Methods of shifting gears

[joelg236]

911 in 2007
http://www.chiefdelphi.com/forums/sh...ad.php?t=59800

[mman1506]

Here is one that one of our team member built http://www.chiefdelphi.com/media/photos/24580

Its a dog shifting design, you can see the the dog in the pictures.

I could get more pictures if you would like. Basically both the gears spin freely from the shaft and the dog is attached to the drive shaft. The dog will latch onto the first or second gear transmitting that power to the driveshaft

[lukedude43]

Looking at the dog shifter if I understand correctly all the gears are free spinning on the shafts and the dog is attached to the driven shaft. the dog meshes with one of the two gears to spin the driven shaft. Is that right?

[mman1506]

Yup, that is correct.

[lukedude43]

Great thanks i'm like 90% sure I understand now

[Tristan Lall]

The one you see most often nowadays is dog shifting: the dog gear is (with the teeth on the side face) engages a recess on one of two output gears. I've built one of these with 188; it worked great. I've also bought these off the shelf.

Captive-ball shifting is once again popular, with the release of VexPro's kit. Previously, 222 used this design. Here, balls are displaced in and out of recesses in the central shaft, to engage gears with slots in their bores.

You used to see axial-engagement ("crash") shifting more often. There, the gear teeth themselves slide in and out of mesh. I'm not a huge fan of this, but implemented with fairly large gear teeth, it works fine.

Rarely, you see tangential-engagement shifting. I tried it once with 188 and it didn't work properly (the gears wouldn't stay in mesh). However, knowing what I know now, I think I could probably make it work. However, I don't see any particular advantage to doing so.

You also used to see planetary shifters more often, when the Nothing But DeWalts modification was popular. (A DeWalt XRP drill transmission, shifted with a servo by sliding the device that locks and unlocks its planetary gearsets.) I've implemented this, again with 188, with two CIMs feeding a single DeWalt. Everything worked reasonably well, except the custom output shaft. (It needed to be hardened alloy steel, not hot-rolled free-machining steel.)

[Michael Hill]

I'd be interested if it were possible to use a linear electric solenoid to shift instead of a servo or pneumatics. The bad part is that they have maximum "on" times (duration the plunger can be "out"), which can be between 1 and 2 minutes or shorter (which is a problem in our application) because they need to cool down. It would be useful for teams who don't want to deal with pneumatics, especially if shifting would be their only use, and want to have fast shifting, because servos are sloooooow (in comparison)

[Dad1279]

Originally Posted by Michael Hill

I'd be interested if it were possible to use a linear electric solenoid to shift instead of a servo or pneumatics. The bad part is that they have maximum "on" times (duration the plunger can be "out"), which can be between 1 and 2 minutes or shorter (which is a problem in our application) because they need to cool down. It would be useful for teams who don't want to deal with pneumatics, especially if shifting would be their only use, and want to have fast shifting, because servos are sloooooow (in comparison)

Could use a window motor/cam linkages to shift both, or a faster motor with a leadscrew.

Dealing with pneumatics isn't that difficult. Don't need an on-board compressor.

[fb39ca4]

Has anyone tried using a shifting bicycle hub? The kind that bikes without derailleurs use?

[Daniel_LaFleur]

Originally Posted by fb39ca4

Has anyone tried using a shifting bicycle hub? The kind that bikes without derailleurs use?

The biggest issue with derailleurs is that it is a single direction gearbox, and most FIRST applications are bidirectional.

[apalrd]

Originally Posted by fb39ca4

The kind that bikes without derailleurs use?

He means something like this


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.

[henryBsick]

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.
1) Project cover page listing team members and showing completed drive base.
2) 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.

[A_Reed]

CVT anyone?

http://en.wikipedia.org/wiki/Continu...e_transmission

Not entirely practical for FRC applications but it certainly would make an interesting design project to see if you can make it work.

[ratdude747]

Originally Posted by fb39ca4

Has anyone tried using a shifting bicycle hub? The kind that bikes without derailleurs use?

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.

[Michael Hill]

Originally Posted by Dad1279

Could use a window motor/cam linkages to shift both, or a faster motor with a leadscrew.

Dealing with pneumatics isn't that difficult. Don't need an on-board compressor.

I realize pneumatics isn't difficult, but it's A LOT of overhead weight (even without on-board compressor). You'll need at least the manual relief valve, emergency relief valve, pressure gauge, storage tank, solinoid, tubing and your actual pneumatic cylinder. That's quite a bit when all you want to do is shift!