1. Vibration Dampers

LO 8-10: Identify and describe the types of crankshaft vibration.

 

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.

   Fig08_64

FIGURE 08-64: Torsional vibration appears as a rhythmically changing crankshaft speed.

 

  Fig08_65

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.

   Fig08_66

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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