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  • #16
    It's true that "conrad" or radial ball bearings have a thrust capacity roughly double their radial capacity and they can be preloaded in pairs to eliminate end float as is done with a wavey washer spring in electric motors. Doing so results in a situation where the ball path describes a cone angle but one too shallow for the precision axial restraint needed in a machine tool spindle.

    One can calculate this cone angle from the ball diameter and half the assembled axial clearance. I took a rough estimate from a 207 radial ball bearing out my collection and got a 1.3 degree cone angle. The cone angle reccommended for Bridgeport angular contact spindle bearings is 15 degrees.

    However axial positioning of a preloaded radial pair is poor in comparison to that of an angular contact pair having otherwise like characteristics. Plot axial deflection per unit of load of the radial pair Vs the angular contact pair and the axial deflection of the one will be several times that of the other. A preloaded radial pair will be downright "spongy" compared to an angular contact pair.

    The spindle bearings commonly found in turret mills built on the Bridgeport pattern are indeed duplex pairs arreanged back to back with a pair of spacers separateing them by an inch or so to enhance rigidity.

    The terms "angular contact" "duplex" and "DB" or "DU" should have loomed large in the discourse between JCD and his bearing supplier. Unfortunately it did not - or - did it? We'll know for sure after JCD reads this and looks futher.

    [This message has been edited by Forrest Addy (edited 02-02-2006).]

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    • #17
      JCD,
      I dont have your email anymore. Send me an email, I might be able to help you with the bearing problem. You can also call me at 360-914-6026.
      Mark
      Mark Hockett

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      • #18
        JCD,

        I am guessing anything further is going to muddy the waters???

        I am not sure if you have the correct angular contact bearings, nor if you have installed them in the correct configuration (ie back-to-back).

        However, there is one thing I am not sure you are clear about:

        The pre-load on these spindles is not "adjustable".
        It is not a Bridgeport secret, because you don't need to know it (unless you have lost the factory spacers, even then I am not sure if that matters**).
        Edit note - it is not a BP secret because the pre-load is built into the bearings. There is no secret to give away...

        When you tighten the nut on the spindle, the bearings and spacers pull up and the preload is automatically set. It doesn't matter what you do, you can't alter this preload.

        To change the preload, you would have to remove or add material to one of the spacers which goes between the paired bearings. Edit note: Do not even think of doing this!

        Question for bearing gurus:
        ** Out of interest, I am not sure how the pre-load is provided - are the spacers identical length (ie ground together) with the preload set by a difference in height in the bearing races - or are the spacers different lengths?

        Edit note - I think I just figured out the answer to my own question. The preload must be in the bearings themselves, because they are sometimes (often?) used in back-to-back pairs without any spacers......

        [This message has been edited by Peter S (edited 02-02-2006).]

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        • #19
          <font face="Verdana, Arial" size="2">Originally posted by Peter S:
          JCD,


          The pre-load on these spindles is not "adjustable".
          It is not a Bridgeport secret, because you don't need to know it (unless you have lost the factory spacers, even then I am not sure if that matters**).

          When you tighten the nut on the spindle, the bearings and spacers pull up and the preload is automatically set. It doesn't matter what you do, you can't alter this preload.

          To change the preload, you would have to remove or add material to one of the spacers which goes between the paired bearings.

          Question for bearing gurus:
          ** Out of interest, I am not sure how the pre-load is provided - are the spacers identical length (ie ground together) with the preload set by a difference in height in the bearing races - or are the spacers different lengths?
          </font>
          That is exactly how it works,you can tighten that nut down until the threads give and not change a thing.That's why when those bearings wear you throw them away and replace them.

          All the b-port spindles I have worked on have had both spacers exactly the same length.

          You can remove material from either sapcer,but that usually isn't required.

          I just need one more tool,just one!

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          • #20
            Darn it, pressed the wrong button...

