Methods of shifting gears
 

I was wondering of anyone has made a two speed transmission and if so how you managed your shifting. From what i understand there are multiple ways of shifting, but the only one I understand is crash shifting due to it's simplicity. I would love explanations on different methods and pictures if possible would be super helpful.

Thanks

Re: Methods of shifting gears
 

Do you want specifically custom shifters or would COTS transmissions work? If you want COTS, you could look into both the AM Super shifter and the VEXPro BallShifter. I think there's also a video or two floating around YouTube that explains ball shifting. Never bothered looking for dog shifting. The CAD for both products are on their respective pages, so if you'd like you can download them and take one apart without actually buying the gearbox.

Re: Methods of shifting gears
 

I was kinda hoping for custom builds just to see how people did it, but thanks to the link to the ball shifter I had only seen the andymarks shifter and the supershifter whih I think are both dog shifting

Re: Methods of shifting gears
 

You might have to do some digging, but you can find some shifting gearboxes on FRC-designs.com

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Somewhere out there is 33's 4 speed transmission.. I know it's in the white papers some where.

Re: Methods of shifting gears
 

Re: Methods of shifting gears
 

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

Re: Methods of shifting gears
 

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

Re: Methods of shifting gears
 

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?

Re: Methods of shifting gears
 

Yup, that is correct.

Re: Methods of shifting gears
 

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

Re: Methods of shifting gears
 

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.)

Re: Methods of shifting gears
 

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)

Re: Methods of shifting gears
 

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.

Re: Methods of shifting gears
 

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

Re: Methods of shifting gears
 

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!

Re: Methods of shifting gears
 

I suggest looking at Team 254's shifters on their website (www.team254.com)

Here's their 2012 transmission: http://media.team254.com/2012/08/gearbox.jpg



Also, a bit unrelated, but did you use to have a MC server?

Re: Methods of shifting gears
 

The question is if the extra weight is worth the extra performance. Pneumatics means faster and more reliable(imo because there is little chance to shear a dog gear) shifts. In addition to that, pneumatics can be very useful for simple, 2 position operations, can be used as springs, and can be actuated after the buzzer. This gives pneumatics tremendous versatility. I personally think pneumatics gives enough benefit to outweigh the price.

Re: Methods of shifting gears
 

I realize that. I've used pneumatics on every robot I've been associated with building. I've just seen a lot of teams staying away from pneumatics because of either difficulty or weight.

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

Re: Methods of shifting gears
 

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.

Re: Methods of shifting gears
 

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.

Re: Methods of shifting gears
 

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.

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Imagine having one of these on your robot. http://upload.wikimedia.org/wikipedi...xtroid_CVT.jpg

Re: Methods of shifting gears
 

190 did a toroidal CVT with power take-off in 2002. (Search the forum for more information, e.g. this thread.)

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The Thunderchickens did a planetary CVT in 2002 and published a whitepaper: http://www.chiefdelphi.com/media/papers/1361

Re: Methods of shifting gears
 

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.

Re: Methods of shifting gears
 

Unless you have a really, really good reason, stick to COTS gearboxes.

Re: Methods of shifting gears
 

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.

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AM shifters are dog sifters

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I don't think dogs appreciate being sifted :D

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Haha you're probably right

Re: Methods of shifting gears
 

Pretty much the history of dog gear shifting in FIRST and the origin of Andy-Mark.

http://www.chiefdelphi.com/media/papers/1306
http://www.chiefdelphi.com/media/papers/1360
http://www.chiefdelphi.com/media/papers/1432
http://www.chiefdelphi.com/media/photos/19082


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.

Re: Methods of shifting gears
 

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.