Types of Vibrations On Ships – Machinery Vibrations

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One of the most common and dreaded problems onboard ships is high levels of vibration. The two most common effects of vibration has on the ship is structural fatigue and discomfort of crew/passengers. An article published in ShipInsight explains about categories of ship vibrations.

Here’s an excerpt from that.

Categories Of Ship Vibration

Ship vibration are majorly categorised into two types-

  1. Machinery Vibration
  2. Hull Vibration 

Machinery Vibration

Vibration originated due to operation of machinery is called machinery Vibration. Different machinery which produce vibration onboard are main engines, propulsion shafts, gearboxes, propellers, pumps, diesel generators etc.

Types Of Machinery Vibration

Machinery Vibrations can again be categorised into three types:

  1. Torsional Vibration
  2. Axial or Longitudinal Vibration
  3. Lateral Vibration

Torsional Vibration

Torsional vibrations may be caused by:

  • Power imbalance of the engine producing a greater variance of torsion produced by the crankshaft
  • Operation of the engine at different speeds than normal, which may be closer to the engine critical speeds than normal
  • Rotary imbalance of the main engine due to loss of a balance weight
  • Damage in the gearing being transmitted to the shafting
  • Propeller damage causing rotary imbalance

The increase in torsional vibrations would produce an increase in torsional stress, result in the generation and growth of cracks in the high stress areas of the transmission shaft and propeller.

Axial or Longitudinal Vibration

Axial vibration is a kind of longitudinal shafting vibration which occurs in the crankshaft because of the radial as well as tangential forces. 

How Axial Vibration Is Generated?

The thrust generated by the propeller depends on the velocity of water incident onto the propeller blades (called wake). Now, due to the curvature of the hull at the aft, the wake on the propeller is not uniform in nature. That is, the wake at the top of the propeller disc is different from the wake at the bottom of the disc. Thrust generated by the propeller is periodic in nature, which often call as alternating thrust. And a periodic thrust becomes the exciting force for the axial vibration of the propulsion system.

Lateral Vibration

This mode of vibration occurs in the direction perpendicular to the axis of rotation of the shaft. 

How Lateral Vibration Is Generated?

Lets consider intermediate and tail shafts as beam, with the shaft bearings as support points. Due to the bending of the shafts, the centre of gravity of the shaft does not coincide with the ideal centreline of the shaft, therefore when the shaft rotates, the centrifugal force on the centre of gravity would cause it to shift further away from the ideal centreline, resulting in a vibratory motion called whirling of shafts.

Why Maximum Vibrations Is Felt Onboard During Engine Start?

When the engine of a ship is started, and its speed gradually increased. During the speeding up, there arrives a point at which the RPM of the engine coincides with the natural whirling frequency of the shaft. Such speed is to be carefully avoided. This speed is called Critical Speed. A ship is never to be operated at this speed range.

Sources Of Whirling Vibration

  1. Propeller Fluctuating Loads- Thrust provided by the propeller is not always along the shaft centreline, but at some angle to the centreline of the shaft. This exerts bending stress onto the shaft, resulting in whirling.
  2. Diesel Engine Excitation- The moving parts of the diesel engine act as a primary source of excitation and gas pressures act as exciting forces for whirling. Radial components contribute whirling vibrations, whereas, the tangential components contribute torsional vibration.
  3. Shaft Alignment Error- If the centrelines of the shaft coupling flanges are not coincided during shaft alignment at the shipyard, it results in loss of the continuity of the rotation axis of the shaft. Also, when all the coupling bolts through the coupling flanges are not equally tightened to the same extent, causing an angular misalignment between the coupling flanges. Such kind of errors will cause the whirling excitation.
  4. Defects in Gearing System- Whenever gearing equipment is being installed, proper mounting and pairing are necessary to avoid whirling induced from improper gearing.

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Source: Marine insight