In a message dated 4/23/2004 5:13:34 AM Pacific Daylight Time,
[email protected] writes:
> The issue of a weakness at the "back" of the VW makes total sense - I have
> seen drawings where the pulley mounting hole is drilled deeper into the
> crankshaft past the narrow sections /grroves etc. I figured that had
> solved the problem.
>
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Removing the sharp metric threads and reaming to remove your tool marks does
in fact remove the stress-risers. The original bore is then deepened and
re-threaded, typically to 1/2-13 and using a rounded thread-form, either SAE or
Whitworth. A special close-tolerance shoulder-bolt must then be made. Ideally
the bolt would completely fill the reamed bore but this is impossible to
achieve since the bolt must have some clearance.
The bolt does not need an exceptional amount of strength since the typical
hub was tapered to the nose of the crank.
Despite all that, the nose still failed. The crankshafts are mild steel and
there simply was not enough metal to withstand the stresses to which it was
being subjected.
Filling the bolt hole and the Woodruff keyway with a weldment using submerged
arc resulted in a solid nose which then had to be annealed, the crank
straightened, the nose reground and the journals re-hardened. A square keyway
similar to that found on the A-series Continentals was used for hub alignment.
That seemed to solve the problem, at least for engines of stock displacement.
But it was fairly expensive even back then (late 1950's); the quality of the
workmanship was a key factor. And you were still left with your prop mounted
to a stubn of shafting barely three-quarters of an inch long.
And of course, the engines kept getting bigger. Prior to the Corvair,
cylinders for motorcycles and a large displacement Wisconsin engine were
available
as war-surplus and could be fitted to the VW crankcase if you were willing to
do the work. Since the crankcases were free, quite a few of us followed that
route. ('Free' because they came from VW's that had been hit in the rear.
Early VW engines had the dynamo tower cast integral with the right-hand half
of
the crankcase. A rear-end collision typically fractured the dynamo tower
causing the crankcase to be discarded. Since conversion to flying status
involved
cutting away the generator tower, there was no shortage of cases on which to
experiment.)
The introduction of the Corvair in 1960 caused larger than stock VW engines
appear on the nose of many airplanes. And for many of them to shed their
props. This lead to the 'long taper' modification. After welding the nose
solid,
the taper was extended across the #4 bearing journal, increasing the contact
area between the hub and the crank by nearly an inch. After being fitted to
the crankshaft the OD of the hub was ground to precise alignment with the other
main bearings and the nose of the crankcase was opened up to accept the fatter
hub, which then used the parent metal of the crankcase as its bearing.
(In so far as I know the long-taper hub was developed by Bob Huggins, who
displayed his engines at Rockford.)
The long-taper hub seemed to solve the fracture problem. But the gyroscopic
loads that caused the fractures were still there and of greater magnitude as
the displacement of the engines increased. (*) There were still enough hub
failures to keep everyone aware of the problem but in the case of the
long-taper
conversion the fault usually lay in the workmanship.
(* Think of the prob as a flywheel. Running in a stable attitude its loads
are nicely balanced and the shaft to which it is attached does not see a lot
of stress. Now do a hard right turn :-) The prop doesn't want to make the
turn. It wants to keep spinning in its original plane. But the SHAFT makes
the
turn... and so does the prop and in doing so some remarkably high loads appear
at the point where the prop is attached to the crankshaft.)
Magnesium is not a bad bearing material for a steel shaft, assuming it has
adequate surface and lubrication. The gyroscopic loads on the long-taper hub
now appeared as wear on the nose of the crankcase. As soon as the thing
accumulated enough wear it would begin to gall and you were soon shopping for
another
case.
This lead to the sleeved-hub conversion, in which the crankcase was machined
to accept a sleeve-type bearing for the long-taper hub. Running in the
sleeve, the hub had significantly more support than when running in the parent
metal
of the crankcase, which is hollowed out for the oil slinger. The main
difficulty was how to ensure adequate oiling. This was finally resolved by
adding a
seal to the outboard end of the sleeve and introducing pressured oil at that
point.
There were a number of other mods involved along the way, such as moving the
thrust bearing up to the #3 main bearing web, but the bottom line is that
after a quarter-century or so of experimentation by American homebuilders we
resolved the crank failure problem. But not cheaply.
Now step outside the box. Why is the prop installed on the pulley-hub?
I submit that if there is no compelling reason to do so there is no
justification for installing the prop on the pulley hub since it is easier and
less
expensive to install it on the opposite end of the crankshaft.
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> What does not make sense is the claim that Great Plains are stating that
> their rear drive, damped engine has done something like 400 hours (I think
> it is 400) without a snag - I will go back to their web site and check.
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Perhaps it HAS done 400 hours. Note the dates of the rear-drive messages in
the AirVW files. Perhaps Steve has found a more suitable elastomer and has
since accumlated 400 hours. Or he may be referring to a smaller engine.
(Hal's
was a 2332, as I recall.)
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>
> I have heard great reports from everyone about steve benet - would be real
> sad f this is true.
>
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True or not, I wouldn't worry about it. Anyone who has ever brought a
product to market knows it's a rocky road. Great Plains provides an invaluable
service to the homebuilt community and is deserving of our support for doing so.
-R.S.Hoover
