How important is your floating end, really?

[salukikev]

On a semi-custom build I'm working on, I have the standard rolled ball screw with fixed bearing block setup in the standard Y arrangement. The ball screw is securely fixed on one end and floating on the other. The Nut is secured to the platform which is in turn aligned by the ways/gibbs along the track. You're all probably familiar.

So this machine has a burly steel plate on the end. When it was previously an acme thread it had a bushing in there on the floating end. Now that the ball screw is there and I've rearranged some things, I have to make something else. But as the rest seems to be right the screw is already constrained by the fixed end and nut & corresponding surfaces. Turning this screw results in a floating end that is pretty near perfectly concentric even if it's literally floating in air unconstrained (as one would hope). I now plan to make either a brass or teflon bushing to put into this steel plate and constrain it anyway- but the more I think about it the more I wonder "does it even matter?"

Interested to hear opinions. Thanks!

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[Jim Dawson]

From an accuracy standpoint I don't think it makes much difference. If you never turn it too fast it will proabaly be fine, but I never liked shafts rotating in mid air. My Haas Z axis screw is unrestrained on the bottom and seems to work fine and it does 400 IPM rapids so spins up pretty good.

[mactec54]

On a semi-custom build I'm working on, I have the standard rolled ball screw with fixed bearing block setup in the standard Y arrangement. The ball screw is securely fixed on one end and floating on the other. The Nut is secured to the platform which is in turn aligned by the ways/gibbs along the track. You're all probably familiar.

So this machine has a burly steel plate on the end. When it was previously an acme thread it had a bushing in there on the floating end. Now that the ball screw is there and I've rearranged some things, I have to make something else. But as the rest seems to be right the screw is already constrained by the fixed end and nut & corresponding surfaces. Turning this screw results in a floating end that is pretty near perfectly concentric even if it's literally floating in air unconstrained (as one would hope). I now plan to make either a brass or teflon bushing to put into this steel plate and constrain it anyway- but the more I think about it the more I wonder "does it even matter?"

Interested to hear opinions. Thanks!

It would depend on how long it is and it's diameter, supported is always better than unsupported and in what position it is in X and Y axes just the weight is going to be more drag in the Ballnut on a Z axes there is not side load on the Ballnut

The term floating is only for expansion, and should be a Deep Grove Ball bearing 2Rs not a bushing, or if you put the Ballscrew under tension, ( which is the best thing to do ) then an Ac Bearing is better, but a regular Deep Grove Ball Bearing is ok in some setups

[salukikev]

In this case, the total length of this screw is 14" including end machining, and its 20mm diameter. What do you mean by "Ac bearing?" I have a floating bearing block I can use that has one deep groove bearing in it that I could use, but it will require some extra machining. I might just drill the bushing hole large enough that I can press the same bearing into the 1/2" steel plate on the end. I'd be surprised if I have to deal with a lot of heat expansion as the whole system isn't going to be running super fast or through a great range often, but maybe I'm underestimating things. The conversation has me thinking about my X axis that I'm still waiting on parts for. That axis involves a 35" 20mm screw (again including ends, and the actual travel/used thread is only ~25"). I came up with a scheme to hold the floating end in a bearing most of the time, but that end has a slot that I can push a pin into to mechanically lock it into the thread cutting drive system. Seems like it should work, but now you've given me some more things to think about. Also- this machine formerly used thrust bearings. Those don't seem to be too popular here, but they seemed to work reasonably well for their purpose.

Edit: I should mention this is a modified shopmaster machine, so combination mill/lathe, so it gets used for all kinds of tasks- sometimes CNC, usually manual, and so that's why thread cutting comes into play.

[mactec54]

In this case, the total length of this screw is 14" including end machining, and its 20mm diameter. What do you mean by "Ac bearing?" I have a floating bearing block I can use that has one deep groove bearing in it that I could use, but it will require some extra machining. I might just drill the bushing hole large enough that I can press the same bearing into the 1/2" steel plate on the end. I'd be surprised if I have to deal with a lot of heat expansion as the whole system isn't going to be running super fast or through a great range often, but maybe I'm underestimating things. The conversation has me thinking about my X axis that I'm still waiting on parts for. That axis involves a 35" 20mm screw (again including ends, and the actual travel/used thread is only ~25"). I came up with a scheme to hold the floating end in a bearing most of the time, but that end has a slot that I can push a pin into to mechanically lock it into the thread cutting drive system. Seems like it should work, but now you've given me some more things to think about. Also- this machine formerly used thrust bearings. Those don't seem to be too popular here, but they seemed to work reasonably well for their purpose.

Edit: I should mention this is a modified shopmaster machine, so combination mill/lathe, so it gets used for all kinds of tasks- sometimes CNC, usually manual, and so that's why thread cutting comes into play.

AC Bearing is short for Angular contact,14" is not very long, if it is as simple as boring out the end plate for a support Bearing it is worth doing around .0002" smaller or a just slide fit is all you need it to be

[cyclestart]

Answered as I was typing.
Reply deleted.

[footpetaljones]

As one data point, the Y and Z axes of Robodrills have a floating end on their ballscrews, roughly 36" long and 16 or 20mm (can't remember). The floating end has a Nachi deep groove ball bearing that sits in a housing that has a groove for a o ring that the bearing OD is pressed against, so the bearing doesn't even touch the ID of the housing. And that thing will fly around at 2100 IPM with 1.5G of acceleration.

[mactec54]

As one data point, the Y and Z axes of Robodrills have a floating end on their ballscrews, roughly 36" long and 16 or 20mm (can't remember). The floating end has a Nachi deep groove ball bearing that sits in a housing that has a groove for a o ring that the bearing OD is pressed against, so the bearing doesn't even touch the ID of the housing. And that thing will fly around at 2100 IPM with 1.5G of acceleration.

It how rigid the drive end is held that counts, It is designed this way to help the Ballscrew when travelling at high-speed the Oring is doing the needed support as well as dampening any resistance and vibrations, saying it's not supported is incorrect the Oring is doing all that is needed to support the Bearing

[AlexGambler]

Thank you for all your opinions. I was thinking about this option working for my site. I found many interesting thoughts and information to realize.

Absolutely agree, thanks to this thread I was able to solve all of my issues.