Gallery, Projects and General > How to's
How tight a fit for caged needle roller bearings?
Lew_Merrick_PE:
Peter,
Not intending to start an "inch vs. meter" argument, one of the advantages of the inch-based system of things (bearings included) is that there has been a single standard for fits, tolerances, and allowances going back to 1914 when the Americas, British, and Canadians got together to standardize things for WWI (the "ABC" specification set). The metric side of the universe has been trying to standardize since 1947 -- and is only now, six decades later, beginning to have "universal" standards. That is because they began with a broader range of incompatible national standards that nobody was willing to surrender or modify.
My base assumption (which may well be invalid) is that your bearing complies with DIN-metric specifications. I am not conversant with DIN bearing allowances and tolerances. The "answer" I gave you is based on US/ANSI specifications codified under the AFBMA (Anti-Friction Bearing Manufacturer's Association) guidelines. I believe that they are close to those defined by DIN -- but that is a "belief" and not a known fact.
My real question is, Why needle bearings? It seems to me that your top rotational speeds are not all that high (i.e. <5000 RPM) and that your non-axial loads are similarly small. A bronze bushing with a central clearance zone being fed a drip-type lubrication of (say) low viscosity silicon oil would do you as well and be much simpler to make and maintain. The issue of heat build-up can be addressed by making the housing as large as possible and letting it be your heat sink. A single bearing-ball thrust bearing should handle any axial loads a small lathe can induce.
Central Clearance Zone Bushing: Let us say that the shaft size fitting into the bushing is ø10 mm X 20 mm long. The "forward" end of the bearing should have (roughly) 5 mm of contact with the shaft as should the "aft" end of the bearing. That leaves 10 mm centrally located where you could have a diametral clearance of (say) 1 mm. A "Gitt's cup" type of reservoir could easily "drip" lubricant onto the shaft through this gap.
???
-
Lew
PeterE:
Thanks a lot Pekka, David and Lew! :thumbup:
Most informative and actually also verifying my thoughts.
Pekka; Your comment on the press fit I totally agree with, that was lurking in the back of my head but needed verificataion. Also the fact that I will be making the tips will also make it possible to adjust to a perfect fit.
David; I did look at the shops web site but could not find any info on this. Only the comment that the cage required support from the housing, but of course I would most certainly get that kind of info if asking, no doubt about that.
Lew; I don´t mind measuring system discussion :D for me I think metric and imperial ´(as well as ETSI and ANSI imperial differences) supports each other, with the odd annoying difference here and there (to keep clear of).
So, to your question on Why needle bearings at all? - That is the core question, and from this short chat I think I probably have over-engineered the thing in my mind. This is a light duty, low speed (mostly) contraption so I guess there is no real need for any other type of bearing than some kind of plain bearing. Perhaps not a PTFE or similar due to material softness, but cast iron, bronze or even brass I think would work quite OK.
Combining that with your idea of making a waist on the tip (or a similar enlargement on the bearing wall) for housing some extra lubrication would work very well, as well as making cooling fins of the oversized housing to further keep the temp down. :beer:
Hmmmm, back to the drawing board ... :coffee:
And again, a big thank you for being sounding boards! :bow:
BR
/Peter
Sorekiwi:
Amongst all this expert technical advice, I'll throw in some practical experiance!
One of my hobbies is playing with a particular brand of old Garden tractors, and they use Torrington caged needle roller bearings in the gearbox.
I have never actually measured the bore, but the bearings are a light press fit in the case. An arbor press would be ideal for fitting them, but due to the shape of the case its awkward to arrange, so I tap them in with a hammer and an aluminum arbor that spigots into the rollers somewhat, and drives on the end of the housing.
(Excuse the color, this picture was taken after I was playing with some home anodising)
I think I have a case without any bearings in it, I'll try to remember to get a measurement of the bore later on.
PeterE:
Hi Mike,
Thank you very much for input, this is exactly the way I would go about it :thumbup: (having learned it the hard way :bang: ). The control is so much better using a "help arbor".
:update:
Have been thinking a bit more and came up with the following idea.
Hope you can see it.
The general design actually benefitted from a smaller thrust bearing and adjoining plain bearing. It will now be placed in the pile of stuff to do.
BR
/Peter
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