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z/x carriage weight.

Posted by TheJones 
z/x carriage weight.
December 03, 2013 06:42PM
So, I'm back at work on my RepStrap build (had to pause it for a while because some life stuff pop up).

I going with a standard rod-type X axis, but my Z axis is a set of 4 roller bearings on each side, being forced apart and into their tracks by two lengths of threaded rods.

Imagine standing in a doorway, with a roller skate in each hand, pressing them out into the door frame. It's kind of like that.

I decided to weight it today, and with almost all of its parts (including the X axis), it comes in at 5.6lbs. (2.5kg). I'm going to be lifting it with a pair of nema17's with integrated ball screws.

Do you guys think that's too much weight for those steppers? What does yours weigh? Should I redesign the Z axis to use rods to eliminate the weight of the roller bearing assemblies?

I'd hate to get all the brackets done and wire it all up, only to have it be unable to move the X/Z carriage properly.
Re: z/x carriage weight.
December 03, 2013 09:01PM
I'm not sure the capacity of a Nema17, but that's awfully heavy. Thats going to limit your top speeds, and its really going to slow down acceleration/deceleration.
Re: z/x carriage weight.
December 03, 2013 09:17PM
Well, the X-carriage is nice and light, and I'm going to use a Bowden variant to keep the moving mass down. The z axis is the heavy part, because of the roller bearings, and that moves relatively slowly (One tick at a time).

I'm not concerned about speed, I'm just worried that the steppers won't be able to lift 5.6lbs/2.5kg reliably.

Edited 1 time(s). Last edit at 12/03/2013 09:22PM by TheJones.
Re: z/x carriage weight.
December 03, 2013 09:19PM
Let's do some off the cuff calculations. A NEMA17 can do 30oz-in easy. You only have to half most of the time but lets assume all the weight is on one so 6lbs. F<=torque*pi/lead or lead<=torque*pi/F

If I plug in for torque and F we are left with a max lead of about 1in. Factor in that screws are only about 20% efficient sometimes then you get a max lead of .2in with a great factor of safty. So get a threaded rod with more than 5 TPI or a pitch smaller than 5mm.

Weight isn't a big issue since z moves slowly. (Assuming this uses the Wallace/Printrbot/Prusa/Mendel style geometry.)
Re: z/x carriage weight.
December 03, 2013 09:59PM
Here are the specs on the steppers that will be lifting the Z.
Quote

200 steps per revolution (1.8 deg/step)
2 Phase, Bipolar, 4 wires
Rated Voltage 2V DC
Rated Current 1.2A
Phase Resistance: 1.7 Ohm ± 10% (20º C)
Phase inductance: 4.5 mH ± 20% (1kHz 1 V rms)
Holding torque: 0.4 N.m Min.
Motor length: 40mm
Acme Lead Screw: 350mm long, Tr8x8(P2

Does your math voodoo predict that two of these will reliably lift the proposed 6lbs?

Also, this uses an i3 type, 90 degree upright frame.

Edited 1 time(s). Last edit at 12/03/2013 10:01PM by TheJones.
Re: z/x carriage weight.
December 03, 2013 10:21PM
Okay so with that screw the efficiency is at least 50%. That motor has a holding torque of around 60oz-in but I like to design for 1/2 of that since you lose torque as the speed increases. The lead is 8mm or .314in

So F<=(efficiency)(working torque)*pi/lead=.5*(30oz-in)*pi/(.315 in)=150oz=9lbs

So each side can lift 9lbs (no factor of safety) independently at a decent working speed. Of course if you get any binding or you get grit in your nut that can drop capacity drastically .

IMHO, you are good to go.
Re: z/x carriage weight.
December 03, 2013 10:47PM
If they can handle 9lbs individually, that means 18lbs combined. Since the actual weight will be 1/3 of that, I think I'll give it a go.

Thanks for the help.
Re: z/x carriage weight.
December 03, 2013 10:50PM
18lbs only if the x is in the middle but I agree that you. If all of your weight is in your x carriage then the max could be 9lbs but even then you have a factor of safety.

Happy printing!
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