Thread: Driving floating end (ballscrew)
i might have missed something obvious here...... it there any reason not to drive the floating end of a ballscrew (600mm) removing the need for the floating bearing
the only things i can think of is backlash from any torsional twist ill get over the 600mm and a slightly shorter life on my stepper nose bearing
thermal expansion... normally you drive the fixed end and the floating end can move to accomodate...
iv designed a slight error into my build and driving the floating end might let me get away with it without to much of a re-work
coefficient of linear expansion of steel is about 13 - 17 x 10^-6 m per m per degC. i.e. a 600mm length will expand max 0.01mm per degC, so allowing for normal working environment say +20degC +/- 10degC you can expect a variation of 0.2mm.
Force = delta_L * E * A/L where E=Youngs modulus (207x 10^9 N/m^2), A = area in m (2e-4 for a 16mm screw), L = length, delta_L = change in length
=0.2 * 207e9 * 2e-4 / 600 = 13800N... not a force your stepper would be happy with!
The Following User Says Thank You to irving2008 For This Useful Post:
jesus irving!! dose your mind ever sleep?
i was starting to sweat over the figures... id have still been at it till the sun came up
0.2mm... thats a bit depressing... i was hoping the aluminium frame would off set some of it but at almost twice the exspansion of steel (with a bit of carbon in it) im still looking at about 0.2mm i think
back to the drawing board
cheers for doing the maths irving xxxxxx
:lol: well I've been compared with worse ;) though James May's 'maths' on Top Gear often leaves a lot to be desired!
My take on this is that there is nothing wrong with getting a bit of rigour into the thinking.... and this is a good example to show how sometimes the little oversight can have big consequences...
more than happy to explain it...
I have had customers wanting to drive from the floating end, and while it is not ideal, it will most likely be ok.
All stepper motors have a wavy washer or two behind the back bearing so this can take up at least 0.5mm.
The coupling will also help.
On another note the thermal expansion would not be that much unless you are really working the ballscrew and it starts to heat up, a Z axis is not that dynamic with most moves happening over a small area, and not up and down over the whole stroke.Visit Us: www.zappautomation.com
Agreed the z-axis wouldnt be.. but this is a 600mm screw... I didnt get it was the Z-axis anywhere...
Though pragmatically I agree with Gary, the coupler will ease some of it... though a typical Nema23 stepper is spec'd at a maximum axial force of 15N so although the coupler will compress out to take up some motion it will still transmit a significant force to the stepper shaft. Those wavy washers might not stay wavy for long! I'd consider putting the screw under slight tension at a lower temp to give some flexibility when/if the temp rises....
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