Worn Shifting Dogs

[Joe Johnson]

From talking to several of the folks who roll their own shift on the fly transmissions, I have gathered that there are significant wear/breakage issues with this type of transmission.

This isn't to say that they are not managable or that the benefits are not worth it, but it is not as simple as some folks may think.

For what it's worth.

Joe J.

[Rickertsen2]

bump

[Tytus Gerrish]

the Technocat tranny's dog is solid steel, as well as the reciving dogd milled into the gears. and it only had to work 2 minutes at a time say 30 times Thats a bit much but i imagine it would hold well never the less

[Andy Baker]

We've been using this type of shifting (the "dog"-style) over the past two years on FIRST robots and have not had one iota of problems in that area.

This style of shifting was not invented by us... these dog engagement gears are frequently used in motorcycle racing transmissions. The benefit with this sort of gear switching is that there is no clutch needed.

Andy B.

[jburstein]

You seem to have gotten a lot replies but few answers. I'm not an expert, but I think the answer is yes, there is the potential for quite a bit of shock load on the dog as it engages. There are two answers as to why the dog does not "wear," I would bet that most teams who use this type of tranny have software that does some type of "speed matching," so that the dog and the gear are moving at approximately the same speed when they engage, reducing the shock. The other answer is that the dog is made out of some pretty beefy steel, so it can handle the load for many cycles. Eventually it will fatigue, but the Technokats at least seem to be saying that after two years of competition/off-season competition exhibitions etc. they still haven't broken anything.

In short the answer is avoid breakage with speed matching and beefy design.

[IrisLab]

Can some describe a "dog" system? Or point to a good white paper? Unforntunately, "dog" is a very common word and google is more than happy to rain my search with cats and dogs!

Any engineering drawings available? Sketches? More importantly, equations!

[RogerR]

the technokat white paper is http://www.technokats.org/documents/shiftonfly.pdf

for the 2003 two motor design

[Matt Reiland]

Last year we were using the Generation 1 Technocats shifter that had the separate dogs mates attached to the gears. In practice we had assembled the transmission too narrow and while at full speed it shifter and ended up in high and low immediately locking up the transmission and shearing off the three screws that held the dog mate onto the gear. We took it apart and welded the gear to the dog mate and it worked for a while longer until it was assembled yet again narrow. This time, under a full power shift it again locked up the transmission but the shock went through it and sheared off the motor pinion on the bosch. We put a new motor in and used it the entire season and post season with no signs of wear on the dog or dog mates. With generation 2 having the dog mate milled into the steel gear it looks to be even stronger. My 'non- expert' opinion is that it is oversized for the loads that are being applied to it by our motors in the kit and for its limited use in a FIRST season it works spectacularly.

[FotoPlasma]

Originally Posted by IrisLab

Can some describe a "dog" system? Or point to a good white paper? Unforntunately, "dog" is a very common word and google is more than happy to rain my search with cats and dogs!

Any engineering drawings available? Sketches? More importantly, equations!

Along with the whitepapers from the Technokats' 2002 and 2003 transmissions, here's a picture of team 716's implementation of this type of "dog" shifting mechanism:
http://www.chiefdelphi.com/forums/pi...&quiet=Verbose

[Stu Bloom]

Originally Posted by IrisLab

... Any engineering drawings available? Sketches? More importantly, equations!

Dave,

The TechnoKats/Andy Baker white paper (referenced above) is probably the best thing you will find to help explain this. If you still need any help with visualization please email me. I can try to put together some additional 3-d CAD views and/or animation of how this shifting arrangement works.

Good Luck!

[Andy Brockway]

We have not seen any wear after the three off-season competitions we ran with the square dogs. In fact the shifting seemed to get easier the more we ran the gearboxes. The square 'dog' is a nominal size, ie 1.25" square, while its mating hole is .004" larger, ie 1.254" square. This gives the needed clearance for the dog to fall into the hole. The corners do not mesh exactly, mostly clearance; the force is applied at a point similiar to a box wrench on a nut.

BTW we do not have any speed matching when we shift. The drivers pull a trigger to shift up and let go to shift down. Shifting down while running at high speed is not good for the gearbox just as it is not good for your father's car