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    Default Large bronze bearing clearance

    I am working on a large shaft and bearing assembly and I am having trouble finding a calculation to determine what clearance I should have between the leaded bronze bushing and the main shaft. My shaft diameter is 37.5" and the journal length is 29.5" long. This is a very low RPM application but high pressure. Can someone give me some guidance here? Most of what I can find is that rule of thumb is roughly .001 to 0015" cl per inch of diameter up to 5" diameter but after that, the math gets fuzzy. Any ideas?
    Thanks in advance

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    I worked on Babbitt bearings for years. We used to use the .0015” clearance rule up to a foot or so long. Only did a couple bearings much longer and still used the same figure. Never had any problems.


    Sent from my iPhone using Tapatalk

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    Thanks for the info. So a total clearance of .056" wont cause any problems for the grease?

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    Could use some more info, like:

    1) Is this a grease lube, pressure oil lube, or dry bearing?
    [Edit: that question answered]

    2) How good is angular alignment and can loads tip (angular shift) the shaft?

    3) Surface smoothness, cylindricity, and hardness of the shaft?

    4) Bearing alloy?

    5) Radial only, or axial loads too?

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    1) Is this a grease lube, pressure oil lube, or dry bearing? (Grease lubed application).
    [Edit: that question answered]

    2) How good is angular alignment and can loads tip (angular shift) the shaft? (Consistent angle alignment)

    3) Surface smoothness, cylindricity, and hardness of the shaft? (The shaft is still being repaired. Hoping for 64 or better finish)35Rc hardness.

    4) Bearing alloy? (938 leaded bronze)

    5) Radial only, or axial loads too? (no axial load. )

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    Quote Originally Posted by DIVEBUZZARDS View Post
    I am working on a large shaft and bearing assembly and I am having trouble finding a calculation to determine what clearance I should have between the leaded bronze bushing and the main shaft. My shaft diameter is 37.5" and the journal length is 29.5" long. This is a very low RPM application but high pressure. Can someone give me some guidance here? Most of what I can find is that rule of thumb is roughly .001 to 0015" cl per inch of diameter up to 5" diameter but after that, the math gets fuzzy. Any ideas?
    Thanks in advance
    I see what you mean... "Fuzzy" the "Math" easy to become :-)

    Journal Bearings - an overview | ScienceDirect Topics

    Just some basics ^^^

    Scaling and eccentricity is a factor especially for 37.5" diameter i.e. things are not linear and start to go "wonky".

    low rpm vs high pressure (non-hydrostatic condition) ?

    The US Navy is a good go to for that type of information but maybe with more sophisticated bearing designs and higher RPMs ?

    I'd be really impressed that any one could reduce that to a simple "metric"/ rule of thumb for a large diameter bearing with a short shaft length. There probably IS a chart out there somewhere that extends to the size and "shape" of the bearing you are dealing with that takes into account the non-linear nature when things are scaled up by that much.

    @DIVEBUZZARDS what is the bearing for ? If you don't mind me asking / if you are allowed to say ?

    [I have more experience with large rolling element bearing and low profile large diameter bearings … but need to get more into hydrostatic bearings + air spindles, so at some point I have to do a deep dive rather buy schtufff off the shelf.].

    I may scratch around over the next few days and see what I can dig up* in terms of a simple reduction of what is otherwise a complex calculus problem that may or may not fit very well to actual real world conditions + your actual application. (Assuming someone else doesn't beat me to it.).

    Does the sleeve/ journal / bushing / bearing have any features or grooves in it at all … ?

    Does it come apart in any special ways ?

    Is this like industrial heritage kind of machine or something that has been in service for the better part of a century ?

    Or something off a massive piece of mining or digging equipment etc. ?

    Ta.

    Eric
    __________________________________________________ _______________________________


    * I'd have to dig up some old Machinery's Handbook for specifics but I don't recall such large plain bearing being mapped out ?

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    This is totally a WAG, so bear(ing) that in mind -

    I'd shoot for a less than formula clearance, if RPM is truly low and heat is controlled by good lubrication maintenance. On the order of .035 - .040". My reasoning is to maintain as large an arc of contact at the load point, and help prevent "rolling up" the sides of the journal, and to give as much support to the "grease wedge" as I can.

    But again, wild ass guess. If Cameraman can come up with real data I'd go with it.

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    IME, Clearance should be determined by minimum lubrication film thickness. This is determined by the eccentricity of the shaft in the
    journal.

    Cameraman's link has a good outline of what that means under " 13-Bearings and seals". The book i have has some calculations to determine this as well as a million other thing i don't understand. To be clear, Ive never done any of these calculations myself, instead i passed them on to our engineer to verify when needed. Also, all of our bearings are either splash lubricated or pressure fed by an oil.

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    Quote Originally Posted by MotoX View Post
    IME, Clearance should be determined by minimum lubrication film thickness. This is determined by the eccentricity of the shaft in the
    journal.

