by less maintenance in general
• harder to implement encoders mildly rectified by less machining required
• harder to replace belts mildly rectified by easier to replace gearboxes
• more expensive(extra gears for extra stage and for encoders) rectified by less machining

Out of all of these things there are a few pretty nice benefits(that still outweigh, at least imo, any negatives associated with them): less machining and less maintenance.

However you lost some one of the most important benefits of a classic WCD to me: the prototype with old drivetrain parts without keeping a large stock of parts(ie belts).

Did you think the trade off was worth it? Do you think for next years you would go with a more conventional belt drive, or maybe even a classic WCD?

Most of these are aren't an issue. They don't have an additional stage of gearing compared to a west coast drive, they just have their shift stage before a reduction (versus a reduction, then shift stage).

[MichaelBick]

I just relooked over the photos and it makes sense now. Nevermind

[Nuttyman54]

Originally Posted by MICHAELABICK

I'm wondering if you think that you would run this drivetrain again considering the following:

...

Out of all of these things there are a few pretty nice benefits(that still outweigh, at least imo, any negatives associated with them): less machining and less maintenance.

However you lost some one of the most important benefits of a classic WCD to me: the prototype with old drivetrain parts without keeping a large stock of parts(ie belts).

Did you think the trade off was worth it? Do you think for next years you would go with a more conventional belt drive, or maybe even a classic WCD?

I'll try to answer all this as best I can. Adam pointed out that we don't actually have an extra stage, we just moved the second stage external to the gearbox and onto the center wheel in a dead-axle setup, which in itself gives us some nice benefits as Roy mentioned.

One thing to keep in mind when you look at 971's designs and design choices with this drivetrain is that it is the result of many years of iteration, and we spent a lot of time trying to make it an integrated package with the team. Our decisions were not based solely on mechanical benefits, because we have some really great software mentors and have developed a driving style along with the physical robots over the years. This robot is the result of designing not just for the game, but for the team that would be using it, and playing to our own strengths. A lot of the tradeoffs and decisions make more sense in that context, such as the outboard motor effect on rotational inertia discussion above. Because we know that our drive style and code mitigates this factor, it benefits us more than it might some other team.

The team would definitely do this drivetrain again. It performed fantastically and we never really had any problems with it that I can remember. It was light, efficient and very easy to maintain.

It's also worth noting that 971 has not done a WCD since 2007. One of the hallmarks of WCD, and what makes it so easy to prototype on old drivetrains is the cantilevered setup and open layout which allows standard parts and easy swapping. In general, sheet metal construction encourages a more integrated approach to achieve an efficient design, but that makes it not great for prototyping. So in that regard, we don't consider it a "loss" because WCD has not been in consideration since the design didn't play to our fabrication strengths.

The 2012 design is the result of many years of various drivetrain iterations and innovations. Each is unique because of the game challenge, but there is a clear progression of eliminating problems while trying to maintain the improvements. One of the big driving forces behind the changes this year was the maintenance. On a traditional WCD, the cantilevered wheels are very easy to access and replace tread on because they are outboard of the frame. Due to our sheet-metal fabrication, cantilevered is not an optimal design so we switched to the dead-axle setup to allow drop-out wheels.

I'm not entirely sure what you mean by "conventional" belt drive, but we found this setup to be a nearly perfect driveline for us this year. We began working belts into our designs in 2011, with the elevator and roller claw both being belt-powered. Our experience with that gave us enough confidence to want to use it on the drive, and our experience this year means we will probably continue. As far as classic WCD, as long as 971 is building with sheet metal, I doubt we will ever run a "classic" WCD. It is far better suited to box tube construction techniques like 254, 1868 and 1323 use, and really isn't a good use of sheet metal resources. I don't think that our design is any better or worse performing than an equivalently designed WCD, but for 971 it's the right choice.

[DampRobot]

MICHAELABICK brings up a good point. What lead you guys to use belts? I've done a few weighted design tables to compare them with chain, and they would only win with an extraneous "quite operation" or "coolness" category.