            [This message has been edited by Peter S (edited 02-02-2006).]

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            • #21
              quote"You can remove material from either spacer,but that usually isn't required."


              what if you could not afford a set of bearings ..would removing material off the spacers take up the wear in the bearings and allow perhaps a few more months/years out of them tired bearings.

              all the best.mark

              [This message has been edited by aboard_epsilon (edited 02-02-2006).]

              [This message has been edited by aboard_epsilon (edited 02-02-2006).]

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              • #22
                <font face="Verdana, Arial" size="2">Originally posted by aboard_epsilon:
                quote"You can remove material from either spacer,but that usually isn't required."


                what if you could not afford a set of bearings ..would removing material off the spacers take up the wear in the bearings and allow perhaps a few more months/years out of them tired bearings.

                all the best.mark

                [This message has been edited by aboard_epsilon (edited 02-02-2006).]

                [This message has been edited by aboard_epsilon (edited 02-02-2006).]
                </font>
                Sure,why not,got nothing to lose.The spindle in question you would remove material from the inner spacer.Not very much thou,we're only talking .0001" or less.Lapping would be the method I would use.

                I just need one more tool,just one!

                Comment


                • #23
                  I don’t know about the rest of the world but, my Bridgeport J head spindle calls for Bridgeport part number (code number) 11190238. I talked with a Bridgeport service technician yesterday, he stated that, from the drawing of the spindle, the lower two bearings are: JM207K-PRB-DB/A/2729. I talked with a customer service representative from Bridgeport today; he confirmed the number of the bearings. The bearings I removed from my spindle are marked MRC 207 S D1 ABEC X. According the information published by MRC the bearings are Special 8 15/32 Dia. ball ABEC 7 with MRC exc. race modifications. According to information published by the bearing manufacturers, the MM207K bearings I installed in the spindle, as replacements are the same.

                  From the numbers above, all of the bearings are CONRAD type bearings. They look like Conrad bearings. When I measure the outer race width with a dial caliper, it is symmetrical on both sides, i.e. the same on both the front and the back of the bearing. When I measure the width of the inter races, it is also the same on both sides of the bearing. I am not measuring the thickness of the races, which is the same as the thickness of the bearing. The width of the race I am measuring is developed by the differences between the outside diameter and the inside diameters of each race.

                  By reason of the above, I am assuming the correct bearings for my spindle are Conrad bearings.

                  After talking with engineers employed by two different bearing manufacturers that have, or do, supply the spindle bearings to Bridgeport for this application, I find that one of the special features of the bearings used in the application is pre set preload. This is accomplished during the manufacturing process by adjusting the relative positioning of the ball grooves to the face of the bearing races so that when the bearings are “at restâ€‌, or not installed, there is an offset between the faces of the races. Then the bearings are installed between the correct spacers in the machine the preload is established. In my spindle the correct spacers between the outer and inner racer are of the same length thaw will establish the correct preload for my spindle.

                  Now, as I did not purchase the “special bearingsâ€‌ from Bridgeport, and have made an error, and purchased $380.00 dollars worth of Timken MM207K “standardâ€‌ ABEC 9 bearings, ABEC 9 was substituted for ABEC 7 at the same price because of availability, from the local bearing supply house, I am still faced with the task of adjusting the preload of these bearings to the proper specifications so that I will obtain the designed spindle performance. As these are “standard bearingsâ€‌ no preload is “built inâ€‌ to the pair.

                  If you can give me some indication as to what preload may give a 20 to 30 degree temperature rise in this application, as noted above. I would sure be grateful. Or even someone could help me understand what a significant amount of preload may be for the application. I just don’t know if .001â€‌ is significant or if .005â€‌ or .0001â€‌ is significant. From the bearing manufacturers as I stated above there specifications are given in pounds of force. I understand why. But that doesn’t solve my problem either.

                  HELP!

