What type of steel are gears commonly made from?

[JessR]

Seems an unspoken part of your analysis approach is to account for the fact that the gear sizing will certainly be driven by the impact loading forces they will see, versus average loading over the service life.

The robot (& gears) have a service life of only few hours (maybe 4 to 20 hours).
You probably already looked at the Boston Gear specifiying material. They use the Service Factor approach & a cookbook, no calculation method. [ http://bostongear.com/ ] Not good for determining this sort of condition, hi impact with short running life.

A few things I would put in the mix for any calculations:
1. The best way to determine the impact forces is probably directly, hook up some instrumentation to a robot and crash it into a wall a few times.
If you want to do it from theory you need to pick some numbers & see how they work out.
Example - pick a number for a max. traction (lbs) your wheels might develop, and multiply it by.. 5? 10? for impact and use that to determing max allowable stress based on yield of the teeth surface (hertzian stress).
Get material specs from the supplier, Boston or whoever. For a first pass I'd use 30 ksi for steel or iron. When you get some actual values you can just scale them to the new yield.

[Joe Johnson]

Unless I know otherwise, I have used 60Kpsi as my yeild for the steel teeth I buy off the shelf.

It is not perfect, but it is probably close enough for my purposes.

As to using the Lewis equation, this is WAY WAY WAY concervative. I approximate the tooth as a beam with the load a the pitch line. I use Mc/I for my max stress condition.

I know these are pretty rough approximations, but they work for me (both in my day job and in 9 years with FIRST).

As to static/dynamic loads, I pretty much just design for stall torque with a whatever safety factor I feel I can sleep with.

Weight matters in FIRST. I try to balance a robust design with one that is not so overdesign it makes me loose a match because I could not put the right enough of sheilding around some soft bit of my bot.

Everything is a balancing act in Engineering.

Joe J.

[Ashley Weed]

Hey, its Greg P, yes, i am on ashley's delphi name, but AH! i do have an answer!

there are typically 3 common materials...

Brass
Steel
Iron

there are also some exotic materials used too...

lexan (plastics)
titanium alloys

yes, yes made gears outa lexan...go ask team 42 about their HUGE lexan gears...

technically, you could make a gear out of anything. the results might not be pretty, although i give you props for tryin'




~Greg!~