The crankshaft is a part that illustrates one of the conundrums frequently encountered in any kind of practical engineering. The stronger the crank, the better. But the crank also needs to be as light as possible. And it is very difficult to “add lightness” without adding cost or sacrificing strength. Ultimately, the strength of the crank depends on the design (especially the journal size), the material used, and the care taken in machining and installation. To discuss the crankshaft as it relates to blower motors we need to analyze what causes a crank to fail.
All crankshafts twist during operation. As each piston goes through a power stroke, a twisting force (torque) is applied to the crank through the connecting rod. As each cylinder fires, torque is applied and relieved and as a result, the crank twists and untwists. Crankshafts also experience bending during operation, but this is usually of less significance than twisting. The most twisting load is on the rear of the crankshaft. The drive belts on the crank snout (power steering and AC, or a supercharger) also adds load, but to the front of the crankshaft. These loads on each end add to the twisting forces working against the crank, and as rpm and power levels increase, so do these stresses. A supercharger adds power and thus increase the stress on the crank with the added loads of driving the SC. When a blower car experiences crank failure, it is often at the snout area due to the added stress of driving the supercharger. Also, since a performance engine is often operated at high rpm, there is further loading the crank.
Repeated twisting of the crank eventually leads to metal fatigue and propagation of microscopic defects in the crystalline structure which progress to visible cracks and failure. What can be done to prevent this short of using an expensive aftermarket crank? The answer is surprising simple. It is to install a high quality aftermarket vibration damper. Sometimes called a “harmonic balancer”. A factory harmonic balancer is made from of a heavy steel ring mounted over a thin layer of rubber on the outside of a center hub which is bolted to the front of the crankshaft. The rubber acts as a spring. the ring twists and “untwists” on the rubber to cancel out crankshaft torsional vibration. The problem is that the capacity of the factory type balancer to absorb vibration is limited and it is typically tuned to work best in the 3,000-5,000rpm range, which is below the peak rpm seen in a high performance motor.
Aftermarket dampers include the Fluidampr, which uses a free-floating ring inside a sealed case filled with a viscous fluid which tunes out crankshaft vibrations over a wide rpm band. Another quality design is the ATI balancer, which is a friction-type balancer. Dampers are available in steel, aluminum, and a variety of diameters. Some are even available anodized in the color of your choice! Kennedy’s highly recommends either the Fluidampr or the ATI balancer for use with a blower motor or any highly modified motor. For high rpm (7,000+) and race motors we recommend the ATI type damper. For street use, Fluidampr is the balancer of choice. A high quality damper will also increase the life of stock cranks and bearings. We can supply a suitable damper for all engine types including trucks, please call for price and application.
A stock type damper may disintegrate at high rpm. This can be very dangerous and at the very least can destroy a lot of expensive equipment. The motor sport sanctioning bodies recognize this and require an “SFI Approved” damper. Dampers meeting SFI specs are much less likely to fly apart and cause damage and/or injury.
To learn more about crankshafts, click here.