As to the OP’s question, I can think of 3 successful team-built shifting gearbox types (this excludes the planetaries in the Dewalts and drill transmissions):
-Mesh shifting (“Crash” shifting) - Sliding gears to mesh. The Killer Bees 4-speed design linked earlier is a mesh shifter, as were many early FRC shifting transmissions. They are relatively to easy to build, and with a large pitch gear and slightly more backlash they can work fairly well. While not space efficient it is possible to have a 3 or more speed mesh shifter with a single mesh. The 4-speed is essentially 2 2-speeds in series as this design is more space efficient. As the paper describes, we really wanted a 3-speed but the 4-speed was easier to build. Note that 2 2-speeds in series requires a double swap shift for 2<->3 shifts and that is not usually a ‘good’ shift to perform (if it’s not perfectly synchronous you’ll end up in 1st or 4th for a short period of time).
-Dog shifting - Sliding a dog to mesh. In this case, the two gears in question ride on the common shaft via bearings, and the mechanical coupling to that shaft is provided via the dog. The dog meshes with one gear at a time to pick a gear ratio. The dog design is limited to 2 gear ratios per dog. You can get 4 speeds by designing 2 2-speeds in series or having multiple dogs on a shaft (this would usually, but not always, require the ability to shift a dog into neutral)
-Ball shifting - Balls sit in cups of the shaft and are pushed out to engage the gears riding on the shaft to engage a gear. This design is capable of any number of gears in sequence adding only another pair of gears and balls and slightly lengthening the shaft, and can shift sequentially very easily. It is impossible to shift non-sequentially (each gear you step through must engage to let the plunger pass by it). For a 2-speed design like the Vex Pro this is irrelevant.
There is also the theoretical option of the planetary transmission. An automotive planetary uses planetaries and lockup/drive clutches to select a power path through the planetaries (which elements are connected to the input and which are grounded). Good designs allow jump-shifting nearby gears with a single clutch-to-clutch shift (exchanging one clutch for another). A drive or lockup clutch in an FRC application could be a part of a dog, mesh, or ball shifter, or a lockup band.
I’ve designed and built a few ball shifting transmission in the past. I really dig the new Vex Pro models that are out. They’ve got many of the ‘wanted’ features I’ve recognized over the years, but never implemented. Kudos to them for a solid design.
Linked are two pictures.
Project cover page listing team members and showing completed drive base.
Shifting detail on the transmissions. The large exploded view is a bit crap (my apologies), but the other simple illustrations outline function showing the shifting shaft, plunger shaft, shift gears, and ball bearings.
I’m happy to share any specific details if anyone is interested in developing their own ball shifter.
thanks to everyone for all the information it’s been incredibly helpful. Something that I didn’t realize I left out is that this is a project to help me learn solidworks and because off that i’m gonna go with dog shifting just because of the easier design. Also if by some chance we actually made this instead of just buying some AM or VEX shifters, dog shifting would be easier for my team to do because they would be our first machined parts and it seems much easier to design than a ball shifter.
Make sure you do it in the off season. Before COTS 2 speed gearboxes were available are team wasted a lot of time making 2 speeds that were mediocre. I’m not sure how well the AM shifters shift but our dog shifters take quite a bit of tuning to shift in a useful fashion.
Team 45’s 2003 gearbox is probably one of my favorite gearboxes. The PDF was released when I was learning to drive. So I was crazy about cars then. The 2003 gearbox is what taught me what a dog shifter was in a manual gearbox.
Being an amateur bike mechanic whose rebuilt a few Sturmey-Archer 3-speed hubs and knows way more about them than I probably should, I can speak on the question. (Long post ahead!)
Most hubs will not work for a drive train. This is because they are designed from the start to only be driven in one direction; the frewheeling ratchets are actually part of the internal shifting design. In essence, such hub has a single planetary stage and locks/unlocks the ratchets to make the planetary run 3 speeds (x:y, 1:1, y:x). Newer designs pushed the concept to 7 speeds (shimano nexus for example) and even 14 speeds (rohloff speedhub, well over the $400 limit @ ~$1000 per hub). However, most FRC robots need a bidirectional drive, so this isn’t going to work out.
However, there are fixed gear planetary hubs… the modern Sturmey archer S3X comes to mind. What I don’t know is whether such hubs maintain the selected gear ratio in reverse.
What I’d be most concerned with would be getting the input and outputs meshed into a FRC capable drivetrain… The non-fixed versions used to and still come in a format for tricycles where one of the hub flanges is replaced with a 2nd sprocket (to go to a sprocket on the rear axle); however, I’m pretty sure the S3X doesn’t come in that format (you’d have to do a gut swap with another hub shell or do some other custom setup)
Shifting could also be tricky; at least the modern designs use an even pull setup (the cable distances between shifts is consistent). Older (pre-2000) sturmey archer hubs had a small gap between 1st and 2nd and a huge gap between 2nd and 3rd, with a neutral zone in between (get the cable tension too loose and you’ll hit it in 2nd under load). In addition, shifting must be done with the input at rest (at least with the non-fixed hubs)
Last, these hubs are lubrication picky. They are usually lubed with oil (I use generic ATF)… and many hubs (older ones especially) could fling oil if overfilled or really old (sheet metal oil caps). Not exactly FRC legal. One could in theory run it on grease but my experience has been that these hubs don’t like being run with grease.
My advice: offseason it if you really dare. It won’t be an purely COTS solution, but to be honest, much more can be learnt from a non-COTS drivetrain when it comes to offseason projects.
