Mechanical engineering question.

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.
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On 08/05/2019 15:30, T i m wrote:

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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On Thu, 9 May 2019 10:12:01 +0100, newshound

Understood.
FWIW, the axle stubs (independent suspension unit, mudguard, length of 'inner' gauge square tube) would 'normally' be inserted to full depth because that still leaves the trailer wide enough for pretty well anything we are likely to put on it. That said, the 'axle' would only be held in at two points (bolts) which are only maybe 1/4 of the length of the axle tube apart.
So, if the wheel was loaded it's possible that (because the bolt holes are quite close), that the inner box could be rotated (on the mating surfaces) until the outer end of the inner tube (or the sides of the M10 bolts / holes AND spacers / holes on the non clamped sides, whichever happens the soonest) and the inner end on the inner tube does the same at the bottom. Once 'settled' however I can't see any reason it would move further (It would look like some very slight camber)?

Ok, thanks. I was hoping that a 20mm diameter hole drilled in one side wall of 50mm x 50mm square tube (that was mainly under a vertical bending type load) would put the hole in the least stressed area of the section?

I made the same by using a 'bubble' screen from a racing bike as a mould for a fibreglass copy, inverting the final form and fabricating a joining strip to fix them both together to form said 'dolphin nose'. This was then mounted on the bike using a home made sub-frame and the fairing was held on with nylon number plate screws with rubber sheet between the two. There was no engine vibration as this was an electric 'motorcycle' but there was quite a bit of road vibration because of the highly inflated tyres (lower rolling resistance on the track). ;-)

Oh.

How much actual movement to you need for said fretting though? Are we talking even at molecular / granular levels?

Again, FWIW, both tubes are galvanised and I'm not sure how 'smooth a surface that gives? Would any high spots tend to get crushed at the fasteners and so help bind the two surfaces together?

Understood.
Thanks for the feedback. ;-)
Cheers, T i m
p.s. If I were to open up one side (the same side <g>) of each hole for said spacer, I could pin one hole in place first (put a captive nut into the inner section) and then drill the larger holes though both tubes at the other point(s), (in the hope of making the holes as concentric / unstressed as possible).
p.p.s. I think I might still go for some form of 'collar' to help resist any movement of the inner tube in the outer and so also minimise any fretting.
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