Heavy 15Kg bed + ball screws = problems with Z axis motor holding?

I will have a very heavy (about 15 Kg) heated bed which will be raised and lowered via screws.
I plan on using ball screws as I figure acme screws might wear quickly, also the friction would make it
harder for the motor to raise and lastly ball screws would be more precise and accurate. I plan on using about a 3:1 ratio on the pulleys driving the screws which will help,
but I am worried the motor ( a NEMA 23) may have trouble holding position with so much weight and low friction ball screws. I read on another
thread someone recommending a motor with a brake. I guess like this one:
Stepper with brake.
Would that be a good idea? It would have the added advantage of stopping the bed slamming down in the event of a power failure.

What would I have to do to configure my driver board (Duet WiFi) software for this type of motor?

Edited 2 time(s). Last edit at 11/19/2017 01:15PM by SplatHammer.

The frame size like NEMA 23 or NEMA 17 doesn't tell you anything about the holding torque of a motor other than NEMA 23 motors are generally stronger than NEMA17 motors. The 475 oz-in NEMA 23 motors on my CNC router wouldn't have any trouble lifting your entire printer.

The bed might lower to the bottom with a power failure, but I wouldn't expect it to slam down.

IMO, plain old trapezoidal lead screws would be better. Don't worry about the wear - the Z axis doesn't travel far, even over the course of several years . Also 15Kg spread over 3 screws is only 5Kg which is bu**er all for an 8 or 10mm screw. Use fine lead screws say 1 or 2 mm. That'll mean single start threads so the pitch will be the same as the lead. This will do 3 things for you. Firstly, the finer pitch will mean that you need less torque to lift the bed. Secondly, the shallow helix angle will mean that the bed will stay put without needing to apply breaks to the motors. Thirdly, you will actually be able to use layer heights that are multiples of 0.1mm without having to rely on micro stepping for positional accuracy (as is the case with those God awful 8mm lead (4 start, 2mm pitch) screws that people will insist on using for their Z axes (sorry but it's a pet hate of mine).

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The motor I am planning on using is a nema 23 180 N.cm which is 256 oz-in. Yeah I guess I am worrying a bit too much!

If it's any help, I use a 59N.cm Nema 17 to lift my (approx) 8Kg bed. That's with 1mm lead screws though.

If a motor is capable of lifting the bed it will have no problem holding it in place.

Brakes are used when the axis will back-drive without power. The brake is designed to close automatically when the machine loses power so that the axis doesn't fall to the bottom of its travel every time you turn the device off.

A brake will not improve performance while the printer is operating, and honestly should not even engage while the machine has power.

Hi Deckingman
Thanks, that does help! I am probably going to go with the ball screws, I worked out that with 16mm diameter screws with 4mm lead and 2 or 4 times advantage with pulley size I shouldnt have a problem
I think ball screws will be smoother and more accurate, plus I get a nice kit with thrust bearings and a mounting block for the ball nut. A 1604 ball screw means that you are only looking at 4mm for about 50mm
of thread (circumference) so its not putting much force on the motor (its like a wedge that goes 4mm higher for 50mm length), especially with mechanical advantage of the pulleys. I would play around with different options
but the time delay and DHL shipping costs (plus Spains €50 charge just for processing import tax) means I want to keep the number of packages to the minimum!

Hi
Thanks for that - I wasnt sure if the brake just came on after the bed had moved or if it was more for power loss. I am learning by reading posts
and unfortunately some people post without really understanding what they are talking about - a poster on another thread wrote as if the brake could be engaged by the software
after lowering the bed a step. I was dubious so thanks for confirming my suspicions! : )

Quote
SplatHammer
A poster on another thread wrote as if the brake could be engaged by the software after lowering the bed a step. I was dubious so thanks for confirming my suspicions! : )

Sometimes a break will engage/disengage during regular operation but such cases are rare. The main situation where you will see this is when you want to lock the axis and then apply extreme forces that would overpower an unassisted servo.

An example would be CNC mill-turn or high speed indexed 4th/5th axes. The axis can be positioned at high speed, then locked in place to withstand cutting forces. (As an example, [ca-en.dmgmori.com] uses hydraulic clamps)

There is no point doing this for a 3d printer because there are no extra forces applied to a stopped axis. You could lock the Z axis for each layer - but the motor would be fine on its own so there is no benefit. As well, engaging and disengaging the brake will take some fraction of a second and might wiggle the shaft a few encoder counts, so it tends to degrade motion quality unless implemented very carefully.

Edited 1 time(s). Last edit at 11/20/2017 03:59PM by 691175002.

Whatever you do dont put your head under the splathammer while adjusting.

Sorry, SplatHammer is my online gaming name (I have a 12 year old son) and given the varied accounts I have I try to keep it simple with minimal usernames!

Thanks for that, having read more (I have flu and bronchitis so I have a bit of time off) I am going to halve the size for the first version so 50x60cm bed and no problems with weight.
Thinking of trying a carbon fiber square tube for the x axis with a 9 or 12mm rail mounted on top.

Zasquatch is struggling with bed drop in a machine that has a 30+ kg bed. See: [forums.reprap.org]
There might be something there for you...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]