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How tight a fit for caged needle roller bearings?

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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  :thumbup:

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.

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