Actually they do both stop - I mean they have to, as they are part of the same
whole - but
the big end of the rod is describing a circle the whole time, while the small
end is
constrained to a straight line by the action of the piston in the cylinder bore
- and both
are rotating around their respective mating parts. The motion we are concerned
with here
is translation, the straight line motion of the piston and connecting rod. The
piston
moves only in a straight line, while the conn rod is both translating and
rotating. The
big end of the conn rod (on the crankshaft journal) is moving through an arc
centered on
the wristpin and rotating relative to the crank journal, the small end of the
conn rod is
rotating relative to the wrist pin and the whole shebang is translating through
a distance
equal to the diameter of the crank journal's rotation - the stroke.
If you plot the motion of the connecting rod/piston combination it becomes
quite apparent
that not only the piston, but the conn rod also comes to a complete stop at
each end of
the stroke. The rotating motions of the wrist pin and the big end of the rod are
relatively mild factors to deal with compared to these sudden halts and
reversals
occurring twice each stroke. This is where your oiling system and your motor
oil's film
strength really matter. Not only does combustion pressure on the power stroke
delivers a
punch that wants to press the bearing surfaces right through the oil film in the
interstitial area between one surface and the other, but the force inherent in
decelerating and re-accelerating the combined mass of the piston and conn rod
produces
similar forces in both ends of the rod at the other end of the stroke.
There are quite a number of relative motion interactions going on in a piston
engine (like
your cat track/ground/dozer example) but the most important in terms of the
forces
affecting engine design is the conn rod/piston combo's motion relative to the
engine
block, and the forces which result from it.
Class dismissed...
Gary McCormick
San Jose, CA
(if only ballistic missile handling equipment was as interesting as automobile
engines -
then I'd be working instead of reading my e-mail!)
snyler wrote:
> Gary McCormick wrote
>
> >I beg to differ - the con rod may still be rotating at the big end, but
> >the small end
> >definitely stops and reverses direction.
> >
> >Arthur wrote:
> Odd to think of part of a moving part stopping and another part of the
> same part not stopping. Does the pivoting at the top end count for
> anything? I once had a discussion with a work friend about caterpillar
> tracks. He said that the tracks on the ground were not moving, I said
> that depends on whether you're on the ground or on the bulldozer
> (relatively speaking)
>
> -marc T.
>
> ===========================================================================
> =========
>
> "Any sufficiently advanced technology is indistinguishable
> from lunacy"
>
> -A Sri-Lankan observing A.C. Clarke struggling with a folding chair
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