Answer EKM Question 7

Q7. Enumerate the causes of vibration in diesel machinery and shafting. Describe the procedure by which it may be reduced by operating personnel, suitable design and devices. State the possible effect of vibration on machinery and crew members.

Answer: Causes of engine vibration:-

• 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.

Possible causes and remedial actions by personnel:

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.

Vibration reduction by means of design and devices:

Static Balancing of Engines 

• For the crankshaft to be in static balance in the transverse plane, its centre of gravity has to coincide with the axis of rotation. Except for a crankshaft with a single crank, static balance is easily achieved by spacing the cranks at equal angles. However, there may still be a static imbalance due to inaccuracies in the forging and machining process. 
• In order to check the balance the crankshaft is placed on two or more knife-edge supports and pushed several times in one direction or the other to give it a slight rolling motion. If it is observed that that crankshaft stops in one particular position each time, it indicates an imbalance. The side that always points downwards when stopped is heavier and correction may be applied by grinding or otherwise removing material from that side. 
• In the longitudinal direction, it is essential to ensure that the crankshaft journals are in one line. In order to achieve this, the bedplate is accurately finished and the bearing saddles are carefully line-bored to accommodate the main bearings such that when installed, the centers of the bearings will be in one line, symmetrical in the transverse plane and parallel to the bedplate upper and lower machined surfaces. In this way, when installed in place, there will not be any bending moment in the crankshaft other than that due to its own weight. 

Dynamic balancing 

• When the crankshaft is in dynamic balance, the conditions for static balance must be present, but in addition, when the shaft is revolving in the bearings or on two sets of rollers the load on each bearing or sets of rollers must remain constant through out the 360° of rotation or during the time the shaft or rotor is turning. If the load on a bearing or set of rollers does not remain constant throughout a revolution it indicates that some mass in one plane is out of balance with another mass in another plane and a couple is set up which causes a vibration in the load on each of the bearings or rollers. 

Additions that may be fitted to overcome primary imbalance 

• In theory, the primary force may be completely cancelled by a system of gear driven weights. A counterweight is attached to the web and another counterweight attached to a gear, driven by the crankshaft at equal speed but in opposite direction. The vertical components of the two weights are in the same phase, but opposite to the primary force and the weights are so chosen that the primary force is completely cancelled. The two horizontal components are in opposite phase and therefore they cancel one another.
• However this is not a practical method due to complexity.

Additions that may be fitted to overcome secondary imbalance 

• The second order or secondary force has a frequency of twice that of the primary and can only be balanced by a weight that moves at twice the speed of the engine. 
• Two wheels fitted with balance weights are chain driven by the crankshaft at twice its speed and in opposite directions. 
• As in the case of the primary balancer, the weights are arranged in such a way that the vertical components are in phase and together they are equal and opposite of the secondary force. The horizontal components cancel each other. The system is known as ' Lanchester second order balancer'. 
• But in a multi-cylinder installation the crank arrangement is such that many of the forces cancel one another, leaving only the residual forces and couples to be balanced. 

Detuning and damping devices:

• The commonest detuning and damping devices used in marine practice ate viscous-fluid dampers and spring-loaded detuners. In each of these both damping and detuning occurs. The viscous-fluid damper consists of a flanged circular casing which is rigidly fixed to the shafting usually at the forward end of the crankshaft. Within the circular casing is a ring which has a large mass and completely fills the casing except for small clearances around it. The ring is not fastened to the casing in any way and is completely free to move angularly within it. The clearance space is filled with a fluid which has the property of retaining its viscosity over a wide temperature range. The ring fitted within the casing is sometimes referred to as the seismic mass. Spring-loaded detuners have been made in many forms but a common form used in merchant ships consists of two parts. One part is rigidly fixed to the crankshaft and has a centrally located shaft projecting from it. The other part, consisting of a large mass (sometimes called the seismic mass), is fitted on the first part in such a manner that it can move angularly. The centre part of this mass is bored out to fit on the projecting shaft. The two parts have a series of slots cut around their circumferences in an axial direction. The two parts are connected by a series of flat springs, the inside ends of the slots being cut with a curve which accommodates the curvature of the springs when they are deflected under load. The moving part of the detuner is enclosed within a casing which is attached to the part rigidly fixed on the crankshaft. The inside of the casing is connected with the bearing lubricating oil supply and drain holes allow a small circulation of lubricating oil through the casing. 

