[Rube]

I'm doing an X3 conversion and have a question on angular contact bearings. I got the bearing pictured below (INA 3200 series). this is a double row angular contact that is is to absorb axial loads in BOTH directions. Now, I must be having a bad day, because I'm confused as to how to pre-load this. If I was using two single row units back to back, I could see pre-loading it by squeezing the two inner races (would need a small space between them) then capture the outer race in the bore. In fact some of the larger double row units I've seen have two separate inner races. So does that mean these do not need to be pre-loaded? I've searched the manufacturers web site for tech info, but couldn't find it. These were "cheap" at $27 each, and I got them thinking I would only need one on one end of the screw. But I think I'd be better off with two single row units, one on each end of the screw, so I could completely remove any slop in the bearings.

Rubes

so thats how I end up with so much stuff...always doing things twice!!!

[HuFlungDung]

I believe you don't add any external preload to these. Whatever axial play these have is what you've got to settle for.

[Rube]

Yeah...thats what I thought (I should learn how to think BEFORE opening my wallet...ha...ha).
I think I'm gonna look for some single units with a steeper angle than these 25* also. Since in this application most of the force will be axial.

[NC Cams]

Double row bearings (pictured in post 1) can NOT, NOT, NOT be preloaded.

IF you want/need preload, you'll need the two single row bearing idea that you already envisioned. With two single bearings, you can shim the outer or inner races or some such similar method to preload as needed.

[rexstep]

Double row angular contact bearings DO come preloaded but I don't know if they come non-preloaded. I have some and they are certainly preloaded.

You can easily tell: try to axially move the inner race to and fro while holding to outer race. If the inner race will wiggle, it's non-preloaded. If not, you're good.

BTW, you can get preloaded ones at www.vxb.com

Rick L.

[Rube]

Thanx Rick...these certainly feel tight now, but for the price I paid for them they are most likely not precision pieces. Besides, they are only 15* contact angle, and in this application most of the force is axial, so I got some single row 40* pieces from VBX last week. They too were extremely inexpensive, but at least this way, I can readjust later if I have too. I'm sure those first ones I got would be fine, but I just like the added adjust ability of the two singles.

[NC Cams]

Having worked in the ball bearing industry for a number of years, I have to take exception to the contention that double row A/C's like that pictured are "preloaded".

The industry typically makes these bearings for taking a combination of axial and radial thrust. As precise as the industry is, they have NOT found/figured out a way to "preload" a double row bearing. THis is why they are sold with clearance codes (IE "normal", CE, C2, C3, C4 etc) as these are radial clearance specs - if you have radial clearance in a bearing, you pretty much HAVE to have axial clearance as well.

When I say preload, I mean to literally run NEGATIVE axial clearance. The reason why you preload a bearing has to do with the load/deflection that takes place until you FIRST remove all the axial clearance that SECOND tke out the initial "spring" or compliance that occurs as load rises. This is readily discernable by making PRECISE load/deflection studies of the bearings.

"Feeling" or 'shaking' the bearing is an irrelevant inspection method. You can have no perceptible play in a bearing but the rings will axially deflect quite substantially when you load them. Recall that bearing clearance/deflection is in terms of MICRONS (0.001 mm). What "feels" like no clearance can actually be a lot. For this reason, preloading is NOT an inconsequential force.

This can be as low as 1%-2% of the radial load and go as high as severl HUNDRED pounds negative preload. It is pretty much impossible to put such preload into a double row, especially when you know/see how they load and space the balls - it simply doesn't happen.

Unless something has changed drastically, I'd think twice about using a double row in place of a true and properly preloaded A/C or better yet a true ball screw bearing.

Before you take my or anyone else's word regarding double row bearings and preload, call any bearing maker's engineering dept (IE SKF, Koyo, NSK, etc) Ask them for a "preloaded double row" and see what they tell you about the availability of same.

In the mean time, caveat emptor.

[Rube]

NC, thanx for your input. As in anything else once you think through all this stuff, it becomes somewhat clearer. I thought I had seen some double row units (they were large ones though) that had a split inner race which I assumed was for applying pre-load.

