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#16
05-11-2012, 02:59 AM
 Adam.garcia Head of Machining FRC #0004 (Team 4 Element) Team Role: Mentor Join Date: Aug 2009 Rookie Year: 1997 Location: High Tech LA Posts: 130
Re: Custom Two Speed Gearbox

Quote:
 Originally Posted by Tristan Lall I tend to design 2-speed gearboxes with the ability to spin the wheels when pushing against an immovable object in high gear, to account for the fact that shifters don't always work right. Failing to do this means that if you can't shift, you'll stall the motors pushing on things, and possibly damage the motors. (Depending on your confidence in the mechanism, this may be an unnecessary tradeoff.)
Can you please elaborate on how I should go about ensuring that I design the high gear to slip at stall?

Are there formulas/a calculator/white paper that somebody has written describing this phenomenon?

I'm using Blue Nitrile Tread from McMaster. However, I do not know the coefficient of friction.

Thanks
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#17
05-11-2012, 03:23 AM
 Tristan Lall Registered User FRC #0188 (Woburn Robotics) Team Role: College Student Join Date: Aug 2001 Rookie Year: 1999 Location: Toronto, ON Posts: 2,291
Re: Custom Two Speed Gearbox

Quote:
 Originally Posted by Adam.garcia Can you please elaborate on how I should go about ensuring that I design the high gear to slip at stall?
If you're designing the wheel to slip instead of stall, the force the gearbox can exert on the ground through the wheel should exceed the maximum frictional force that the ground exerts on the wheel. The maximum gearbox force is a function of the motors' torque at stall. The maximum frictional force is a function of normal force (itself a function of weight) and frictional coefficient.

If you're designing the gearbox to always shift into low in case of a failure, you should characterize the possible failures and try to deal with them. This isn't easy for shifting mechanisms that don't return to a home position (e.g. servos) during a malfunction, and is also difficult if you can't walk up to the robot to override a malfunctioning valve (which you can't, at least not during a match). Since redundant actuators and control channels aren't really feasible under most circumstances, taking this design approach will invariably lead you to compromise—but that might be good enough.

Quote:
 Originally Posted by Adam.garcia Are there formulas/a calculator/white paper that somebody has written describing this phenomenon?
John V-Neun wrote one, and has updated it over the years. It's very useful. Others have written similar tools (like this and this), but I haven't tried them.

Quote:
 Originally Posted by Adam.garcia I'm using Blue Nitrile Tread from McMaster. However, I do not know the coefficient of friction.
#18
05-11-2012, 09:47 AM
 sdcantrell56 Registered User AKA: Sean FRC #2415 (Wired Cats) Team Role: Mentor Join Date: Jan 2006 Rookie Year: 2004 Location: Atlanta, GA Posts: 979
Re: Custom Two Speed Gearbox

Quote:
 Originally Posted by Tristan Lall Probably about 1.0 on carpet (an educated guess). See here for a bit more information.
Any type of roughtop seems to be more like 1.2 to 1.3. I would try to ere on the side of caution rather than pushing things so definitely use the higher coefficient of friction.
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