installing ball screw bearings

[dgalaxy]

does anyone no where i could find information on reinstalling the ball bearing in my screws i had to remove the bearings to take my y axis screw to get trued
from an engine block crash i just want t0 make sure that i install thhe bearing the correct way

[zoeper]

I've done that only the other day and described how i did it here:
Cleaning of Ballscrews
It worked for me but there might be easier/better ways??
Pieter

[NC Cams]

Are you trying to reassemble the ball nut or the end support bearings? Big difference.

If it is the end support bearings, did you mark them for position and orientation when you took them out? If not, WHY???????

Ball screw support bearings traditionally have a chevron ( > ) etched into the OD. When properly aligned, they are merely tightened back into place using the same orientation they were in when you took them out.

You did mark the orientation during disassembly, didn't you????

[zoeper]

NC,
Why are you only telling me this now??
I took my whole slant bed apart oblivious of this fact!

PS. Are the bearings not supposed top be perfectly round?
and how do they first determine the location of the ^
Some background would be much appreciated.
Pieter

[shannbailey]

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[NC Cams]

ZOEPER: Why? Because you didn't ask before you took it appart.

I also bet you never looked at the owners manual for any "be carefuls" either. Although to be fair, I haven't seen many warnings about issues like this in ANY owners manuals. If you're like some folks I've seen, I suspect you may have torn into it like a kid ripping open Christmas presents and may have screwed up more stuff than you even suspect as a result.

The OD chevrons orient the bearings so that if the faces that have been offset ground for preload, they are properly oriented at installation. If some hack did the offset grinding and didn't mark the chevrons, your guess is as good as any as to the proper assembly procedure. You have a 1 in 3 chance of installing them properly if you blindly guess at the procedure.

Roundess and eccentricity are NOT the same thing. A wheel can be perfectly round but if the hole that mounts on the rim to the spindle pin is off-center, it will rotate in an eccentric manner. All the balancing in the world won't fix it either.

In the days of less than concentric raceways to OD/s and ID/s, the chevrons and 'pip' marks on the inner race sides denoted the eccentricity high points. By mounting the spindle bore eccetricity high point opposite the bearing eccentricity high points, you could minimize the amount or even the need to grind the ID after installing bearings to bring the ID concentric to the axis of rotation established by the ball paths.

[zoeper]

NC,
I still do not understand the 'offset grinding for preload' part, would you mind explaining how this is acchieved? Now that the damage has been done, what is the best way to recover from it?
Pieter

[dgalaxy]

right! the damage has been done. i was told to at a shop that to straighten the screw everything had to be removed obviously it is a right on the spot decision .so off i took them off the screw is bare i had it straightend and now i need to reinstall them. i have no knowledge of screws but i geuss im going to learn there is a guy in michigan that said he would do both of them for 550 but i would like to try this myself if anyone could point me to some links or info on a what is called a level1 rebuild i would totally appreciate it
People have to learn somehow and i think it would be great knowledge and i could help people learn from my mistakes
John

[NC Cams]

Zoeper: please do a 'bearing preload' search via Google.

Barden or one of the other bearing makers has a foto that explains it better than a 500 word paragraph. You might also do a "bearing preload" search on this website. I recall explaining it with pictures, once before. Again, pictures make it oh so much easier to understand.

Regarding reloading ball nut: you do NOT just dump in the balls. You first have to segregate them by size as they usueally use 2 different ball sizes to assemble the nut. They install them ...oOoOoOo... so that they track properly.

Recover from botched bearing assembly practice ??? Depends on how bad you've botched up the beariings and if you botched them up. Can not tell via message board thead if you did or did not hurt anything. If you installed them backwards and tried to shove the balls over the side of the raceways, the bearings could easily be junk.

As far as fail safe recovery, can you spell "new bearings"???

[NC Cams]

ZOEPER: at this point, I am confused which may be causing even MORE confusion on your part.

So we need to straighten some things out.

Did you take appart the BALL NUT or did you take appart the BALL SCREW SUPPORT BEARINGS. Please advise.....

