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As experts in crankshaft torsional vibration for more than 57 years, we at Vibratech TVD, makers of FluidamprŪ StreetdamprŪ and Heavy Duty Diesel dampers, feel it necessary, to clear up some common misconceptions and set the record straight regarding torsional vibration and the role of a crankshaft damper.
Does a Vibration Damper ("Harmonic Balancer") Really Balance an Engine?
It is a common misconception that, "A damper is supposed to balance an engine". In most automotive circles, the torsional vibration damper is referred to as a "harmonic balancer", a trade name of General Motors given to a leaf-spring friction damper they introduced in 1925. This misnomer may have, today, led to a misunderstanding, where the desire to correct rotating mass imbalance to avoid damaging flexural vibrations, is being confused with the role of a torsional vibration damper. Dampers can be designed to include a counterweight, or in some similar way permit the balancing of an engine, but the damper itself does not balance the rotating assembly, the counterweight does.
Crankshaft vibration dampers are designed to reduce torsional vibration, the small but sometimes dangerous angular oscillations a shaft undergoes, as the alternately ignited cylinders deliver torque spikes to the shaft via the pistons and connecting rods. Crankshaft torsional vibration is inherent in all internal combustion engines and can be especially harmful in high horsepower, high rpm and torque applications, if a properly designed damper is not utilized.
Another common concern is that fluid and mechanical dampers contain moving parts, which are believed to impede proper balancing of the crankshaft assembly. A fluid-type damper, such as FluidamprŪ, contains an internal inertia ring that is self-centering, even at idle speed, due to the balance of pressure and shearing forces in the viscous fluid that surrounds the ring. Also, every precisely machined inertia ring that goes into a FluidamprŪ is separately high-speed balanced during its manufacture process.
In elastomer dampers, the outer inertia ring is fixed relative to the mounting hub through a ring (or rings) of elastomeric material (usually rubber). These units must be balanced after assembly to correct imbalance due to any offsets between the inertia ring and the hub that are created by the imposition of the rubber ring between them (note the large balance holes on many of these dampers). An elastomer damper that is not balanced out of the box is not self-correcting at any speed.
Damper Inertia Rings - Do They Help or Hurt?
Another misconception is the role of inertia rings in vibration dampers and their behavior during rapid engine accelerations and decelerations. The role of inertia rings in elastomer dampers, such as OEM stock dampers, is different from that of inertia rings in fluid dampers.
In an elastomer damper the inertia ring / elastomer system is tuned to vibrate (act as a tuned absorber) out-of-phase with the fundamental (lowest) torsional natural frequency of the engine rotating assembly. The properties of the elastomer allow it to absorb the vibration, converting the mechanical energy into heat energy to achieve damping. The limited capacity of an elastomer damper for dissipating vibrational energy without overheating can result in a very short life, especially in high horsepower race engines running at high speeds. In addition, the main rotating assembly to which the damper must be tuned includes the crankshaft, pistons, rods, flywheel, and any rotating mass that may be directly coupled or geared to the crankshaft. Changing any one of these elements from the stock conditions reduces the effectiveness of the tuned stock damper. Also, an elastomer damper may actually increase the crankshaft's torsional vibration amplitude if it is not tuned to the vibration frequency of the crankshaft, due to alterations made to the system or due to an improperly specified stiffness (durometer) of the elastomer. Under such conditions the inertia ring could oscillate in phase with the crankshaft, increasing the stresses in the shaft.
In a fluid damper, the inertia ring is independent of, though closely surrounded by a hermetically sealed casing, with a thin film of viscous silicone fluid filling the engineered shear gaps between the two. During engine startup the fluid drags the inertia mass up to speed. In the absence of torsional vibration, the inertia ring and casing rotate at the same speed. The effect of any torsional vibration that is present at any given speed is to oscillate the casing as it rotates, while the inertia ring continues to rotate at the average rpm. The resulting relative motion between the two results in shearing of the fluid, where the mechanical energy is again converted to heat energy to achieve damping, as in the case with elastomer dampers. For the same size damper, a fluid damper can dissipate more energy and run cooler than an elastomer damper, without any significant deterioration of performance.
Since the inertia ring of an elastomer damper is directly connected to the main rotating system through the elastomer, the crankshaft will feel additional loading and higher stress as the engine is rapidly accelerated. The additional loading would be directly proportional to the inertia of the ring. The inertia ring of a fluid damper is only "connected" to its casing through the silicone fluid. Because of this, only about one-half of the ring's inertia is felt by the crankshaft, reducing any loading to the crankshaft that might be caused by rapid changes in engine speed.
An elastomer damper works well at the fundamental natural frequency of the main rotating assembly to which it is tuned and for the vibration excitation level for which it was designed to absorb energy. That, coupled with their relatively low cost of manufacture, is why OEM stock dampers are elastomer-type dampers. In diesel engines reconfigured to provide higher horsepower and greater torque, the fundamental natural frequency of the main system may change substantially, and a number of other natural frequencies can be excited in the typically extended engine operating range. Even through rapid accelerations and decelerations these high frequency vibrations can reach damaging amplitudes. Fluid dampers (sometimes called self-tuning dampers) are able to reduce torsional vibration amplitudes at any frequency occurring at any engine speed, something elastomer dampers can't do.
Vibratech TVD, formerly Houdaille, invented the fluid damper in 1946 and has been manufacturing and selling its dampers for Heavy Duty diesel since and high horsepower, high rpm race engines since 1985. Many racers rely on Fluidampr race after race, for longer crankshaft and bearing life.
Many racers have told us "FluidamprŪ is the best performance race damper on the market."
That's a great endorsement, but maybe you should decide for yourself… from the facts. To learn more about crankshaft torsional vibration, a good technical paper to read from the Society of Automotive Engineers is "A Practical Treatise on Engine Crankshaft Torsional Vibration Control" by Robert C. Bremer Jr. (SAE Paper SP-445, June 1979).