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Originally Posted by dlavery
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Good advice, Dave. I'll post the details of this below, in chronological order.
2000 servo-shifting gearbox from myself & the TechnoKats (45)
This is the "granddaddy" of these print and CAD packages, released for everyone to re-create. While it was not the first dual-speed shifting gearbox, it probably had the most impact on transmission design. Over 100 teams used this shifting style, and the NBD designs are an evolution of this.
2002 2 speed shift-on-the fly transmission from TechnoKats (45)
This was one of 3 (I think) shift-on-the fly transmissions in 2002. It worked well, and is also a lesson in design overkill. This version is the 1st version of what is now the
AM Shifter 2 (AM Shifter 2 is the 5th generation of this design).
The dog-shifting style is not easy to create, so Dave Lavery and team 116 came up with
this gear meshing design. It could be made without CNC usage, so that was advantageous to many teams.
Also, in 2002, Paul Copioli and team 217 came up with their
Crazy Chicken Transmission, which is a modified CVT, utilizing a motor-driven ring gear for optimal torque. This was a revolutionary design, and inspired many.
(Since the 2002 game let teams lift up over 300# of weight onto their robots, shifting transmissions were all the rage. Many teams had good ones, but these above were publicized via CAD and detailed prints for all to see. Countless students and adults learned how to design gearboxes from these 3 examples.)
In 2003, both
the TechnoKats and
Epsilon Delta posted their improved designs from the 2003 season. Both saved weight, through making their designs more simple and efficient.
Also in 2003,
Andy Brockaway and team 716 gave their version of the shifting gearbox to the FIRST community. Their style is another version of the dog-shifting gearbox, using an effective style.
In 2004,
team 716 updated their shifter.
Also that year,
Jim Zontag & team 33 released their 4-speed gear mesh shifter to FIRST. This was an evolution of team Epsilon Delta's shifter, but used much smaller gears with smaller ratios of incremental shifts.
Yet another groundbreaking paper released in 2004 (in the fall, I believe) was the
Nothing but Dewalts white paper from Joe Pavliga and Joe Johnson. This utilizes a purchased Dewalt gearbox and makes it available for usage on the kit motors. Speed shifting is done by using a servo arm to push the gearboxes speed selector arm.
Again in 2004,
this ball-locking design by Brad Rigdon and team 222 was released (a 2-page paper at least) to show their spin at shifting. They have proven that this method also works, and a good handful of teams have used this technique over the last 2 years.
In 2005 Mark Koors and I figured that there is a market for shifting gearboxes, so we began
AndyMark, Inc.. This was done for teams who did not want to re-invent the wheel and for teams who did not have the machining resources to create these transmissions. Like I said above, teams are getting a tried and true design in the AM Shifter.
As for cost... sanddrag is right. If you just look at purchased parts (gears, bearings, materials), you can make a shifting transmission for $140-$180 (building them 1 or2 at a time). But, this does not count in fabricated costs. If you have the fabrication resources, you can download CAD designs from above or from the
www.andymark.biz website and make your own. That is one of the reasons we offer the CAD. However, this also takes time. This time is greatly used if it translates into student inspiration.
Some general tips:
20 dp gears are a good size for dog-style shifting transmissions
16 dp gears (or larger) are good for gear meshing transmissions (I have seen these gears get whittled down quickly from the meshing in and out)
For center to center gear spacing, I use a 0.002" addition to the calculated center distance. I previously used 0.0005", but that did not allow for manufacturing tolerances.
It is better to have a slightly "looser" transmission as opposed to a "tight" transmission. I believe that this 0.002" addition for center distances is good. Also, I don't trap the gears along the length of the shaft, as I would rather let them float on the shaft as opposed to being bound tightly. Some AM customers are suprised that the gears can slide on the shaft 0.03-0.04".
Press fits for high rpm shafts is ok (10,000 and higher).
As the rpms get lower, the torque gets higher, the shafts get bigger, and the importance of the keyway (or square drive hole or hex drive hole) rises.
Hex shafts are your friend. A design is simplified if you can drive hex shafts, utilize a hex broach and not use keys.
Good luck,
Andy Baker