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Vibration and Forces - Engine vibration and critical speed

  • constantly changing firing pressure.
  • unbalanced forces, couples & moments due to reciprocating & rotary masses.
  • pulsation due to gas forces including exhaust gases.
  • guide force moments.
  • axial forces due to in place bending of crank webs.
  • torsional vibration caused by varying torque and propeller thrust.

Amplitude:- It is the maximum displacement of vibration from the point of equilibrium.

Node:- It is the point in the vibratory system at which the amplitude of vibration is zero.

Order of vibration:- It is the number of vibration cycles in one revolution of the engine.

Vibration Mode:- It is designated by the number of nodes in a system.

Natural vibration:- It is the vibration caused by the elastic forces of the crankshaft material and the inertia of its masses in the absence of external forces.

Forced vibration:- It is the vibration of the crankshaft and the shafting coupled to it, which is induced by a variable engine torque.

Resonance:- It is the coincidence of the frequency of the natural vibration and the frequency of the forced vibration. It results in vibration, local overheating and over stressing of the shafting.

Vibration due to start up:- 
Balanced engine tend to vibrate during starting and gradually the vibrations die our as more cylinders develop their own power. This is due to intermittent fuel delivery and misfiring of some cylinders giving rise to unbalanced inertia forces and moments. After a while, the combustion pressure is the cylinders level up and the imbalance is reduced.

Torsional crankshaft vibration:- 
The engine crankshaft, its flywheel gears and different elements of the propeller shafting form an elastic system. Incapable of being absolutely stiff.
Application of torque to the crankshaft causes it to 'twist' within elastic limits. Removal or reduction of the torque causes the crankshaft to twist or un-twist in the opposite direction.
This state will recur, for the crankshaft will be urged by the elastic forces of its material and the inertia forces of its material and the inertia forces of its masses to vibrate at a certain frequency.
Torsional vibration is the relative vibration of the masses of the elastic system causing it to twist and untwist.

Critical speed:-
It is the crankshaft speed at which resonance may occur. There may be more than one critical speed range for an engine. It manifest itself by a sharp increase in the amplitude of torsional shaft vibration. Critical speed can be measured by a torsiograph, which automatically records the torsional speed.

Severe engine vibration within a certain speed range
1. Possible causes of such vibration;
Critical speed: In multi-cylinder engine there will be a peak for each cylinder. In two stroke cycle engine the number of peaks per revolution will be equal to the number of cylinders. If the peaks per 360° of crank rotation are multiplied by the engine revolutions per minute, we can get the frequency per minute. If this frequency is similar to anyone of the various natural frequencies of the engine shafting system, we have a resonant condition.
    Barred speed range is the speed of the engine in rev/min at which the resonant condition occurs is referred to as the critical speed. The high speed associated with resonant conditions start to build up as the engine speed approaches the critical speed, and do not come back to some safe value until engine speed is increased beyond the critical speed.
The unsafe stresses either side of the critical speed are referred to as the flank stresses. The barred speed range of an engine is the range of the speed from the beginning of build-up of unsafe flank stresses to the dying away of these stresses at some higher speed. Obviously the engine must not be continuously operated at speed within the barred range. The barred speed range or ranges are often marked on the engine room and bridge propeller shaft speed tachometers.
Vibration in this speed range is normal but a severe vibration can not be neglected and causes for the same are to be investigated. 

Possible causes are:

A. Engine power balancing
Engine having multiple cylinder is so design that every cylinder produces equal power. In the event when there is an imbalance between the power output of the units the engine may experience vibration. The different power output can be because of fuel or air supply quantity as well as timing.
The fuel pump condition and timing of the particular unit which have a different power output to be inspected. Also fuel injection valve to be inspected. Scavenge port is to be checked for cleanliness. Exhaust valve Inspection for the positive closing and proper timing. Piston rings are to be checked to ensure they are properly sealing with liner and so utilizing the fresh air charge completely also should not causing blow past.
B. Lubrication system
Improper lubrication between the running gears can be a cause of noise and vibration. The job of lubrication oil is to reduce friction between the rubbing surfaces, additionally it act as to dampen the noise and vibration produced at the mating surfaces. Inspection to be carried out for the adequate lube oil supply in the main engine bearings, gears, chains, crosshead sliding surfaces and cylinder lubrication. Thrust bearing and stern tube bearing lubrication also equally important to check.
C. Clearances
Increased or insufficient clearance both are not acceptable for proper operation of an engine. Bearings, Guides, Bushes, Gears, Piston rings etc. are to be checked for proper clearances as per manufacturer's recommendation.
D. Chain and Gears
Chain tightness must be checked and kept within proper limit. Slack chain not only cause vibration in the engine but also cause damage to its associated parts. Additionally Slack chain will disturb the timing of engine. Gears are to be inspected for the wear or damage.
E. Tie rod and Holding down bolt slackening
The slackening of tie rod may be an outcome of a scavenge fire. If left unattended it can cause damage to main bearing, crack the bedplate especially by way of main bearing saddles and thereby tending to bearing failure. Holding down bolt may slackened due to the heavy seas, broken bolt or failure of chocks.
F. Propeller and shafting
Main propulsion engine is permanently and directly connected to the propeller. Vibrations that have risen in propeller and shafts are also required to be investigated. Propeller imbalance due to damage to surfaces including fouling by fishing nets etc. leading to such vibration of the engine-shafting-propeller system. Lubrication failure of an oil lubricated stern tube (or insufficient cooling) and bearing damage will greatly increase the frictional force, within the bearing leading to vibrations. Cargo and ballast distribution are responsible for the propeller immersion in water, this factor is also be considered.


2. Consequences of operating the engine under such vibratory conditions;
If the engine is continued to run in this vibrating condition, this could lead to damage to machineries, structural damage, power loss, increased fuel consumption and hazard to crew health. It may start initially with loosened foundation bolts, proceed with loosened tie-rods and end up with fatigue cracks appearing on the shaft at critical locations. It will damage the healthy components of the engine also other machineries. This will be transmitted through the hull and cause discomfort for the crew and passengers onboard. The discomfort caused will impair the efficiency of crew and deteriorate their health. Cargo and store lashing/securing arrangement may become ineffective and can cause accidents.
3. Procedure to be implemented in order to investigate and rectify the problem.
When the engine was initially running in stable condition and started severely vibrating in this range of speed, the following factors can be responsible and need to be investigated:

A. Engine power balancing
• Check the power output and peak pressure of every cylinder by taking a power card and drawcard.
• The fuel pump condition and timing of the particular unit which has a different power output to be inspected. If found in good condition then the fuel injection valve is to be inspected.
• Scavenge port is to be checked for cleanliness.
• Exhaust valve Inspection for the positive closing and proper timing.
• Piston rings are to be checked to ensure they are properly sealing with liner and so utilizing the fresh air charge completely also should not cause a blow past.

B. Lubrication system
• Inspection to be carried out for the adequate lube oil supply in the main engine bearings, gears, chains, crosshead sliding surfaces and cylinder lubrication.
• Thrust bearing and stern tube bearing lubrication are also equally important to check.

C. Clearances
• Bearings, Guides, Bushes, Gears, Piston rings etc. are to be checked for proper clearances as per the manufacturer's recommendation.

D. Chain and Gears
• Chain tightness must be checked and kept within proper limits. Gears are to be inspected for wear or damage.

E. Tie rod and Holding down bolt slackening

F. Propeller and shafting
Noise and vibration onboard ship
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