On Thursday, July 21, 2016 at 9:45:47 AM UTC-7, Dan Marotta wrote:
Very informative, thanks!

How about this for the use in question: Drill the holes in the trailer
tongue oversize and use a steel bushing the exact length of the outer
tube of the tongue. Then use a high strength bolt to hold it together.
Or simply a trailer hitch pin with a spring clip or lock to hold it in
place.

On 7/21/2016 10:23 AM, Craig Funston wrote:
On Thursday, July 21, 2016 at 7:43:44 AM UTC-7, Dan Marotta wrote:
A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.

--
Dan, 5J
For typical steels used in bolt fabrication, shear strength correlates pretty well with tensile strength. Current guidance from the American Institute of Steel Construction use a shear strength equal to 60% of the tensile strength (ultimate values, not yield). This is for a bearing type (not friction) connection with threads included in the shear plane. High strength bolts are allowed shears up to 75% of tensile if the threads are excluded from the shear plane.

Craig
7Q

--
Dan, 5J

Drilling oversize holes in the tongue for a bushing just makes the bushing the bearing member and takes away any possibility of developing clamping force on the tongue which is where it's needed.

The bushing could be welded into the tongue or shorter bushings that are fed in from the end of the square tube to brace the sidewalls against compression from the bolt could be used

Craig