[slow-poke]

I'm on a quest to get the backlash on my machine under 0.001" or as close as reasonably possible.

This is an older 8x30 knee mill with new Chinese rolled 1605DFU ballscrews and double nut.

With the ballscrew/double nut out of the machine and in my vise, I could feel some backlash between the two nuts, I was able to squeeze a 0.002" feeler gauge between the nuts. By adding a single 0.001" shim between the nuts the backlash is now about 0.001" so I think I have the ballscrew nut about as good as I can get it (it is what it is). That leaves the bearings......

With the original 6001Z bearings in the BK12 blocks and everything assembled, actual backlash is 0.004", so about 0.003" axial movement in the bearings. I ordered a random set of 7001AC bearings. These are not a matched set. so as I understand it I will need to shim them to get close to zero. I measured all five bearings and in all cases the inner race is wider than the outer race by between 0.0006" and 0.0022" Seems to me like it would make more sense to ship these with narrower inner flanges so tightening the retaining nut would allow the end user to torque these to zero or near zero backlash. Does that make sense?

So at this point, with what I have, I think my only option is to add a shim between the outer rings?

If I try that I'm thinking of using say a 0.005" shim, and then in theory I can take out the remaining 0.001" or 0.002" by torquing the retaining nut, does that make sense?

I realize ordering a matched set of ACB's might be a more sensible approach, but I would like to give the shim approach a try.


Comments, suggestions, expertise is most welcome as this is all new to me.

Any wild guesses on how much torque will be required on the retaining nut to take up the remaining slack?

[joeavaerage]

Hi,
why not use a DF matched pair, after all thats why God made them.?

Craig

[slow-poke]

Hi,
why not use a DF matched pair, after all thats why God made them.?

Craig

Search for matched 12x28x8mm angular contact bearings is not yielding much, source?

[RaderSidetrack]

One possibilty that may help is to put "matched" in quotes in your Google search. Example search phrase:

"matched" 12x28x8mm angular contact bearings


Some of the results:
https://bepltd.com/products/xc7001ct...aring-set-of-2
https://simplybearings.co.uk/shop/p2...duct_info.html
https://www.ebay.co.uk/itm/195162697244

The word/phrase in quotes -- in theory -- means "required" in search syntax. I say 'in theory' because sometimes the required match is buried in the fine print and irrelevant etc, but quotes can help narrow search results.

[joeavaerage]

Hi,
this pair is a little cheaper:

https://www.ebay.co.uk/itm/335228849...Bk9SR-j4zI3UYw


As is this pair:

https://www.ebay.co.uk/itm/182914737...Bk9SR-j4zI3UYw

Craig

[slow-poke]

Hi,
this pair is a little cheaper:

https://www.ebay.co.uk/itm/335228849...Bk9SR-j4zI3UYw


As is this pair:

https://www.ebay.co.uk/itm/182914737...Bk9SR-j4zI3UYw

Craig

Craig,

Thank you for the links.

I still have one more question....

My understanding is that these bearings will either have slightly wider inner races, or slightly wider outer races, and when properly installed the preload force on the bearing will move from the (unloaded distorted shape) to the preloaded non-distorted shape where the inner and outer races of the matched pair will be in full contact and some predetermined preload will exist. If my understanding is correct, that would mean to get the bearings to the preloaded non distorted shape, the clamping force would have to be applied to:
- the outer races if they were narrower than the inner races,
- the inner races if they were narrower than the outer races.

So in a application where the ballscrew retaining nut is applying the clamping force only to the inner race, the bearings would have to have slightly narrower inner races style. If the inner races were actually wider it would not be possible to preload them with the ballscrew retaining nut, because it would simply clamp the already touching inner races while leaving the outer races loose.

Is my understanding correct, or am I missing something?

[joeavaerage]

Hi,

So in a application where the ballscrew retaining nut is applying the clamping force only to the inner race, the bearings would have to have slightly narrower inner races style. If the inner races were actually wider it would not be possible to preload them with the ballscrew retaining nut, because it would simply clamp the already touching inner races while leaving the outer races loose.

That is my understanding also. The inner races are a few um narrower than the outer races and so when the inner races are clamped together there exists some preload. The issue is that 3um of preload might be perfect,
but 5um would cause rapid wear and overheating and 2um would have insufficient preload. Trying to make shims with that level of precision is beyond me. I prefer to buy matched sets. Mostly they are available
as 'universally' matched which means you can use then in DF, DB and DT arrangements.....but as you see they are not cheap.

Often angular contact bearings are placed at each end of a shaft...and therefore matched pairs make no sense. In that circumstance you either have crush tubes to set the preload or you have to use precise torque
on the nut to set the preload. If you have ever fitted opposed taper bearings in a trailer or car axle then you use the nut to adjust the preload. Angular contact bearings can be treated in a similar manner.

Craig

[slow-poke]

Thanks,

That's helpful.

I ordered the P4's off eBay, the seller accepted my $50 offer, so they should be on their way shortly.

One of my buddies tells me the way he sets the preload on these is to slowly increase the torque on the ballscrew retaining nut a little at a time and then measure the torque required to turn the ballscrew, when the torque increases to between 5 and 7 in-lbs stop and lock the retaining nut.

Have you ever herd of anyone using this technique?

[joeavaerage]

Hi,
I've not heard or seen it done with angular contact bearings but the idea is OK. When doing axle bearings the common thing to do is tighten the nut until the 'lash' just
disappears, ie surface to surface contact within the bearing. In the case of a car or trailer you might back the nut off 1/8th turn so there is just a little clearance
so any heat build up does not cause any extra wear. For a ballscrew where you want preload then you would do the nut an extra 1/8th turn or whatever. Measuring the increase
in frictional torque is a reasonable way to estimate the preload. Next question is how much is right?

That's why I buy matched bearings, the manufacturer has already done the critical part....al I have to do is fit them properly and they should perform as the specs suggest.

Over the years prices for matched bearings has come down, I presume the use of CNC grinding machinery has made that possible. Unless you want extreme precision then
universally matched bearings of modest to moderate cost are entirely good enough for me.

Craig