Operation of Viscous damper:

• The effectiveness of any mass on the natural frequency of a shafting system vibrating torsionally is dependent on the mass moment of inertia of the mass. A viscous damper is made up of two masses: the comparatively light casing and the heavy inner ring which has a high mass moment of inertia. When the engine is started the inner ring lags behind the outer casing, which moves with the crankshaft. The viscous fluid drags the heavy ring round and it quickly comes up to the same speed as the casing. When the inner ring is moving with the speed of the casing the effective value of the mass moment of inertia of the damper is the summation of the value for the casing and the inner ring. As the speed of the engine is increased and a resonant condition is approached, the crankshaft and outer casing commence to vibrate torsionally. When this occurs the viscous fluid separating the casing and the ring allows some slip to take place and the effective mass of the damper is reduced to that of the casing only. The reduction in the mass alters the frequency of the system by increasing it. When slip occurs the energy within the vibrating system is dissipated by the movement of shearing action within the viscous fluid, and the amplitude of the vibration is kept within safe bounds. As the amplitudes are kept low the stresses remain low. This will occur at any critical speed.

Operations a spring-loaded detuner:

• The connection between the fixed and moving elements of a detuner commonly takes the form of flat strip springs, or laminated springs made up from a series of flat strips. These springs fit into slots which have a curved profile. The curved profile is such that when the detuner operates with very small vibration amplitudes the distance between the spring supports in the slots is nearly a maximum. When the vibration amplitudes are large the distance between the spring supports is reduced. If we liken the flat strip spring to a beam we can see that as the distance between the supports is reduced the stiffness of the beam or the spring increases. This change in stiffness of the spring in effect alters the mass moment of inertia of the moving part of the detuner and so varies the natural frequency of the shafting system. When the engine is in operation and the speed is altered so that it coincides with or approaches a critical speed, vibration of the shafting system commences. The element of the detuner fixed to the crankshaft vibrates with the crankshaft. The moving part of the detuner then lags or leads the vibratory  movement of the crankshaft and the natural frequency of the shafting system is changed or detuned and the resonant condition vanishes. While the moving part of the detuner is lagging or leading the part attached to the crankshaft, the energy used to reverse or change the loading on the flat springs dissipates a large amount of the energy of the vibration, and damping occurs. The damping effect reduces the amplitude of vibration of the crankshaft and keeps the stresses within safe bounds. 

Detuners and dampers designed for use with main engines will usually operate continuously. The heat generated from the damping action of the viscous fluid or the load reversals on the flat strip springs must be continuously removed so that no serious temperature rise takes place in the damper or detuner. Viscous-fluid dampers are sometimes fitted external to the engine crankcase in such a manner that sufficient air circulates around them to remove the heat generated. The lubricating oil allowed to circulate through a detuner is usually of sufficient quantity for adequate heat removal. 

Detuners and dampers are if fitted at the nodal points in a shafting system subjected to torsional vibration there is no torque variation or amplitude arising out of the vibration. As there is no torque variation or amplitude nothing is available to activate the movement of the seismic mass in the detuner or damper. No detuning or damping can therefore take place. Detuners and dampers are most effective if they are fitted at the anti-nodes. In practice this may not be possible so they are often fitted at the forward end of the engine. There is nearly always enough vibratory movement at the forward end of the crankshaft to activate fully detuning and damping devices. In some special cases detuners have been fitted at the aft end of the engine and on the intermediate shafting. 

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.