On 10/05/2019 00:26, T i m wrote:
On Thu, 9 May 2019 10:12:01 +0100, newshound
wrote:
snip
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.
[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.
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)?
Havn't quite got my brain around the geometry (but I will come back to
this later)
I don't think I would worry about the loss of strength caused by the
larger hole.
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?
That's correct.
snipped
But ordinary "chassis" aluminium is particularly bad for both fretting
and fatigue.
How much actual movement to you need for said fretting though? Are we
talking even at molecular / granular levels?
Not very much movement. There is no clear cut definition. On very big
structures (say, tunnelling or earthmoving equipment, or large mobile
cranes) you might get what tribologists would recognise as fretting at
maybe a millimetre stroke. More conventionally, we'd think of it as
movements of maybe tens of microns. Sometimes you will see fretting
between parts that are a close fit, for example ball bearing races on
their shafts or housings, and the movements there might be only a micron.
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.
The effect of fretting would be to loosen a joint that you thought was
tight. I can't see any real prospect of getting a fatigue crack that
would propagate right through a tube. And even if it did, my picture of
your chassis is that each movement is defined by a pair of rails, not a
single rail (assuming you don't have adjustment on the "tow-bar" part).
So if a single "joint" failed the structure would still be held together
by the parallel joint. I'm not quite clear if your axle stub is on a
single square tube, or a pair of square tubes.
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?
Just about the only atomically smooth surface over relatively long
distances is freshly cleaved mica. Hot dip galvanised surfaces are
normally rougher than the substrates, they even feel "spikey" to the
touch. As you say, the high spots get crushed when surfaces are clamped
together. But actually the real area of contact under a tightly clamped
joint (for example between the washer and the main component for a bolt
tightened up to its yield point) might only be 10% of the nominal area.
For metals, the only place you see high real areas of contact is along
the contact line between an olive and the cones in a compression
fitting. I can't immediately find a link to a really good image, but
here's something that shows the idea (it's a bit misleading because it
is talking about lubricated contacts).
https://www.google.com/url?sa=i&sour...571050 365796
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.
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).
Sorry, a bit confused about this too.
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.
Collars are good, I'd try to do that too.
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus