Since we're picking even the tiniest of nits, I will throw out the comment that
piston height as a function of crank angle is not 'simple sinusoidal motion',
but some weird thing of the following form, for a rod length R and crankpin
radius r (i.e. stroke 2r), and crank angle a,
height = r * cos(a) + sqrt(R*R - r*r*sin(a)*sin(a))
from which you can _clearly_ see that the longer the rod is, the more closely
you approximate a sinusoidal height variation, but you don't get true sinusoidal
motion.
I have to go now, my tomato soup's boiling. Hey Dr. Mayf, what are you using for
a firewall? Space shuttle ablative tiles? You're really going to sit _behind_
this nuclear furnace you've built?
Theo Smit
tsmit@novatel.ca
B382002705
> -----Original Message-----
> From: DrMayf@aol.com [SMTP:DrMayf@aol.com]
> Sent: Thursday, October 08, 1998 2:54 PM
> To: tigers@autox.team.net
> Cc: mayfield@traveller.com
> Subject: Re: More Rants on Rods...+ Rod stress and Strains
>
> Excuse me on this one folks. I got tangled up in an AOHell flash session and
> this went out befor I completed it. But, the bottom line is that even the
> ch*vy guys test things strangely and prognisticate get results. I'll finish
> this one day and sent it out again.
>
> I find fallacy in some of the comments to Dr. Bob. When the rod piston is
> reaching the top and bottom of the stroke, the velocity is approaching zero,
> but the acceleration is approaching the maximum. Simple sinsusoidal motion. At
> max acceleration you get max stress/ strains. Why would we expect this? Well
> as the piston is driving upward it has lots of inertia and so it has to be
> stopped or it goes right out through the cylinder head. Same on the down
> stroke, lots of inertia to load the rod in compression yeilding high stresses,
> plus if this is the power stroke the forces on the rod are additive (eg
> inertia plus pressure times piston area). This I would think would be the
> design load condition that automotive engineers consider. And since the piston
> is pinned to the rod as is the crank, all forces go through the rod centerline
> in the direction of rotation. Uneven pressure on the top of the piston could
> cause bending of the rod in the cross axis. When the engineer considers this
> he/she would likely do a vector addition of the forces for combined bending
> and column buckling. I can take some time to develop this if there are non-
> believers out there.
>
> Class dismissed.
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