Which comes first, the shaft or the bearing?

I have a bearing who's bore is .8735. That's smaller than a .875 shaft that it's suppose to fit over. So which is manufactured to fit which? The shaft to fit the bearing, or the bearing to fit the shaft?


In other words:
I have to design a shaft that will have a sleeve bearing fit over it. If the manufacture of the bearing says its bore is 1.50", do I have to machine the shaft to fit the bearing, or is the bearing oversized just a tad? How does that work?

Look up bearing fit...it will tell you what you need to inow

Anti friction bearings are made to standard sizes. Shaft and housing fits are specified by the bearing manufacturer to suit different operating conditions. Sleeve bearings can be made to suit specific shaft sizes.

Is the bearing a press-fit in the hole? If so, install it before finishing the bore. I always just figure sleeve type bearings/bushings are going to need finishing to size, either boring or reaming once in place.

If a press-fit and you've already pressed it in, that's most likely why it measures undersize.

To answer your question though, I always make the shaft standard and adjust fit at the bushing/bearing. If the shaft was made elsewhere or to some odd spec for a reason, the same still applies. Fit the bearing to the shaft.

Mechanic Bill said:

I have a bearing who's bore is .8735. That's smaller than a .875 shaft that it's suppose to fit over. So which is manufactured to fit which? The shaft to fit the bearing, or the bearing to fit the shaft?


In other words:
I have to design a shaft that will have a sleeve bearing fit over it. If the manufacture of the bearing says its bore is 1.50", do I have to machine the shaft to fit the bearing, or is the bearing oversized just a tad? How does that work?

Click to expand...

What are you measuring the bearing with, a caliper or an inside mic? I highly doubt it is .8735" diameter.

Ball bearings in inch and metric are typically right on nominal diameter or maybe a couple of tenths under. Those
would be standard bearing configuration without any setscrew or eccentric locking extension.

The ball bearings with locking provision are typically looser fits, maybe .0005 to .001" over but some will be as small as nominal dimension. For the most part, these will always fit over precision ground shafting, which might be as large as nominal dimension to .0005" under. Sometimes precision ground shafting is not round enough to get a bearing on, even though it measures up like it should go, there will be a 3 lobed shape to it that will not fit inside a round race.

Yes, the bearings are made to standard sizes. You make the shaft to suit with required clearance or interference fit. Bearing makers aren't going to grind a bearing to your special size unless you pay out the wazoo for it. I'm thinking along the same lines as Hu for this one. What are you measuring with? Please tell us it's not the "I.D." jaws of a caliper, of which many are notoriously inaccurate.

If it is a bronze sleeve bearing( like an oilite sleeve bearing), it is usually .001 to .002 oversize so when you press it in,the shaft should still fit. May have to ream it if the press is to tight.

I am using a caliper. A really good one from The Home Depot (a little sarcasm there). Maybe that's where my confusion lies.

I'm just a lowly mechanic, not an engineer or a machinist. But I've taken on the task of trying to modify a machine that has had an issue from its inception 60 years ago. The issue has never been addressed. I had the idea several months ago and to date I've made a prototype assembly by hand from scraps of material I had lying around my shop. It was very crude and only worked twice before it failed, but it worked. The theory is sound.

So now I'm at phase two of the project. Build a precise prototype from the correct materials and with the correct dimensions. I'll walk into a local machine shop and hand them my drawings and they'll make my four major parts for the prototype. When they hand me my parts and I assemble them onto my machine and it doesn't work, it'll be my fault because of something I overlooked or was ignorant about. They made the parts to my specifications so I have to know what the specifications are.

The first thing I've learned from y'all is that I will not trust the caliper I am using. I'll use it as a guide but for nothing more henceforth.

The part in question, to which this thread relates, will have a 1.5" ID plastic sleeve bearing pressed into it. As I understand it, once the bearing is pressed into the part, the ID will be at the nominal 1.5", so I will have the shaft made accordingly. What I don't know, because I'm not an engineer, is the inside diameter of the part in which the bearing is pressed into. The manufacturer of the bearing gives an OD of 1.656". If you were going to bore a hole in the part for the OD of the bearing, what bore diameter would you use? And once the bearing is pressed into the part, and its ID diameter should now be 1.5", what OD of the shaft on which the bearing will pivot should I have the machine shop turn it to?

The shaft will take about one second to pivot 90 degrees. Then another second to return to its home position. But during normal operation, the shaft and bearing will turn in unison. It's only when something jams on the machine that the bearing will need to pivot on the shaft. There's not a tremendous amount of load. The bearing needs to be a slide fit on the shaft but with a close tolerance. And, if the design works as planned once I get the bugs out, and I actually get to produce and sell the assemblies once testing is complete, I need a design that I can just press the bearing in place and assemble it onto the shaft. I won't have the luxury of boring each bearing to fit each part. So I need a good design with close enough tolerances so I can just slap (so to speak) the parts together into an assembly and ship them off to consumers.

If anyone has anything else they care to offer, I would greatly appreciate any advice you care to give. I came here to seek the advice of professionals. I think I'm in the right place. You may have already answered my questions so I will go back and carefully read each response again. Thank you all very much...

Plastic sleeve bearings are unlikely to be precision in their tolerances. You'll likely have to settle for somewhat loose, would be my guess, or else ream them to size after installation. I think you just have to buy some and get them measured up or let the shop you use measure them up and get the fit you want. They may have to scrap a part or rebore it to find the right amount of interference to make the sleeve bushing work as is. Plastic bushings can worm their way out of a hole if they are not tight enough to stay retained, and getting the right interference is troublesome if you are seeking a precision bore after it is assembled. But you could retain them with snap rings.

One possible good source of accurately made sleeve bushings are called guide elements. These are used in hydraulics, for guiding piston rods. They come in quite a variety of sizes. They are split for installation because they are held captive in a groove with closed ends. They more or less float in position (not a press fit because they are split, but their wall thickness is accurately controlled), but they do keep the rod from contacting the metal gland.

Another poor thread title...What is wrong with "bearing to shaft fit"

Proper thread title is so others can search a subject...perhaps having questions down the road...

michiganbuck said:

Another poor thread title...What is wrong with "bearing to shaft fit"

Proper thread title is so others can search a subject...perhaps having questions down the road...

Click to expand...

....and maybe combining all (4) separate threads into one, all dealing with the same shaft/bearing fit problem ?

Good the Op's home depot measuring tool did not read to five places or the hole might be .87352

Good to know only a knife sharp edge would measure very close to an ID .. no body makes a knife edge on a caliper so the

ID measure with such often it is difficult to get 4 place measurements.

Yes one can trig that out to see a .020 inch flat on two jaws would produce a .xxxx error in the results.

A very good radius would be better than a flat..

For plastic bearings, my favorite is Igus.com
They offer a ton of different polymer choices each optimized for a different set of conditions. Which does not sound essential for your app.

The other thing they offer is detailed info on shaft and housing fits. They are the only company I have found where they say, if you bore the housing to this size and tolerance, the bearing ID will install AND have this give size and tolerance. They then go on to tell you what size and tolerance the shaft works with the bearing.
https://www.igus.com/product/86 click on technical info tab. Also call and talk to a tech rep. very helpful

Ditto on the Igus. On a lot of plastic bearings the amount the bore shrinks when the bearing is pressed into the housing can vary. Sometimes they have to be reamed to get the shaft fit.