VIBRATIONS

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Types of Vibration

Resonance is when Excitation frequency meets naturally frequency.

Excitation –   Primary     

                 –  Secondary

Primary Excitation : Combustion Forces

                                  Rotating forces

                                  Reciprocating forces

these are part of engine specifications calculated in advance characteristics of engine at given speed and power.

Secondary excitation: Sub structure resonance becomes source of excitation can be within or out of engine.

            eg.  Transverse girder

                    Bridge wing

                    Main engine platform.

Its calculated at the design stage of specific propulsion plant.

Different type of vibration

  • Externally unbalanced forces– 1st and 2nd order
  • Guide force moments– H and X force moments
  • Axial vibration shafting
  • Torsional vibration shafting

Axial vibration damper

  • Engine induced–   expansion and contraction of crankshaft Firing load.
  • Propeller induced—due to pitching and improper immersion of propeller blades.

Where S = stiffnes

M = Added mass

Therefore, the natural frequency is very high so that excitation frequency never matches.

Torsional Vibration shafting

  • Engine induced– Connecting rod makes different angle at different points
  • All units fire at different point
  • combustion pressure fluctuation.
  • Propeller Induced–  P =T*W

damaged propeller blades and fluctuations in torque

Twisting and untwisting of shaft gives torsional vibration

Natural frequency of shaft =

Torsional vibration dampers

Externally unbalanced moments

These are generated in engine

First order– same frequency of engine

Second order– twice the frequency of engine

Rotational component:

V & H component

Reciprocating masses– F= M * A

We have only V component.

1st order 2nd order
W 2w
Reciprocating & rotating Only reciprocating
V & H Only V
1,2,3 and 4 4,5 and 6

Correction of first order (v) component

These may reduce the vertical moment to an insignificant value (although increasing, correspondingly, the horizontal moment) so this resonance is easily handled. A solution with the zero horizontal moment is also available.

Correction of first order (V) component

Correction of 2nd order (V) component

  • No compensators, if considered unnecessary on the basis of natural frequency, nodal point and size of 2nd order moment.
  • A compensator mounted on the aft end of the engine, driven by the main chain drive.
  •  A compensator fitted on the fore end, driven from the crankshaft through a separate chain drive.
  •  Compensators on both aft and fore end, completely eliminating the external 2nd order moment.

PRU: power rated unbalance =  external moment / engine power

                                  second order vertical moment (N-m) / engine power (kW)

Below 120 Not likely
120-220 Likely
Above 220 Most likely

Guide force moments (Y)                                         

Guide force moments (X)

Summary