"Dave Plowman (News)" wrote in message
...
In article ,
Dave Baker wrote:
Casting this sort of thing gives a more rigid result than forging -

Unfortunately no, not by any measure. Cast iron has a lower modulus of
elasticity (the amount it bends for a given load) than steel as well as
lower ultimate tensile strength. A cast iron crank, of the same shape,
is both more flexible and weaker than a steel one but as I said
previously it is less notch sensitive so less likely to break from
stress cracks and has a longer fatigue life which is important in high
mileage engines like truck diesels.

Sort of goes against what I was taught - although it could be if designing
both for exactly the same load the cast one becomes in effect more rigid?

Quite possibly yes if you have to make the sections very much bigger to
allow for the lower tensile strength. In fact that's why you generally only
see cast iron things in bloody great lumps because it's basically weak and
brittle, especially in thin sections. However in bloody great lumps you
still ain't going to break it and it's cheap.

http://www.engineersedge.com/manufac...s_strength.htm

Even the good stuff, malleable or nodular (SG) cast iron, only has about 70%
of the modulus of elasticity of steel and half or less of the tensile
strength. The crap stuff, basic grey cast which you would use for an engine
block or gearbox casting, is much much weaker than that. If you tried to
make a coathanger out of cast iron wire instead of steel wire it would snap
like a carrot with anything heavier than a shirt on it. Like I say though,
in bloody great lumps it's perhaps understandable that people might think
it's strong but the same size bloody great lumps of steel would be much
stronger and also much more rigid.

In simple terms cast iron is used because it suffices for the job at
less cost than steel. Even with very highly tuned engines putting out
several times their stock power I've never seen a stock cast iron crank
break.

Crikey - broken crankshafts used to be quite common on thrashed engines -
the B series BMC unit was famous for it. Early A series too. Of course
with modern ignition/injection systems it's easy to limit the maximum revs
to a safe limit.

I'm talking 'proper' engines Ya know, like anything designed after the
Ark?

The A series, and the early B series, only had three main bearings on the
crank which is why they bent like bananas under high rpm loadings and broke
so easily. Anything more modern will have five mains and be very hard to
break.
--
Dave Baker