Al-Ko Trailer Tongue failure

On Thursday, July 21, 2016 at 9:12:54 AM UTC-7, AS wrote:
On Thursday, July 21, 2016 at 10:43:44 AM UTC-4, 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

Dan - bolting theory 101: Never design a bolted joint where the bolt(s) are going into shear! A bolt is supposed to clamp two or more members together and the whole thing is supposed to hold together by friction. If the joint moves, the axial pre-stress or clamping force provided by the bolt(s) was not high enough! Short, stubby bolts will not hold their clamping force for long. That's why long, slender bolts that can be elongated up to their yield point and act as axial springs are preferred over short, stubby ones. Going up in bolt diameter does in most cases not solve the problem.
If you really want to improve the ALKO design, use stand-off bushings (NOT stacks of washers!) and longer bolts. Figure out what the torque rating for that bolt grade is and precisely torque it to that value. Use a good torque wrench and NOT Bubba on a 3ft cheater pipe!

Uli
AS

Uli,

Use of bearing vs. friction for bolts in shear depends on which industry you're operating in. For building structures it's quite common to use bolts in bearing since often the loads are relatively static. Friction bolting is advantageous in applications where the loads are cyclic and fatigue is a concern. It's also advantageous for applications that require accommodation of loose field tolerances since the bolts can be tightened to clamping friction and the bolt holes can be oversize or slotted.

I agree the ALKO tongue connection is subject to cyclic loads and fatigue and would benefit from a properly installed friction bolted connection. A compression bushing between the inside walls of the square tube is needed to develop the proper clamping force. Drilling a larger hole for a bushing and pushing it through the square tube won't accomplish the goal.

Craig
7Q