- Crankshaft Torsional Absorbers -
Often mistakenly called "crankshaft harmonic dampers"
The previous page of this section addressed the issue of the torsional excitation which a piston engine applies to whatever device it is driving. However, there is the separate issue of the torsional vibration of the crankshaft itself within the engine.
A crankshaft, like a plain torsion-bar, has mass and a torsional spring rate (see TORSIONAL VIBRATION). That causes the crankshaft system to have its own torsional resonant frequency. The torque peaks and valleys described above cause the engine crankshaft itself to deflect (rotationally) forward and backward while it is operating. When those pulses (excitations) are near the crankshaft resonant frequency, they can cause the crank to vibrate uncontrollably and eventually break.
Understand that the resonant frequency of the crankshaft system is different from the resonant frequencies encountered in the PSRU system. The torsional resonant frequency of the crankshaft system is a function of :
- crankshaft length;
- crankshaft torsional stiffness;
- crankshaft stroke;
- bobweight mass;
- moments of inertia of rotating items attached to or driven by the engine.
Many engines, including virtually all V8’s, V6’s and inline 6’s, use a device on the free end of the crankshaft to attenuate the amplitude of what could otherwise become destructive torsional oscillations of the crank. These end-to-end torsional vibrations of the crankshaft are caused by the alternating compression and combustion pulses described above. Without an appropriate absorber, V8 crankshaft life can usually be measured in minutes at full power. (The flywheel-less Sprint Car V8 engines with the water pump attached to the nose of the crankshaft are indeed a special case by virtue of the substantial alteration in natural frequencies by the lack of a flywheel, coupled with the natural damping effect ot a water pump.).
Many of the automotive 4-cylinder engines don’t require such an absorber, primarily because of their inherently higher stiffness-to-mass ratios. However, several automotive manufacturers have initially omitted a torsional absorber from early engine runs, only to find that crankshaft life was unacceptably short. The Nissan folks discovered this with the early 240-Z engines, which didn’t have an absorber, and therefore lasted only about 100 hours in automotive (i.e. VERY LIGHT DUTY) service.
Often, the vibration attenuating devices on the free end of an engine crankshaft are incorrectly referred to as "DAMPERS". In most cases, they are ABSORBERS. (That's not semantics. A damper dissipates energy, typically as heat. An absorber alternately stores and releases energy to counteract vibration. See VIBRATION BASICS.)
The elastomeric ("metal-ring-on-rubber-spring") devices used by the automotive industry (as well as by Teledyne Continental Motors on the GTSIO-520) are ABSORBERS which are tuned to counteract vibration at the frequency where the particular engine generates its worst torsional excitation. There is a certain amounmt of internal hysteresis in the elastomer, which provides a small amount of damping, but the primary effect is that of countering the vibration at and near the resonant frequency.
Both Continental and Lycoming also use internal ABSORBERS in all their high-output engines. These absorbers are tuned to counteract particular orders of excitation. The internal absorbers consist of pendulous counterweights attached to the crankshaft cheeks by loose pins in hard bushings. The clearance between the pins and bushings establishes the torsional order which each counterweight absorbs. The mathematics of this type of absorber are fiendishly clever, and presented in great detail in ref-5:3:284-288.
One aftermarket device, the Fluidampr ™, really is a DAMPER. It dissipates energy by transforming it into heat by shearing action in a high viscosity fluid. Some race car people seem to like it, but it is banned from the top levels of NASCAR racing, reportedly because with extended use at high levels of energy dissipation, the polymers in the shearing fluid become rearranged and settle off-scenter when the engine is stopped. Next time it is run, the nose of the crank is horribly out of balance, and there's a short run from there to cranksaft failure..
That type of damper is mildly effective over a wide range of excitations, but contrary to intuition and hype, it is significantly LESS effective in reducing vibration in a specific, targeted frequency range, the exact situation you have in an aircraft engine. (There is ample research in the engineering literature showing exactly that fact.)
Whatever device is used to absorb crankshaft internal torsional vibrations, it has an effect (usually small) on the excitation produced at the loaded end of the crankshaft around resonance. That device, together with the Mass Moment of Inertia of the devices attached to the output flange of the crankshaft, will affect not only the value of the crankshaft resonant frequency, but also will influence the longitudinal location of the torsional node on the crankshaft.
