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Thread: Mechanism with vertical shaft

  1. #1
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    Mechanism with vertical shaft

    Hi,

    First off, I am an artist, not an engineer! I'm building a spinning artwork...


    I have built a mechanism that has a vertical shaft with a bracket at the top. An artwork bolts to this bracket with a load of approx 5 kg. The artwork then spins at approx 120rpm.


    The pictured design (photo) works fine, except there is some play in the shaft / bearings that I would like to eradicate. Since building this version I have done a lot of reading and updated the design as shown in the diagram.


    Currently I have a clearance fit on the shaft/bearings. I understand that I can heat fit the bearings and this will result in zero play. However, I am confused about how the load is distributed. So here is my question...


    If the radial bearings are heat fitted to the shaft, doesn't this remove the load from the thrust bearing? My understanding is that I want the weight on the thrust bearing and to use the radial bearings for guidance.


    Any advice will be gratefully received !
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  2. #2
    My compliments to an artist trying to do engineering. Your work looks good.

    "some play in the shaft / bearings". Quantify that. How much play? Can you measure it with calipers?

    You must also quantify what is acceptable and what is not. There is no such thing as "no play". You might achieve "no visible play" but there is still some play. Engineers work with numbers. Give us some.

    Is this "play" due to imbalance? If so, the best solution is going to be in balancing.

    Are you sure this play is actually in the bearings? Are you sure its not in small deflections of the support structure itself? One reason I ask is that the support material appears to be 3D printed, which means plastic. Am I wrong?

    I seriously doubt that your situation is severe enough to require heat fitting the bearings. You only see that in high speed, high accuracy spindle assemblies. Not in a 5kg 120 rpm rotating framework.

    And yes, all thrust load should be carried by thrust bearings. Depending on their design features, radial bearings cannot generally tolerate significant thrust loads.

  3. #3
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    Hi, thanks for your response...

    1.) How much play?
    Currently I can wobble it with my finger. The shaft is smaller than the internal diameter of the radial bearings. I believe this is a clearance fit.

    2.) You must also quantify what is acceptable and what is not?
    I don't want any visible play.

    3.)
    Is this "play" due to imbalance?
    No, it's because the shaft is smaller than the internal diameter of the radial bearings

    4.)
    Are you sure this play is actually in the bearings?
    It's not in the bearings, it's because the shaft is currently smaller than the internal diameter of the radial bearings.

    5.) I seriously doubt that your situation is severe enough to require heat fitting the bearings.
    Maybe not, however, fitting a h6 shaft means there is no play (certainly nothing visible) and is relatively easy for the results.

    To expand upon my original post:

    I can machine a shaft that has no visible play (a transition fit?) But then the internal ring of the radial bearing is not connected to the shaft, so logically I am not making correct / full use of the bearing.


    Heat fitting the radial bearings to the shaft physically connects them, which leads me back to my question. If the radial bearings are connected to the shaft, how is the full load on the thrust bearing?

    I am using the following deep groove radial bearings:
    https://www.skf.com/sg/productinfo/p...6201-2RSH%2FC3


    The catalogue states, "They accommodate radial and axial loads in both directions..."


    Unless there is something I don't know (very likely!) it would appear my options are:


    1.) A transition fit where the internal ring of the bearing does not engage.


    2.) Heat fitting the bearings (see question re: load on thrust bearing)


    Thanks!

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    On the SKF webpage you linked to there's a "Download PDF" link that provided specifications.

    The tolerance specified is a "C3" which is a reference to the Internal bearing clearance of the bearing component.

    The tolerances for a C3 according to SKF is equivalent to a ISO Group 3 tolerance. Design the shaft and bearing retainer feature to a ISO Group 3 tolerance for the stated size.

    ISO has established five clearance classes for many bearing types. SKF uses designation suffixes to indicate when the bearing internal clearance differs from Normal.


    ISO clearance class SKF designation suffix Internal clearance
    - C1 Smaller than C2
    Group 2 C2 Smaller than Normal
    Group N - Normal
    Group 3 C3 Greater than Normal
    Group 4 C4 Greater than C3
    Group 5 C5 Greater than C4

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  5. #5
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    @Kelly_Bramble Thanks for your reply. I have read up on internal clearances, however, I don't understand how the internal clearance is relevant to my question(s)

    Perhaps a better way of phrasing what I am trying to achieve is:

    To spin a vertical shaft with as little play as I can realistically achieve should I ...

    1.) Machine a shaft with a transition fit as per the design in my first post.
    2.) Heat fit the radial bearings to a h6 shaft as per the design in my first post.
    3.) Redesign and if so, how?

    If (2) can you (anybody) explain how the load is on the thrust bearing when the shaft is connected to the radial bearings as well.

    Thanks!

  6. #6
    Try to get bearings with extended inner races. They have set screws that allow you to attach them directly to the shaft. No need for highly special tolerances.
    4768k16p1-d03c-digital@1x_637051245798149900.png

  7. #7
    There is a type of bearing that has an "extended inner race". These bearings include set screws that allow you to fix the inner race to the shaft. I would recommend those for your application. No need for the super high tolerances you're anticipating.

  8. #8
    Technical Fellow Kelly_Bramble's Avatar
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    Let me re-phrase your questions into something non-leading..


    This is what I think you question should be:
    “What bearings do I need and what are the mating part fits and tolerances?”


    The internal bearing tolerances provide guidance on the fit and tolerance requirements of the particular bearing. Application, loading and size define fit and tolerance requirements.


    For a C3 bearing or ISO Class 3 related installation tolerances are available in the ISO or ABEC standard (Internal tolerances relate to installation tolerances and fits).


    Do not heat roller bearings up and install – seriously.


    Thrust bearings normally come with wave washers or other shimming features and do have specific requirements for installation.


    CONSULT with the bearing manufacturer or related ISO/ABEC standard for installation design requirements (shaft and housing).
    See the following for General Bearing Installation fits and tolerances.


    Bearing Shaft and Housing Installation Tolerances Size and fit tolerances for bearing mating shafts and housing

    Bearing Engineering and Knowledge Application Menu


    Tolerances, Engineering Design & Limits & Fits
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  9. #9
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    I tend to agree with Joe_boggs. That will be your simplist solution. Further more, with a load of 5kg and a speed of 120 rpm I'm questioning why you need a radial load bearing and a thrust bearing at the bottom. A deep groove ball bearing or angular contact ball bearing can handle some combined thrust/radial load.

    If you really want to redo the clearance between the shaft and the inner race of the bottom bearing then take note of the fact that for the same outer diameter you get bearings with different internal diameters.

    A 6208 bearing will have a outer diameter of 80mm and inner diameter of 40mm.
    A 6307 bearing will have a outer diameter of 80mm and inner diameter of 35mm.

    So you if take that shaft and machine a shoulder onto the bottom section you can accomodate a bearing with a smaller bore. The shoulder can then mate with the inner race of the bearing, thus making your roller bearing take the thrust instead of the thrust bearing. This will give you the opportunity to machine the reduced section of the shaft to the correct clearance. For instance, a bearing with an inner diameter of 35mm will be a clearance fit onto a shaft with an outer diameter of 34.988mm assuming the h6 clearance mentioned above. The tolerances for even a "clearance fit" are incredibly tight.

    The grub screw obtion mentioned by Joe_boggs might really be your best option though.

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