Ball bearing spindles

Several small lathes and milling spindles are supported by a pair of deep-groove ball bearings, often pre-loaded by 'wavy washers' or Belville spring washers. I don't know what grade bearings are generally used. If the bearings are to be replaced - but not by converting to taper roller bearings - is it worth the expense of C2 bearings?

OK, it's a 'how long is a piece of string?' question, I suppose. However, it's worth knowing the answer because C2 grade bearings seem to be made of Unobtainium in NZ. Upgrading just for the fun of it can get quite expensive...

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you can use angular contact bearings which will be better on a lathe , I wouldn't bother on a mill . There are many that have used the origional bearings for years and have had no issues. Never seen belville or spring washers used ever and i would be removing them if they were fitted.

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If the bearings in a lathe headstock are pre-loaded, either by adjuster nut on the spindle or by wavy washers, the clearance is taken up, so how would the extra clearance in a C2 be of any relevance?

The usual trick is to upgrade to angular contact bearings or even better taper rollers. ISTR the ArcEurotrade website has an article on doing the conversion.

Edited By Hopper on 30/12/2018 12:17:31

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The design of that headstock is terrible , the axial load from the cutting tool is baring against a circlip not a machined shoulder in the headstock .

The wavy washer only controlls the end float of the spindle in one direction which is fine if you never want to machine anything feeding the carriage away from the chuck.

I am glad i never bought a unimat if their headstock set up is the same through all the small models they make !

Surely the difference in the radius the balls run on will be so minimal in a bearing with, what, half a thou less clearance than standard? Probably not worth spending extra money to obtain the C2 bearings.

Cool, furnish us with your engineering reasoning for that!

Read my last post.

I think it's true to say that a Belville washer is an elegant economy driven feature that in practice, I've never found to be an issue on my Compact 5. If I was turning propshafts for an aircraft carrier then I'd want a substantially rigid machine but for light model engineering work, I've never yet taken a heavy cut away from the headstock that has overcome the force exerted by the washer. Even if it made itself known, one can always turn the item around in most cases and alter the cutting direction towards the chuck. Most tools require a little adaption of work practices to their idiosyncrasies. I've owned mine for maybe eight or nine years and it's never been an issue.

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Spring-loaded ball-bearing spindles have a long and noble history. Precision grinding spindles have often used such a method to pre-load the bearings, to ensure stability, although rather better engineered than Emco's circlip abutment scheme. Prof. Chaddock used one such design (copied from Hoffmann) in his Quorn. Angular contact bearings may be the most commonly used in this application, but a deep-groove bearing, when axially loaded, is behaving like an angular-contact bearing. However, this is where I start wondering about bearing clearance grades...

An axial force on a deep-groove bearing causes displacement of the inner race wrt the outer, which causes the tangent at the ball contact points to 'swing round', so that the line of reaction force, normal to the tangent, swings towards the axial direction, to oppose the applied axial force. The geometry of a 'tighter' bearing will (presumably, all other things being equal) restrict the amount by which the tangent can swing round. Thus, for a given axial force, it seems to me that a greater radial force is induced in a 'tighter' bearing, and the resultant of the axial and radial components will be greater, i.e. greater forces at the ball contact points. Perhaps, if a deep-groove bearing has to resist axial forces, it should be a 'slack' grade, so it can approximate more to an angular-contact bearing when loaded. Or are the forces so far within the allowable range that 'tight' bearings are OK? They should provide greater radial stability in any case. Or have I got myself completely confused?

Edited By Kiwi Bloke 1 on 31/12/2018 19:50:58

We used preload washers in gas turbine gearboxes when I worked on them 40 years ago. The reasoning was that by preloading them you wouldn't get skidding between ball and track during periods when they were only lightly loaded. These were straight spur gears. These were revolving at tens of thousands of rpm with pumped oil jet lubrication

I'm a bit disappointed that no-one responded to the last para in my last post, above.

It seems to me that, counter-intuitively, in, say, a small lathe headstock, a properly preloaded pair of slacker grade deep groove ball bearings might be preferable to a pair of C2 bearings because the loaded slack pair will adopt a configuration more like a pair of angular contact bearings, and be stiffer in the axial direction. I suppose, however, this is at the expense of radial stiffness. Or am I getting old and stupid? (I know what my wife thinks...)

It's an interesting hypothesis ...

I suspect [without any substantiating evidence], however, that there may be another factor at play

The slack bearing is probably made with wider tolerances throughout; so the effect of your hoped-for change of geometry might be 'lost in the noise'.

MichaelG.

I suspect the same, and there's always a possibility the gain isn't worth the pain. The problem with up-rating any machine is that some other factor comes into play and you have to fix that as well to get any benefit. Perhaps many upgrades will be needed.

I'd welcome a before and after comparison showing the effect of changes on actual work. If you can't show the modification made a difference, it wasn't worth doing!

However, I have a utilitarian view of tools - I am a barbarian. No way would I ever bother to replicate an old lathe's original colour scheme! But that's just me - if improving a lathe makes the owner happy that's a jolly good reason for doing it.

Dave

Exactly what I was thinking. Close tolerance bearings are pointless if you are going to apply any worthwhile axial preload.

Personally, I'd just go straight to taper rollers or, on a small lathe, angular contact bearings.

Interesting how 'cost cutting' becomes 'elegant economy' depending on the provenance of the machine...

Neil

P.S. Looking at the geometry, with ever reducing clearances the static radial load increases significantly under any preload, which is not what you want for good bearing life.

Neil

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Could you explain that one please, Neil ... I'm lost

MichaelG.