Internal clearance vs Precision rating of bearings

[everettdavis]

Internal clearance vs Precision rating of bearings

Internal clearance is the total distance one bearing ring moves relative to the other radially (radial internal clearance) or axially (axial internal clearance).

Let me cite some information from, then point you to a more detailed PDF posted at MachineDesign.com written by Keith E. Meyers of SKF which will help you differentiate between the two.

Bearing clearance is categorized as unmounted and mounted clearance. The bearing manufacturer establishes the initial, unmounted bearing internal clearance. It is the clearance in the bearing right out of the box. In the late 1940's that was the way bearings were classified and folks associated the description "Precision" to them.

If you think of the Shopsmith Quill Assembly, with two bearings mounted, they measured what is called Run-Out of the assembly to determine if the 'assembly' had more than 0.0015" of total run-out (wobble if you prefer). If it did, they were rejected. Manufacturing logs of the day indicated those that were measured and passed were frequently 0.0008 on average. Those vintage Fafnir bearings were C0.

The original 10E/10ER bearings - Fafnir 202KLL3 are still available today. https://www.motionindustries.com/produc ... u=00051563

It is a Radial/Deep Groove Ball Bearing - Round Bore, 16 mm ID, 35 mm OD, 14.4 mm Width, Double Sealed, C0

They are just not as commonly available in all markets.

The specifications for it list both Internal clearance and Precision rating data in today's specs. In the 1940's the precision rating had not been standardized by ISO, ANSI or AFBMA so it was not listed as a standard at that time even though engineering manuals and product application guides did get into that area for the engineers who were designing equipment to insure they chose the right bearing for the right application.

These specifications get into the mounted clearances which are smaller due to the fittings, clamping, press fitting (interference fit they call it), thermal expansions and the metallurgy and mechanical expansions that change due to the physical diameter 1/2", 5/8", 3/4", 1", 2" etc. of the shafts they run on. (Different strokes for different folks so to speak) Thus you now have a table of clearances for the various specifications which vary in clearances for the same bearing classification based on the size of the shaft, its clearance or interference fit on the shaft, its clearance or interference fit within the housing it is retained or pressed into.


Specifications: Fafnir 202KLL3 Used in 10E /10ER Quills

Bore diameter:16 mm

Outside diameter:35 mm

Overall width:14.4 mm

Bore type:Round Bore

Closure type:Double Sealed

Row type & fill slot:Single Row Non-Fill Slot

Internal clearance:C0

Precision rating:Not Rated

Finish/coating:Uncoated

Cage material:Steel

Inner ring width:14.4 mm

Outer ring width:11 mm

Fillet radius:0.6 mm

Maximum rpm:14000 RPM

Series:200K

Dynamic load capacity:1930 lb

Static load capacity:830 lb

For further reading I would suggest reading Keith E. Meyers PDF on Understanding bearing internal clearance which I uploaded to My Google Drive under the link at http://www.shopsmith.com/ss_forum/viewt ... 90#p185690 since you have to register and become a member to download it otherwise.

I agree it is confusing as terms are occasionally misused by various folks and I did not sign up to become a mechanical engineer. My goal was to find out what Magna / Shopsmith originally used, and replace the bearings with the same bearing when I restore a machine.

The Real Questions are:

1) “Do you need less than 0.0015 run-out when cutting or turning wood?” No. Wood expands far more.

2) “Do you need sealed bearings?” I ‘believe’ yes if you have a choice.

• Seal at least on the outside bearings on the quill and drive sleeve assembly, the belt sander, the jointer, and both Mark V mounted Planer or Professional Planer

• The manufacturer’s today state Rubber Seals are the most resistant to dust and contamination. The rubber seal is my preference today.

Shopsmith moved away from C0 when they introduced the Mark 5 in 1953, and those were the standard bearer (and bearing) for the next six decades in Mark 5 / Mark V's.

All of us will have to contend with what they used vs. what we can find / afford / justify as we restore machines to service. If you charge to restore others machines, that is a factor you must consider, and price accordingly. Our late friend Bill Mayo had his preferences, Jacob Anderson, and Hog Winslow have theirs as they provide (or provided) services for others commercially.

Realizing that others may have serviced your unit before you bought it, the bearings in place now may or may not be original, or made to the original specifications as the OEM. The older it is, the more likely that would be.

Dimensions of the quill shaft design, machining including housings have been changed both by the OEM folks through the decades, and by 3rd parties performing aftermarket alteration services. That's why I originally wrote the Shopsmith Bearing Guide.

The right size bearing with the right seals or shields for your application is what's required, and to know what you need you have to measure what you have, and consider what might should be there. The Bearing Guide will help you determine, not define what you need.

Everett

[Dansmith]

Thanks for the very informative post.

[badtheba]

Thanks a lot. That clears up my previous confusion.