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#81
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drill/tap in frames
On 7/11/2018 11:16 PM, Jeff Liebermann wrote:
.... If they break at the same tension, then I'll declare the Rivnuts are safe to use. If there's a substantial difference in tension, then I'll declare the Rivnuts weaken the frame. Note that your two stated possibilities are not mutually exclusive. Your logic is a sort of mirror image of the fallacy applied to all sorts of weird bicycling schemes: "This makes bicycling a bit safer [or at least, we think so], so everybody should use it." And in its worst version: "We must make it mandatory." -- - Frank Krygowski |
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#82
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drill/tap in frames
On Thursday, July 12, 2018 at 8:05:59 AM UTC-7, AMuzi wrote:
On 7/12/2018 2:31 AM, sms wrote: On 7/11/2018 8:43 PM, Jeff Liebermann wrote: On Wed, 11 Jul 2018 17:24:40 -0400, Frank Krygowski wrote: Exactly. Strong enough is strong enough. OK, so let's pretend that the tube with the Rivnut bent at 10% less tension. Is that "strong enough"? There's no way to tell without the original design calculations, or reverse engineering the frame with an FEA model. Too bad Autodesk killed their online ForceEffect web app. http://blogs.autodesk.com/inventor/2017/01/17/autodesk-forceeffect-family-retirement/ I think I could have modeled the problem using the program. "Rivnuts are great for low stress areas, but they rely on expanding in addition to crushing to grip the surrounding material, therefore a rivnut will impose a tensile stress around the hole which isn't good news since this will add to any load stresses, not to mention the concentration effect brucey speaks of. I'm not sure there is any place on a bike frame that I'd be happy to use them. Any time I've needed to attach something it's been with a properly machined alloy 2-bolt clamp around the tube with a thin [1mm] thick rubber shim between clamp and tube." Why then don't more airplanes fall out of the sky? https://www.skygeek.com/rivnut-tool.html http://spenceraircraft.com/hardware/...ivet-nuts.html from that page: " Our rivet nuts are manufactured to meet the National Aerospace Standard. " So much for zero tolerance eh? With a broken bicycle you could walk home! The rivnut industry has been hiding the truth for almost a century. See this? https://tinyurl.com/yd5s33m9 Massive rivnut-related failure. Apollo 13? Rivnut. It is so much worse than you know. -- Jay Beattie. |
#83
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drill/tap in frames
On 7/12/2018 10:39 AM, Jeff Liebermann wrote:
On Thu, 12 Jul 2018 00:22:57 -0700, John B. Slocomb wrote: On Wed, 11 Jul 2018 20:16:33 -0700, Jeff Liebermann wrote: Nope. I'm not trying to measure if the tubing is strong enough. Assuming identical lengths of tubing, I wanted to see if the presence of a Rivnut significantly changed the tension required to bend or break the tubing when compared to the identical tubing that did not have a Rivnut inserted. If I'm able to pull hard enough, I should be able to eventually break both tubes. If they break at the same tension, then I'll declare the Rivnuts are safe to use. If there's a substantial difference in tension, then I'll declare the Rivnuts weaken the frame. I'd think it obvious that any hole drilled laterally into a tube would have an effect on the bending strength of the tube. The question wouldn't be whether the rivnut changed the strength of the tube but whether the tube was strong enough with the rivnut installed in the tube. Agreed. Reading between the lines, what others are apparently suggesting is that even with a hole drilled into the frame, the tubing is still sufficiently strong to consider the bicycle rideable. In other words, if the drilling a hole and installing a Rivnut decreased the bending strength by 10%, I would agree that the hole and Rivnut don't pose a risk. However, if it decreased the strength by 50%, I would consider it a hazardous modification. I'm not too sure what to do about numbers in between or even if the 10% is realistic. To complicate matters, there's the problem of the notch required by Rivnuts to prevent rotation. That's a stress riser by anyone's definition and will probably be the start of any break during testing. I'll try to position it where it will do the least damage. Now all I have to do is find a donor steel frame and a way to bend it without producing a crimp. Drivel: The CNC conversion is about 80% done with cables running everywhere and no way to close the controller box: http://802.11junk.com/jeffl/pics/CNC-conversion/ Work on the conversion came to a screeching halt when one of the participants went in for major surgery, and the owner got a big order that required dropping everything and switching to making parts. The good part is that it appears that profits from this first job will more than pay for the cost of the hardware and parts (about $3,000USD total). We even bought a new VFD that really wasn't needed. The only problem is that I can't get any machine time to make my toys. Seamless 4130CrMo aircraft tube is cheap if you want to pursue it with two sections cut from the same tube. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#84
