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| Temperature Stabilised Bearings - how is the stabilising done ? |
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| David Jupp:
I've just been advised that temperature stabilised bearings will no longer be supplied in particular sizes due to 'lack of demand'. Manufacturer takes the view that 'standard' bearings ought to be OK for almost all applications, and for others it is just tough. The application I'm thinking of uses S2 bearing in the cooler part of the system and S4 in the hotter part. These bearings are moderately large and expensive, so stripping and re-working standard bearings may not be as daft as it sounds. Does anyone have any detail of how rolling bearings are heat stabilised? Can anyone point me to a resource on this matter? My other thought is to work with a specialist company to move to ceramic bearings. A secondary question - the temperature stabilisation is claimed to be about dimensional stability, BUT what exactly does this mean? What happens if the bearing gets too hot (apart from thermal expansion)? I ask because the application in question is surrounded by 'myth and magic', but nobody seesm to really know on what basis the original design decisions were made nor if they are critical. Any input is valued! |
| NeoTech:
Can only relate to what we do when making custom knifes during heat treat.. Then a temperature stabilizing process is done by packing the hot item in Co2 ice. Its also called a Subzero heat treat or zero stabilisation.. It makes crap really frickin hard.. especially 440C steels. And when the steel is heated backup again it really doesnt loose any tolerances. Unless its closing in on temperatures that would omit the heat treat completely. (800C and above in my case with knifes). |
| David Jupp:
Thanks - re-heat treating a finish ground part isn't really very practical. I've since found some information that suggests that different steels may be used for the higher S grade bearing races (not sure about the balls). Either way it isn't looking very promising to 'upgrade' an existing bearing, it seems the temperature stabilisation happens much too early in manufacturing to be able to do it as an after thought. Looks like the options are find an alternative manufacturer, or move to ceramic/hybrid bearings. |
| Lew_Merrick_PE:
David, I am not sure about the process of which you are speaking. I know of bearing elements designed and made for operation at a specific temperature, but I have never heard that referred to as stabilization. I am more familiar with the term as applied to plastics and aluminum materials where you "soak" them at a somewhat elevated temperature as part of a stress relieving process. Most steels have a Coefficient of Thermal Expansion (CTE) that runs in the close neighborhood of .000006 in/in/°F. Thus, at an operating temperature of 570°F, they would expand (500 X .000006 =) .003 in/in of dimensional length (glossing over some geometric relationships for the sake of simplicity). If the (say) inner bearing race had a neutral axis diameter of 3.250 inches (not uncommon in 3.000 ID bearings), then the race would expand to (pi X 3.250 + 3.250 X pi X .003 =) 10.2408 inches in length making for a (10.2408/pi =) 3.2598 neutral axis diameter -- which could really screw up the fit of your bearing elements. It has been many years since I have done this, but bearings are designed for such operating conditions. There is a lot of art in such design work. But I have never heard this referred to as heat stabiliziation... |
| David Jupp:
Hi Lew - that is what the bearing suppliers we have talked to seem to call it, though there may be other names for it. The one thing that should be universal is the S number (regardless of what you call how it is achieved). Ratings up to S5 exist, but not necessarily for all bearing types/sizes. S0 for operating temperature up to 302ºF S1 for operating temperature up to 392ºF S2 for operating temperature up to 482ºF S3 for operating temperature up to 572ºF S4 for operating temperature up to 662ºF S5 for operating temperature up to 752ºF The clearance class is important too and is specified as C3. In this application the temperature comes from the environment not really from any localised heating of the bearing elements, so thermal expansion shoudln't be too much of an issue. The bearings used are double row shperical roller bearings of moderate size - used to be able to get S2 and S4 without problem, but other applications which needed this have apprently disappeared or have been dealt with differently. Our application is now left 'out on a limb'.... |
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