pic: 1503's Off-Season Project

[Pat Fairbank]

Originally Posted by s_forbes

Very cool chassis design! The four speed transmissions look really neat, what kind of shifting mechanism are you guys using? I wasn't aware of any issues with the AM dog shifting design, but I'd like to see what you came up with.

I'll preface this post by stating that I don't mean any criticism at all to AndyMark shifters; we've used them unmodified in a few of our robots with great success. Most of the "improvements" we've made are because of stuff we want to do which most teams don't (such as shift at 60 psi and shift two transmissions with one cylinder), or just to make manufacturing easier for our student shop, since, with the exception of the 12 and 40 tooth gears, this design requires gear and shaft sizes that AndyMark doesn't offer.

That being said, here's a list of the issues we encountered and the solutions we came up with. Here's an Inventor screenshot of the shifting interface.

Issue #1: The split pins connecting the dog and shifter block to the shifter shaft (the small shaft that slides inside the output shaft) are hard to add/remove during maintenance, and tend to shear when shifting at 60 psi.
Solution: Our shifting shaft has a #8-32 thread on one end which screws directly into the dog (and is prevented from unscrewing by thread-locking compound), and a #10-32 thread on the other end which goes through the 3/16" bearing which is held on with a nut. This has the added benefit of being much easier to machine, since you don't have to drill perpendicular holes in a tiny shaft.

Issue #2: The AndyMark dog gear is difficult to machine, as are the surfaces on the gears with which it mates.
Solution: Replace the dog gear with a piece of 1/4" steel square bar, 1" long, with a hole in the middle tapped to #8-32. Since all our gears come with hubs that we have to machine off, leave 1/8" of the hub and cut a cross into it slightly wider than 1/4".

Issue #3: We need longer equivalents to the AndyMark output shaft, which is difficult to machine (for our students who do the work on manual lathes, anyway) since a hex shaft needs to be parallel-turned to round along most of its length and then have a keyway milled into it.
Solution: Use pre-keyed 1/2" hex shaft, and just mill a slot into it where the shifting dog needs to go. To space our driven gears apart like the hex in the original shifter does, we instead have a ring-shaped spacer that fits around what's left of the hub of the smaller gear, and rubs against the larger gear (right now it's aluminium, but the plan is to make it out of bronze later).

Issue #4: The tiny screws that hold the shifter block on to the 3/16" bearing are very difficult to access on the robot when the transmission needs to be disassembled for maintenance. Also, the 1/4"-28 thread tends to strip from the non-axial forces when we shift both transmissions with one cylinder.
Solution: Replace the aluminium shifter block with a custom steel one (not pictured). The screws that hold the 3/16" bearing in now come in from the top, and it's their shank that holds the bearing in instead of the head.

Originally Posted by s_forbes

How often did you have to change out chains on your 08 robot, and how loose did they get before you changed them?

I don't remember exactly, but it was something like once or twice over three competitions. They weren't anywhere near loose enough to come off the sprockets, but enough to be occasionally contacting the ground.