On Fri, 10 May 2019 11:51:55 +0100, newshound
wrote:

snip

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.

Thanks.


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.

Interesting, thanks.


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.

Because material is effectively moved / removed between the
compressive dimensions (and hence forces) of the fasteners?

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.

A picture speaks 1000 words (hopefully). ;-)

http://tinypic.com/r/2zfociv/9

To help break down the picture, the main X member is 50x50 tube with a
smaller section stub welded in the middle facing forwards. The main
50x50 spine tube then bolts to that (and that bit is then 'fixed' as
such).

Into both ends of said main X member go small(er) section stubs that
also carry the suspension units and mudguards etc. These are held in
place by 2 x M10 bolts in one of possibly 2/3 different 'widths'.

Into the main spine section slides an equally long length of smaller
section that carries the tow hitch, bow snubber and jockey wheel and
that is also held in place (lengthwise) by one or two M10 bolts.

You might also be able to see from the picture the green plastic
collar / bushing that helps align / quieten the telescoping interface.


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.

Interesting, thanks. I was sort of envisioning that with any form of
insert that was transferring the compressive forces on the joint from
the bolt though to the inner surface. Eg, if no washer was used (as
with an external nut / bolt) would the 5mm 'material' of said spacer
(M20 OD, M10ID for M10OD bolt) have as much area in contact with the
inside box section as the skirt of a nut might (had you been able to
get one in there etc)?

Thinking on from that ... would a countersunk 'Nutsert' set into the
inner tube offer as good a fastening as said spacer (and would remove
the need to open the opposite sides up to 20mm etc, but would require
the need to replace the bolts with machine screws but would allow the
use of a 'locknut' on the thread sticking out the other side)?

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

Looks 'deep'. ;-)

snip

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.

Sorry ... an alternative to the above (goal) would be to have a 20mm
drill bit on a 10mmOD mandrel to ensure the holes though the faces of
the outer and inner box sections on one side remained concentric with
the M10 holes on the other? I'd drill them though in-situ because I
have no idea how accurate (relative to the geometry of the sections
themselves) any of the holes actually are. Pinning the two sections
together at one bolt hole would at least ensure some of it was held
still / straight etc. ;-)

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.

Yes, I think I'll have to get the 3D printer out. ;-)

Cheers, T i m

p.s. If using spacers, should I just use mild steel or would it be
worth using stainless (and how with they react with the mild steel /
galv / zinc of the existing components)?