Rotating Ballnut - design ideas

[JAZZCNC]

HI JAZZ thanks for your insight regarding my question just talk with my ball screw supplier he suggest me to use 4040 for 3.2meter axis and 3232 for 2 meter axis..and i might follow their suggestion regarding that if budget is not constraint..anyway maybe i opt for 3232 all axis 4040 is really expensive

Do they know you are rotating the nut and tensioning the screw.? Makes big difference.
This is the first 10x5 machine using rotating ball-nut I've built, I've built plenty of 8x4 and the difference between them really surprised me.

The secret is keeping the nut rotation speed down and getting a good tension on the screw with nice straight pull.

Good luck

[Kitwn]

Yes, 32mm pitch with 2:1 ratio will give the same linear torque as 16mm pitch 1:1 so torque is canceled by pitch difference like you say.
However, when I say Torque is doubled so the smaller motor can be used, what I'm referring to is that if 1:1 the 32mm pitch would require a larger motor to give the same linear force as 16mm pitch. So the ratio doubles the torque and allows the smaller motor to have the same linear force as 16mm pitch.

Aha! The point I had missed is that your 'smaller motor' required to drive a 32mm pitch screw with 2:1 pulleys is NOT smaller than the motor required to directly drive the 16mm pitch screw. In fact it may well be the same motor, which is at the heart of what I was trying to say. Thanks for clearing that up.

Is there a case for using the direct driven screw with a bigger motor? You are saving the cost and complexity of the 2:1 drive and halving the speed required from the motor, so running it in it's higher torque region anyway?

Kit

[teamloks]

Do they know you are rotating the nut and tensioning the screw.? Makes big difference.
This is the first 10x5 machine using rotating ball-nut I've built, I've built plenty of 8x4 and the difference between them really surprised me.

The secret is keeping the nut rotation speed down and getting a good tension on the screw with nice straight pull.

Good luck

do you use BK/Fix unit on both end side?

[JAZZCNC]

do you use BK/Fix unit on both end side?

Don't need end bearings because not spinning the screw. The ends need to be either externally threaded or internally-threaded so you can tension the screws. I've used both methods on different machines and prefer externally threaded as can get more torque.
The force required to take Sag out over 10ft is considerable so make sure the brackets holding the ends are substantial and adjustable so can align screw in two planes.

[JAZZCNC]

Is there a case for using the direct driven screw with a bigger motor? You are saving the cost and complexity of the 2:1 drive and halving the speed required from the motor, so running it in it's higher torque region anyway?

Kit

Can do either but using a larger motor usually doesn't cost less because requires larger drive and often more volts so larger PSU.
However, on long machines rotating nut wins hands down and requires a belt connection, so the ratio is no extra cost. Rotating the screw on the long machine means much larger ballscrew is required and this really starts to ramp up the costs because everything scales up with it, Bearings, couplers, motors, drives, PSU
Even then a rotating the screw system cannot achieve the feeds a rotating nut can.

[Voicecoil]

Reading this thread with interest (as I may need to go this route on my "big" machine soon), I noticed the concerns over rotation speed of ballnuts. This made me recall some stuff I read in the TBI catalogue some months back about different types of ballnut (differing mostly in the configuration of the return paths) and their relative speed capabilities, worth a read and maybe doing the sums they outline there.

[JAZZCNC]

Reading this thread with interest (as I may need to go this route on my "big" machine soon), I noticed the concerns over rotation speed of ballnuts. This made me recall some stuff I read in the TBI catalogue some months back about different types of ballnut (differing mostly in the configuration of the return paths) and their relative speed capabilities, worth a read and maybe doing the sums they outline there.

Certainly worth a read but what helps with rotating nut is to get shortest length Nut possible.
It's also critical that the Shaft as a Flange which is machined perfectly concentric to the Nut otherwise concentricity to the screw is misaligned so causes big vibrations.

[Voicecoil]

Certainly worth a read but what helps with rotating nut is to get shortest length Nut possible.
It's also critical that the Shaft as a Flange which is machined perfectly concentric to the Nut otherwise concentricity to the screw is misaligned so causes big vibrations.

What's the thinking behind the shortest possible nut please?

The concentricity is an obvious thing and might rule out some of the cheaper offerings if my experience on the machine I've just completed is anything to go by. Having set everything up and got it working smoothly I thought I'd spin one ballnut through 180deg to get better access to the lube nipple. Major fail, that side of the axis got really stiff, it turned out that the casing/flange of the ballnut was something like 1 or 2 degrees out of alignment with the axis of the screw.

[JAZZCNC]

What's the thinking behind the shortest possible nut please?

access/interference and balance/inertia mostly, it's no big deal and depends on the design to some degree but shorter is better in my experience.

The concentricity is an obvious thing and might rule out some of the cheaper offerings if my experience on the machine I've just completed is anything to go by. Having set everything up and got it working smoothly I thought I'd spin one ball-nut through 180deg to get better access to the lube nipple. Major fail, that side of the axis got really stiff, it turned out that the casing/flange of the ball-nut was something like 1 or 2 degrees out of alignment with the axis of the screw.

It's not always the case obvious to everyone that's why I mentioned it. However most ball-nuts, even the cheap ones, have a machined reference surface on the outer body which you can chuck upon and skim flange if needed. Thou I've not had any issues if honest.

[chrismolloy]

First post on this bulletin board; I enjoyed this thread...or at least what I had a chance to read using the spotty WiFi signal at work anyhow.

I find myself in need of a couple of rotating ballnuts for a 3000mm-long 2505 ballscrew, and have come across this older thread on the subject. I was inspired by the designs depicted here, and worked to duplicate/improve upon them when I became curious as to why angular-contact ball bearings were used instead of tapered roller bearings.

I'm trying to keep the unit as compact as possible and I suspect that the tapered roller bearings will help in that respect. The force on the hollow rotating shaft will be primarily axial, along the ballscrew axis with barely any radial load being applied by the tension of the timing belt/pulley.

I came up with the following design (sorry for the seemingly random view angles)--Any comments/input regarding it's viability?:








Thanks,

--Mark

Do you have design drawings to share for this?