Bearing clearance

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JohnHudak
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Bearing clearance

Post by JohnHudak » Sat Apr 10, 2021 5:19 pm

I ran across this on a print for a 1.6” mikado..
It’s the bearing clearance for the front truck, look at the tolerance for the fit.. I had never heard of mounting bearings like this, what’s the point?
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Greg_Lewis
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Re: Bearing clearance

Post by Greg_Lewis » Sat Apr 10, 2021 6:25 pm

It depends on how things need to move. I have some riding car trucks I made to a design by the late Cal Tinkham. The ball bearings for the axles ride in holes in the side frames that are .010± over the o.d. of the bearing so when one side of the truck lifts, there will be some flex between the side frames and the axles. These trucks are very nimble and derailments are rare even on rough track. Cal's cars have several thousand miles on them without a bearing failure.
Greg Lewis, Prop.
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RET
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Re: Bearing clearance

Post by RET » Sun Apr 11, 2021 8:44 am

Hi John,

Its as Greg says, it depends. Usually, its the shaft that rotates, and in that case, the press fit of the bearing is on the shaft, not the housing. If the shaft is stationary, then the press fit is in the housing. As they come from the manufacturer, most rolling element bearings are a bit sloppy because the sloppiness is supposed to be taken up by the press fit on assembly. That is why the amount of interference is important. If the interference is too great, then there is no play in the bearing and it doesn't turn freely. In that case, the bearing will fail quickly.

If the shaft rotates and there is no press fit, the bearing inner race "walks" on the shaft under the applied load and the shaft wears. Initially, the rate is slow, but it increases as the clearance grows.

The bottom line is whichever part rotates has to have a press fit on the bearing. Treat rolling element bearings like a watch, leave them in the package until assembly, and above all, keep everything clean! For most of our uses, sealed bearings are preferred unless you are going to have some form of lubrication like car ball joints where the new grease pushes the old grease and dirt out of the bearing.

Hope this helps a bit.

Richard Trounce.

Pontiacguy1
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Re: Bearing clearance

Post by Pontiacguy1 » Mon Apr 12, 2021 7:42 am

All ball bearings have some amount of radial play (sloppiness) built into them from the factory. It has nothing to do with the amount of press-fit they can take. Ball bearings that are made to have either Locking collars or setscrew locking inner rings all have the same amount of radial play set at the factory, at least all of ours did when we assembled them. They are designed to take some amount of misalignment as-is, and also there is an assembly tolerance which helps with making them faster to assemble. You do not want a heavy press fit on any radial ball bearing, because you will crack the inner ring. Bearing rings are heat treated, and most of the smaller sizes are through-hardened. Makes them very hard, but somewhat brittle. A small bearing like what would be used in the drawing needs to be a bit under 0.001" (typically half a thou), or something like that on small radial bearings, or you'll hear a very slight 'snap' when you press it home, and then that bearing is done. Again, it depends on the Diameter of the bore, AND the thickness of the inner ring. Some of these wide inner agricultural bearings can have a lot of meat around the bore and can stand a heavy press fit.

Like RET said: Whichever member is rotating, that one should be locked to the bearing. The non-rotating member can have some slop in there to assist with misalignments from side-to-side. Just finished putting some bearings on a small 4-wheel critter chassis this weekend. They are radial ball bearings, approximately 15mm bore, 30mm OD, and 9mm wide. A light press fit on the bore, and 0.010" oversize bore on the axle boxes is exactly what I used.

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JohnHudak
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Re: Bearing clearance

Post by JohnHudak » Mon Apr 12, 2021 8:03 am

Thanks to everyone for the very informative replies.. This old dog learned a new trick..!
The bearing specified for the axle boxes is a New Departure Z99-3L03 which has a O.D. of 1.377, so the print is calling for a .005"-,010" oversize bore..
When the print says " to prevent damage to ball bearing" do you think they're referring to the possibly of too much of a press fit destroying the bearing??
Thanks, John

Pontiacguy1
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Re: Bearing clearance

Post by Pontiacguy1 » Mon Apr 12, 2021 8:21 am

New Departure 'departed' in the late 1970s- early 1980s timeframe. They actually were bought by the Fafnir Bearing Company back at the same time. We had a few machines in the plant where I worked that were old New Departure-Hyatt (or NDH as we called them) machines. New Departure and the Hyatt bearing companies had merged a decade or two earlier to form NDH. We were a Fafnir ball bearings plant, and actually made some of the old NDH product, albeit with a Torrington part number. Look them up, you'll find I think that it is 17 mm ID x 35 mm OD x 10 mm Wide. There are a lot of companies that have that bearing now. Sadly, unless you get some New-Old Stock bearings, they will all be made in China now.

