DoN. Nichols wrote:
On 2008-03-11, Tim Wescott wrote:
More design questions:

I need a stick-like thing, about 30 inches long, to go from a pivot
point to a little electric motor. It needs to be moderately rigid,
light and inexpensive in lots of 50 each or so. It would also be quite
nice if it's moderately attractive -- mill finish aluminum without too
many scuff marks is just fine for this. Prettier is better.

Hmm ... does it need to be more rigid in one direction of
rotation than the other (e.g. more load with clockwise rotation than
with counter-clockwise?) If so, then either carbon fiber or fiberglass,
laid with the right fiber orientation would probably be stronger. (Note
that this is presuming that you are transferring torque through the tube
from the motor's shaft.

If you need it stronger in compression or tension, then other
choices might be better -- though tension would do well with either
fiberglass or carbon fiber.

I'm thinking that thin-wall aluminum* tube of about 5/8" diameter will
work well, and I'm getting ready to be shocked once again by how much
things cost these days. While I'm waiting for quotes, I thought I'd
solicit the group for suggestions.

Thin wall? Is it being used for torque (as I suggested above)
or just to hold a motor tensioned against the belt or gears?

Neither. The tube is pivoted on a horizontal axis, the motor has a
propeller on it that turns the assembly on the pivot. The only working
load that the tube has is the weight of the motor and any transitory
loads from the thrust of the propeller while it is accelerating the
assembly.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html