Plasma Attempt #2

[Robin Hewitt]

use the formula torque=angular acceleration * moment of inertia, where acceleration is in rad/s^2.

In post #4 I calculated the moment of inertia of your 10g (well not quite, see below) tube using the formula, courtesy of Wikipedia:

I'm sorry to do this to you, good try and all that, know you only want to help, but you can't bamboozle me with your arithmetic and Wiki-whatsit

force = mass * acceleration

If you want torque you add a distance component

Torque = mass * acceleration * radius

Of course the acceleration here is peripheral acceleration, not radians/s/s

If you insist on radians/s/s and moments of inertia... One turn is 2 pi radians and moves you by 2 pi radius so they simply cancel out after you have gone all the way round the houses.

The 1/8" wall tube weighs 2.35 kg, the peripheral acceleration is the same 2.94 m/s/s as the gantry, the worst case radius is 0.019 m

2.35 * 2.94 * 0.019 = 0.131 Nm

But geared 3:1 down the motor only needs to find an extra 0.044 Nm

[Jonathan]

Fair enough, that's a nice intuitive way to work it out for a thin-wall tube since you can assume the mass is concentrated at the radius. This clearly is an approximation, since the tube does have thickness, so to work it out accurately you have to consider the integral of mass*radius^2 and you end up with the formula I quoted after touching upon real maths! In this context the approximation is of course valid (error is <10%), however this method wont get you very far if the tube was substituted for a simple shaft.

The 1/8" wall tube weighs 2.35 kg

Now your numbers make sense, it must be 2700kg/m^3 and therefore aluminium tube...I thought it was steel! Hmm to be fair you did say it was aluminium originally. Whoops...