There is some amount of radial play in each bearing, and each bearing can withstand some amount of misalignment. If you try to twist a ball bearing sideways too much, you can destroy it, damage the seals, cause premature wear, damage to the retainer, etc... So, the clearance allows the bearing to float, have more misalignment, etc... You can make your axle boxes where they will misalign more, and then the OD can be a tighter fit. Either way, if one wheel goes over a 1/2" hump in the track or stick or something, and the other one stays in the same position, the bearings/axle boxes combination has to be able to deal with that misalignment without tearing stuff up.

John Hasler
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Re: Bearing clearance

Post by John Hasler » Mon Apr 12, 2021 8:25 am

If you secure both ID and OD of two bearings at opposite ends of a shaft the bearings can be damaged by excessive axial loads due to differential thermal expansion, flexing of the frame, etc.

Pontiacguy1
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Re: Bearing clearance

Post by Pontiacguy1 » Mon Apr 12, 2021 10:16 am

Which is the reason we made some housed units that had a bearing with a flat OD instead of a Spherical OD, and had some sideways movement inside of the housing. If you had a shaft that went in a furnace, oven, etc... Then you would use a Spherical OD ball bearing housed unit on one end, and the 'floating' flat OD one on the other end. Both would be locked to the shaft. If installed correctly, it would move outward when the shaft grew so that there wouldn't be any binding. There were some other ways to do that as well, but those were made for that application.
For High-temp and harsh applications there were housings that had the bearings completely contained inside and which ran in oil. Some of those could float sideways as well. We also made some larger housed units that had bores of 6", 8", 10", etc... Those were large, and one housed unit was all that you could mount on a pallet.
There are literally hundreds of specialty applications for bearings, and there is a solution for pretty much any bearing application situation you could come up with. It's all really fascinating, I think.

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Re: Bearing clearance

Post by Kimball McGinley » Mon Apr 12, 2021 2:28 pm

I have a slightly different take on the OD clearance drawing note. One common way to ruin a new BB is when the press-fit load gets transferred through the balls themselves during assembly. The press fit force is usually many times the allowable axial thrust load. Such bearings can be heard/felt to "ratchet" afterwards. OOPS!
So, an ID press must use a press ram that does not touch the outer race, and vice-versa. Be vigilant here; one race or the other may be just slightly proud of the other, even if they are supposed to be co-planar.
In the case of a loco with bearings already assembled on the axles, whether pressed or Loctited, there would be no way to contact just the outer races to press them into the axlebox. Any OD resistance/interference would have to be taken through the balls, potentially ruining the bearing. This is yet another reason why the OD must be a clearance fit in this case.

The drawing says "Burbank, Calif."; I presume Chet's Railroad Supply?

Pontiacguy1
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Re: Bearing clearance

Post by Pontiacguy1 » Mon Apr 12, 2021 4:01 pm

That is called Brinnelling, or where you have dimples or divots in your races now because of blunt force. That happens a lot more often than you'd think. Never a good idea to press it through the balls. That 'ratcheting' effect can also be felt if you crack a raceway while installing it because of too heavy of a press fit. Some ham-handed person trying to install a bearing with a hammer can also ruin one in short order. Like KM said: try to press on the member of the bearing that is actually going to be taking the press fitment.

If you bend or deform the cap and seal, then it will start purging grease, and the bearing will fail quickly. A lot of customer complaints we used to get were about the bearing purging grease when it was running, and you could see where someone had hit and deformed the cap when they were trying to install it. Another by-product of trying to install a bearing with a hammer.

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