    Cameraman's link has a good outline of what that means under " 13-Bearings and seals". The book i have has some calculations to determine this as well as a million other thing i don't understand. To be clear, Ive never done any of these calculations myself, instead i passed them on to our engineer to verify when needed. Also, all of our bearings are either splash lubricated or pressure fed by an oil.
    I'm digging around on and off (almost like a fools errand), and really seems that companies that specialize in this keep a LOT of that information as to in-house technique and knowhow for companies that specialize in large diameter bearings / babbitt / Kingsbury etc. etc. [No published clearances for more standard sizes. ]

    Seems that any assumption made can dramatically shorten the life of the bearing and maybe trash hardware that costs millions of dollars +++

    http://edge.rit.edu/edge/P14453/publ...l_Bearings.pdf

    (Through RIT Rochester Institute for Technology ).

    ^^^ This a really interesting study _ " Understanding Journal Bearings Malcolm E. Leader, P.E. Applied Machinery Dynamics Co. Durango, Colorado "

    At least you get a sense of the multidimensional nature and how things can go pear shaped but again smaller bearings and higher RPMS.

    The Do's and Don't s section is kinda interesting.

    Don't -->

    "4. Installing bearings too tight or too loose is a recipe for disaster. Never set clearances in a tilting pad bearing in the field - this must be done in a qualified shop. "



    + many others

    " Don’t ever disassemble a machine without measuring the bearing clearances “as-found”. This is the only chance you get to obtain this data which is an invaluable diagnostic tool."



    "Don’t use the low bid to buy or repair bearings. Reusing babbitt from old jobs or overflow is forbidden. Tiny contaminations can lead to early bearing failure.


    There are some good references that could be sourced through interlibrary loans (really old books):

    like...

    1. Machine Design, Part III by International Textbook Company, 1907
    2. Bearings and Their Lubrication by L. P. Alford, McGraw Hill, The American Machinist, 1911
    3. Lubrication of Industrial and Marine Machinery by C. L. Pope and W. T. Everitt, John Wiley and Sons, 1954, LC number 53-9023

    __________________________________________________ ___________________________________

    I used to subscribe to a channel that uploaded a ton of declassified films (US gov films/ archives) that had a lot of videos that show how folks in the Navy maintain and diagnose problems with Kingsbury/ Babbit type shaft bearings.

    Interestingly rolling element bearing by comparison like from KAYDON, are relatively transparent about everything + very good documentation.

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    Quote Originally Posted by Milland View Post
    This is totally a WAG, so bear(ing) that in mind -

    I'd shoot for a less than formula clearance, if RPM is truly low and heat is controlled by good lubrication maintenance. On the order of .035 - .040". My reasoning is to maintain as large an arc of contact at the load point, and help prevent "rolling up" the sides of the journal, and to give as much support to the "grease wedge" as I can.

    But again, wild ass guess. If Cameraman can come up with real data I'd go with it.
    That's extremely kind , in this case if I do manage to stumble on anything conclusive it would come with a massive ass covering disclaimer from me lol.

    The application IS really critical , is it from an 1830's Beam engine or a critical component in an ageing nuclear power station ? etc. ;-)

    Sometimes that does underscore the difference and styles of engineering between aerospace and heavy machinery and heavy transportation, occasionally I work with someone from the later, and it does throw me for a loop for a while how different everything is.

    __________________________________________________ ______________________________________


    1116_md_ht_pl_bearings_diamclearance-1-.jpg

    One graphic I stumbled across ^^^ but only goes to 8" and still a lot of stuff missing.

    Odd the original machine has no guidance ?

    + fitting to a shaft that has to come back from repair ?

    I am assuming some sort of asymptote is reached pretty quickly for maximum film thickness / lubricant (for larger diameter but the relative shortness of the shaft (relative to diameter + large diameter) is a major concern in terms of loading and pressure gradients and profiles along the short shaft (not just radially) ? .).

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    Type in the search phrase " boundary layer lubrication" in a DuckDuckGo search.

    The journal that you are working with is supported by boundary layer lubrication rather than hydrodynamic lubrication. The design rules for hydrodynamic bearings which involve shaft eccentricity, radial clearance, surface feet/minute, oil viscosity, diameter/ length ratio, and oil temperature do not apply. Boundary layer lubrication is what occurs in a greased pin -steel sleeve assembly such as in a backhoe linkage.

    A very brief introduction to the subject:

    http://www.me.utexas.edu/~bryant/cou...ubrication.pdf

    The design goal for this type of bearing is to keep the journal contact pressure as low as possible. The journal is supported by metal to metal contact with the bearing surface and by oil trapped in the contact surface roughness. The radial clearance between the journal and bearing should be kept small to maximize the contact surface area. There still needs to be some clearance to allow grease to be injected into the bearing gap at the lowest anticipated start up temperature. If the grease pump is unable to force lubricant into the bearing the clearance is too small.

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    I'm going to offer some comments but, just to be clear, I am not an expert on this subject. However, my intuition
    whispers in my ear that 50 thou, or even the 30 or 40 thou mentioned above is way too much clearance. The
    assumption is that once the shaft reaches a certain size you're not going to need more clearance for grease as
    the shaft gets bigger. A film of grease .010" to .015" should support and protect the shaft but I can't see a layer
    of .040" to .050" even existing--the weight of the shaft and its load will compress the grease layer to a thinner
    thickness in any case.

