Re: pic: 971 Friction Clutch Transmission Insides
 

Low was geared at 14.3:1 and high 4.9:1 with a 3.5" wheel. (5.8 and 17 free speeds). In hindsight, we would have made our low a bit higher. It took 0.5 seconds to get to top speed in low, which made the shift point a little too early. One of the things we learned from this gearbox is that the optimal speeds for accelerating to high gear assuming shifting are different than speeds chosen for pushing. We have a student who is currently doing the simulation effort to see what the best speed pairings are for an auto shifting robot. That and how fast we can make our low gear before we start blowing breakers in low gear also.

The clutch pads are made of a machinable brass. The drums are something like a 4140 steel. We really didn't know what the best friction pairing to use would be, so we ended up settling for brass and steel because it has a decent coefficient of friction and because we have seen this pairing used in other friction applications. We ended up designing the clutch based on a friction factor assuming that it was greased brass on steel because we figured we couldn't garuntee that we wouldn't have grease get in there, and we figured that it might help with wear if we preemptively greased it. So far we have been pleased with the clutch material chosen, however we have in no means run it through the paces of an entire FRC competition season. If anyone knows of a better clutch material pairining, please let us know!

Part of why we built the transmissions was because the theory behind the design was solid, but there were enough uncertain factors like this that we really couldn't tell for certain if it would work without building it.

Re: pic: 971 Friction Clutch Transmission Insides
 

Pretty cool, I've been thinking about how to implement a clutch transmission for a few months now. Pretty awesome to see. Nice job!

Re: pic: 971 Friction Clutch Transmission Insides
 

On the note of clutch material, did you ever try any of mcmasters clutch lining? It seems like using something like clutch lining would let you use a much weaker cylinder and save some weight.

If you haven't already, I would suggest looking into moped and scooter clutches. They use very small pieces of clutch lining glued to the shoes and operate with very little force between the shoes and drum.

Thanks for posting these pictures and fantastic job on the transmission! you've definetely given me some food for thought.

Re: pic: 971 Friction Clutch Transmission Insides
 

33 made this point a few times when talking about optimal acceleration (here's a link). I second Adrian's suggestion of trying McMaster's brake/clutch lining. (Their thinnest is 1/8".) On another note, are you worried about deformation of the steel drum? It looks like your wall is around .050, and with the amount of force you're applying with the cylinder, I could imagine that the wall would get bowed out over the course of a season.

Re: pic: 971 Friction Clutch Transmission Insides
 

Something (semi) interesting to note, we did some extra testing of this drum transmission today at 254's lab. When we removed all air pressure, and ran the robot against the wall, we could slip the tires without slipping the clutch. There seemed to be more enough friction in the clutch, even without normal force from the piston, for the clutch to remain fully engaged.

Further testing at low but non-zero air pressures failed because... I broke their robot removing the breaker from the compressor spike.

Re: pic: 971 Friction Clutch Transmission Insides
 

How does the weight compare to a dog gear shifter?

Re: pic: 971 Friction Clutch Transmission Insides
 

Someone else also recommended the McMaster clutch material to us right as we were finishing up the design. Unfortunately we didn't end up getting a chance to figure out how to use it at this time, but would definitely investigate if there was ever a next time. Our main concern was we didn't have good ideas about how we would go about bonding the clutch material to a substrate, and we were also concerned about the tolerance stackup that this approach would cause. With the linkage geometry, we have only 10 thou of allowable wear in the shoe height (or tolerance error) before the angle goes from 10 degrees to 0, at which point the design stops working. This was one of the main risks that we wanted to investigate when building it. We were also concerned that if we had a wide range of lengths from the top of the shoe to the center axle, then under pressure the selector would be pushed into the center axle and bind, thus reducing the axial force that got turned into clutch friction.

We went with the thin wall for weight savings (and packaging). We had the luxury to spend a bit of time doing FEA on the drums and concluded that the steel drums have a factor of safety of about 3. It also helps that we are using a relatively higher strength steel. We haven't seen any issues with the drums deforming, but then again we haven't done much rigorous testing.

We haven't actually weighed the shifters (wasn't really one of our concerns), but we are pretty sure they are less than the 3 CIM dog shifters they replaced.

Re: pic: 971 Friction Clutch Transmission Insides
 

Is there any particular need to have a 2 speed propeller? Huehuehue :D :p

This is really cool, and a unique/very-untraditional way of shifting in FRC