LO 8-10 Identify and describe the types of crankshaft vibration
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 8-10 Identify and describe the types of crankshaft vibration |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:44 PM |
1. Vibration Dampers
LO 8-10: Identify and describe the types of crankshaft vibration. |
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Vibration Dampers Torsional vibration is observed as the crankshaft decelerates and accelerates during rotation and is caused by the alternating combustion and compression pressure in each cylinder Figure 64. Cylinder pressure applied against the piston and connecting rod assembly is intense enough to also bend, deflect, and twist the crankshaft while it turns. Even the ends of the connecting rod are distorted under heavy engine loads. Different from torsional vibration, a crankshaft torsional twisting event in the crank throw results in a rebound of the crankshaft throw as the journal snaps back in the opposite direction of rotation Figure 65. The effect of this additional torsional stress is telegraphed through the geartrain and driveline, and if not properly controlled, can cause rapid main bearing and geartrain wear and possible breakage of the camshaft and the crankshaft itself. Twisting and bending forces can also produce harmonic waves in the crankshaft. Harmonic waves are like the vibration of a guitar string or clothesline, with pressure waves moving back and forth and end to end along the crankshaft. The appearance of these waves is similar to sound waves; in fact, when some engines are operated at idle, the sound of a harmonic wave (like a ringing bell) can easily be heard. This wave motion, called harmonic vibration, is another potentially destructive force to the engine crankshaft. All waves have points in between the peaks called nodal points . In the crankshaft, nodal points are areas of potential breakage due to harmonic vibration. Because diesels encounter tremendous harmonic and torsional vibrations, unique vibration-damping mechanisms are used to control the destructive effects of both Figure 66. |
FIGURE 08-64: Torsional vibration appears as a rhythmically changing crankshaft speed. |
FIGURE 08-65: Torsional twisting versus torsional vibration. A crankshaft will actually deflect under power impulses, producing torsional twisting. Torsional vibration involves rhythmic speeding and slowing of crank speed. |
FIGURE 08-66: Power take-off drive flanges are now commonly placed at the rear of the engine, where more torque can drive the device and reduce transmission of torsional vibration. |
The mass of the flywheel can substantially suppress torsional vibration at one end of the crankshaft. At the flywheel, changes in crankshaft velocity are minimized by the inertia of the flywheel, which resists speeding up and slowing down with each change in cylinder pressure. To reduce torsional twisting at the opposite end of the crank, a vibration damper is located at the front of the crank, where it is needed to reduce both torsional and harmonic vibrations.
Five types of damper construction used on diesel engines are:
- Elastomer damper
- Viscous or fluid damper
- Combination damper
- Rotating pendulum vibration absorber
- Balance shafts
| Technicians Tip | ||
| Whenever accessory drive belts are spontaneously thrown at specific revolutions per minute (rpm) or alternators, A/C compressors, and other accessories have loosened and fallen off an engine, technicians should suspect a problem with the vibration damper. Excessive torsional twisting of the crankshaft will result in broken cranks and camshafts only near the front of either shaft. Torsional vibration complaints are also reported as unusual driveline noises, excessive gear rattle, and gear wear. |
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1.1. Elastomeric Dampers
LO 8-10: Identify and describe the types of crankshaft vibration. |
Elastomeric Dampers
Elastomer dampers refer to vibration dampers that use rubber as the primary material to absorb harmonic vibrations and reduce torsional stress on the crankshaft. These are made of three parts: The inner hub is attached to the crankshaft, which connects to a heavier, outer iron inertia ring through a rubber or elastomeric ring Figure 67. The damping action occurs as the outer ring oscillates alternately with speed changes in the inner ring’s movement. For example, when the crankshaft slows, the inner ring slows too, but speed inertia of the outer ring will stretch the rubber ring that connects the two metal rings. When the inner ring slows down, the outer ring transmits stored inertial force through the rubber ring to speed up the inner ring. The opposite action occurs as the inner ring accelerates and transmits the sudden speed change through the rubber ring to the outer ring. The outer ring will resist the speed change, thus slowing down the inner ring. The elastomeric damper is tuned to vibrate out of phase with the lowest torsional frequency of the engine. The rubber ring inside the damper also has the ability to absorb harmonic vibrations, much like placing a heavy glove over a ringing bell. Alignment and verification of the proper operation of the two inertia rings is confirmed by a mark scribed in the face of the outer and inner rings. If the marks are not aligned, the rings have debonded from the elastomer ring and the damper is defective Figure 68. Damper runout, when measured with a dial indicator, should be compared with OEM specifications. A damper with a rubber ring that is twisted or falling out of the inertia ring is also likely defective.
FIGURE 08-67: An elastomeric damper’s outer inertia ring and inner hub move in opposite directions during a torsional impulse. The movements help minimize changes to crankshaft speed by accelerating the crank when the ring slows and slowing the crank when the ring speeds up. |
FIGURE 08-68: When inspecting a vibration damper, alignment marks on an elastomer damper must line up. |
| Technicians Tip | ||
| Timing marks, located on the outside of elastomer vibration dampers, may be incorrect if the outer hub has slipped relative to the inner hub. Oil on the damper and age are common deterioration factors causing slippage, and a technician must carefully inspect the damper for evidence of these factors. |
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1.2. Viscous Dampers
LO 8-10: Identify and describe the types of crankshaft vibration. |
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Viscous Dampers Silicone fluid is used in viscous dampers to tune out harmonic and torsional vibrations. Higher horsepower (hp) engines, those over 300 hp (224 kilowatts [kW]), possess higher vibrational energy, which can lead to fatigue from overheating and shorten an elastomer damper’s life. In a viscous damper, the inertia ring rotates inside a sealed case connected to the crankshaft Figure 69. A thin film of highly viscous silicone fluid fills the small gap between the inner inertia ring and the outer case. During changes in crankshaft speed, the silicone fluid drags the inertia ring, permitting it to rotate at approximately the same rpm as the crankshaft. If no torsional vibration is present, the inertia ring and outer casing rotate at identical speeds; however, changes to crankshaft speed (due to harmonic and torsional vibrations) cause the inertia ring and outer case to shear the silicone fluid. This converts the relative motion between the inertia ring and the outer case into heat energy, which effectively dampens torsional and harmonic vibrations. The oscillating movement between the ring and the case allows the viscous damper to dissipate more energy than the same size elastomeric damper, permitting more effective damping of torsional vibrations. To determine whether this type of damper is defective, it should be visually inspected for leaks, excessive runout, or dents that could prevent movement of the inner ring. If an engine is operated for about half an hour, the viscous damper will be warm or even hot to the touch. This can be measured with an infrared thermometer. The increased temperature is due to silicone fluid shearing and is one of several quick test methods that indicate that the damper is operating correctly. |
FIGURE 08-69: A viscous damper uses a heavy inertia ring that moves and shears silicone fluid inside a steel case. |
| Technicians Tip | ||
| Dampers should never be removed with a puller applied to the outer inertia ring. Most dampers are designed to be removed with hand force only. Pulling on the outer ring or the case of a viscous damper destroys the component because distortion of the case or bending of the inner inertia ring will prevent proper mechanical functioning of the damper. |
Combination Dampers
As the name suggests, combination dampers combine the damping advantages of both viscous and elastomeric dampers. Elastomeric dampers provide good low-speed harmonic damping in comparison to viscous dampers. The reverse is true at higher engine speeds, and viscous dampers provide better damping of harmonic vibrations.
2026 - CDX |
1.3. Rotating Pendulum Vibration Absorber
LO 8-10: Identify and describe the types of crankshaft vibration. |
Rotating Pendulum Vibration Absorber
Unlike regular balancers, a pendulum vibration absorber provides torsional control by producing forces that directly cancel forces that produce torsional vibration. Steel rollers, called centrifugal pendulums , fit loosely into a specific number of holes in either a harmonic balancer or an overhead cam drive gear. The rollers store and release energy back into the crankshaft rather than converting the mechanical energy into heat energy the way dampers do Figure 70. A patented mathematical algorithm is used to calculate roller, hole, and gear size in order to control how the rollers move forward during compression strokes when the crank slows and rolls backward during the power stroke. Movement of the rollers back and forth minimizes engine speed changes, keeping torsional vibration low. Manufacturers claim that the pendulum damper provides a smoother-running engine, increased valve train stability, and more accurate injection timing. Pendulum dampers have the advantage of being lighter than conventional dampers.
FIGURE 08-70: A rotating pendulum vibration damper is used by Caterpillar in overhead camshaft Advanced Combustion Emissions Reduction Technology (ACERT) engines. |
©2026 - CDX |