The Knowledge - Velocette Owners Club

gear must be more than the wattage produced - a few thousand volts times a few milliamps! I think it
will absorb rather less power than a regular dynamo producing the same wattage, and considerably
less than a dynamo or alternator powering daytime lights. The two main causes of premature fibre gear
failure are 1. slack in the magneto bearings and 2. the fact the the same teeth gets the load
on every revolution, both from the magnetic flux, and from the operation of the contact breaker. Both
these causes are preventable by diligent maintenance: 1. by reshiming the bearings as necessary and
2. by periodically repositioning the atd on the magneto spindle. OK, we know neither task is easy. In
fact it's rather less trouble to wait till it fails and replace the whole thing. But if that routine is followed
one will get trouble free service for tens of thousands of miles.
# VOCNA 308 Other then striping the teeth off caused by loads in excess of the shear strength of the
material the gear tooth is made of there are generally just two types of gear tooth failure. One is when
the load, or torque, on the gear is uniform and not high enough to shear the teeth off but the contact
pressure on the tooth surface exceeds the fatigue strength of the material. In this case "spalling" occurs
caused by sub-surface fatigue of the tooth resulting in small chunks of metal coming off the surface.
(This is very common with Velocette transmission gears.) Eventually the tooth surface erodes to the
point not enough thickness remains to take the load and the tooth sheers off. The other is when cyclic
overloads, or hammering, occur that are large enough to bend the tooth. Depending on the amount of
overload the tooth bends back and forth 100s or 1,000s or 1,000,000s of times until the tooth fatigues
at it's root, or base, and breaks off. Spalling can occur in cyclic load failures too. However, the failed
tooth of a cyclic load failure is usually near full thickness where as the uniform load failure tooth is
usually eroded thinner. When we think of vibration in or Velos we usually think only of reciprocating
balance factor. However, there is another vibration caused by the power pulses of the engine called
"torsional vibration". Simply put torsional vibration is caused by the flywheel speeding up every time the
engine fires and then slows down until the next fire when it speeds up again, all in the same direction of
rotation. All piston engine exhibit torsional vibration. The more pistons the smoother, or more constant
the velocity of the flywheel rotation. Single cylinder engines are the worst. This is one of the reasons
why our Velos have round flywheels in addition reciprocating balance correction and cushion drives on
the crank shaft. For four stroke engine the torsional "impulse" occurs in the second order, or every
other revolution. At 4,000 rpm the flywheels accelerate and decelerate 2,000 times a minute. In
automotive work an un-flywheeled V-8 engine crank shaft commonly instantaneously accelerates 6 to 8
degrees ahead of it's continuos rpm at each cylinder firing then slows back to it's continuos rpm before
the next firing. I have no idea how many degrees the crank shaft accelerates in our velos. But, given
the large bore, high compression, and low mass of the flywheels I'm sure it is a lot. Where is all this
breeze going? Here goes: Our engines exhibit torsional vibration, the gears have mass, and the device
on the end of the gear train has mass and in some cases a load or resistance to turning. Since all the
gears in the timing chest are positively locked to the crank shaft by their teeth they are accelerated and
decelerated at the same rate as the crankshaft torsion vibration. Since the gears have mass and
maybe a rotating weight at the end of the gear train the load on the gear teeth is a function of the mass
times acceleration. In rotating machinery kinetic theory the toque to accelerate, or decelerate, a
flywheel possessing polar moment of inertia approaches infinity as the time for velocity change
approaches zero. So the gear teeth can "feel" high cyclic loads due to engine torsional vibration simply
because of their own mass, weight, even though there is a low load at the end of the gear train. In the
case of our aluminum gear a way to tell if tooth fatigue occurred is the look at the broken surface of the
tooth under a magnifying glass. If the failed surface looks grainy or crystalline it is likely a fatigue failure
from tooth bending has occurred. Never use aluminum or fiber gears in a motorcycle!
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