Advice request from mechanical engineers

   / Advice request from mechanical engineers #1  

mwayne

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Maybe someone knowledgeable in mechanical engineering can explain a situation to me. I used some 1/2" all thread on a project and was told that the maximum tensile load I could allow for is 550 poundsper all thread. This was based on the information in a chart that I later found on engineering toolbox.com


As it was explained to me, since the allowable load on a 1/2" threaded rod is 1100 pounds per inch (according to the chart) and I was using a nut that was about 1/2" thick then I was only able to put a 550 pound load on that all thread.
When the chart was presented to me the note at the bottom was cropped out, that note states "The maximum loads in the table above are based on allowable tensile stress of 12 kpsi - reduced by 25% to 9 kpsi allowable stress." My understanding is that all thread is based on 60kpsi material. For background what I did with the all thread and nuts is this: I rolled a 1200 pound transformer into extremely tight quarters on a dolly then used the all threads (2) that were attached to an overhead beam as a screw jack to lift the transformer off the dolly and then lower it down in place. The task was performed successfully and no strain or imminent failure was noted.
Can someone please explain why this chart is different from my expectation? Is hanger rod different from all thread? Did I come as close to fouling up as the ME said? Thanks in advance for your replies.
 

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   / Advice request from mechanical engineers #2  
I'm not sure what the basis for the chart is, but you wouldn't want to design with 60 ksi for a mild steel hanger bolt. That is the failure stress. The yield stress is typically half the ultimate strength for low carbon steel. 9 ksi seems to provide a lot of margin, but given the potential for bending or impact in hanger rods, it might be prudent.
 
   / Advice request from mechanical engineers #3  
I'm no engineer but I do know there's different grades of all-thread just like there's different grades of bolts. The stuff I buy from the hardware store is probably the equivalent to a grade-2 bolt. The difference is primarily stretch or elongation. I would not use that for critical jobs even though it's probably 58-60ksi. The better grades used in construction can go up to 120ksi or more.
 
   / Advice request from mechanical engineers #4  
Maybe someone knowledgeable in mechanical engineering can explain a situation to me. I used some 1/2" all thread on a project and was told that the maximum tensile load I could allow for is 550 poundsper all thread. This was based on the information in a chart that I later found on engineering toolbox.com


As it was explained to me, since the allowable load on a 1/2" threaded rod is 1100 pounds per inch (according to the chart) and I was using a nut that was about 1/2" thick then I was only able to put a 550 pound load on that all thread.
When the chart was presented to me the note at the bottom was cropped out, that note states "The maximum loads in the table above are based on allowable tensile stress of 12 kpsi - reduced by 25% to 9 kpsi allowable stress." My understanding is that all thread is based on 60kpsi material. For background what I did with the all thread and nuts is this: I rolled a 1200 pound transformer into extremely tight quarters on a dolly then used the all threads (2) that were attached to an overhead beam as a screw jack to lift the transformer off the dolly and then lower it down in place. The task was performed successfully and no strain or imminent failure was noted.
Can someone please explain why this chart is different from my expectation? Is hanger rod different from all thread? Did I come as close to fouling up as the ME said? Thanks in advance for your replies.
Sorry, i get lost in the Imperial units...

Though the higher the fastener grade, the less difference there is between yield strength and break strength. Or actually, between when plastic defornation starts to occur, and the max strength. Unless you count on plastic deformation to absorb shock energy, you better calculate with the yield strength. Then a safety factor of 3 in alternating loads and a factor of 2 in static loads. And a factor of 5 if people get to walk underneath.

There are professors who promoted on fasteners, most of us have no clue about the amount of calculations you can do to optimise a bolt connection.

In most of the industry we just follow the standards and not spend a day to calculate a single bolt...
 
   / Advice request from mechanical engineers #5  
Static load or dynamic load. I'd worry more about strain (stretching and necking) that strain. Rods like compression, not tension, if that's their use.
 
   / Advice request from mechanical engineers #6  
Static load or dynamic load. I'd worry more about strain (stretching and necking) that strain. Rods like compression, not tension, if that's their use.
There are large differences in the design of Rods for compression as compared to tension.
 
   / Advice request from mechanical engineers #7  
Yes, tension is a lot better application for rods. In compression they fail by buckling at a relatively low load.
 
   / Advice request from mechanical engineers #8  
Sounds like you did a one time lift and not a permanent installation.

Different requirements for different applications.
 
   / Advice request from mechanical engineers #9  
Static load or dynamic load. I'd worry more about strain (stretching and necking) that strain. Rods like compression, not tension, if that's their use.
I use M36 rods (1" 9/16) as adjustable feet under my workbench made of 10mm (3/8") deck and UNP 120 (4" C channel)

I dont want them to buckle when i yank on stuff in the bench vise...
 
   / Advice request from mechanical engineers #10  
Static load or dynamic load. I'd worry more about strain (stretching and necking) that strain. Rods like compression, not tension, if that's their use.
This is literally the opposite of the truth. Rods are tension members, not compression members. Rods in compression buckle. You may be confusing the term "rod" from a practical standpoint, such as thinking "connecting rod", but the subject is allthread or threaded rod, which is also described perfectly in tension, like a true rod from an engineering standpoint.
 
 
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