Bearing Questions

ddickey

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I plan on trying to make a live center sometime this winter. I was going to use three angular contact bearings 7204 with a needle bearing on the end.
Do i want three single row or two double and one single?
Back-to-back I think is the setup. Is a double row also called a duplex?
Are single row made to be mounted as pairs or three?
Explain the factory preload.
Are all angular contact bearing preloads set?
 
you can use 3 angular contact bearings, but that may be overkill especially if you have a thrust bearing in the design too.
2, single row angular contact bearings would be sufficient
you will want to stack the bearings with the cup opening facing out to get the most thrust protection
i generally only see 2 bearings in the applications that i have seen angular contact bearings used.

preload is the measure of clearance between mounted bearings on a shaft.
generally speaking, you tighten a locknut to a predetermined torque or torque angle and then loosen the locknut a couple threads and re-tighten the locknut(s) to ZERO CLEARANCE.
too much preload will result in early bearing failure, not enough preload, the shaft endplay may be excessive and shorten bearing service life.

the angular contact bearings i have run across are generally kinda loose running, being retained with circlips or mounting nuts with a locknut
 
So the duplex or double row have the preload set because they come as a pair? Single row would have to have proper preload set by a bearing lock ring or nut?
I read this and it had me wondering. Is preload different than clamping load?
https://nhbb.com/reference/ball-roller-bearings/preload-duplex-bearings.aspx

  1. Specify factory “built-in” preload sets. Since each set is checked for preload level as it is built, these duplex bearings provide the greatest control over preload. The clamping load can be applied either with a single nut tightened to a specific torque, or a clamping ring with multiple screws tightened to a prescribed torque.
 
for angular contact bearings set, the preload will be given by the thickness of the bearings. Have seeing a skf specification for this preload somewhere. not sure if is easy to find bearings set with specified preload to buy. otherwise you will need to grind bearing face...
 
If I had to make a live center I would use a couple of 6200 series ahead of a 7200 series. Instead of the run of the mill 6200, I would use a better tolerance bearing, I've forgotten the designation letter/number, more $$$. I've been retired for three years. While they are still just a ball bearings, it would be tighter than a 7200 and it would take lateral loading better than a 7200. The 6200s should float enough in the housing so there is no axial loading on them, let the 7200 take that. While they say 6200s can take axial loading, I've never seen it work out too well.To do bearing clearance reduction, you would have to rely on the shaft fit.
 
I plan on trying to make a live center sometime this winter. I was going to use three angular contact bearings 7204 with a needle bearing on the end.
Do i want three single row or two double and one single?
Back-to-back I think is the setup. Is a double row also called a duplex?
Are single row made to be mounted as pairs or three?
Explain the factory preload.
Are all angular contact bearing preloads set?

Consider this to symbolize a center and one angular-contact bearing, with the center bearing on
the inner race and the outer race held stationary: " < ) "
As I understand it, identical bearings stacked back-to-back "< ( )" (or is it face-to-face?)
will take on the factory preload (the inner races being optimally resistant to both directions of thrust).

Preload just means that the 'slack' in the bearings is taken out (it takes some pressure on the
BBs to put their contact track in the correct part of the bearing race, and that force is the preload).

The preload then determines the centering accuracy of the bearing when it is not under thrust load
from the workpiece. The leftmost takes on extra load (thrust) only when you PULL on the center.
For LOTS of thrust, the addition of a third bearing in the correct orientation "< ( ) )" will share the load
in its resists-thrust direction. The bearings, to share, must be identical and in contact on
the outer races (i.e. not just slip-fit, but pressed, glued, or tightly clamped in a housing) as
well as on the inner races.
A double-row bearing is a "( )" or ") (" pair that cannot fail to have the races in the
factory-set preload. Adding a third race to the double-row means using a non-identical
thrust bearing, so maybe you don't want to do that. Two double-row would be good.

Non-angular-contact ball bearings (6200 seris) are accurate regardless of load, and can be put into the mix
just about anywhere (they have matched race lengths, so thrust just goes through to the next bearing
in line). Matched spacer rings (inner-race and outer-race with same length) can be tossed
in without changing the preload.

Needle bearings don't have races that match diameters with a stack; those confuse me.
 
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All of my "home made" live centers have off the shelf standard ball bearings in them. Nothing fancy. The one live center I grew up using for many years was made up of three 203 bearings, with the one facing the outside having the seal still on the bearing. No super precision bearings here. I have another revolving center made up of similar bearings that support a 3" 3-jaw chuck. Cannot detect any run-out in it for the work I do..

In general, to use super precision bearings, the shafts are generally harden or case harden and precision ground. The housing the bearing goes into may or may not be harden, and are generally ground to size.

I'm just saying, it may be a over kill trying to use 7200 or even a 6200 series bearing for a live center. I will agree with one of the post saying to use a 6200 series bearing. They are available with or without seals. With seals, makes them more adapted to making a live center.

Ken
 
Remember that the bulkier you make the bearing center, the more difficult it will be to get tools around it and into the work. Look at some good ones like Royal and note how they leave as much room around the actual center to work with as possible while having a solid, rigid assembly. Hanging the center itself far out of the assembly just makes it more flexible, and less rigid.
 
In thinking about the making a live center, I realized the challenges. Interesting. On my wood lathe ,it came with a cheap?? metal lathe live center, MT 2 . It got stiff, so I changed out the bearings, nothing at all special. But when you get expensive live center, you should be getting better grade of bearings and a high precision shaft. Some plants use better grade bearing just to avoid having to change them as often. I don't understand bearing pre-load , I've heard of it being used on carnival rides, but not in industry. When you install a bearing, they come from the factory loose, you then take up of some of that clearance, not all 1/2??, by expanding the inner race. On most machinery they use what I call K-taper sleeves. On the bigger bearing we would use 12" feeler gauges to pick both sets of rollers. It's painful on your fingers, BTW, pushing it over them. On smaller bearings the fit will reduce the clearance, sliding fit versus heat shrink fit. The latter is where it gets interesting, as in 1/10s.

If you use sealed 6200 bearing, pop the inner seal off, you have to lube the 7200 any way. Use good grease, CV joint grease is better grease than what comes in bearings, it's a moly grease.
 
This spindle rebuild company has a some great information on bearings used in spindles on the "Blog" portion of their web site..
https://highspeedtechnologies.com/blog/
I think there are 4 sections to this bearing explanation. I think this will answer all of your questions.

A few weeks ago I needed to replace tapered roller bearings that were no longer available for the spindle of my mill. I called (as the web site said to do - even if you are just looking for information), told the guy I couldn't afford to have them rebuild my spindle but he still spent over 30 minutes with me. He guided me in the selection and installation of radial contact bearings.
I couldn't have rebuilt my spindle without the info here and their help on the phone.
 
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