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drill/tap in frames
On 7/11/2018 11:43 PM, Jeff Liebermann wrote:
On Wed, 11 Jul 2018 17:24:40 -0400, Frank Krygowski wrote: Exactly. Strong enough is strong enough. OK, so let's pretend that the tube with the Rivnut bent at 10% less tension. Is that "strong enough"? There's no way to tell without the original design calculations, or reverse engineering the frame with an FEA model. Too bad Autodesk killed their online ForceEffect web app. http://blogs.autodesk.com/inventor/2017/01/17/autodesk-forceeffect-family-retirement/ I think I could have modeled the problem using the program. Maybe this will work: http://structural-analyser.com Nope. Won't do tubing. And BTW, the test you're describing would be much, much easier to do in a proper tensile testing machine. Find an engineering student, get him interested, have him get permission to do it as a class project, and your data would be much better. Yep, but my use of UCSC equipment has turned into a complex hassle. I'll spare you the details, but at this time, it's not an easy options. I was wondering how I would do it on a proper machine. Probably support the tube at the ends and push in the middle behind the Rivnut. The problem with that is I'll probably crimp the tube where it's pushing. It's likely I'll tear the tube before it bends. So, I'm stuck with securing one end, and pulling (or pushing) on the other. But on the other hand, tensile strength of the tube isn't really the concern. The concern would be fatigue strength, and if we're talking about the down tube, it would be under repeated, reversing torsional stresses. Good point. However, it will take too many tubing samples to test all the possible combinations of forces available. Shall we keep it simple and just bend a tube or two? Well, it depends on which you want: Test results? Or good and applicable data? I strongly suspect that you'd find no significant difference. One feature of the Rivnut is that its clamping action on the parent metal applies compressive stress. Fatigue cracks start in regions of tensile stress. The Rivnut may even make the object stronger. Clamping action requires equal compression at all points around the Rivnut hole. That's not going to happen in tubing where the Rivnut is being crimped onto a curved surface. At the peak of the curve, there will probably be plenty of compression force holding the Rivnut in place. 90 degrees to either side, there may be an air gap with zero compression force. In order to make it stronger on the curved surface of the tubing stronger, the Rivnut would need a matching curve. I agree there would be variations in compression around the circumference of the hole. I doubt that they would be important. Here's the situation with dozens of details of bicycle design: Because of the geometric complexities, plus the uncertain and variable loads, we can't be precisely sure of the stress levels or safety factors. We could ensure nice high safety factors only by adding considerable weight, but that's usually undesirable. So what has happened in practice over the last 150 years? Effectively, it's been evolution by trial and error. A framebuilder may try a new way of fabricating (say) the connection of the seat stays to the main frame tubes, one that saves two ounces. Others notice and wonder if it will break. If it doesn't break, others copy it. If it breaks, it's not used again. It's survival of the fittest designs. These days, some can streamline the trial and error process by use of FEA. But A) that usually happens only in big firms like Trek, Cannondale or Specialized; and B) it's still normally done only for main design features, not for details like Rivnuts. How does this evolution process apply to Rivnuts? They're used a lot. They work. They're acceptable. Anything more is Scharfian nonsense or navel gazing. -- - Frank Krygowski |
#85
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drill/tap in frames
On 7/12/2018 10:59 AM, Frank Krygowski wrote:
On 7/12/2018 11:39 AM, Jeff Liebermann wrote: On Thu, 12 Jul 2018 00:22:57 -0700, John B. Slocomb wrote: On Wed, 11 Jul 2018 20:16:33 -0700, Jeff Liebermann wrote: Nope. I'm not trying to measure if the tubing is strong enough. Assuming identical lengths of tubing, I wanted to see if the presence of a Rivnut significantly changed the tension required to bend or break the tubing when compared to the identical tubing that did not have a Rivnut inserted. If I'm able to pull hard enough, I should be able to eventually break both tubes. If they break at the same tension, then I'll declare the Rivnuts are safe to use. If there's a substantial difference in tension, then I'll declare the Rivnuts weaken the frame. I'd think it obvious that any hole drilled laterally into a tube would have an effect on the bending strength of the tube. The question wouldn't be whether the rivnut changed the strength of the tube but whether the tube was strong enough with the rivnut installed in the tube. Again, I expect some reinforcing effect from the Rivnut's clamping action. For an analogy: Did steel frames fail in significant numbers when water bottle bosses were brazed on and tapped? I doubt it. While a crimped-on Rivnut wouldn't add as much strength (as a guess) I think it may add enough to get the strength back up to that of the un-drilled tube. Also, I'm pretty sure a bike down tube sees little if any bending stress, except perhaps in a crash. The stresses of concern are torsional. Agreed. Reading between the lines, what others are apparently suggesting is that even with a hole drilled into the frame, the tubing is still sufficiently strong to consider the bicycle rideable. In other words, if the drilling a hole and installing a Rivnut decreased the bending strength by 10%, I would agree that the hole and Rivnut don't pose a risk. However, if it decreased the strength by 50%, I would consider it a hazardous modification. I'm not too sure what to do about numbers in between or even if the 10% is realistic. To complicate matters, there's the problem of the notch required by Rivnuts to prevent rotation. That's a stress riser by anyone's definition and will probably be the start of any break during testing. I'll try to position it where it will do the least damage. The notch is small enough to be enveloped in the crimped portion of the Rivnut. I doubt that it's effective as a stress riser. Really, I doubt that it feels any significant stress, other than compression from the clamping or crimping action. In practice, crashed frame tubes don't deform at the bottle bosses. The general truism is that a brazed joint is as strong or stronger than the steel tube so a brazed insert poses no risk. That may not be exactly correct but we work with it. Some builders at the cusp of change between 'no brazed bits' and 'braze every possible thing' fashions (like Galmozzi) brazed bolts on the tube and so nuts secured the bottle cage. Odd looking but worked as well as anything. -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#86
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drill/tap in frames
On 7/12/2018 3:01 AM, sms wrote:
On 7/11/2018 8:43 PM, Jeff Liebermann wrote: On Wed, 11 Jul 2018 17:24:40 -0400, Frank Krygowski wrote: Exactly. Strong enough is strong enough. OK, so let's pretend that the tube with the Rivnut bent at 10% less tension.Â* Is that "strong enough"?Â* There's no way to tell without the original design calculations, or reverse engineering the frame with an FEA model.Â* Too bad Autodesk killed their online ForceEffect web app. http://blogs.autodesk.com/inventor/2017/01/17/autodesk-forceeffect-family-retirement/ I think I could have modeled the problem using the program. It's a bad experiment because it doesn't take into account whether or not the hole was properly drilled and the Rivnut properly installed. You also have to do it with the same aluminum tubing used on a bicycle frame, but even then it isn't accurate because there's no way to factor in metal fatigue which isn't an issue as much in steel as it is in aluminum. Remember, just because you can often get away with doing a really stupid thing, it doesn't mean that you should still do it. Remember, just because one person declares a practice to be stupid, it doesn't mean the practice really is stupid. In fact, if the practice (like the use of Rivnuts) is generally very successful, the stupidity probably lies elsewhere. -- - Frank Krygowski |
#87
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drill/tap in frames
On 7/12/2018 11:54 AM, Jeff Liebermann wrote:
On Thu, 12 Jul 2018 00:41:36 -0700, John B. Slocomb wrote: Then, of course, the question arises as to what is a significant difference? Someone mentioned a bit ago about deciding that a TIG welded frame joint was strong enough to work, ignoring that a properly made sweated joint is always much stronger. Bicycle tubes and thus frames, are not all of the same strength. Columbus XCR material has an ultimate tinsel strength of 1350 MPa (195,800 PSI), Nobium is 1050 (152,289 PSI) and 25CRMO4 is 80 MPa (116,030 PSI). One can only speculate on the effects of drilling, oh say, a quarter inch rivnut hole in a (approximately) 200,000 psi strength tube and doing the same thing in a tube roughly half the strength? Good point. I plan to use the weakest possible steel tubing as excavated from the junk pile. I think it unlikely that I'll find any manner of exotic metals or even double butted tubing. At this time, all I care about is that the two tubes are reasonably identical. If you really proceed with this, you should understand that under static loads, low strength steels are largely unaffected by stress risers. Their ductility enables microscopic high stress areas to yield and distribute stress away from the discontinuities. High strength steels are usually less ductile, so stress risers are more of an issue. All steels are affected by stress risers in fatigue situations. -- - Frank Krygowski |
#88
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drill/tap in frames
On 7/12/2018 11:26 AM, Frank Krygowski wrote:
On 7/12/2018 3:01 AM, sms wrote: On 7/11/2018 8:43 PM, Jeff Liebermann wrote: On Wed, 11 Jul 2018 17:24:40 -0400, Frank Krygowski wrote: Exactly. Strong enough is strong enough. OK, so let's pretend that the tube with the Rivnut bent at 10% less tension. Is that "strong enough"? There's no way to tell without the original design calculations, or reverse engineering the frame with an FEA model. Too bad Autodesk killed their online ForceEffect web app. http://blogs.autodesk.com/inventor/2017/01/17/autodesk-forceeffect-family-retirement/ I think I could have modeled the problem using the program. It's a bad experiment because it doesn't take into account whether or not the hole was properly drilled and the Rivnut properly installed. You also have to do it with the same aluminum tubing used on a bicycle frame, but even then it isn't accurate because there's no way to factor in metal fatigue which isn't an issue as much in steel as it is in aluminum. Remember, just because you can often get away with doing a really stupid thing, it doesn't mean that you should still do it. Remember, just because one person declares a practice to be stupid, it doesn't mean the practice really is stupid. In fact, if the practice (like the use of Rivnuts) is generally very successful, the stupidity probably lies elsewhere. Goes both ways. Sometimes, the crowd looks to The Left while visionaries look Right: https://www.zerohedge.com/sites/defa...20nato%204.jpg Other times the 'different drummer' is trouble: http://www.latimes.com/local/lanow/l...122-story.html -- Andrew Muzi www.yellowjersey.org/ Open every day since 1 April, 1971 |
#89
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drill/tap in frames
Frank Krygowski wrote:
Then you should definitely not just drill and tap the frame tube itself. Why not? -- underground experts united http://user.it.uu.se/~embe8573 |
#90
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drill/tap in frames
On Thu, 12 Jul 2018 10:05:56 -0500, AMuzi wrote:
On 7/12/2018 2:31 AM, sms wrote: On 7/11/2018 8:43 PM, Jeff Liebermann wrote: On Wed, 11 Jul 2018 17:24:40 -0400, Frank Krygowski wrote: Exactly. Strong enough is strong enough. OK, so let's pretend that the tube with the Rivnut bent at 10% less tension. Is that "strong enough"? There's no way to tell without the original design calculations, or reverse engineering the frame with an FEA model. Too bad Autodesk killed their online ForceEffect web app. http://blogs.autodesk.com/inventor/2017/01/17/autodesk-forceeffect-family-retirement/ I think I could have modeled the problem using the program. "Rivnuts are great for low stress areas, but they rely on expanding in addition to crushing to grip the surrounding material, therefore a rivnut will impose a tensile stress around the hole which isn't good news since this will add to any load stresses, not to mention the concentration effect brucey speaks of. I'm not sure there is any place on a bike frame that I'd be happy to use them. Any time I've needed to attach something it's been with a properly machined alloy 2-bolt clamp around the tube with a thin [1mm] thick rubber shim between clamp and tube." Why then don't more airplanes fall out of the sky? https://www.skygeek.com/rivnut-tool.html http://spenceraircraft.com/hardware/...ivet-nuts.html from that page: " Our rivet nuts are manufactured to meet the National Aerospace Standard. " So much for zero tolerance eh? With a broken bicycle you could walk home! I might be interesting to know that the "rivnut" apparently was first developed to hold the deicer boots on the leading edge of an aircraft's wing in 1930, so we can safely say that they have been successfully used in the aircraft industry for the past 88 years.... strange how they could have been so successful in one industry while (apparently) a failure in another. -- Cheers, John B. |
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