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#1
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On Friday, July 8, 2016 at 2:43:03 AM UTC-5, Per Carlin wrote:
On Friday, July 8, 2016 at 4:31:04 AM UTC+2, wrote: Remember that "tensile strength" measures a bolt under a longitudinal load. What you are seeing in this instance is a failure under "shear load." Two entirely different situations, and one of the main reasons that aircraft bolts (AN) are not the same as "Grade 8" or the European (DIN) equivalent.. High tensile strength bolts often exhibit less than desirable brittleness under shear load. I think Mark is close to the root cause of the failing bolts. It is not the strength of the bolts itself that makes in brake (sounds funny, I know). In the bolt-configuration in the initial post is the function of the bolts to hold the Al-Ko tongue tight to the square bar, the friction between the tongue and the bar makes the strength. If the friction coefficient is low (fat, grease, dirt) or bolts is not tight (loosen by vibrations, deformations etc) is the friction between the tongue/bar low and a shear stress occurs on the bolts. The bolts are not dimensioned for this and will brake by fatigue. This is a common problem on Cobra trailers(the nose-cone / spare wheel holder), the bolts a not tighten enough from the factory and / or they vibrate loose on the road. Per is right, a properly engineered bolted connection is never meant to hold shear loads. The longitudinal tension in the bolted connection should press the pieces together with a force that causes friction sufficient to withstand the bending moment. |
#3
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On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote:
wrote on 7/8/2016 7:36 AM: Per is right, a properly engineered bolted connection is never meant to hold shear loads. The longitudinal tension in the bolted connection should press the pieces together with a force that causes friction sufficient to withstand the bending moment. Bolts are routinely used in shear, from toys to airliners. On my glider, the tail wheel, landing gear, and propeller are fastened with bolts in shear, along with many other items. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) - "A Guide to Self-Launching Sailplane Operation" https://sites.google.com/site/motorg...ad-the-guide-1 - "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm http://soaringsafety.org/prevention/...anes-2014A.pdf Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does. |
#4
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On Friday, July 8, 2016 at 9:05:24 AM UTC-7, wrote:
On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote: wrote on 7/8/2016 7:36 AM: Per is right, a properly engineered bolted connection is never meant to hold shear loads. The longitudinal tension in the bolted connection should press the pieces together with a force that causes friction sufficient to withstand the bending moment. Bolts are routinely used in shear, from toys to airliners. On my glider, the tail wheel, landing gear, and propeller are fastened with bolts in shear, along with many other items. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) - "A Guide to Self-Launching Sailplane Operation" https://sites.google.com/site/motorg...ad-the-guide-1 - "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm http://soaringsafety.org/prevention/...anes-2014A.pdf Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does. Herb, There are some cases where friction is what is relied on for a bolted shear connection, but these are very special cases and require specific installation procedures. In building structures, these bolts are designated ASTM A325 SC or ASTM A490 SC. The "SC" designating slip critical. In order to perform as slip critical connections there needs to be enough capacity in the bolted sandwich to allow the bolt to yield slightly in tension during tightening. This provides a consistent preload to develop the friction. This type of connection is impossible to achieve when bolting though a hollow tube unless there is a bearing sleeve through the tube to allow development of full tension in the bolt. Without the bearing sleeve the bolt simply crushes the tube and sufficient clamping force is never achieved. Many buildings and other structures are designed and constructed using bolts in shear without relying on friction between the faying surfaces. Slip critical bolting (friction bolting) is a nice solution when cyclic loads and fatigue are a consideration because it eliminates the movement that happens when bearing bolts (bolts in plain shear) are cycled from one load direction to another. Cyclic loading can also be accommodated by interference fit bolts, but the degree of accuracy required in fabrication is too expensive for all but specialty applications. Best regards, Craig Funston, P.E. , S.E. , P.Eng. |
#5
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On Friday, July 8, 2016 at 9:29:13 AM UTC-7, Craig Funston wrote:
On Friday, July 8, 2016 at 9:05:24 AM UTC-7, wrote: On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote: wrote on 7/8/2016 7:36 AM: Per is right, a properly engineered bolted connection is never meant to hold shear loads. The longitudinal tension in the bolted connection should press the pieces together with a force that causes friction sufficient to withstand the bending moment. Bolts are routinely used in shear, from toys to airliners. On my glider, the tail wheel, landing gear, and propeller are fastened with bolts in shear, along with many other items. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) - "A Guide to Self-Launching Sailplane Operation" https://sites.google.com/site/motorg...ad-the-guide-1 - "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm http://soaringsafety.org/prevention/...anes-2014A.pdf Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does. Herb, There are some cases where friction is what is relied on for a bolted shear connection, but these are very special cases and require specific installation procedures. In building structures, these bolts are designated ASTM A325 SC or ASTM A490 SC. The "SC" designating slip critical. In order to perform as slip critical connections there needs to be enough capacity in the bolted sandwich to allow the bolt to yield slightly in tension during tightening. This provides a consistent preload to develop the friction. This type of connection is impossible to achieve when bolting though a hollow tube unless there is a bearing sleeve through the tube to allow development of full tension in the bolt. Without the bearing sleeve the bolt simply crushes the tube and sufficient clamping force is never achieved. Many buildings and other structures are designed and constructed using bolts in shear without relying on friction between the faying surfaces. Slip critical bolting (friction bolting) is a nice solution when cyclic loads and fatigue are a consideration because it eliminates the movement that happens when bearing bolts (bolts in plain shear) are cycled from one load direction to another. Cyclic loading can also be accommodated by interference fit bolts, but the degree of accuracy required in fabrication is too expensive for all but specialty applications. Best regards, Craig Funston, P.E. , S.E. , P.Eng. I was about to start in with the shear versus friction discussion on bolted joints when I saw Craig's discussion. He is absolutely correct. I have been in the crane design industry for many decades and use the friction joint technique when using many bolts in a connection but when one bolt is used, shear is usually the design mode. Lynn Wyman, P.E. |
#6
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Thanks F1 and company--love the shared knowledge here.
I've had one nose cone bolt and one front handle/top bolt fail, and carry all spares. Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh. Anyone successfully upgrade the Cobra suspension to help with the sharp jar and also big dips? I run 13" trailer tires at high psi, original steel rims. Does Cobra offer a heavy duty option for insert-state-here? Thanks in advance. |
#7
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On Monday, July 11, 2016 at 1:43:56 PM UTC-7, Auxvache wrote:
Thanks F1 and company--love the shared knowledge here. I've had one nose cone bolt and one front handle/top bolt fail, and carry all spares. Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh. Anyone successfully upgrade the Cobra suspension to help with the sharp jar and also big dips? I run 13" trailer tires at high psi, original steel rims. Does Cobra offer a heavy duty option for insert-state-here? Thanks in advance. That bolt did not fail because it was under-specified for the application - there is something else going on here. Adding a bolt with greater strength will not resolve the issue. Just think about it: what forces are being applied to the bolt by towing that would have caused it to fail? As an engineer, I have seen so many times when people become fixated on the wrong cause of the problem (I have done it myself). You may have just have had a nut that came loose because of vibration. Tom |
#8
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On Monday, July 11, 2016 at 4:43:56 PM UTC-4, Auxvache wrote:
Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh. I can only second that, 'At the cows'! I just returned from Moriarty on I-40. West Memphis has a long bridge or elevated highway build with concrete sections which are all sagging in the middle. That excited my van and two-axle trailer to the point where I felt the front wheels coming off the ground.. The whole rig was galloping and the only way to make it stop was to slow down to about 35mph, which did not please the truckers behind me. I did inspect the Cobra trailer, ALKO tow bar assembly and the contents of the trailer afterwards and did still find everything being tight and in place. Road/Highway maintenance does not seem to be important in certain states. Uli AS |
#9
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Keep in mind (not really directed to Herb....) if you upgrade the bolt rating, it ALSO requires a higher torque value to maintain the correct clamping force within the elastic limits of the fastener.
Common issue is, "well hey, I put in better hardware, but it still failed!!", person never properly torqued the better hardware to an appropriate torque valve within elastic range. I'll stay out of lubed vs. dry threads, rusty vs. clean threads........ |
#10
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Just checked my trailer year 2011 and the same cross bolt was sheared at the nut. The bolt was still in but the nut was gone and you could see the shear plane. and yes the trailer is towed with a motor home.
David ASG29E BV |
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