What benefits (other than the above, which shouldn't really be considered in a design) led you to use belts on your bot this year? As I see it, they add a lot of lead time and cost for parts that need to be ordered and can't really be reused in many situations (unlike chains and sprockets). I believe I read a report a few months ago that concluded that the efficiency benefits from belts over chains are marginal at least for FRC. If belts cost more, take longer to get, and don't perform much better than chain, was there some other factor that led you to chose them?

[Cory]

Originally Posted by DampRobot

MICHAELABICK brings up a good point. What lead you guys to use belts? I've done a few weighted design tables to compare them with chain, and they would only win with an extraneous "quite operation" or "coolness" category.

What benefits (other than the above, which shouldn't really be considered in a design) led you to use belts on your bot this year? As I see it, they add a lot of lead time and cost for parts that need to be ordered and can't really be reused in many situations (unlike chains and sprockets). I believe I read a report a few months ago that concluded that the efficiency benefits from belts over chains are marginal at least for FRC. If belts cost more, take longer to get, and don't perform much better than chain, was there some other factor that led you to chose them?

Weight is an obvious reason. Pulleys are all aluminum and belt is vastly lighter than chain.

[PAR_WIG1350]

Originally Posted by DampRobot

MICHAELABICK brings up a good point. What lead you guys to use belts? I've done a few weighted design tables to compare them with chain, and they would only win with an extraneous "quite operation" or "coolness" category.

What benefits (other than the above, which shouldn't really be considered in a design) led you to use belts on your bot this year? As I see it, they add a lot of lead time and cost for parts that need to be ordered and can't really be reused in many situations (unlike chains and sprockets). I believe I read a report a few months ago that concluded that the efficiency benefits from belts over chains are marginal at least for FRC. If belts cost more, take longer to get, and don't perform much better than chain, was there some other factor that led you to chose them?

Depending on the belts' construction, the issue of stretching can also be eliminated (or at least minimized to a negligible level) so you can design for exact center to center distance without the need for tentioners to compensate for the stretch. This reduces complexity and can save additional weight.

According to the Gates website, belts, especially GT2 belts, reduce backlash significantly which makes the robot more responsive and allows for more precise control in autonomous mode.

[roystur44]

Belts have better damping characteristics under load. Try running a chain setup vs a belt setup over 12 inches to a high speed shooter wheel and you will see a big difference in the speed control, efficiency and the damping.

Belts are quite. There is less friction loss with belts which means higher motor efficiency. Chain tends to stretch over use belts do not. Lastly belts are easy to align they are more forgiving in long runs and when properly tensioned don’t require chain guides

[Brandon Holley]

Originally Posted by Cory

Weight is an obvious reason. Pulleys are all aluminum and belt is vastly lighter than chain.

To go along with this- we ran our belts without tensioners at all this year, dead center to center distance and that was it. Zero issues all year.

You also don't have to 'make' chains to certain lengths, you can just buy the belt and design around it. This usually saves at least a few hours later in the build season. I know once we had all our components in, the assembly of our drive system took less than an hour.


Damp- would you mind sharing your W.O.T.? I'm just curious to see what metrics you value versus my own.

-Brando

[MichaelBick]

After realizing that your gearbox had two reductions instead of three(that was really nice designing by the way, as expected from 971), it became clear why the belt drive had some really nice advantages over the chain. I really thought that you were running a three stage gearbox for a second, which really threw me off.

Thanks for the great explanation of your drivetrain. It was fantastic that you emphasized building to a team's strengths, and I think that is one of the most, if not the most, important lessons for all the new teams.

[DampRobot]

Originally Posted by Brandon Holley

Damp- would you mind sharing your W.O.T.? I'm just curious to see what metrics you value versus my own.

