[crjohnson]

Hi guys,
I am starting to assemble and test some of my cnc parts. One thing that I have not been able to find is the amount of tension/torque that I need to put on the ballscrews locknut on fixed-end mount. Can anybody please give me the correct method for tightening this?

I have THK BIF2005 screws and BK15 fixed end mounts.
Cheers!

EDIT: I did actually find a THK document from 2001 that states 2350Ncm (23.5Nm) for an RN15 model, thats the only info found so far.

[crjohnson]

Edit no. 2: I also found this Hiwin document that only states 8Nm on the M15 size lock nut. I Feel this is closer to the mark than the 23Nm in my first post?

https://hiwin.sg/wp-content/uploads/...nformation.pdf

[mactec54]

Edit no. 2: I also found this Hiwin document that only states 8Nm on the M15 size lock nut. I Feel this is closer to the mark than the 23Nm in my first post?

https://hiwin.sg/wp-content/uploads/...nformation.pdf

23Nm is not much for a 15mm nut that is around 17 Ft-lbs. and for a Bearing lock nut, that would be about right, this would depend though on the thread pitch and the thickness of the nut.

[crjohnson]

M15x1 is the locknut thread. I thought 23Nm would put a fair bit of preload on the 2 bearings in the support block thats all.

I also found a figure of 790N.cm (8Nm) in an NSK document, so thought it was strange that THK says 23Nm

[mactec54]

M15x1 is the locknut thread. I thought 23Nm would put a fair bit of preload on the 2 bearings in the support block thats all.

I also found a figure of 790N.cm (8Nm) in an NSK document, so thought it was strange that THK says 23Nm

It would depend on the Bearings you are using, if they are a matched pair, then the preload is predetermined, and you can't change that no matter how much you torque the nut.

8Nm =5.9Ft-Lbs is not much for a 15mm lock nut, you can try any of these numbers and see which works best for your application, you will see backlash if the lock nut is not tight enough.

[crjohnson]

7002 bearings that coming with THK bk-15 mounts as far as I know. I didn't know the preload is predetermined actually. I just didn't want to overload them and cause any damage. I'll see how 8Nm feels and go from there.

[mactec54]

7002 bearings that coming with THK bk-15 mounts as far as I know. I didn't know the preload is predetermined actually. I just didn't want to overload them and cause any damage. I'll see how 8Nm feels and go from there.

If they are genuine THK they will have P5 precision grade bearings and these would normally be matched pair, being a matched pair, you can't over torque them. this may help on how to set them up, if you don't have this PDF.

[crjohnson]

Thanks again Mactec!

I did have a copy of that document but didn't see the comments on the second page about them being "supplied with an appropriately supplied preload". I usually skip over the marketing material on the first or second pages but this time it bit me..

I'm still suprised none of that is mentioned in the mounting procedure or the torque spec for the nut. Hopefully someone in future sees my oversight and it saves them some time.

[mactec54]

Thanks again Mactec!

I did have a copy of that document but didn't see the comments on the second page about them being "supplied with an appropriately supplied preload". I usually skip over the marketing material on the first or second pages but this time it bit me..

I'm still suprised none of that is mentioned in the mounting procedure or the torque spec for the nut. Hopefully someone in future sees my oversight and it saves them some time.

It's only when you use regular Angular contact bearings that you have to set the preload to suit the Bearing, for me even a regular AC Bearing, I Grind them as a matched pair and give them the preload that I want the Bearings to have, so anything is possible if you know how.

Any Precision matched pair of Bearings, always has a preset preload, that you can't change, no matter how much you torque the Nut, so it is really what the Nut / Thread can take, in this case not the Bearings.

A way that Precision Bearings can be over loaded is if the shaft or the housing, is not the correct size for the Bearing.

[badhabit]

If they are genuine THK they will have P5 precision grade bearings and these would normally be matched pair, being a matched pair, you can't over torque them. this may help on how to set them up, if you don't have this PDF.

[....]Any Precision matched pair of Bearings, always has a preset preload, that you can't change, no matter how much you torque the Nut, so it is really what the Nut / Thread can take, in this case not the Bearings.

Sorry for reviving an old thread here, but i've searched the forum for a similar question and found this to almost answer the question i have...so... i have a 32mm ballscrew with M25x1.5 thread on the end. The bearings says "7205A DF P5" on the side...looking in the THK document you supplied it looks right for the 25mm shaft, so far so good, it matches what i see in the garage... now you say i can't overtorque the bearing lock nut, but how much torque should i apply then? If the thread should dictate then M25x1.5 is rated for something like 1000Nm/800ft.lb. but that seems crazy to me, is that really the case or am i way of here...?

Side question: does some sort of "torque wrench lock-nut spanner adapter thingy" exist or do i have to macgyver something with the wifes bathroom-scale to torque the thing with a regular spanner?

[joeavaerage]

Hi,
I have 32mm THK BNFN ballscrews in my machine and they have FK25 fixed end bearing mounts. It has precision matched angular contact bearings. Whenever I have done up the nut I just torque them
to' conveniently firm' and then locked them with the grub screws. As Matec says the actually preload of the bearings is fixed by the precision grinding that 'matches' them when being made. Provided the
nut torque is sufficient to hold the two faces of the opposing bearings firmly in contact with each other such that the designed preload is applied and doe not fluctuate...I cant see the the nut torque
matters. In effect I'd apply something like 20 ft.lb, ie conveniently firm, but no more. More does not achieve any useful purpose.

Craig

[crjohnson]

I once found an old THK document with locknut torque figures, now I cannot find where i saved it.. BUT I do have an NSK document that states 20.6Nm for an M25x1.5 lock nut and 5Nm for the set screw. Those figures are for "light load lock nuts" and WBK-25 size blocks.

The high-speed, heavy load blocks (WBK25DF-31H) state 85Nm!

[peteeng]

Hi all - A more precise description of this issue is that in a matched pair of bearings the bearing clearance is set by the matching. So No matter what torque (nearly) you can't crush the bearing as the load is resisted via the inner races. If the bearings are unmatched then the preload may crush the bearing or leave a clearance. The preload on the ballscrew is another matter. I have ground down a spanner so it fits inside the mount and I do the nut up snug, then do up the lock screw, then apply loctite. I grip the ballscrew with vice grips that have a timber collet. The screw slips in the collet at a snug tension.. Peter

[joeavaerage]

Hi,
the only time I could think that such a high torque of 85Nm would be required were if the reversal of load caused the axial load be transferred to one bearing
and moves marginally 'leaving the other bearing behind'. Thus the high torque is to accommodate the elongation of the 25mm land of the ballscrew under tension.

Cant help but think if I can 'stretch' a 25mm section of a hardened ballscrew I may have bigger things to worry about than the fluctuating preload of the bearings!.

Craig

[badhabit]

Ok, cool, so the consensus is that matched bearings should just be "nice and tight" and no need for the torque-wrench on this one...

Thanks!

[crjohnson]

On another topic - does anyone here rebuild there fixed-end ballscrew mounts? If so, how often? Simply clean and re-pack with grease?