On 08/05/2019 15:30, T i m wrote:
I mentioned the adjustable (telescoping) dinghy trailer the other day
and how it used to have plastic inserts / collars taking up the small
(1mm or so) gap between the inner and outer, galvanised box steel
sections.
I quickly tried using tape the other day but found when you put enough
on to fill the gap, you couldn't actually slide the sections together
past a couple of inches and by the time you removed enough to allow
said, you were back to quite a gap again. ;-(
I still think a small band of fibreglass at the key points could be a
workable solution but I had another more mechanical solution /
thought.
What if instead of the M10 bolts that currently located the adjustment
by going though both inner and outer sections (and in some instances,
also hold other brackets in place), what if I drilled though one side
of both tubes say 20mm (the outer is 50 x 50 box, the inner smaller
obviously) and turned up some suitably long x 20mm OD, 10mm ID spacer
tubes that could be fitted though said 20mm holes and then stop up
against the inside of the opposite inner, when nipping them up it
would now pull the 'inner' section hard up against the inside of the
outer section, whilst the interference fit of the spacer would also
keep the other two tubes aligned on the opposite side?
This is the same process I've used many times when fitting towbars to
cars where the only fastening was to the bottom of a hollow box
section and they provide suitable spacers in the kit to avoid you
crushing the box (drill bolt hole right though, open top one up, drop
spacer into hole etc).
In the case of this trailer, you could actually run it with all the
nuts and bolts finger tight as the insertion of one tube into the
other seems to be sufficient to provide the strength (and in the case
of the axle halves, the turning resistance) so my only real 'concern',
is the opening up of the holes on one side from M10 to M20 (say) [1]?
I understand the stresses are at their least in the middle of the box
sections (especially when the holes go though horizontally and most of
the loads are vertical) but what does the panel think?
My goals here are to retain the ease of adjustment / takedown (you can
then carry it in an estate car / MPV or even on a roofrack / boot) but
to remove any of the clanking, especially when the trailer is lightly
loaded.
Cheers, T i m
[1] The spacer and suitable washer could stay on the bolt, effectively
forming a bolt with a bigger shoulder.
That could work. The issue might be fretting, because you are always
going to get some "flex" at the clamped joints.
I don't think I would worry about the loss of strength caused by the
larger hole.
I once fitted a fibreglass "dolphin" fairing to an old British bike
without any of the proper fittings. I used fittings fabricated from
aluminium sheet, clamped with rubber sheet and penny washers but, even
so, they regularly failed from fretting fatigue at the contact sites.
But ordinary "chassis" aluminium is particularly bad for both fretting
and fatigue. I think you would be unlucky to fail your steel tubes in
the same way. I would, though, regularly look out for fretting if you do
this. It is worth putting grease on the joints IMHO. Although this will
reduce the friction (and increase the chances of movement) it will also
provide a barrier to air access and reduce the risk of fretting wear.
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