-Brando

Sure thing. I've always designed belt setups just like chain, with a tensioner/sliding endpoint, so this effected my WOT:


Category Weight Chain Belt
Availability: 4 4/16 3/12
Weight: 4 3/12 4/16
Manufac.
Speed: 3 4/12 4/12
Total: 11/40 11/40

The thread I was referring to in terms of efficiency can be found here: http://www.chiefdelphi.com/forums/showthread.php?t=76964&highlight=belt+chain+effici ency. I believe their methods were quite scientific and methodical, and they concluded that belt was 6% "faster" then chain over a given distance.

6% used to seem small compared to the difficulty of obtaining belts that can only be used for one application. I suppose that not having to do sliding tensioning blocks would be a plus for chain, but I'm really not that sure. Although teams certainly have been successful with it, center distance design with belt or chain has always seemed like a technique that could tend to cause problems when you actually try to put it together.

[AdamHeard]

Originally Posted by DampRobot

Sure thing. I've always designed belt setups just like chain, with a tensioner/sliding endpoint, so this effected my WOT:


Category Weight Chain Belt
Availability: 4 4/16 3/12
Weight: 4 3/12 4/16
Manufac.
Speed: 3 4/12 4/12
Total: 11/40 11/40

The thread I was referring to in terms of efficiency can be found here: http://www.chiefdelphi.com/forums/showthread.php?t=76964&highlight=belt+chain+effici ency. I believe their methods were quite scientific and methodical, and they concluded that belt was 6% "faster" then chain over a given distance.

6% used to seem small compared to the difficulty of obtaining belts that can only be used for one application. I suppose that not having to do sliding tensioning blocks would be a plus for chain, but I'm really not that sure. Although teams certainly have been successful with it, center distance design with belt or chain has always seemed like a technique that could tend to cause problems when you actually try to put it together.

6% is substantial when teams are operating on fixed power.

The argument that the lead time of belts is a disadvantage isn't really the entire story there. Do teams really build their entire robots with no parts that have a lead time? We have several suppliers of belts, and we KNOW that we can get any given belt we need in reasonable time. We also make sure to design around belts that exist and are in stock, which is trivially more effort than designing around a chain spacing you know.

[scottandme]

For those who are using/have used belts in their drivetrain, what belt/pulley combinations have worked well? I'm assuming everyone is using the 5mm GT2 profile, but I've seen a good amount of variance in belt width and pulley diameter.

We're currently working on a drivetrain based around 15mm wide 5mm GT2 belts, and 20T pulleys. The gates website has a lot of information regarding the rated load capacity of different profiles/belt/pulley combinations, and it looks like 20T pulleys should be sufficient, but some real world experience would be helpful.

[Jared Russell]

Originally Posted by scottandme

For those who are using/have used belts in their drivetrain, what belt/pulley combinations have worked well? I'm assuming everyone is using the 5mm GT2 profile, but I've seen a good amount of variance in belt width and pulley diameter.

We're currently working on a drivetrain based around 15mm wide 5mm GT2 belts, and 20T pulleys. The gates website has a lot of information regarding the rated load capacity of different profiles/belt/pulley combinations, and it looks like 20T pulleys should be sufficient, but some real world experience would be helpful.

Having a number of teeth that is evenly divisible by 3 lets you hold a pulley with a standard 3-jaw chuck more easily, just in case you want to do something to the bore.

[scottandme]

Originally Posted by Jared341

Having a number of teeth that is evenly divisible by 3 lets you hold a pulley with a standard 3-jaw chuck more easily, just in case you want to do something to the bore.

The tentative CAD design for now has us running a 6WD Franken-WCD with the two 15mm belts running inside a 2"x2"x0.125" tube. Not really ideal from a maintenance standpoint, but I'm hoping the belts will reliable enough.

I'm also fond of the divisible by 3 trick, but I think the 18T is the minimum Gates recommends for the 5mm profile, and bigger pulleys have higher load capacity. The 20T is about the biggest pulley size that fits comfortably inside the side rail tube, accounting for the center drop. The next pulley divisible by 3 is the 24T, which is too big. Setting up the 4-jaw isn't too big of a hassle, but I would probably just make a fixture plate for 8 pulleys, probe the existing bores and have the mill drill the bores to 0.5" before broaching them.