In any case, I decided that two single row units are the answer for me in this application. I'm trying to get the best precision I can, and I dont want to have to wonder if ANY of it is coming from those bearings. An added benefit (although I already spent the money on both) is that two singles are cheaper than one double. Thanx again for your input.

[NC Cams]

A double row UNSPLIT outer ring with a set of SPLIT ROW inner rings could very well be preloaded - it depends what sort of "preload" is applied if/when the inner rings are shoved together until tight.

The bearing pictured in post 1 is definitely a traditional, non-preloaded double row with, typically a combination of radial and axial clearance.

[Robin Hewitt]

Axial race at either end and a die spring trying to stretch the ballscrew, that way you never have to compensate for wear. Things will move around a bit when everything goes horribly wrong but even so, might save overloading your nut

[project5k]

might save overloading your nut

insert crude joke here....

[rexstep]

Hi NC Cams,

I readily admit that I am not a bearing expert but I know that Universal Thread Grinding Company, makers of a very high-end preloaded leadscrew product, offer their assemblies with just such a preloaded double row assembly.

Moreover, I called VXB several months ago with this same question and was assured that their double row angulars were preloaded. To my understanding, no play detectable to the fingers indicates at least a light preload, but I remain open to learning.

Rick L.

[NC Cams]

Be carefulf of vernaculars.

The bearing pictured in post one is a classic "double row conrad bearing assembly". They are built with one piece inner and outer rings. To assembe the bearing (as all such bearings are assembled) they use the "conrad process" wherein they literally plastically deform the outer rings, drop in the balls, then undeform the ring and move the balls into even spaces and then pop in a cage.

Not until or unless they find a precision way to measure the raceways, calc a way to establish EXACTLY the ball size that will generate the fixed preload for the counteracting opposing forces in the raceways from the preload, could they come up with a "preloaded" bearing.

Now, it IS possible to put in oversized balls, Iit is possible to load them with C2 clearance which has a pretty "tight" feeling but this is technically NOT a preload.

Now, if the bearnigs ARE in fact two single rows of bearings mounted as a pair, these are technically NOT "double rows". Rather, they are duplexed - yes there are two, yes they are mounted in matched pairs, NO they are not double row bearings at least as the industry defines the.

Guys have "preloaded" or more appropriately TRIED to preload double rows by running real heavy press fits. This does make them feel tighter due to the reduction of RADIAL clearance. But it does not take care of removing the AXIAL clearance which is what the member is trying to remove.

I've seen many a "duplex" bearing incorectly identified as "double rows" for the reasons already cited - sadly, this is also done by people in the industry who should know better. When/ if you REALLY want to absorb high amounts of axial thrust with NO deflection potential, the best/most appropriate way is with a high contact angle, heavily preloaded, DUPLEXED pair of A/C bearings.

Duplexed bearings have two rows of bearings but they are not NOT NOT the same as a double row conrad bearing.

[Robin Hewitt]

they literally plastically deform the outer rings, drop in the balls, then undeform the ring and move the balls into even spaces and then pop in a cage.

Amazing, so if I removed the cages and pushed all the balls to one side I still couldn't fish one out?

Conrad must have been one heck of an engineer

[NC Cams]

Unless there is a "filling slot" or the shoulders of the rings are quite low (as in/on the NON thrust absorbing side of the A/C's) yes, you could NOT remove the cage(s) and simply remove the balls without first deforming the outer ring. Try it and see for yourself.

A/C's are not assembled that way.

They put the cages on the balls and then set the inner ring atop the outer and then heat the outer ring with induction or convection heat (hot plate). When the outer ring swells up adequately, the inner ring and balls and cage "drop" into place.

If you should, perhaps shove the inner ring apart by pushing in the wrong direction, you have a 99.99999% chance of hurting the balls and/or raceway by shoving the balls over the edge of the raceway. This typically irrepairably cuts the balls and/or deforms/brinnels the raceway.

And, yes, Conrad was a pretty clever engineer as every ball bearing made uses the "conrad method" (ring deformation) to construct bearings.

When you know how conrad style bearings are made (double row bearings like that pictured in post 1 are definitely conrads), it becomes intuitively obvious that axially preloading them is essentialy all but impossible to do properly or consistently.

For the application in question, buy a pair of properly sized A/C's that are already preloaded (the heavier the better), bolt them in so that the offsets are proper, tighten them down until the offset rings touch and go to town machining.

