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jbeale

understanding specs on ball bearings

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I am playing around with some "deep groove" ball bearings. I have read that "Deep groove ball bearings mainly take radial load as well as moderate axial load." But what does that mean exactly?

For example, one particular bearing (6203Z, OD=40mm) has a spec for static (4.8 kN) and dynamic (9.55 kN) loads but I think that is radial only. Since there is no spec number provided for axial load, is there some rule of thumb for "moderate" like 1/10 or 1/100 of radial load, or should I assume it really cannot handle any axial force at all? Pushing in on it just by hand, it feels pretty solid, though. This size bearing is used in some machine tools, motorcycle crankcase, etc.

http://www.amazon.com/Bearing-6203Z-17x40x12-Single-Shielded/dp/B002BBAWUE

EDIT: Ok, maybe this is my answer; 1/2 to 1/4 of rated static load Co:

 

If deep groove ball bearings are subjected to purely axial load, this axial load should generally not exceed the value of 0,5 C0. Small bearings (bore diameter up to approx. 12 mm) and light series bearings (Diameter Series 8, 9, 0, and 1) should not be subjected to an axial load greater than 0,25 C0. Excessive axial loads can lead to a considerable reduction in bearing service life.

http://www.skf.com/group/products/bearings-units-housings/ball-bearings/deep-groove-ball-bearings/single-row-deep-groove-ball-bearings/axial-load-carrying-capacity/index.html

 

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Note that a little bit of axial load is actually benefical to the lifetime of the bearing as it makes sur the balls are always in contact somewhere and not wobbling around. Generally this is done on high speed applications (universal motors etc) with a ring-shaped wave spring ("Smalley spring").

Normally you should allow one of the bearings to float axially a tiny bit to compensate for thermal elongation. If your shaft and chassis do not expand at the same rate it will generate axial loads.

One thing to think about: the deeper the groove, the less tolerant the bearing is for misalignment (again, pressing the balls sideways and eating lifetime).

Normally you should also lock radially the race that rotates seen from the load's point of view (sometimes the load turns with the shaft, sometimes the load is always in the same direction compared to the chassis. This is to avoid that the bearing starts doing "Hula Hoop" in its housing (or the shaft doing it in the inner race depending on the load case).

But for DYI all this is overkill...

Normally the relevant info you look for are dimensions and maybe load.

Outside tolerances are usually pretty standard and not needed if you don't manufacture an engine yourself. (we are talking microns of tolerances...)

Usually the inner race bore is tolerance H7 which means it will slide on a standard ground shaft rod and the outer race is so that you will have to slightly tap it into a standard bore.

The load will help you determine the statisical lifetime (L10, expected lifetime after which there is a 10% chance of failure) This is counted in milions of turns, so if you don't put huge overload or fill the bearing with sand you will never experience a bearing failure in your lifetime.

Inside tolerances will give you an indication of how much play there is in the bearing itself. Tighter means more expensive. If you don't have a good reason to have a precision bearing (high speed and/or accurate positioning like a high rpm mill spindle) then stick to the regular ones.

Again, we are talking a couple of microns here, so if your machine is made of plywood or if you didn't manufacture the whole bearing arrangement within microns, you don't need accurate bearings or carefully calculate load case and liftime, it would be like putting a Ferrari motor in a Lada.

 

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