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| Temperature Stabilised Bearings - how is the stabilising done ? |
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| Lew_Merrick_PE:
Hi David -- I suspect that we are, once again, people separated by a common tongue. Is it a wrench or is it a spanner, etc? If you notice, your classes of bearings have 90°F operating spans. I suspect that (say) an S5 bearing would rattle like a castanet at room temperature. There is almost certainly a mechanical/thermal preload allowance in each class such that the shrink-fit assembly will take up some (if not most) of the CTE/thermal expansion load in a manner similar to (mechanical) pre-loading of a bolt which reduces the rattle at lower temperatures -- at least that is how I remember it. Most of the things I have done requiring thermally specific bearings in recent years have gone on high-altitude aircraft and spacecraft which have opposite direction in their operating temperature requirements. There the issue is less CTE/thermal contraction than it is low-temperature embrittlement of steels. I suspect (but do not know directly) that you could approach it from the other side of the interface by adjusting your shaft and bore sizes to create the correct preload. That would certainly be simpler than diassembling bearings and making new race components. ??? |
| David Jupp:
Lew, thanks for your thoughts. I'm talking specifically about temperature stabilisation NOT thermal expansion issues. This is to do with the steels retaining hardness and hence not distorting at the operating temperature. I.E. it's a heat treatment and/or steel composition issue (and maybe some other 'magic'). The S value and the C value (these bearings have both specified) relate to different things. CTE shoudln't really be a problem as steel balls in steel cages in a uniform temperature environment (albeit hot) should not alter clearance very much from that at room temp - the brass cage may be affected of course. Am trying both alternate supplier of conventional bearings and a move to hybrid (steel race ceramic rollers, rated for 450 C - 842 F). It actually turns out that S3 would probably be adequate for our application - the S4 spec comes from original design which envisaged operation at up to 350 C, in reality it is under 300 C. |
| Lew_Merrick_PE:
David, 1) Just the fact that you have steel races and bearing elements all with the same CTE does not make the delta-T issue irreverent. The sizes of each element is different, so the expansion of each element is different. At least that was the primary issue back when I was building and designing (in the evolution of my career) rotary elements in hot gas-flow environments. 2) Think about the heat treating process. First there is hardening where you heat the metal up to some soak temperature, increase the temperature to a second soak temperature (usually just above the Curie Point), and then quench it to force the crystal structure of the metal to remain in the (high temperature modified) form. This is usually followed in modern commercial practice by a quench & soak at some seriously sub-zero (cryogenic) operation that, as I was taught in the 70's, converts more of the crystal structure into the high temperature modified form before being returned to room temperature. We now have a part with high hardness, high (stress-based) strength, and high brittleness because of the internal strains (i.e. stresses) inherent in the unbalanced high temperature modified crystal structure. Those stresses (strains) are relieved by tempering. The higher the soak temperature used in tempering, the more of the stresses are relieved and the hardness, strength, and brittleness are reduced. (And, in modern practice, tempering often includes more cryogenic processing.) The variations in temperatures and time of soak vary by the chemical composition of the alloy. High-speed steels are a class of alloys that retain their hardness, strength, and reduced brittleness (toughness) to higher operating temperatures than traditional carbon steels. Other classes of alloys (hot-work, tungsten alloy, etc.) have been developed for similar reasons. Thus, if you need a part to retain (say) Rc-62/60 hardness at some elevated temperature, it is more a matter of selecting the appropriate alloy (with the concomitant heat treating regimen) than applying a process to an alloy without the ability to maintain physical properties at the operating temperature. It would not surprise me (not being an expert in this field) that there is some secondary thermal processing required to establish properties at a given elevated operating temperature, but I have never heard of such a thing being called stabilization. At the risk of sounding like a hopeless pedant, I looked up heat stabilization in my 1984 Edition of the (26 volume) ASM Metals Handbook and found nothing appropriate to bearings. (Most of what I found deals with alloy composition for stability at various operating temperatures.) I have the standard machinist/tool & die/blacksmith background in metallurgy of my apprenticeship coupled with 2+ years of having a formally trained and well-experienced METALLURGIST (who led metallurgical R&D for the US Army for more than two decades) as my adviser in college. I have held my own with metallurgists over the years in a wide array of projects -- but I am not a metallurgist -- only a general-purpose mechanical design engineer. |
| David Jupp:
Lew, I have a degree in Metallurgy (for whatever that might be worth) - however I was not at all clear on what 'temperature stabilised' actually MEANT (that's why I posted here). It is a term usd by the bearing manufacturers, but they don't to my mind explain it at all clearly. Like you, the term did not match with my education in these matters. I take your point about different scale of rolling elements vs races. I'll consider further. I have subsequently found a little bit more information from manufacturers, but not a lot. It seems likely that in this context the term refers to a mixture of material selection and heat treatment. |
| Lew_Merrick_PE:
David, One of the companies that was a bread & butter client back in the 80's was The Rocket Research Company. They had bumper stickers printed up that read: But what if you are a rocket scientist and you still can't figure it out? Unfortunately, I was not working on site for them when they handed them out... |
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