[devincox]

I am staring on the x axis on my IH Mill and have been looking at designs for the bearing blocks and have some questions with regard to the bearing block's construction.
I have seen 3 different designs with regard to placement of the AC Bearings in the bearing block (driven side of the ball screw)
1. a shoulder built into the bearing block to separate the back to back AC bearings
2. a spacer/washer placed in between the AC bearings to separate the back to back AC Bearings
3. nothing between the AC bearings

I am curious as to which design is better? Or is it 6 of one half dozen of another?


Thanks,
Devin

[SCzEngrgGroup]

Any of them will work, the difference is in ease of design/fabrication, and adjustment. With #2, those "washers" are shims which are used to set the bearing pre-load, so when you install the shaft, you can just tighten up the nut and get to work. With the other two designs, the nut torque determines the bearing preload, so assembly is more critical if you want to avoid backlash (if the nut is too loose) or rapid bearing failure (if the nut is too tight). Personally, I'd go with #2. Bearing shims are available from McMaster-Carr.

Regards,
Ray L.

[davo727]

If you buy a matched pair of preloaded ac bearings they are ground with the offset built in so you just put them face to face and torque the shaft nut and when the center races are clamped together you are all set with correct preload and no slop.

[Mike Nash]

#1 I figured to be pretty difficult to get accurate. That inner step thickness needs to be very uniform.

#3 is only going to work with matched bearings, you won't have any preload otherwise because the inner and outer races are the same width.

I made two spacers on the lathe for my X and Y. It dawned on me when doing the Z axis that the outer race of a cheap standard bearing of the same OD as the angular contact bearings should be a very uniform spacer, so that's what I'm doing. A bit thick maybe, but cheap.

Is anyone else finding that the time to make all of these parts without a CNC machines is very time consuming?

[devincox]

I already have the bearings and they are not a matched pair, so I am going with #2.
Now a few questions:
Which shims do I get? How thick? My bearings are 15mm x 35mm x 11mm.
What torque do I set the nut to?
I am not sure exactly how to know when the proper preload has been achieved.

Thanks,
Devin

[BobWarfield]

I already have the bearings and they are not a matched pair, so I am going with #2.
Now a few questions:
Which shims do I get? How thick? My bearings are 15mm x 35mm x 11mm.
What torque do I set the nut to?
I am not sure exactly how to know when the proper preload has been achieved.

Thanks,
Devin

Knowing when the proper preload is achieved is a real trick as far as I have seen. NC Cams and other bearing experts seem to indicate there is no easy way to do this and you should send the bearings off to a specialist to grind them so they become a matched pair.

It occurs to me that one could build a test cell that would enable you to set a recommended preload and then measure the displacement caused by that preload and thereby determine the required spacer thickness.

I have some notes on how to create such a test fixture on my web site here:

http://www.cnccookbook.com/CCMillBeltDrive.html

I haven't tried it. I'll be interested to see if someone else has a better idea, or whether the suggestion is to just take a wild guess on the spacers and get things approximately right.

Cheers,

BW

[philbur]

I think the technically correct method for angular contact bearings for precision applications is to buy a matched pair.

In my limited experience I have never come across preload by shims for angular contact bearings, I have always assumed that the difference between no preload and to much preload on small angular contact bearings is a few tenths, making shims impractical. This of course may be a wrong assumption.

I think the generally accepted "not correct" but "workable" solution for things like ballscrew thrust bearings is to use a short outer spacer together with a jam nut or some other nut locking arrangement. Personally I prefer a single nut with a radial setscrew locking arrangement, fine adjustment is a bit easier.

Phil

Knowing when the proper preload is achieved is a real trick as far as I have seen. NC Cams and other bearing experts seem to indicate there is no easy way to do this and you should send the bearings off to a specialist to grind them so they become a matched pair.

It occurs to me that one could build a test cell that would enable you to set a recommended preload and then measure the displacement caused by that preload and thereby determine the required spacer thickness.

I have some notes on how to create such a test fixture on my web site here:

http://www.cnccookbook.com/CCMillBeltDrive.html

I haven't tried it. I'll be interested to see if someone else has a better idea, or whether the suggestion is to just take a wild guess on the spacers and get things approximately right.

Cheers,

BW

[BobWarfield]

A preloaded pair is definitely best, but given that option was already ruled out, shims are what's left.

