Bearing Fits?

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warmstrong1955
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Re: Bearing Fits?

Post by warmstrong1955 »

Actually, it would be J6 to K5. Fit tolerances are based on diameter.
I'm sure there are folks here who who are much more qualified to answer your question than me. FYI, I wouldn't know a 4-8-4 Northern if one bit me in the butt. My experience with rail equipment is all in the underground mining world. A harsh, nasty, dirty mean environment, and is the same for the loads, duty, vibration, & shock. Things I doubt you will see.
In a mine, if I was to use that 6208 bearing for axle, I would go for a K5. A tight K5. But that's for a mine.

Maybe you should post in the Live Steam forum.

:)
Bill
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spro
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Re: Bearing Fits?

Post by spro »

This has been good information. I bought some precision bearings for a mill spindle and the code needed some investigation. Well heck I bought them anyway. These are super tight and could deform a hollow spindle.
EOsteam
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Re: Bearing Fits?

Post by EOsteam »

I decided to consider the k5 tolerance and check the math just to get my head around this. If I'm reading the charts correctly they are calling for a 21 micron interference fit if the shaft is between 30 mm and 50 mm diameter. The bearing ID on the 6208 2RS is 40 mm. Converting to Imperial we get a bearing ID of 1.5748".
21 microns is .00082677 or rounding out to .0008".

.0008/1.5748 = X/1
After algebra we get
X= .0005"

So in my head at this dimension of 1.5748" the interference is .0005" per inch of diameter. Yes, I know that the dimension is 1.5748" and I will be allowing .0008" of interference fit but I needed to do the math to compare to X per inch to see if it made sense. "I've been using .001"/ inch of diameter for shrink fits and I know how tight that is.

Some background: 1000 microns in 1 mm. If you want an easy online calculator then do a Bing or Google search for "microns to inches"
Also, if you want to convert from mm to inches then divide X mm by 25.4 because there are 25.4 mm in an inch. Example: 40 mm divided by 25.4 = 1.5748"

I've left the post in this category of of "General Discussion" since whenever I check the forum it is usually by clicking "Active Topics" and I figure I can't be the only one doing that. I also hope that I haven't offended with the math discussion. I was raised in the U.S. but in our local school system our science classes were all metric and even though I'm used to using it I still had to verify that there were 1000 microns in 1 mm.

Again, thanks for all the help. I'm not sure how long this would have taken me without the fantastic support this forum offers.

Harper
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Atkinson_Railroad
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Re: Bearing Fits?

Post by Atkinson_Railroad »

Harper,

Your choice of the k5 tolerance fit is a good one. It’s the same tolerance listed in the earlier posted chart.
Exceeding the maximum shaft diameter of 1.1573 inches as shown in the chart will result in “over kill” for the fit.
A 6208 bearing is considered a “light service” bearing as it is. A tighter fit than expressed in the chart will preload
the bearing eventually causing a premature failure.

“But”, when considering your application… it would likely take quite a while for the bearing to exhibit
any noticeable sign of degradation because it’s not being used at say, 1800 RPMs during an everyday 8 hour service demand.

One other very important “thang” related to bearing fits. You have to be certain your measuring tool(s) is/are
known to be accurate or your efforts will not be productive.

I can send you an inside industry PDF on the subject of searching for the “perfect” bearing fit
if you PM your e-mail address to me.

After reading it though, you’ll still be left with unknowns. The subject being an “art form” has so many factors
outside the scope of what an Internet forum can simply cover adequately.

The other even more controversial subject related to bearings?

Lubrication.

Don’t even get that one started : )

John
John Hasler
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Re: Bearing Fits?

Post by John Hasler »

John writes:
> One other very important “thang” related to bearing fits. You have to be certain your measuring tool(s) is/are
> known to be accurate or your efforts will not be productive.

Given that he has the bearing in hand his instruments need only be repeatable to a sufficient degree of precision.

Measure the bearing with a repeatable transfer gage or inside micrometer (don't use calipers). Measure that with your outside micrometer (do so even if you use an inside micrometer). Use the result of that measurement in your calculations (even if it differs from the specification). Use the same micrometer to measure the shaft. Your fit will then be limited by the repeatabiity and precision of your instruments, not by their accuracy.