                  P.S. I am assuming I have a preload problem, because I have .005â€‌ vertical movement, .0015â€‌ horizontal movement, .0006â€‌ eccentricity, and when spindle is loaded sideways, by hand, it rattles, the spindle chatters when machining, and when I measure the clearance between the outer bearing spacer and the outer bearing races, in the assembles spindle, a .008â€‌ gap is present.

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                  • #24
                    Just had a think about this.
                    tread carefully .........once you grind them spacers , they will be useless if you ever have to go back to the genuin type bearings.
                    Think when you send the spindle to bridgeport or a reconitioner, they re-use your spacers.

                    all the best.mark

                    Comment


                    • #25
                      <font face="Verdana, Arial" size="2">Originally posted by Peter S:
                      Question for bearing gurus:
                      ** Out of interest, I am not sure how the pre-load is provided - are the spacers identical length (ie ground together) with the preload set by a difference in height in the bearing races - or are the spacers different lengths?
                      </font>
                      The usual way to do it is to buy a duplex pair and clamp them together in a suitable housing. The preload will then be whatever was ordered, if the bearing manufacturer did his job right. Most commonly the outer races are clamped together, and the inner races will then press against each other, take up the play, and provide the designed preload.

                      However you get the same effect if both raceways are separated by spacers of identical length. The outer races get clamped down, and the inner races are then squeezed by exactly the same amount that they would be if they were in direct contact.

                      The tolerances involved are very tight. Think of it as a spring preload with a very stiff spring - the elastic constant of the steel. So standard practice is to buy pairs of bearings from a competent manufacturer, and grind the spacers together in one setup so they come out the same length.

                      Note that spacer separation is not something done casually - it is an integral part of the spindle design. Spindles made with duplex bearings may have a duplex pair at one end and a single bearing at the other, or may have duplex pairs at each end, or may have a single pair with long spacers positioning the halves of the pair at opposite ends of the spindle. Those are the simple arrangements, I've seen others too. As usual in design, there is no "best", the choice is a balance of tradeoffs in a specific application.



                      [This message has been edited by sauer38h (edited 02-03-2006).]

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                      • #26
                        Take the thing apart and check the spacers with a tenths reading mic,they should be both THE SAME LENGTH.Do this before going any further.If one or both spacers is spun,has a ding or is just dirty you will not get the proper preload no matter what you use for bearings.

                        After doing that if you still have up and down motion you will have to measure than and correct the grind on the spacers.The easiest way to accurately measure the endplay is to bring the spindle nose into contact with the back jaw of your vise and clamp the nose in it.Position a dial indicator to read off the end of the quill housing and use the quill handle to move the quill up and down.The reading you get from this is what must be removed from the inner spacer ONLY.Once that amount is removed,reassemble the quill and make a test cut,at that point your problems should be over.The spindle should have a slight drag,this will disappear once the machine is up to operating temp because the spindle WILL expand in length as it reaches temp.

                        The temps should never climb above 150-160*f,if this occurs you must remove(by lapping)material from the outer spacer only.

                        Am I to assume that you paid $380 for off the shelf bearings when the exact replacements are availible here for $303.00,click on part #52.

                        http://www.alternativemachinetool.co...9290f9e08e554b
                        I just need one more tool,just one!

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                        • #27
                          On second thought,since your bearings are"standard"that should indicate standard flush ground races,which would provide the minimum contact angle of 15*.Since the greater the contact angle equals the greater radial load capacity it is possible that the factory bearings were custom grinds.If your old bearings were factory spec grinds,then they won't be flush ground.You may be able to measure any amount of offset grind by simply using a mic to measure the width of the inner and outer races on the old bearings.That dimension would yield the correct preload setting for the new bearings.Basically it would tell you how much difference you would need to machine into the spacer set.I'm betting the inner races of those bearings(old ones) are narrower than the outer races.If thats the case your inner spacer will have to be ground shorter by twice that amount to provide the preload you need for the flush ground bearings you have.
                          I just need one more tool,just one!