Interestingly, Slocum's Precision Machine book mentions a method similar to my test cell method:

http://tinyurl.com/6pd7uu

I got that off a quick search. He does refer to shims as less desireable but doesn't rule them out.

I think it's a case of, "Relax, it's a hobby not an aerospace/nuclear reactor quality production tool."

One other consideration is that every account I've read says that trying to figure out the torque on a nut as a way of setting preload just doesn't work. Be careful not to over-torque or over-space too or you may damage the bearings, though that seems less likely in this application than a spindle.

Cheers,

BW

[Cruiser]

I have found that the biggest deal with finding or setting a proper pre-load with a mechanical system would be heat and what happens to the components of that system. If this is a cold system which would only be affected by ambient conditions then taking the bearings to zero and then adding .002" pre-load would be considered safe for the bearings. something like a spindle which gets hot and lengthens in use would require more pre-load as when it gets hot the bearings get sloppy and the pre-load helps to compensate. you may find the opposite to be true if it is the housing of the mechanism which expands and causes the bearings to get tighter ! There is a lot of math and rates of expansion of different matr's per their sizes, or mass etc to calculate anything which would be usable, of which very few people have the knowledge or information to work with. In short, your designing so you decide using your best guess then test the system for a period of time and be ready to redo it if necessary ! My guess is that it will not take much to keep the bearings doing the job you expect. And, I have found that bearings are somewhat tolerant of mistakes more so in a low energy system, but as the energy within the system rises so does the risk of destruction rises. This information is borrowed from the Mickey Mouse school of shade tree engineering written by professor Donald himself !

[philbur]

Yes but he seems to recommend against it. Quote:

"Using shims is a sure way to create difficulty in getting the preload just right so that there is no variation in tare torque. The reason is that a difference in preload caused by tens of microns can be too much. Controlling shim thickness to this level can be extremely difficult. ….. If a manufacturer tries to sell you leadscrew support bearings that require you to make measurements, determine the required shim thickness, and then use shims, tell them that you do not want their bearings."

The Tormach uses a pair of angular contact bearings with jam nuts on each leadscrew. It seems to work OK, but as you say it is a bit hit and miss to adjust properly. I guess for amateur use a bit to much preload on the leadscrew support doesn't show-up as a problem until you have a couple of thousand + hours on the machine, which maybe never. So probably shims or jam nuts are both a bit borderline but we get away with it, providing it is done with a bit of care.

Phil

PS: Interesting reference, I just ordered the book, an early Christmas present to myself.

Interestingly, Slocum's Precision Machine book mentions a method similar to my test cell method:

[philbur]

Strange, I can't find that reference anywhere on Amason.com.

You are probably right, but it is nice to understand what's going on. However I think that 0.002" on small angular contact bearings is probably going to kill them stone dead.

Back to back match bearings are good for applications where heat/expansion may be a problem as the dimension exposed to the differential expansion is only one bearing width.

Phil

This information is borrowed from the Mickey Mouse school of shade tree engineering written by professor Donald himself !

[devincox]

Thanks for the excellent information and resources. I think the best way to proceed, for me at least, is to use the bearings I have now (unmatched) and shim. This is just a hobby for me and I would be happy just to get .001 with the machine. I will however, save up for the matched bearings.
How strange would it be to ask for bearings for Christmas? (c:

Devin

[devincox]

another question:
What is the fit between the AC bearings and the bearing block itself? Interference, press..??

[Cruiser]

