Plasma Attempt #2

[Robin Hewitt]

Where's the maths, I can only see arithmetic?

I passed the 11+ and went to a grammar school where they taught us maths, arithmetic was for those plebs down at the secondary modern

The motors drive 3:1 down on to 24 tooth T5 open belts. I got some 15mm wide, steel reinforced belting which is probably massive overkill but I'd rather it stretched as little as possible. Nice and light to.

There is a 2.5m axle across the back, I think 1.5" 10g aluminium tubing should be fine for that. At <2rps it should not get the shakes .

The unsupported rails are 30mm x 1600mm with the longer LUU type bearing blocks, one at either end. There are no cutting forces to contend with so they only need to support 10 kg each. The bearings are kind of chunky at about 1.25 kg each, but I think that is mass well spent.

[Jonathan]

There is a 2.5m axle across the back, I think 1.5" 10g aluminium tubing should be fine for that. At <2rps it should not get the shakes.

That's to link the belt drive I assume? If so the inertia of that tube will be immense; I=pi/2*7850*2.5*((38.1/2/1000)^4-(31.3/2/1000)^4)=0.0022kgm^2. To put that into perspective that's only half the moment of inertia of a 1.5" solid bar.

You'd probably be better off with bar. For 2rps critical speed and 2500mm length that's only 5mm diameter, so use anything between 5mm and 33mm solid bar and and have less inertia than the tube yet still have high enough critical speed. Tube will have higher torsional stiffness so could do with comparing that.

[Jonathan]

Forgot to say, 0.0022kgm^2 will add about 0.45Nm to the stepper torque requirement - angular acceleration of shaft is 4*pig/0.6. So torque=4*pig/0.6*0.0022=0.45Nm. So there goes your 2* factor of safety!

If instead of the tube you used 12mm steel bar it's only 0.0082Nm added, which is about the same torque as a 24T HTD pulley will require. Critical speed of 12mm bar is fine, but to avoid needless vibration you could pop a bearing in the centre to support it.

Those powers of 4 really do give some surprising results.