Driving floating end (ballscrew)

[blackburn mark]

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

[irving2008]

thermal expansion... normally you drive the fixed end and the floating end can move to accomodate...

[blackburn mark]

thermal expansion... normally you drive the fixed end and the floating end can move to accomodate...

cheers irving... i need to do some research on how much exspansion i might get... my couplers are the slitted aluminium type and will allow for some exspansion

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

[irving2008]

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!

[blackburn mark]

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

[AdCNC]

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!

Bloody ell irving, it makes me chuckle every time i read a post off you like this, it remind me of James May off top gear when i kicks in with his maths :lol: not thats theres anything wrong with that... i wish i could understand all that lot.:tup:

[irving2008]

: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...

[Gary]

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.

[blackburn mark]

cheers gary,, im going to push on with the revised plan... it solves enough of a problem to take the risk

as far as the temp goes im sure the screws will see a 20 degree shift in a shed in the uk

soooooo...... with the wavy washers plus an easy .5mm in the coupling im kind of confident

[irving2008]

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....