You will also know a bit more about the accuracy of your instruments since the bearing dimensions can be assumed to be accurate.
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mcostello
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Re: Bearing Fits?

Post by mcostello »

Just a heads up, there are many flavors of say 6202, ID's and OD's vary from the norm, do not assume. Measure what You have. Have been bitten with this a few times, and I have a Friend that sold bearings over the counter and He has seen this a lot. The Manufacturers don't always change the numbers on bearings.
John Hasler
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Re: Bearing Fits?

Post by John Hasler »

If I buy a new bearing and the dimensions don't match the ones in the manufacturer's catalog it's going back.
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warmstrong1955
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Re: Bearing Fits?

Post by warmstrong1955 »

mcostello is correct.

From my bearing guy friend, some years ago:

"What you will find it’s all bets are off for 6203 and under. If you look in BCA & FAFNIR AG books you’ll actually see modifications from 6204 through 6206 also but not as many and they stay closer to the original in most dimensions. We taught our guys to measure anything up to a 6206 since we saw so many AG bearings.
And that’s the truth."

Bill
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Harold_V
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Re: Bearing Fits?

Post by Harold_V »

Atkinson_Railroad wrote: Mon Jan 22, 2018 9:01 am One other very important “thang” related to bearing fits. You have to be certain your measuring tool(s) is/are
known to be accurate or your efforts will not be productive.
Man, you said a mouthful!
That's the point I've tried to convey to those who think using calipers (digital, dial or vernier) is acceptable for creating acceptable fits. Sure, you can use them for that purpose, but you hit the desired combination of sizes strictly by chance, as they are not capable of providing reliable readings.

I've often wondered how many guys, who have struggled to establish fits they desire, blame their lack of skill when the real problem is they are not working to the proper dimension, because they simply don't know what it is? That's the typical results of using calipers for precision measurements.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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BadDog
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Re: Bearing Fits?

Post by BadDog »

Speaking of measuring the bearing to start with, that seems a necessity for most home shops that don't have a calibration schedule. How else do you know if the mic you are measuring with is going to help you produce a hole within a few tenths? That's assuming you are measuring with t-gages.

Two weekends back I was asked to upgrade the pilot bearing in a Cummins 6BT flywheel. Stock (Dodge sourced) flywheel has a needle bearing pilot which is a known failure point, basically considered an 50k service interval replacement (not officially, but anecdotally). My buddy put a new one in when he converted his 72 Suburban to Cummins power (I did the machine work), and it just went out at 80k, ruining the input shaft as well. Anyway, it appears the "fix" is to upgrade from a (26 mm? Didn't measure) needle bearing that rolls on the input shaft to a high quality 1.750 sealed bearing. The spec fit was 0.0010 to 0.0015 undersize. I was using t-gages, so needed my 2" mic to be spot on for a 5 tenths tolerance range, so I started by measuring the bearing. I could have checked the mic against gage blocks, but why bother? The goal is to fit the bearing, not the gage blocks, and my mic had been referenced to the gage blocks some time back. So I measured the bearing and found it 0.0001 under stated size, according to my micrometer. Several checks all produced the same value. Rather than try to adjust my mic for the 1 tenth, I just adjusted my target range. Let me tell you, that was a tense job for me as it's something I very rarely do. But in the end, after cleanup/debur and light sanding to improve finish for accurate measuring, I landed it 1 tenth under the max size. Considering the still less than fine surface finish, probably right on the high side. But I was shooting for the low side. Tough (for me) working to tenths with a mill and boring head. I included an 0.030 deep 0.001 oversize starting bore to align the bearing, which pressed in exactly as it should.
Russ
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warmstrong1955
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Re: Bearing Fits?

Post by warmstrong1955 »

That's what I do Russ.

I used to get a lot of work done at the local machine shop when I was still in the underground biz. Invariably, he always asked if I could bring him the bearings, pins, or whatever when he did jobs for me. Got the same requests from machine shops from Arizona to Alaska too.
A lot of merit for doing that I think!

Bill
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Bill Shields
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Re: Bearing Fits?

Post by Bill Shields »

There is a modern, easy solution to all of this discussion.

It comes in a red bottle labelled "Loctite Bearing Fit"
Too many things going on to bother listing them.
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