OT: How Are Torque Specs Calculated? (Hitch Install)

I put a hitch on my daughter's SUV this morning. There are a total of 6 fasteners.
4 each M10 - 1.25 x 30 Class 10.9 bolts (Class 10.9 is basically Grade 8) 2 each 7/16 x 14 nuts for a 7/16 x 14 U-bolt (the nuts are zink-colored, might be Grade 8, the U-bolt is silver)
As shown in the instructions linked to below, the M10 bolts are used to attach the hitch to the frame via the vehicle's weld nuts and the 7/16 nuts are used with the U-bolt to secure the center of the hitch to the vehicle's tow loop.
Per the instructions, the M10 fasteners should be torqued to 49 ft-lbs and the 7/16 fasteners should be torqued to 70 ft-lbs.
What factors are used to determine those 2 values? Is it the type of fastener? The stresses (shear vs. pull?) The thread sizes? A combination of everything?
The instructions, for your reference:
https://www.curtmfg.com/masterlibrary/13555/installsheet/CM_13555_INS.pdf
Thanks for the lesson in torque settings.
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On Jun 22, 2019, DerbyDad03 wrote

The usual rule is to torque to something like 70% of yield limit for the nut and bolt, to prevent loosening from road shock and vibration. With the conical washers (if Belleville type; not clear from instructions), it would be enough torque to flatten the washers. One installes Belleville washers with cone point at the bolt head.
Having this come loose would be a big problem. I would also use Locktite thread-locking compound on the various threads. Available in hardware stores and auto parts stores.
Joe Gwinn
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On Sat, 22 Jun 2019 14:45:54 -0700 (PDT), DerbyDad03

Don't know the reasons - but when I installed the hitch on the wife's Jetta < for bicycles only > the frame bolts were torqued to a decent value - I forget the exact number - but 2 of the other bolts were exhaust mounts and they were torqued very lightly ... The one other attachment point was the "drain hole " at the bottom of the trunk cavity - using a large heavy washer - and that was the one attachment point that didn't fit perfectly - maybe 1/2 inch off but I made it work .. I'm no auto mechanic < to say the least ! > I usually hate working on cars - and usually screw up - but this job made me think that ... if I can do it - anyone can ! John T.
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On Saturday, June 22, 2019 at 7:40:53 PM UTC-4, snipped-for-privacy@ccanoemail.ca wrote:

The hardest part was cleaning the welds nuts. I had to tap 2 of the 4 before I could even get the M10 bolts started. One of them was right above the muffler and it was a bear getting the tap in there.
That was also the hardest one to torque. I'm sure glad it was only 49 ft-lbs. My torque wrench is much longer than my regular ratchet handle and it barely fit where I needed it. I could only get one "click" of the ratchet mechanism on each return swing. Trying to hold the socket up on the bolt while tightening the bolt "one click" per swing was a real pain. I sure was happy when the torque click finally happened.
The previous hitch I installed was on my Odyssey right after I bought it used from the dealer. Either the previous owner took off the hitch that was on it or the dealer did. The van obviously had a hitch on it before (I could see the marks) and the weld nuts were perfectly clean. I could snug the bolts up by hand. Hitch, but no wiring, so I'm assuming a bike carrier was its purpose.
I also installed a tranny cooler on that vehicle. Ody trannies used to suck and you didn't want to stress it even more by towing without a cooler.
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On 6/22/2019 4:45 PM, DerbyDad03 wrote: ...

...
In short, yes. :) Hopefully, the manufacturer designed the loading based on some engineering calculations. A simple-to-read summary of how torque is computed for given fastener and application given a design loading is at <http://www.zerofast.com/proper-bolt-torque . That proper loading specification for the application is where the design engineer has to do the right thing.
Of course, as the article indicates, how what loading you actually get for a given torque compares to that design specification load that was used to provide the torque spec is all dependent upon how close your installation matches the assumptions made in the design calculation--particularly the K (friction) factor. The fastener dimensions and properties are pretty standard.
--


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On Saturday, June 22, 2019 at 7:45:15 PM UTC-4, dpb wrote:

I have to assume that the person that came up with the spec also knows the specs of the weld nut on the vehicle. Imagine spec'ing a tightening torque for the bolts that are 10 ft-lbs higher that the weld nut can handle.
Man that would suck!
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If you say so .. I doubt that my $ 175. hitch was subjected to any rigorous engineering evaluation .. :-) Copy & paste perhaps ? John T.
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On Saturday, June 22, 2019 at 9:16:52 PM UTC-4, snipped-for-privacy@ccanoemail.ca wrote:

Somebody is doing the engineering. Curt hitches for different vehicles (Ford F150, Honda Odyssey, etc.) all show different torque specs in their install instructions. Unless they are just making them up as they go along, the specs must be based on some amount of engineering research.
The hitches I looked at (and installed) are all under $150. Hitches are one of the most reasonably priced accessories you can buy, considering how useful they are.
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On 6/22/2019 10:45 PM, DerbyDad03 wrote:

...
While I've not done any looking, I'd be pretty certain there are DOT requirements that will cover manufacturer having needed to do such engineering or hired it done to be compliant.
--


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On Sat, 22 Jun 2019 14:45:54 -0700 (PDT), DerbyDad03

Correct. The size, material, and thread pitch all come into play along with the type of stress (which basically determines what bolt size, thread, and alloy are used)

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