.002" too much ? Then just go to what you think is zero ! When i put those buggers in my spindle, I had to info to go by other than experience with other mechanisms, such as the tail stock on a machine i ran. Seems i did that thing three times in 25 yrs ! On my spindle i set it up initially to zero by feel and then tightened the lock to the first position. This proved to be too little ! I ended up having to go two more positions on the lock ring. I don't remember the thread pitch, but let say 12 pitch. Three positions on the lock ring would be from 1/3'd to 1/4 turn therebouts. So, 12 pitch is .0833" lead and 1/3'd of that is .0277" of preload ! Before i reached this load the spindle would loosen up and get a bit wobbley. Now it runs quiet, cool, and stays rigid ! I has been going on for many hours now, and yes i have spare bearings to redo it if necessary. So when .002" is applied to the mechanism, do you think it all goes into the bearing ? The answer is NO it does not. It goes into the whole of the mechanism between the points of containment ! or compression ! The purpose of the test cell or mockup of the mechanism or machine is to see how much the mechanism obsorbs the applied tensions, and how much heat changes the whole of the set-up. What i'm impying is that your making rocket science out of basic mechanics ! Your drive will not need much but will obsorb much more than you give it credit for. The aluminum will compress, the core of the ball screw is quite soft, and after machining to size will stretch under a load. It is the outer workhardened area which is toghened up on the ball screw, and you will see when you have to grab it later, or cut it later ! Those bearings are not woosies ! test your system and see what ya come up with. after all, how good is your ability to measure the amount of differances your worried about ? can you measure to the millionths in your shop ? Do you have a grinder capable of microns ! I think not, so your going to have to keep it basic ! The people who wrote the books on these things are writing to cover everything within the fisics the science and the mechanics to keep from being sued ! later on. As stated, I put together many differant things which used some very expensive and complicated bearing arrangements and did so by the book ! They generally took more applied preload than I would have guessed at otherwise. And I recall also, the assemby of used bearings in some of these systems required a reduced preload than that of new bearings ! This was to end up with an equal runnability ! What your setting up is NOT that critical ! All you want to do is reduce friction, and illiminate backlash ! Your not trying to control tonnage when touching down to a runway or keep the energy of 60,000 rpm from desintegrating everything and killing people ! Don't over complicate what your trying to achieve or over spend in trying to accomplish it ! Thats all i'm saying here. You can read the books, Hell, I read some of the books and took the test afterwards, and I keep coming back to basics because thats the way it is here in the real world !

[philbur]

Sorry Cruiser but there are so many things to discuss here. For example: Elasticity and hardness of a material are two different properties. All steels have the same degree of elasticity regardless of how hard or soft they are. Reference: Machinery's Handbook - 27th Edition - page 474 -Table 13 - column "modulus of elasticity"

Anyway, as long as you are happy why let a few facts get in the way of a good story.

Phil

.....the core of the ball screw is quite soft, and after machining to size will stretch under a load. It is the outer workhardened area which is toghened up on the ball screw

[Cruiser]

I was not comparing hardness to elasticity, they are only mildly related, and should we include toughness, density, alloy, temper, etc to your direction of logic ? We could get into the alloy & carbon content of the screw including the degree of workhardness left from the rolling process or the alloy & silicate content of the aluminum along with the sections or sectional density's of the specific block design of which we do not have and can only theorize and speculate upon. Besides that you only picked one item to flame me with and refered to several, its a cold day here so warm me up a little !

[Cruiser]

I'm back Phil, I have read the definition of "modulus of elasticity,E," and (also called young's modulus) "is the ratio of unit stress to unit strain within the proportional limit of a material in tension or compression" ! NO where is there a statement of "All steels have the same degree of elasticity regardless of how hard or soft they are." And If this was the case then we wouldn't need to worry at all about the application of say grade 3 bolts v/s say a grade 8 bolt ! If they indead were equal in their degree of elasticity ! OH wait we left out their shearing stress qualities should we put that in here too ? maybe we should include their resistance to electrolosis as well, since we are discussing a machine running in an ambient wet environment of coolants of which build up salt's ! ooh, now we must define and limit the term salt too. Actually, I'd prefer to stop it here and wait to see what this happless gentleman comes up with for his bearing block arrangement. Not to assume he will even want to tell us !

[philbur]

Hi Cruiser. You need to read up (and understand) the difference between tensile strength of a material and it's modulus of elasticity. Two steel bolts of a different grade will have different tensile strengths but the same modulus of elasticity. You will see this if you look in Machnery's handbook at the reference I gave you.

Tensile stength is about the applied load at failure. Modulus of elasticity is about how much the material stretches (within it's elastic limit) for a given load.

Phil

And If this was the case then we wouldn't need to worry at all about the application of say grade 3 bolts v/s say a grade 8 bolt !

[Cruiser]

Phil, NO I don't need to read up on modulus of elasticity, I don't need it don't use it. all i need to know is it will stretch and effect the system. You Read all you want, My object is to get some work done ! You are the one who brought up modulus, not me ! I was saying that after the ball screw is peeled and turned down to the shaft size needed for his mechanism that it can and will stretch under stress and heat and effect to some degree what he is trying to accomplish ! AS WILL EVERYTHING in the confines of the unit. OK, it feels warmer now !

[devincox]

If the arguing is over, I will repost another question:

What is the fit between the AC bearings and the bearing block itself? Interference, press..??