    If the shaft heats up enough to significantly change the diameter then allowance has to be made for that but I
    suspect the thickness of the layer of grease will remain constant once you get over a certain size...

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    You have to allow some room for the grease and account for the heat generated by the shear action within the grease. I'm no expert either, but I think it's usually better to be a little loose than a tiny bit too tight in these situations, as the worst thing is to get metal on metal contact and generate a "runaway" heating and seizure.

    It would be helpful to know what the original clearance spec was for this bearing, as it seems to be a repair of an existing mate, not a new design. That would be a good starting point for the new fit.

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    I would think the extra clearance would be for thermal expansion. Am I right in thinking even in normal circumstances that size shaft might grow .01 in a 50 degree temp change or is my math wrong?

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    Default Very complex problem.

    Erik

    Thanks for the feedback. This is a vintage 1930's forming roll for steel. The original bearing was wiped out so no numbers can be ascertained from the original parts. Rpm for this application is in the range of 1 to 2 RPM's. and this is for short cycles.

    Thanks
    Gary


    Quote Originally Posted by cameraman View Post
    I see what you mean... "Fuzzy" the "Math" easy to become :-)

    Journal Bearings - an overview | ScienceDirect Topics

    Just some basics ^^^

    Scaling and eccentricity is a factor especially for 37.5" diameter i.e. things are not linear and start to go "wonky".

    low rpm vs high pressure (non-hydrostatic condition) ?

    The US Navy is a good go to for that type of information but maybe with more sophisticated bearing designs and higher RPMs ?

    I'd be really impressed that any one could reduce that to a simple "metric"/ rule of thumb for a large diameter bearing with a short shaft length. There probably IS a chart out there somewhere that extends to the size and "shape" of the bearing you are dealing with that takes into account the non-linear nature when things are scaled up by that much.

    @DIVEBUZZARDS what is the bearing for ? If you don't mind me asking / if you are allowed to say ?

    [I have more experience with large rolling element bearing and low profile large diameter bearings … but need to get more into hydrostatic bearings + air spindles, so at some point I have to do a deep dive rather buy schtufff off the shelf.].

    I may scratch around over the next few days and see what I can dig up* in terms of a simple reduction of what is otherwise a complex calculus problem that may or may not fit very well to actual real world conditions + your actual application. (Assuming someone else doesn't beat me to it.).

    Does the sleeve/ journal / bushing / bearing have any features or grooves in it at all … ?

    Does it come apart in any special ways ?

    Is this like industrial heritage kind of machine or something that has been in service for the better part of a century ?

    Or something off a massive piece of mining or digging equipment etc. ?

    Ta.

    Eric
    __________________________________________________ _______________________________


    * I'd have to dig up some old Machinery's Handbook for specifics but I don't recall such large plain bearing being mapped out ?

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    Yes, its in that range of expansion at 50 degree increase.

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    Why would the shaft increase in size with heat and not the housing? They should both grow together to some degree.

    Agree with this:
    Quote Originally Posted by LKeithR View Post
    However, my intuition
    whispers in my ear that 50 thou, or even the 30 or 40 thou mentioned above is way too much clearance. The
    assumption is that once the shaft reaches a certain size you're not going to need more clearance for grease as
    the shaft gets bigger. A film of grease .010" to .015" should support and protect the shaft but I can't see a layer
    of .040" to .050" even existing--the weight of the shaft and its load will compress the grease layer to a thinner
    thickness in any case.

    If the shaft heats up enough to significantly change the diameter then allowance has to be made for that but I
    suspect the thickness of the layer of grease will remain constant once you get over a certain size...

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    Babbitt specs don't apply here. They are almost always run with pressure fed oil lube. This app is using grease and a leaded bronze bearing. Based on similar fits I've seen in large shops I would tend more toward .0008" per inch clearance, so about .030" larger on the bore than the shaft. Where's Forrest when you need him?

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    If this hasn't been posted already.

    http://www.nashua.edu/paradisem1/Mac...7_Mach_11A.pdf

    ^^^ relevant chapter in Machinery's handbook (27th edition) . (at least most of you / us have a copy of some vintage of MHB kicking about.)

    It's surprisingly complete considering it is not super user friendly though lol.

    Like what folks are saying in this thread, seems in terms of bearing design a lubrication analysis (and selection) is carried out first then the actual design of the bearing derived from that. [Takes into account friction and heat temperatures generated (Delta t's of 40 degrees Fahrenheit )].

    (munching through this over coffee),

    Quick read / crib sheet - pages 2218, 2222, 2238, 2239 thru 2242.

    Takes a bit of effort to comb through that all but does show conditions for use of various greases (high load low RPM condition).

    And does have some worked through simple calculations and formulae towards pages 2240. (Once you have figured out the lubrication scheme and range of clearances.).

    There is one mention of large bearing condition and application.

    __________________________________________________ __________________________________________________


    Might comb through and simplify (time willing) at least to reduce what Machinery's handbook advises to a minimum... (take a razor to it to close in OP's original question … "How to calculate … " . Or determine what may be missing in this case...

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