You can buy some unpreloaded bearings and shim them (PITA to do if you don't know what to do or how) or you can buy some A/C's and have them preloaded by a rework house. One such house is KAF MFG in Stamford CT.

They do exceptionally good work, I have NO affiliation to/with them and they are NOT necessarily cheap. The work is STELLAR - I have my bearing preload work done there and have NEVER been disappointed.

Send them your bearings, tell them how much preload you want, whether you want DF or DT or DB mount and pay the bill when it comes. They should be ready to go out of the box - simply wash carefully, do NOT air spin them, grease them and install them properly.

[aubrey]

Hi all
Why are "ball" bearings used in situations where the bearing has to take a not inconsiderable loading sideways?
Surely taper roller bearings are far more suited to the job.
OK, the design of the assembly would be a bit more complicated having to have an add a method of "adjusting" the play and seperate seals but apart from that it should be a far better solution.
Furthermore, roller bearings are able to take far greater loads in the direction we are putting them in a leadscrew situation and would be infinitely adjustable in the way that they were designed to handle.
I'm not sure that ball bearings were actually designed to be jacked together to reduce slop. Doing so is a short term thing and causes accelerated wear leading to more "slop".
Also, due to the larger contact area, rollers would last forever if no dirt got in.

Just a thought
Aubrey

[Rube]

...taper roller bearings are far more suited to the job...

Thats the stance I originally took myself. I made a very meager attempt at finding some small tapered roller bearings, but couldn't find anything small enough. Again, just a very cursory look though. I then decided that for this particular application that was overkill anyway when a set of $10/pr 40* A/C ball bearings were available. I also think that part of the reason would be that there is more power needed to turn a tapered roller?

NC...what is "matched" when two single row A/C bearings are installed in a duplex arrangement? For my Y axis in my X3 conversion I was going to use two of the 7200B's back to back (or is face to face better for any reason?) in a fixed /floating arrangement (the far side of the screw left hangin'). Is there a real need to "match" them in this application? Why would they not need to be matched if they were installed on opposite ends of the shaft (like I plan on doing with my X and Z)?

[NC Cams]

The reason you "match" the contact angle is so that you have the same thrust and/or radial capacity regardless of the direction of load/thrust. Check out the Fx and Fy loading factors for the bearings and you'll probably see a/the difference.

I guess you could run mismatched pairs - this is sometimes done in triplex or quads to gain radial or axial thrust capacity as needed. However, I can't say that I've seen any cases where an A and C contact angle were matche as a duplex set.

DB gives better resistance to overturning moments applied to/thru the bearing.

DF gives better allowance for misalignment that may exist to/thru the bearing.

Single fixed versus non-floating ends depends on the length of the screw and/or whether or not you have a lot of thermal growth potential to deal with. A long slender screw will stretch easier than a short stubby one so you''d tend to use one method over the other depending on the needs. It all gets taken into the mix when the machine/application is being engineered.

Tapered rollers do not have as low a rootating friction in thrust as ball bearings for a very simple reason - ball bearings absorb thrust via pure rolling whereas tapered rollers ultimately absorb thrust by the ends of the rollers rubbing/sliding against the flanges.

Simply because the the tapers have so much metal carrying load over long/wide rollers, they have a HUGE capacity for load. Just because a tapered roller has high radial capacity does not make it unilaterally better/best for any specific application, especially axial thrust in ball screws.

[Rube]

Thanx, I appreciate your input...I think I'm on my way

[aubrey]

Tapered rollers do not have as low a rootating friction in thrust as ball bearings for a very simple reason - ball bearings absorb thrust via pure rolling whereas tapered rollers ultimately absorb thrust by the ends of the rollers rubbing/sliding against the flanges.

Simply because the the tapers have so much metal carrying load over long/wide rollers, they have a HUGE capacity for load. Just because a tapered roller has high radial capacity does not make it unilaterally better/best for any specific application, especially axial thrust in ball screws.

Thanks for that. Makes good sense for the home hobby mill seeing as they are not pushed as hard as the commercial ones.
May be an alternative if you are for ever having to adjust for excessive backlash AND you can locate suitably sized units.

Go Well
Aubrey