                          Comment


                          • #28
                            <font face="Verdana, Arial" size="2">Originally posted by wierdscience:
                            Since the greater the contact angle equals the greater radial load capacity </font>
                            Huh? Where are you measuring contact angle from? Zero contact angle means the balls are running in the center of the raceways, and radial capacity is maximized. They can't take any axial load and stay in that configuration, of course.


                            [This message has been edited by sauer38h (edited 02-04-2006).]

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                            • #29
                              JCD; right now you have what you are calling "conrad" bearings, that is 207 (plus modifiers) ABEC Grade 9 measuring 1.3780 ID 2.8346 OD, and 0.6693 width.

                              Did you look at the link on angular contact bearings I posted? Have you compared the thickness (Not the width which should be somewhat less than 0.669) of the outer race on one side to that of the other? The back of the race ahould be probaably 0.220 thick measuring radially and the front about 0.090 or there abouts. If your bearings have equal thickness then they are radial bearings not angular contact.

                              If this is true, I regret very much to say you have been sold the wrong bearings and there is no way to use them in your Bridgeport spindle and have nromal function. There is no way to preload them to get the axial stiffness you need for the spindle to function properly.

                              To silence the inevitable comment, yes the replacement "conrad" bearings can be "preloaded" but because of the very low contact angle attainable they will not have adequate axial stiffness for machine tool service.

                              Do NOT alter the width of the existing spacers regardless of conflicting advice you may receive. Get the correct bearings ABEC 207 angular contact Grade 7 or superior ground DU for low or medium preload and install them using the unmodified spacers.

                              I am eMailing you my home phone number.

                              Possibly you can salvage the situation with a minuimum out of pocket. I hope you kept notes and the replacement bearing packaging. From them, work up a chronology of telephone and other contacts with notes on what was said and the neame of the people you talked to. Contact the people who sold you your bearings and notify them they sold you the worng ones. Tell them you discovered the error only after you installed them.

                              Here you'll have to consult your chronology and determine your line of negotiation. If I understand correctly, You did some research with Bridgeoport's successors and gathered some basic information. Then you told the bearing supplier the original bearing numbers and more or less dumped the selection decision in their lap counting on their expertise to provide you with the correct bearings for your milling spindle. There are probably a hundred thousand Bridgeport mills and their clones in service. Replacement bearings is a routine inquiry for most any bearing supply housee and they usually have a spindle and precision bearing specialist who catches these problems.

                              I would think you chould look to your supplier for remedy ie: replacement of the incorrect bearngs for the correct ones at no charge to you.

                              [This message has been edited by Forrest Addy (edited 02-04-2006).]

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                              • #30
                                The obsession with whether or not they're conrad bearings seems unproductive. The point is that Bridgeport bearings should be replaced with whatever Bridgeport says should go in their spindle. Concerns with whether they're conrad, roller, angular contact, or anti-gravity bearings seems superfluous. If Bridgeport thinks they're the right bearings, then that should be good enough for us farmers.

                                Unfortunately, the rosy scenario of suppliers screwing up and selling the wrong bearings doesn't appear to describe the situation as sketched out in the first post. I gathered that it was a failed experiment in flying cheap (and no implied criticism there, I'm as big a fan of cheap as anybody - when it works). The local bearing shop took the trouble to contact Bridgeport and found the correct story. The customer bought other bearings, regardless. A followup post says, "According to information published by the bearing manufacturers, the MM207K bearings I installed in the spindle, as replacements are the same." I have already explained why the bearing manufacturer's claim that they are the same is valueless. Only Bridgeport's claim is relevant, and Bridgeport says they're proprietary. That means they can't be bought off-the-shelf from the bearing manufacturer.

                                I agree that messing with the spacers is not a great idea in this case. All that will do is add the price of replacement spacers to the cost of the project.

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