On 21/07/2016 18:35, Craig Funston wrote:
> 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.
From my understanding of bolted joints, cyclic shear loads can induce
fatigue failure and using a higher strength steel is not going to
prevent fatigue. I suspect the designers underestimated the number and
magnitude of the fatigue cycles these trailers are subject to when towed
behind motor homes.
Note I have not looked at one of these in person to know if these
suggestions are practical. But I would look for a simpler fix:
- Drill out the 12mm bolts and fit 16mm bolts, and make the problem "go
away" for a long time, hopefully forever.
or
- Drill additional holes for 2 extra 12 mm bolts, one adjacent to each
of the existing ones. Then fit two extra bolts, but with one head of
each pair of bolts on opposite sides of the trailer. Biggest advantage
of this is there are now four bolts and any 3 will easily carry the
load. So if you check it once in a while and find a bolt missing or
damaged, you will have ample chance to replace it and check the other 3,
before things deteriorate to a point of potential catastrophic failure.
In reality the redundant bolts will probably not take any load, but
hopefully with pairs of bolts fitted from opposing sides, their will be
an un-threaded portion bolt to take the load in each major shear plane.
Ian