How tight a fit for caged needle roller bearings?

[PeterE]

I hope that the experts on this forum may help me with a constructional detail.

I have some caged needle roller bearings aimed for a small project and in the instructions/information in the catalog it says that these bearings will get their support from the housing. To me this means that they require some sort of press fit, but how tight??? A light push fit, light press fit or more? I guess that a too tight fit will affect the bearings free rolling.

BR

/Peter

[Lew_Merrick_PE]

Peter -- The answer (as always) is it depends.  What is the Pressure*Velocity (PV) loading?  How large is the shaft?  How large is the bore?  What is the hardness and surface finish on the surfaces upon which the needles roll?  How are they being lubricated and with what?  A properly designated tolerance set will account for all these factors.

Now, that said, a tolerance of +(0.1 + 0.008*Shaft Diameter)/+(0.05 + 0.005*Shaft Diameter) in mm is a general rule-of-thumb answer for lightly loaded, moderate speed/moderate pressure, lightweight oil lubricated applications.  Be very careful about any fit where the total clearance at assembly is less than 0.04 mm as the needle rollers will not be free to turn or pass lubrication.  Hardened (Rc-45 or harder) and ground (8 micrometer) surfaces will work best.

Does that help?

[PeterE]

Thanks a lot Lew I think it actually helps 

There are of course several factors to take into account as very well pointed out. What I sort of lock on to is that one should be careful about the fit, this since I understand as the cage can quite easily be compressed and then actually hinder the rollers from turning.

Let me explain a bit more about my intended use as it may give some more info.

It started some time ago when making a light duty exchangeable live center for my Unimat clone lathe. The idea and drawing came at the time from Bob Loaders book on Unimat accessories. It was made as a housing with a "hole" of the same diameter as the existing tail stock but with three bearing balls in the bottom. By making different and exchangeable tips this little live center became extremely useful, see pic below. So useful that I had in mind to do something similar for my C3 mini-lathe.



In the picture we see the housing at the back to the left. The tips are all steel apart from the two items in the front row to the left. Those are the original live center and the original dead center. A small wooden stand helps me keep them in order. They are all made on the small lathe.

For the slightly larger contraption I thought that using needle roller bearings would give a better turning motion and less heat build-up, and by trapping a thrust needle bearing at the bottom I would get a better bearing than just three balls.

So, I started off by getting a blank end arbor for the housing, two caged needle rollers with 15 mm ID and 22 mm OD, and a thrust bearing with 21 mm OD. The tips I intend to make from a good steel, close to silver steel but not hardened as my steel bits have proven to wear very little even without hardening.

So, here is my origin for the question. The tips will of course be made to a suitably close fit in the roller bearing and to run very close to the housing to prevent swarf as good as possible. The lubrication will be the same type as used in front hubs on cars, that is a high pressure, high temp, high quality bearing grease.

But back to the original quesstion, how tight a fit? As I understand it the fit ought to be a light push fit so the cage does not spin on its own, but not so tight that the rollers are stopped. Possibly one could use some loctite stuff to glue the bearings in place to secure them a bit more?

Am I completely up the wrong tree here? (And yes it is kind of an extension to the Stevensonīs small live center )

BR

/Peter

[PekkaNF]

This might be oversimplified, but if I were making that this would be my take:

This outer sleve is relatively weak and therefore you can (and should) fit this bearing unlike any other rolling element bearing. To my knowledge generally other bearing types really shouldn't be coaxed to fit they are not intended. But this type needle roller bearing outer sleeve is thin and the most important thing is to prevent it moving under load. Of-course you don't want crush it but don't want sliding fit either.

1) I would aim decent press fit for needle roll sleve. Reasoning is that i would not like the sleeve roll under load. It would make mess of the bearing, journal and housing. This should give you nominal clearance for the journal. I would not overly worry about the exact ideal value, because you will have control over the journal with large margin.

2) Then I would make the journal to proper clearance. These parts you will be producing several and they are wearable/intercangeable by nature, it should be only a little bit trial and error before you'll reach the clearance mentioned above.

PekkaNF

[David Jupp]

Try the bearing manufacturers web site.  For example the SKF site has an extensive publication which makes recommendations for tolerances on shaft and housings for different types of bearing - including various different syles of needle roller bearings - interestingly they give different housing tolerances depending upon housing material.

[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! 

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 donīt mind measuring system discussion    for me I think metric and imperial ī(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. 

Hmmmm, back to the drawing board ... 


And again, a big thank you for being sounding boards! 

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    (having learned it the hard way  ). The control is so much better using a "help arbor".

 

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