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Several of the transmission designs from last year (including ours, which is posted in the White Papers section) used pneumatic pistons to position the primary drive gear specifically to deal with the "gear mis-mesh" issue. If a piston moves the primary drive gear between the Low Gear and High Gear positions, then it acts as a spring if the gears do not perfectly mesh when they are shifted. If the gears do not perfectly mesh when the system is shifted, then the drive gear and driven gear just contact each other on their side-faces (beveling the corners of the gear teeth helps prevent any severe wear problems).
The piston/spring just applies side forces to the primary drive gear to hold it in place until the gears rotate sufficiently to properly mesh, then the drive gear drops into place. Again, beveling the corners of the gear teeth makes the meshing action even smoother. If this sounds confusing let me know and we can post an animation of the system that shows how it works.
On most long-term applications, which may typically go through millions or tens of millions of cycles, you would probably not do it this way. But in the type of systems designed for most FIRST robots, which are typically subjected to maybe a few thousand cycles, wear is not too big of an issue.
-dave
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