[SEL] Curved spokes ...engine rotation

Rob Skinner rskinner at rustyiron.com
Wed May 24 08:47:25 PDT 2006


>If the flywheel runs the opposite way the outer 
> rim will tend to wan't to fold in on itself.

Right you are, Lindsay.  Renowned engine builder Sheldon
Brown, partner of Jeremiah Cochran, studied the issue in
great detail.  Here is what he wrote in his Famous Flywheel
Treatise of 1904:

Are your flywheels spoked right, or are they giving you
trouble?
Many engineers would answer with a rueful "Right!"
 
Spoke Cracking Leads to Flywheel Breakage!
Thanks to superior American technology, modern engines are
more powerful than ever. At the same time, the trend to
lighter flywheels and fewer spokes continues. 

As a result of the higher stresses and cutting-edge
construction, a fundamental design flaw in existing
flywheels comes to light. Millions of engines are suffering
cracked or broken spokes usually on the left side flywheel,
leading to poor fuel economy at best, or getting left behind
by your competition, or, in more severe cases, injury or
death!

Older flywheels were so over-built that they made up for
incompetent design by having lots of heavy curved spokes and
heavier-than-necessary rims. Even today, most right side
flywheels hold up alright, because they still have more
spokes than are needed to cope with the usual loads
experienced by flywheels...but what about the all-important
left flywheel? 

While the right side spokes tend to hold up alright, the
left spokes are often left with insufficient strength. If a
left spoke is cracked, there's no way that the engine can
run right...you'll suffer catastrophic flywheel breakage due
to metal fatigue on the left spokes.

Go right now to wherever you left your engine, and check the
spoke tension of your right wheel by plinking the spokes
with a sledge hammer and listening to the tone. You'll
probably find that the right side spokes are at the right
tone, but that the left spokes will be less melodious. When
you plunk the left spokes you'll notice that the pitch of
the plunk is considerably lower than that of the right
spokes. Why is this? 

The Myth of "Bent Spokes"
Believe it or not, many of the unscrupulous con-artists that
are so common in the engine industry will try to get you to
believe that this is a "feature!" Yeah, right!

They'll give you some spiel about a mystical property called
"curve", which supposedly has something to do with the fact
that the belt is on the right side. This is, of course,
complete nonsense, since the belt never touches the
flywheel, as long as your machinery is aligned just right.

History Lesson Part 1.
These idiots must have been left back in school when it was
time to study history. Everybody knows that in the early
days of engines, there was a problem with the left flywheels
falling off. 

This was solved by the Wright brothers, when they invented
the system of using a left-hand thread on the left flywheel
bolt.

Precession
You might think that this would not be right, and that the
Wright brothers had it backwards, because the friction of
the main bearings would tend to turn the right flywheel to
the left and the left flywheel to the right. Thus, the
right-handed right flywheel should fall right off, while the
left handed left flywheel would be left in place...but it
doesn't work that way; instead an interaction with the
Earth's magnetic field sets up eddy currents right in side
the crankshaft, leading to a tendency for the flywheels to
tighten themselves, as if by magic.
 
History Lesson Part 2.
When Nicolas Otto and Tullio Crossley independently invented
the engine, back in the olden times, they used normal
right-handed threading for the crankshaft main bearing caps.
This worked alright for the left caps, but it was discovered
that the right cap would tend to unscrew itself in use,
unless the engine was left out in the rain, where rust could
help to immobilize the threads.

The great British engine inventor Dugald Clerk cured this
problem by making the right crankshaft main bearing cap have
left-hand threads, so that, even if it starts a little bit
loose, it will tighten itself right up.

The French were slow to pick up on this improved technique,
and many German engines are still made with wrong-way
threading, but the rest of the world has picked up on this,
and the vast majority of engines now have main bearing caps
that are threaded the right way: right threads on the left
side, left threads on the right side.

The Lessons of History
Like many great inventions, bilateral obverse threading
seems completely obvious once it has been explained; the
solution to the problem of left flywheel breakage is, in
fact just a simple matter of using a left-hand threading on
the clamps that run to the left side of the flywheel!

This completely eliminates the precession effect that tends
to loosen left side flywheels in old-fashioned engines that
use right-hand threading!

Flywheel Building Issues
Since I'm a practical mechanic as well as an inventor, I
couldn't overlook the confusion that having two different
types of bolts could cause. I've come up with an ingenious
solution to the problem, however:

As an optional accessory item to bilateral obverse thread
systems, we are making available special color-coded bolts
to help the engine builder keep track of which is which.
Since left hand bolts look just like ordinary bolts, we
offer special bright red bolts to fit the right-hand
threaded spokes still used on the right side of the wheel.
This prevents them from becoming confused with the left-hand
threaded bolts.
 
What About the Southern Hemisphere?
Well, what about it? It is well known that in the Southern
hemisphere, the problem of left flywheels loosening up does
not occur, but rather it is the right flywheels that loosen
up if left to their own devices. 

Hence, engineers who run engines mainly in the Southern
hemisphere should use left hand threads on the right side
flywheels, not the left.

Honesty compels me to admit that for engineers who live and
run engines within 50 miles of the Equator, any bolts will
offer little functional value.

Engineers who regularly travel back and forth between the
Northern and Southern hemispheres would be well advised to
swap the flywheel bolts side-to-side after crossing the
equator.







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