REASON: THERE IS A HUGE DIFFERENCE BETWEEN THE TWO and, hence, the methods to (re)-solve the problems that occurred due to the improper disassembley and reassebly there of.

DGALAXY: I'm more convinced that you have a ball nut disassembly/reassembly issue. Again, please confirm.

I don't want to confuse either you or zoeper anyfurther than you may already be.

[zoeper]

Nc:
I have disassembled the whole machine. Ballscrew support bearings was removed because coolant and other muck was feeding though the support bearings and into the servo gear housing. Also if you look at the post that i have referred to earlier and you look at the photo you will also see why i had to take the ball nut apart.
Cleaning of Ballscrews
I had i look at the bearing preload grinding procedures and setups , but i am still in the dark as to why the orientation is so important (other for high point identification) in which case you would install the marks 180 degrees apart
If not, then they must have been tested while seated inside the machinetool and that does not sound practical either.
The machine was bought from a "reputable" machinetool dealer in Cape Town, but some monkey did a repair job on it, doing some real nasty stuff like replacing hardened taper pins with mildsteel round bar, leaving out bolts and seals and reassembling with cracked castings etc.
What i'm saying is that this is a 20 year old machine that has been compromised. I want to make it work so that it can start paying for itself again.
Your guidance would be much apreciated.
Pieter

[NC Cams]

A complete discussion of all the why's and wherefore's of bearing preload is far too complex for a venue such as this. For this reason, I can only refer you to a general explanation as found below:

http://www.bardenbearings.co.uk/index.cfm?articleid=717

The three most common methods of preloading are described in the illustrative PDF's. They can be downloaded via fig 1, 2 & 3 from the Barden website.

I suspect your machine uses preloaded bearings in some positions (IE: ball screw support bearings) but they may use position preloaded bearings in the spindle if it is so equipped - they might also use spring preloads, especially if the spindle is a high speed one or one that has a lot of thermal growth potential.

THe "offset" that I"m referring to is literally ground into the face of the bearing ring - usually by the bearing MFR. at the time of production.

Essentially, the brg mfr takes into consideration the external loads and the thrust capacity of the bearing and computes a reasonable preload for the bearing. When the bearing is assembled, the "offset" is taken up and the design preload is applied as the rings are AXIALLY clamped together at assembly.

The preloading is TOTALLY different and a separate entity than the eccentricity high point markings.

This "preload reasonableness " takes into consideration internal friction/loading, durability and the external loads and the stiffness that is needed for the bearing to support the applied loads without deforming - this deformation is what can cause unplanned for tool deflection or chatter. Ball screws are especially critical in their need for preload as they are typically long (thus can stretch) and position accuracy necessitates that the support bearings be VERY rigid - thus they are typically highly preloaded.

Sometimes, there are bearing rework facilities that can do this type of preload work and or rework but they are few and far between. The only one I have worked with is KAF Mfg. in Stamford CT in the USA.

If the bearings are assembled and lubed properly, they will give hours and hours of essentialy trouble free service. If the bearings are not installed properly & the loadings are misapplied, the bearings can be damaged irrepairably merely by tightening them into place.

Although bearing suppliers will provide info on the generalities of preloading, they do NOT provide it as "do it yourself" information - the subject is simply beyond the realm of DIY as you have to take a lot of factors into considertation that most people a not equipped to do - especially swing wrench monkies who fubar machine tool "repairs" as you can attest to.

I catch a lot of flack sometimes about my 'high end' attitude toward bearing applications work. However, it is situations like this is what I try to prevent. Namely, to prevent some IDIOT who shouldn't be allowed near a machine to FUBAR it by doing things in a half-fast totally wrong "this ain't rocket science" fashion. What a waste of human DNA.

Sadly, you are the victim of miscreant DNA. WIth luck, perseverence and research, you may be able to minimize your losses.

HOWEVER, let there be no mistake about this, unless you can fix the damage and return a reasonable level of accuracy to the bearing fits and preloads, the machine will never run to even a fraction of its original design intent and potential. Sadly, in some cases, the only fix you may have open is to buy new bearings and, if they are ball screw support bearings, these tend to be VERY, VERY, VERY expensive.