5 WAYS TO BREAK THE NOSE OF A CRANKSHAFT
In recent months we have had numerous questions and
issues raised about the nose of the crank and what can cause it to
break.
What we have found is that in most cases the crankshaft gets the blame
for such failures. But in fact, it is the parts that are being used in
conjunction with the crank, it is the additional machining done to the
parts that are being used with the crank, it is 99% of the time NOT the
crank that caused the crank to break.
1.
IMPROPER MACHINED CRANK GEARS
A.
Champher machined at wrong angle.
B.
Champher
machined with too small an angle.
C.
Belt drive
gears.
The seal
sleeve bottoms to the face of the main before the interface of the gear
bottoms against the step in the nose of the crank. All of the above
prevents the crank gear from bottoming against the step on the nose of
the crank. This leaves a gap between the gear and the step, which allows
the crank to flex ...A fatigue crack starts. SNAP!!!! The crank breaks.
2. DAMPERS WITH MOVING INERTIA WEIGHTS
- .Fluid,
balls, springs, inertia rings with rubber O-Rings, etc. Can you balance
a wheel on your race car if the tires are flat???? How can your rotating
assembly be balanced if to quote one manufacturer," These units
(Dampers) should not be on the crank for balancing as the inertia weight
may not be centered until the engine starts. "NEWS FLASH!!!! Centrifugal
force will always take the inertia weight off center no matter what RPM.
Your assembly is never balanced. TELL TALE SIGN!!!! Metal transferred on
nose outside diameter and damper internal diameter ...A fatigue crack
starts. SNAP!!!! The crank breaks.
3.
EXTERNAL BALANCE vs. RPM
-
Rotating weight multiplies as RPM increases. Engines have heavier or
lighter balance weights and larger or smaller noses. RPM above 5500RPM
is more risky on a Small Block Chevy than a Big Block Chevy. However, as
RPM’s go up, the weight more and more wants to leave the crank due to
centrifugal force. Do not be surprised if at some point fatigue sets in
and the nose comes off.
4.
DRIVES EXTENDING BEYOND THE NORMAL DISTANCE ON THE NOSE -
Multi-stage oil pumps, blowers, etc all have belt drives that require
torque taking off at 90 degrees to center line of the crank. More torque
is necessary for driving these things and further away from main bearing
support all leads to multiple of leverage wiggling the nose. Fatigue
sets in, nose breaks, blower stops. The Small Block Chevy has the
smallest diameter nose and the weakest of all. Note: Blowers take
substantially more 90 degree torque than dry sump pumps, therefore, more
likely to break noses. Not recommended for Small Block Chevy. If a
blower is being used, use a crank with a Big Block nose.
5. IMPROPER BALANCING TECHNIQUE -
The counterweights on a crankshaft are designed to work all together as
a system within a certain bob weight range. To correct the balance on a
crank where the counterweights are too heavy the following should be
followed:
Internal Balance:
If more than 2 holes are required in each end, the outer diameter of all
the counter weights should be turned in a lathe to correct the out of
balance condition in all the counterweights. If you try to drill more
holes, you will create a secondary wave which will lead to crank flex
and eventually a fatigue crank.
External Balance:
The crank is spun with the external balance and flywheel. If it is
determined that the assembly is too heavy where the weight is on the
damper and flywheel, do not make the correction on the end
counterweights of the crank. The out of balance condition is in the
damper and flywheel, which is where it should be corrected. It is very
simple to alter the bolt on weight of the damper and drill the balance
weight on the flywheel. If these components need to be replaced simply
bolt on the proper weight to the damper and match balance the flywheel
which has to be balanced anyway. If you correct in the end
counterweights, you will create a wave in the crank which will wiggle
the nose of the crank which well eventually start a fatigue crack which
will snap the crank.
Stroker kits.com can balance
your assemblies on our in house CEMB digital Balancer. All rotating
assemblies will be with in 1/2 grams both ends combined call us for
prices and details.
Bessel
Motorsports