# bearing question



## John Rudd (Oct 22, 2010)

Guys,

On my minimill, I replaced the standard bearings taper rollers some years ago....

Recently things havent been so good with the machine so I decided to strip it... 

I find the bottom bearing has parted..That is the inner race has come away from the cage and rollers... 

I have two questions for the collective..

1)...Does the above mean that the bearing is broke and I should fit a new one or clean/grease/reassemble/refit the bearing? ( the top brg is fine, the one that is at the motor end...)

2).. The outer race is housed in the mill body, does this mean that the inner races should be a tight sliding fit on the spindle to enable the bearing preload to be set? 

I'm guessing at no.2 that if the brgs were tight on the spindle then I wouldnt be able to adjust them for correct preload and this has maybe led to the (early demise ? ) bearing coming apart?

As always your help appreciated....  :bow:

( now be quick to answer me wont you... ;D )


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## robwilk (Oct 22, 2010)

If i understand you correctly if the rollers on a taper bearing have come away from the inner race then the bearing is well worn out the cage part should hold the rollers in place.
Both outer race and inner race should usually be hard up to a boss then you should be able to set up the end float of the shaft depending on what holds it all together.
I personally don't have a mill so don't know what con trolls the preload on the bearing but as a agricultural mechanic i do use taper bearings a lot in gear boxes and many other applications so if i can help any more i will.
Hope this is of some use to you 

Rob......


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## John Rudd (Oct 22, 2010)

Rob,

Thanks for your reply..

What worries me is why the bearing has expired...... ???


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## robwilk (Oct 22, 2010)

If a bearing has prematurely worn out then either you have had to much load on it or to little lubrication if the bearing was to loose i think you would have noticed.
When setting up taper bearings that are set by a nut you would usually tighten the nut up until you start feeling resistance then back the nut of a bit just to take the pressure of the bearing.
if the bearings were held in place by say a circlip then you would add shims to the outer race to get the desired end float usually a few thou .
Rob.....


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## John Rudd (Oct 23, 2010)

Now I have it all stripped, everything looks fine.....Except the bottom bearing isnt seating fully.

I think this is where my problem stems from.

I did a bit of research and found the following text over on one of the files at the 9*20 lathe group...Ok the text refers to the lathe spindle but applies equally to the mill setup..I hope it is of use..


_Quote.._
Dependant upon the difficulty in removing the Inner Races and bearings it is essential to
make sure the new bearings are a hand press fit on the spindle bearing journals.

The fit of the bearings on the spindle bearing journal is CRITICAL! A tight fit is acceptable,
but a too loose fit or non-concentric spindle is a rejected lathe! Because these lathes are not
made to high quality tolerances, the bearings are intentionally made too tight on the Spindle
shaft journals. This prevents you from adjusting the Pre-Load. Before replacing the bearings,
you should make sure that you can slide the left side inner bearing race (closest to the pulley)
over its journal with medium to firm hand pressure.

The right bearing inner race may be a bit firm as once it is in place it doesn't move in relation to
the spindle. Only the left side moves when adjusting the preload. But the left end needs to
move when *cold* with a gentle tap of the mallet in order to make adjustments.

When sizing the spindle journals, how tight is too tight? Well, if you can't move the inner race
on the spindle with a gentle tap from a nylon mallet, then how do you think you are going to
adjust the preload? That inner race needs to move along the shaft. When setting the preload,
you may find you are only turning the spindle nut 1 or 2 degrees to get the proper tension.
How loose is too loose? The races should not slip easily. They should be a firm interference fit
so there is no play when things warm up. The fitting must be approached very cautiously. If
you go too far you will have to take the spindle to another lathe and knurl the spindle journals
or replace the spindle.
When everything is assembled, be sure to do a run-in at low speed for 30 minutes to an hour.
Check the spindle and head stock every five minutes during this run-in period to make sure it is
not too hot. If you detect heat, stop the lathe and back off the preload. If things are just warm
after an hour, increase the speed for 5 minutes and check again.
After using the lathe for a few hours, the preload may need additional tightening as the bearings
seat. The temperature should never get warmer than a warm cup of coffee.
_Unquote_.


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## steamer (Oct 23, 2010)

Hi John,
Can you post a picture of the setup/situation?

Some good shots of the bearing race and the installation would go a long way towards understanding 

Dave


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## John Rudd (Oct 23, 2010)

Dave, 
A bit late I'm afraid...I've just come back from the garage after commencing the rebuild....

I've worked the spindle now the new bearings are a tight sliding fit, so setting the preload shud be straight forward.....

New FAG bearings cost me £20 ( cash ;D beer money no doubt for tonight) from a local bearing factor..So decent bearings are going in, mind the ones I took out were Timken... :-\

I think I know what went wrong....

Some years back I removed the ball bearinged races from the mill and replaced them with taper rollers. I reckon the mill spindle was machined for ball races not tapers...So the bearing lands would have been an interference fit on the inner races.
As tapers require lateral movement for preload adjustment, the bearing lands would be too tight, so when re-assembled, I dont think I had the pre-load set correct. I think this is the cause of the bearing issue..

One other contributory facture is knocking 7 bells out of the drawbar to release the taper on the MT3 taper on the chuck/collet holder...

I need to find another method of breaking the taper....

How do others remove the drawbar from their mills?


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## Troutsqueezer (Oct 23, 2010)

I find that very few foot pounds of pressure are needed when tightening the drawbar in the first place. I tighten it just a little way past snug. When I use my plastic hammer to break it loose, I don't have to tap very hard.


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