pic: 2012 frc971 transmission

Solidworks shot

Are those custom gear’s or are they ones from andymark?
Also, Where do you get these special pneumatic pistons, bimba? and how are they different from the standard ones?

I loved your simple solution to saving space this year, along with the pancake cylinders for shifting. Are there any tips or tricks you might have when making an “inverted” gearbox? Were there any issues you encountered that were unique to this type of setup? What would you do differently?


Is there any measurable benefit to the fan above the CIMs?

The pistons are from Bimba. They are shorter than a standard piston (albeit wider). They also generally ouput less force, as the rod is smaller than the commonly used shifter pistons.

Do you have a part number for the pistons? I was looking at a couple of different ones a couple of weeks back. Just curious which ones people are using.


They really helped on the practice bot. I was talking to Austin at Davis this year and was unsure about trying out fans. After Davis we put fans on the practice bot, it really helped us get longer practice time.

On the comp bot in 2 minute matches, totally unsure 971 would have to answer that one.


I don’t have any experience with them on CIMs, but I added one to the KOP air compressor on our demo bot and it dropped the running temperature and idle cooling time significantly. We had been having issues where the compressor would get so hot from running constantly it would melt the hose coming out of it (causing an “explosive” loss of pressure). Since adding the fan we have yet to see such a problem occur again, even after a 5+ hour demo that was outside on a hot day and in direct sunlight.

Somewhat related, on this years robot we used two FisherPrice (I think) motors to run our shooter system. We had been having an issue where after moderate use, the motors would heat up and then, the lower one in particular, would stall when trying to shoot a ball. The motor had been mounted to the robot using an aluminum collar we made for it (which covered about 60% of the motors surface area and was roughly 3/4" thick). To fix the problem, I took the collar off and cut small groves around it (like a computer heat-sync, and since putting it back on the robot we have yet to have the launcher stall on us again.

I could see a fan on a CIM being useful particularly in elimination rounds where you have a lot of matches close together with very short breaks between them. In years past our motors have gotten very hot (one year we even used a can of compressed air upside-down to cool down our drive motors) during finals, so an active cooling system could be a good way to alieviate such problems and maybe squeak out that little extra bit of performance during the most important matches.

TL/DR version: Temperature matters, cooler running motors will perform better. :wink:

I’m not seeing how this is single reduction. I can’t get the numbers even close to those speeds, and in the picture it looks like the gearshaft is above the wheelshaft (or the wheels would hit the CIMs).

Is there another gear reduction external to this gearbox that directly drives the center wheel?

You are correct; A 15 tooth gear coming from he transmission drives a 50 tooth gear which is on the same axle as the wheel.

In the back of the picture, you might be able to see that some of the wheels had gears on them. These were the center wheels that interfaced with the transmissions.

The use of the final gear allows us to make the transmission removable while the belts and wheels can stay on the chassis all tight. Let’s see a WCD do that.

We need to make the inside side plates a little stiffer by insert some bends into the plate. This will give us a flange to mount the plastic grease guards.

We need to insert alignment pins from the transmission to the chassis to ease the assembly and get the optimum gear spacing. We had to manually tune the alignment and if the screws became loose the transmission would have power loss.


Fans help out on other motors too. Try running a pair of Fisher Price motors at high load and run them intermittently in forward and reverse. They don’t like that and you will see how smoking hot they get. Attach a pair of small fans to direct continuous air flow over the motor cooling slots and casing and you will see a tremendous improvement of the power output over a time frame.

I love the space savings with this “inverted” setup, but in order to place the CIMs outboard of the gearbox, you risk a higher moment of inertia. This will have the effect of making the robot harder to turn.

Some quick calculations tells me that moving the CIMS outboard will increase the total moment of inertia by about 5% (for a “long” chassis) to 8% (for a “wide” chassis) for an otherwise equally-distributed robot. For a constant turning torque, this results in a similar percentage less angular acceleration.

Takeaway: for a snappy turning response, mount your heaviest items as close to the center of rotation as possible.

This is a good point, and 5% is something that will be noticeable to most drivers. There are a few other factors as well that go into the decision for us:

  1. With most 6wd dropped center setups, turning is usually really really snappy already, sometimes too much. Especially with a robot geared above 15fps, turning response can be almost uncontrollable without a lot of practice.

  2. The drive code that 971 uses (along with 254 and maybe some other teams) is set up to do controlled radius turns in normal operation, rather than controlled turn rate, which mitigates the “snap” turn response characteristic of 6wd. This makes the robot much more controllable at high speeds (the 971 2012 robot was geared to 17fps no load). Further discussion on this style found here. Because of this, we try to avoid needing to turn in place when we drive, so the moment of inertia aspect isn’t really an issue for us.

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).

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

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.

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?