Locking Bearing preload nut

I am in the process of adding a 10mm LH ACME leadscrew to my Taig lathe.

The two thrust (and one roller) bearings for the motor end of the screw are mounted in an aluminium plate. On one side (left in image) is one half of a small dog clutch which is pinned to the leadscrew (turned down to parallel).

On the other side (right) of the bearing set I have an ACME nut on the leadscrew that can be used to preload the thrust bearings to eliminate backlash.

To lock the preload, I will drill the ACME nut and put in a grub screw to fix it once loaded. The alternative is to saw the ACME nut in half and use the two halves as conventional locknuts.

The grub screw looks easier to do, but before I go too far, can the combined wisdom of the forum see any gotcha's I have not spotted? Or a better idea for the preload, ACME LH Nyloc nuts seem a bit rare.

Edited By Peter Cook 6 on 19/06/2021 17:45:06

When you say you will drill the nut, will the grub screw bear on the leadscrew?

An option is to saw the nut approximately 50% through (if really keen drill at the end of the sawcut so it does not create a stress riser) and then drill and tap parallel to the leadscrew (saw cut maybe 25% along the length of the nut. The flexing of the thinner bit due to the axial pressure of the grubscrew will stop it undoing.

The idea is shown as slide 2, letter N here: https://slideplayer.com/slide/8842407/ . You can either compress or expand the nut.

Where did you find an ACME screw in 10mm dia.? Is it an imperial or a metric pitch? Is there a possibility it could be trapezoidal thread?

Thanks DC. That Idea looks useful and potentially easier. I was a bit worried about positioning the grub screw so that it fitted either between the threads of the lead screw or on a land and didn't end up half and half.

You are correct ( I just checked) the thread is trapezoidal not ACME - my mistake. But the nut(s) and rod are all matched from the same source, so (hopefully) no issue.

If there is room, you could put a nylon plug under the locking grubscrew to prevent the scarring that locking screwsusually cause.

+1 for Old Mart's suggestion, or alternatively a brass disc.

Rob

Given the nut is pressing against a bearing that will spin freely it will have little tendency to unscrew.

As the bearings are "solid' the preload will not need to be continuously adjusted.

Simple solution, if it was me doing it, light or possibly medium threadlock.

Dave

Hi Peter in industry they use a copper slug between the grub screw and the thread of the shaft. Use a slightly over size diamiter, only a few thou bigger diamiter than the grub screw hole, tap it in until under flush and then push it in with the grub screw till it is just flush with the thread inside the nut. Then run a tap down the nut to cut the thread in the copper slug. You will be able to adjust the play at any time without damaging the shaft thread. The shaft thread is always smaller than the main screw thread to help assembly and use smaller bearings. You can use two standard taper roller bearings, face to face, which will give you better stability.

David

Edited By David George 1 on 19/06/2021 22:47:06

Is there space for a jam-nut? KISS principle at work.🙂

With such small bearings I would be inclined to use a spring or spring washer for preload as otherwise it will be tricky to set otherwise.

Neil

Jam nuts are difficult to use when setting preload. You always end up slightly rotating the one providing the preload, even though you are trying your hardest only to rotate the one doing the jamming.

A good engineering solution will separate providing the preload from providing the locking action.

The screw thread in question is a LH trapezoidal one, so the jam nut is unlikely to be found in the odds and ends bucket.

Hi DC31k's, slitting the the nut and using a grub screw to wedge the slot apart, has been used in industry for very many years for similar applications.

Regards Nick.

Peter,

Using an ACME nut to set the pre-load can also be tricky as it is a course pitch thread. If the adjusting nut is on the turned down parrallel section at the end, you can use a nut with a fine thread. (drawing below of a ball screw set-up) As well as using 2 nuts, split nut or a grub screw, removable nut retainer (eg blue Loctite) can be used. The dog clutch could be screwed onto the same threaded end as the lock nuts and pinned as shown in your drawing.

Paul.

A question:  Not knowing your design, You are using a trapezoidal lead screw and lock nuts to eliminate backlash. You will still have some backlash with the running nut. Why not use a ball screw ?

Edited By Paul Lousick on 20/06/2021 14:13:44

Thank you for the received wisdom of the forum. More Ideas for me to ponder and experiment with.

David, I did look for taper rollers - but with a 7mm ID they seem to be unobtainium and if do find any probably out of my price range.

NDIY - One option was to create a jam nut by cutting the nut in half, but I take the point about the difficulty of holding the preload still enough - especially with a 2mm pitch. 5 degrees of shift is 0.028mm change in the preload position.

Paul, I know will have some backlash on the running nut, but I can probably live with that on the little lathe as long as the leadscrew bearings don't add to it. The idea of a fine thread on the leadscrew to make adjustment easier is interesting. Perhaps thread the dog clutch on, adjust and lock with a grubscrew. A ballscrew is serious overkill for the purpose.

Thanks to everyone for the ideas. I have a couple of nuts on order with the leadscrew, so may try the grubscrew with a brass or copper slug, DC's approach with a slit in the nut and threading the inside end of the leadscrew. Just to see which works best.

Edited By Peter Cook 6 on 21/06/2021 12:51:48

Jam nuts are difficult to use when setting preload.

Yes, possibly, but a lot manage to use jam nuts when setting gibs, so not impossible. I would agree that slitting the nut and using a grub screw would be easier, but I still like jam-nuts as a simple alternative.

Dog clutch side can be immobilised, as can the main nut, so only the second nut to be tightened (possibly loctited as well?).