Thermal expansion (was tensioner rebuild)

["Darren Leung" <darren@xxxxxxxxxxxxx>]

I've never really fully bought into the thermal expansion argument, but just
for reference, common metals expand on the order of magnitude of 10ppm/deg C.
In the simplest case of a foot (about a third of a meter) of such a metal
increasing in temerature from 25C to 125C,  that's just 100*10ppm*0.3, or
about 0.3mm.
In a simplified case of the V6, you have a sort of triangle.  Aluminum is
around 25ppm, and that works out to be around 0.75mm across the top, and about
0.6 for each side, or a (very) rough belt lenthening of 3mm.
That's not huge, and it's not a piece of paper, but it is on the order of,
say, a cog in the wheel.

During low-speed operation, the belt is subjected to tremendous tension
between the crank, the 4-5-6 bank, and the 1-2-3 bank.  Naturally, the
tensioner is there to close the loop of tension between 1-2-3 and the crank,
and bring equilibrium.
But what seems apparent is that the the tension level goes up so dramatically
on acceleration that it would take some kind of tremendous spring power to
close that same loop, and only hydraulics could have the power to bring
equilibrium dynamically.  But NOW, 3mm _could_ make a difference.

OTOH I don't know how the Zat unit works.  AND Martin's turbocharged engine
obviously hasn't proven it wrong.  But others have also pointed out that
there's a lot more purchase around the cogs of modern cam belts than on our
v6's.  So ya makes your choice and ya drives your car.

*As a side note, I'm surprised that there isn't some kind of device that works
on polymer compression, instead of spring tension.  Confine an ultra-high-CTE
polymer inside the chamber of the hydraulic unit.  As the temperature rises,
the polymer expands the only way it can, outward, pushing the piston and the
bearing against the belt.  As temperatures go down, the poly returns to
original state.  It could be a solid replacement for short-motion hydraulics.

*While I'm dreaming, what about a ferrofluid-based hydraulics, just add
electric field....


Darren
'83 GTV6
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