And I forgot wrist pin offset...
Assume a crankshaft going into the screen (actually something I feel like doing
sometimes), with the piston assembly oriented vertically (moving up and down)
and the crank rotating clockwise through angle a. Positive 'x' wristpin offset
is to the right. I'll denote the square of a value as value^2.
height = r * cos(a) + sqrt(R^2 - (r*sin(a) - x)^2)
Is this topic bludgeoned to death yet?
Theo Smit
tsmit@novatel.ca
B382002705
> -----Original Message-----
> From: Theo Smit [SMTP:TSmit@novatel.ca]
> Sent: Thursday, October 08, 1998 3:58 PM
> To: DrMayf@aol.com; tigers@autox.team.net
> Cc: mayfield@traveller.com
> Subject: RE: More Rants on Rods...+ Rod stress and Strains
>
> 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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