lead screw whip ? bearing to use and why?

[technomage]

Preventing lead screw whip ?What bearing to use and why?

What bearings are recomended for pretensioning my 1/2 8 2 Start x 60" leadscrew to minimize whip.
From what I have read in posts angular contact bearings are the way to go.
But all the angular contact bearings I have been able to find online are either metric or megabucks.
I know that Joe of "Joes Machine" and others have used standard radial bearings with a moderate preload and are getting resonable life out of them.
In looking I found these would one of them be better suited in terms of preload etc.


http://www1.mscdirect.com/CGI/NNSRIT?PMAKA=03379823
http://www1.mscdirect.com/CGI/NNSRIT?PMAKA=03380607
http://www1.mscdirect.com/CGI/NNSRIT?PMAKA=03379831

the machine should be sturdy enough to hold up to a hefty preload if needed
Thanks for any input.

[RICHARD ZASTROW]

You might want to search "whip" on the CNCzone search.

[sdantonio]

Try looking at these, you just need to look for ones with 1/2ID

http://www.vxb.com/Merchant2/merchan...Category_Code=

[sdantonio]

Also check out

http://www.cnccookbook.com/CCCookbook.htm

Eliminating Backlash parts 1&2, this will tell you a lot about AC bearings.

[technomage]

Originally Posted by sdantonio Try looking at these, you just need to look for ones with 1/2ID http://www.vxb.com/Merchant2/merchan...Category_Code=

I still am only coming up with metrics for angular contacts I had browsed Vxb and others for 1/2" ID bearings before posting. Am I missing something?
Ps. Thanks for Cookbook link an extra reference source never hurts
Thanks

[technomage]

Since I can't find any resonable priced 1/2" angular contact bearings I will most likely go with a "deep grove Bearing" Any Feedback on how well they hold up with axial preloading ? I belive that they are used on "Joe's Machine"?

[RICHARD ZASTROW]

Whats "resonably priced"? Equal quality deep groove radial bearings ain't cheap either. Deep groove radial bearings have SOME axial thrust capacity but preloading might cause excessive wear. NC Cams had a pretty good explanation some time ago on another thread. BTW, if your using 1/2" thread, won't you have to turn the ends down to fit bearings? I don't think you will want to locate on the thread OD. Try Peer or AST for low priced Chinese bearings.

[martinw]

Dear technomage,

I'm no expert on this, but I did go through the design loop that you are going through now about a year ago. My gadget is, still conceptual... I'm ashamed to say.

1) Keep the screw in tension?

If you want to reduce whip, you can do a number of things, and putting the screw in tension is one of them. The problem is that, without an extremely rigid and thermally stable frame, the screw is unlikely to remain in tension at all times. To avoid this, tension might need to be applied to the screw by a really powerful compression spring. This may put a considerable axial load on the bearings at each end of the screw, certainly more than the axial load that you might wish to apply to a deep row radial bearing.

OK, instead of using standard bearings, you could use angular contacts, but I still doubt that you could maintain sufficient tension, and at reasonable cost, as the frame distorts thermally.

2) Increase the end fixity of the screw?

Another way of reducing the tendency to whip is to reduce the "effective length" of the screw by ensuring that the screw is " positionally and angularly restrained" by the bearing arrangements at each end. You can do this by using a pair of bearings at each end. Space each bearing apart by about two bearing ODs (I think... can't find the reference..sorry).

I opted for a couple of double-row AC bearings at the motor end , and a couple of standard radials at the far end. The motor end DRAC bearings should have a couple of nuts on the leadscrew to pre-tension the inner rings of the two bearings against each other. This should prevent any axial loads , (and movement) being transmitted to the motor.

At the other end of the screw, the screw is free to move axially through the inner rings of the standard bearings. The bearings at this end do not attempt to prevent axial movement. The main span of the lead screw is not under tension.


3) Does it work?


Dunno guys...all comments welcome...



Best wishes


Martin