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Why no hollow ballscrews?

S

Strostkovy

Titanium
Joined
Oct 29, 2017
Any ballscrew system has the inherent tradeoff of rigidity vs inertia (aside from rotating ballnut systems). I was wondering why nobody (that I am aware of) makes hollow, large diameter ballscrews? It seems to me that would reduce inertia while keeping rigidity. I know mass at a large radius has more impact that mass near the center, but gut feeling still tells me this would be an improvement.

I know that rolled ballscrews probably can't be made this way, but I see no reason this wouldn't be desirable for ground ballscrews. Anyone have any insight on this?
 
Strostkovy said:
Any ballscrew system has the inherent tradeoff of rigidity vs inertia (aside from rotating ballnut systems). I was wondering why nobody (that I am aware of) makes hollow, large diameter ballscrews? It seems to me that would reduce inertia while keeping rigidity. I know mass at a large radius has more impact that mass near the center, but gut feeling still tells me this would be an improvement.

I know that rolled ballscrews probably can't be made this way, but I see no reason this wouldn't be desirable for ground ballscrews. Anyone have any insight on this?
Click to expand...

Expense. Since most ballscrews are relatively small diameter (under 50mm) the inertia is in many cases not enough to worry about.

It also limits what sorts of drive or attachment options you'd have, which would further increase costs or constrain applications.

But some fancier machines use chilled screws, which in those cases do have gundrilled holes down their length for coolant to flow through.

Saying that, I recently picked up a large (~6" or so) diameter screw that weighs over 600lbs. In that case, a 4" bore would be a significant savings in inertia with a fairly small loss of stiffness and capacity.
 
Stiffness has another name in structural engineering: Second moment of inertia. Without mathing it out, I couldn't tell you which does better.

But I'm not inclined to, because such screws are typically loaded with thrust loads, which uses the whole area equally. Bending is what primarily relies on the outside surfaces. So the core material can add thrust stiffness and contribute less to the rotational inertia than a wider hollow unit of the same cross-sectional area.
 
BoxcarPete said:
Stiffness has another name in structural engineering: Second moment of inertia. Without mathing it out, I couldn't tell you which does better.

But I'm not inclined to, because such screws are typically loaded with thrust loads, which uses the whole area equally. Bending is what primarily relies on the outside surfaces. So the core material can add thrust stiffness and contribute less to the rotational inertia than a wider hollow unit of the same cross-sectional area.
Click to expand...

I mean torsional rigidity, i.e. twistiness. Thrust stiffness is a pretty valid point though.
 
Milland said:
Expense. Since most ballscrews are relatively small diameter (under 50mm) the inertia is in many cases not enough to worry about.
Click to expand...

Agree with the expense part.
I think that screw inertia is a very big thing to worry about in cnc machine design. In most cases a huge limitation.
The screw has too much. Fine use a bigger motor. That motor now has more inertia in itself added.

Removing the core has not so much effect on the inertia since the mass at center so again cost vs performance.
A tad torsional is given up in hollow but not very much and screws all wind up in use with load.
On a normal machine screw the benefits not great and the possible out of balance if the hole not perfect a concern.

Of note. Some car makers want hollow on front and rear axle shafts. Others buy solid.
When you get way out there you can affect the allowed torsional and wind before shear.
When drag-strips went to concrete launch pads driveshaft failures occurred and this the fix.
Bob
 
CarbideBob said:
Agree with the expense part.
I think that screw inertia is a very big thing to worry about in cnc machine design. In most cases a huge limitation.

Removing the core has not so much effect on the inertia since the mass at center so again cost vs performance.
A tad torsional is given up in hollow but not very much and screws all wind up in use with load.
On a normal machine screw the benefits not great and the possible out of balance if the hole not perfect a concern.
Bob
Click to expand...

Hi Bob,
I'd argue there's reasons why you don't want too fast a screw acceleration, perhaps most specifically risk of ball skidding. Also, loads on the trust bearings and mounts goes up with increased rotational acceleration, so you have to upsize and increase stiffness there.

As far as axial load capacity, I'd suspect that most such loads are limited (again) by thrust bearings and their mounts. I'd bet money (not a lot) on that being a higher-order factor over the loss of core material in the screw as far as system stiffness goes.

But mostly I think it's cost and loss of mounting flexibility from not being able to easily add threads and smaller bearing mount diameters when a cored screw is used.
 
Milland said:
Hi Bob,
I'd argue there's reasons why you don't want too fast a screw acceleration, perhaps most specifically risk of ball skidding. Also, loads on the trust bearings and mounts goes up with increased rotational acceleration, so you have to upsize and increase stiffness there.

As far as axial load capacity, I'd suspect that most such loads are limited (again) by thrust bearings and their mounts. I'd bet money (not a lot) on that being a higher-order factor over the loss of core material in the screw as far as system stiffness goes.

But mostly I think it's cost and loss of mounting flexibility from not being able to easily add threads and smaller bearing mount diameters when a cored screw is used.
Click to expand...

You might be surprised. Took me less than 5 minutes browsing McMaster to find a thrust bearing that can handle over 20k pounds of thrust for a shaft under 2" in diameter. Considering a 2" solid shaft would have about pi square inches of cross-sectional area, we'll call it 3 because it's a screw, a 20k lbf load would compress or extend this spring we call a ballscrew by 0.00023" per inch. A machine with 20" of screw between bearing and load would experience 0.0046" of deflection from the load, which is unacceptable.

Therefore, I'm reasonably confident that thrust bearings are good at what they do, machines don't typically experience loads that high under normal operation, and the thrust bearing is not the limiting factor.
 
Wouldn't the mass that the screw is moving/Accelerating have a much larger
effect than the mass of the screw?

I know at some point of performance, every gram matters, but are CNC machines
at that point?
 
Perhaps, but that's a dynamic load and resultant deflection. So your worst case loads are likely to start damaging the balls of the thrust bearing if they were a constant, which doesn't usually happen in a machine tool. There will be deflection of the bearings, the mounts, and all the related hardware under such high thrusts.

I'll agree that in extremes you'd lose load and accuracy, but it's already accepted in have higher-end machines with cooled screws to have a hole down their length. So if it's an issue, you upsize the screw.
 
Ballscrews that are hollow for oil cooling have small holes through them. Why would that be? I mean, if you're going to drill a long hole why not make it big one to lighten it up if it's such a good thing?
 
Garwood said:
Ballscrews that are hollow for oil cooling have small holes through them. Why would that be? I mean, if you're going to drill a long hole why not make it big one to lighten it up if it's such a good thing?
Click to expand...

I just replaced one in a Fadal it had a big hole and a insert that went all the way down and flushed out the same hole. Just one big hole
Don


Sent from my iPhone using Tapatalk Pro
 
Garwood said:
If Fadal did it, it must be good!
Click to expand...

They did a really good job with that cooled ball screw mess..

They used a water based coolant, and when, NOT IF!!! the seals
leaked, the WATER based coolant went directly into the thrust
bearings.

That was the one fancy high end thing the Old Fadal tried.. Spectacular failure.
And its not like a Fadal is fast enough to have ball screw issues due to heat..

If they had used velocite or some other light oil (apparently some have gone to that) and
made it so it didn't leak DIRECTLY into the thrust bearings, might have been a good thing.

Fadal was good at 'simple'. They nailed 'simple'.. Not so good at fancy.


And just for fun.. The chiller pump on a Fadal is a soda fountain pump.
 
You can buy nice axle shafts that are gun drilled from place like Mark Williams Enterprises. But....that's for an application where weight is an issue. For a stationary machine....not worth the trouble and money.
 
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