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On Sun, Feb 12, 2023 at 04:45 PM, Julius Henry Marx wrote:

No problem, C[E]3 is the standard all around bearing type most everyone uses.

According to the SKF handbook, and my experience, C3 is not the usual bearing everyone uses, although it is being widely offered on Ebay and similar places. Clearance are described as C2, Normal, C3, C4 and C5. For a 17 mm bore bearing such as the 6003 the clearances are:

C2? 0 - 9?μm

Normal 3 - 18?μm?

C3? 11 - 25?μm

C4? 16 - 33?μm?

C5? 25 - 45?μm

A Normal bearing has no extra markings on it. All the others are marked C3, C4 or whatever. A Normal bearing will be installed with one race being an easy fit in the housing or on the shaft, and the other a tight fit on the shaft or in the housing. Fitting the bearing then closes up the clearances. If the bearing is tight on the shaft AND in the housing a C3 bearing is used, as the clearances are closed up a bit more. In some circumstances, usually due to temperature differences, the clearances will be closed up even more and a C4 bearing will be use. I've never seen a reason for using C5 bearing, but someone somewhere must need them. Clearances are important as bearings work best with a bit of preload on them - this stops the balls skidding on the races, and causing premature wear. However, too much preload will overstress the races and cause spalling, and short bearing life.

Radial bearings are designed, as the name suggests, for radial loads. With extra clearance the bearing can take significant axial load as well, as the line of contact between the balls and races tilts, making it more of an angular contact bearing. The more the tilt, the higher the axial load that can be applied. On the U3 the bearings have an axial preload applied by the disc springs, so I would look for a C3 or C4 bearing. Until recently it was unusual to be offered much choice of clearance off the shelf. The DB/SL uses E13 magneto bearings. These are a very old specification of angular contact bearings, ideal for the job but not available sealed, and sometimes hard to find, which is probably why this type were not used in the U3. Unfortunately angular contact bearings (according to SKF) are not available in 17 mm bore and 35 mm OD, otherwise they would be good for the U3. There aren't any magneto bearings the right size for the U3. Taper roller bearings would also be very good for the job, but are not made in these smaller sizes.

I would use sealed bearings (2RS). SKF say that 50% of bearing failures are due to too little grease, too much grease, the wrong kind of grease, or dirt in the grease. Their sealed bearings are supplied with the right amount of the right kind of grease, and they claim they are greased for life. I'm willing to believe them. I would always look for a respectable make - SKF, FAG, NSK, Koyo come to mind - bought from a proper bearing supplier.

I've just looked on Ebay and I find the SKF 7003CD bearing will fit. They are described as super-precision spindle bearings - ideal you might think - but they cost about ?120 each, so you might think twice about using them. Not sealed either.