MattFurness wrote:
You would think that if buckling in this
plane was an issue you would see wear on the edge of the bearings
where the oil film had broken down.
To be clear: buckling would happen exactly once. And you’d be lucky
to find enough pieces of the bearing afterward to do such an
analysis. Buckling is not something that can occur on an ongoing
basis.
Also…the clearance of the piston in
the bore and the length of the piston skirt by the gudgeon pin
probably prevents the application of any moment (along the plane of
the crankshaft) into the rod via the small end bearing. If this is the
case then the piston is pretty much applying direct thrust down the
rod centreline…
All true, and all unrelated to buckling failure. A buckling failure
presumes pure compression load, no moment; the sideways buckling is a
result of the pure compression.
Let’s see if I can clarify buckling theory a bit. Presume the center
of a column under compression loading is deflected a small distance
to one side. The compression loading then works to increase this
deflection. There is also a “restoring force” generated within the
column itself, as a result of its stiffness, that tends to pull it
back straight. As long as the restoring force exceeds the
compression’s tendency to bend the column further, the column won’t
buckle.
The amount of deflection is the issue. If the center of the column
is deflected a lot, it doesn’t take much compression to overpower the
restoring force and buckle the column. The higher the compressive
load, the less you can deflect the center of the column before you
get into trouble.
There’s a limit, though; once a particular level of compression is
reached, any deflection at all will cause buckling – which means, in
essence, that no deflection is required, it will generate its own
deflection. The column will buckle under pure compression with no
bending moments or sideways deflections input. This is the failure
mode I’m talking about with conrods subjected to tens of thousands of
G’s.
Buckling is a catastrophic failure. Once buckling commences, it very
quickly accelerates to complete folding of the column because the
deflection reduces the load the column can carry. The very first
time a conrod tries to buckle, you will have a bent or broken conrod.
I was wondering if the H design can have better radii
down onto the big end journal and thus improve the fatigue situation
at the change of section.
Possible, I suppose, but I’m not sure what’s wrong with the radii on
the I configuration.
Normally with this sort of stuff the
original designers get it pretty right which is why the standard rods
all look pretty similar. Then along comes the ‘‘H’’ rod and it is
supposed to be better. It may well be an improvement in some second or
third order consideration which the original Engineers weren’t worried
about.
There’s also a design criteria difference. The engineers designing
the production conrods are looking for long fatigue life at moderate
loads and speeds coupled with ease of manufacture. The aftermarket
rods, though, are typically designed with competition in mind, so the
emphasis is on high load and high speed with less concern for ease of
manufacture.
It wouldn’t surprise me at all if the I conrod is significantly
easier to mass-produce than the H rod. Of course, it may just be
that the big manufacturers are geared up to make I rods and don’t
want to retool to make H rods.
Maybe rotational inertia…
Offhand, I’d suspect that H rods have larger rotational inertia. The
I has one broad rib and two short ones, while the H has two broad
ribs and one short one. Of course, presuming all these ribs are
roughly equivalent in thickness might be a mistake.
maybe windage losses???
Might the H form little scoops to catch air?
maybe something else???
Of course. We’re just guessin’ here.
– Kirbert
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