Thursday, June 18, 2020

Dampers, Detuners and Compensators

  • Every running machine has a tendency to vibrate because of several moving parts incorporated within it.
  • When in motion, the machine will have an oscillatory motion around an equilibrium point.
  • The natural frequency of vibration is always present in marine engines, but the effect can be dangerous when the vibration frequency reaches high levels. This happenswhen the natural frequency of vibration from an external source integrates with the engine vibration or when there are out-of-balance forces generated inside the engine which create 1st and 2nd order movements.
  • Such effects can result in severe damage to the marine engine’s internal moving parts, cracks in the structure, loosening of bolts and securing and damage to bearings.
Dampers:
  • Dampers are used to damp or reduce the frequency of oscillation of the vibrating components of the machine by absorbing a part of energy evolved during vibration
  • Axial vibrations: When the crankthrow is loaded by the gas force through the connecting rod mechanism, the arms of the crank throw deflect in the axial direction of the crankshaft, exciting axial vibrations which, through the thrust bearing, may be transferred to the ship’s hull
  • Torsional vibration: The varying gas pressure in the cylinders during the working cycle and the crankshaft/connecting rod mechanism create a varying torque in the crankshaft
Axial Damper:
  •  The Axial damper is fitted on the crankshaft of the engine to dampen the shaft generated axial vibration i.e. oscillation of the shaft in forward and aft directions, parallel to the shaft horizontal line.

  • It consists of a damping flange integrated to the crankshaft and placed near the last main bearing girder, inside a cylindrical casing. The casing is filled with system oil on both side of flanges supplied via small orifice. This oil provides the damping effect.
  • When the crankshaft vibrates axially, the oil in the sides of damping flange circulates inside the casing through a throttling valve provided from one side of the flange to the other, which gives a damping effect.
Torsional Damper: 
  • It is a twisting phenomenon in the crankshaft which spreads from one end to other due to uneven torque pulses coming from different units ‘pistons.
  • Consist of an inertia ring added to the crankshaft enclosed in a thin layer of highly viscous fluid like silicon.
  • The inertia ring is free to rotate and applies a lagging torque on the crankshaft due to its lagging torsional motion.
  • When the crankshaft rotates, the inertia ring tends to move in radial direction but the counter effect is provided by the silicon fluid damping the vibration
 De-tuners
  • De-tuners  are used to alter the frequency of the vibrating machinery reducing the vibration of the engine
  • Guide force moments: When the piston is not exactly in its top or bottom position, the gas force, transferred through the connecting rod, will have a component acting on the crankshaft perpendicular to the axis of the cylinder. Its resultant is acting on the guide shoe and, together, they form a guide force moment
  • Side Bracing: Normally fitted on the top of the engine which increases the stiffness and raises the natural frequency beyond the working range
  • Flexible Coupling:If the engine has a Power turbine connected to its crankshaft via a reduction gear, then flexible coupling is used to compensate for the vibration occurring during motion transfer. The Flexible elements are mainly spring or special material rubber for de tuning the vibration.
Compensators:
  • Compensator comprises two counter-rotating masses running at the same speed as the main engine crankshaft
  • The external moments are known as the 1st, order moments (acting in both the vertical and horizontal directions) and 2nd order moments (acting in the vertical direction only, because they originate solely in the inertia forces on the reciprocating masses.
  • The counterweights on the chain wheel produce a centrifugal force which creates a moment, the size of which is found by multiplying the force by the distance to the node.

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