Answer EKM Question 11

Q11. Describe the phenomena of vibration in marine diesel engine. Explain the terms:

(a) Axial Vibration

(b) Torsional Vibration

(c) Transverse Vibration

(d) Resonance

(e) The role of vibration dampers.

Answer: There are three forms of vibration:

• Axial vibration in which a particle moves forward and backward in line of the axis of bar or shaft.
• Torsional vibration in which the particles moves in small circular arcs around the centre of axis of the shaft.
• Transverse vibration in which the particles move in lines perpendicular to axis of the bar.

Axial vibration

These stresses arise from the crankshaft being alternately compressed and stretched along its axis. 

• Axial vibrations are longitudinal shafting vibrations and are excited by:  
• Radial and tangential components of the combustion pressure and mass forces in the individual cylinders, causing unbalanced couple. 
• Torsional vibration induced by propeller thrust. 
• Propeller working in the non-uniform wake field.
• Axial or longitudinal vibration of the crankshaft may excite the hull to vibrate through the thrust bearing. The gas pressure in the cylinder is transferred to the crankshaft as connecting rod effort. The tangential component T turns shaft and generates the shaft power. The radial component R deflects the crank webs in axial direction; In large two stroke engines, with their heavy static and dynamic load on crank pin the crank webs systematically open up and close in during one revolution of the engine. Thus, at any instant of time the number of cylinder at different phase angles are executing same type of motion but of different amplitudes. 
• These may be added up to excite the shaft system to resonant vibration along the axis of the shaft. An additional excitation source may come from torsional vibration and the effect of their coupling together may be more pronounced near torsional and axial critical speeds coming closer. Although shafting axial vibrations alone are rarely the cause of severe shafting damages, they are usually the cause of a vessel's hull vibration, excited by the variable force acting on the engine's trust block. 
• To minimize unfavorable side effects of the shafting axial vibrations, most engine builders of the low-speed diesel engines integrate an axial vibration damper into the engine casing. That way, the axial vibration damper becomes a standard building block of modern low-speed diesel engines. 
• An axial damper with the Torsional Detuner at the free end of the engine is shown as an example taken from engines built by New Sulzer Diesel. The transfer of oil between two chambers of the Detuner through throttle valve limit the axial amplitude of vibration. The extent of movement of the crankshaft can be measured and accordingly the throttle valve is adjusted. 

Torsional vibration
Torsional stressing being a result of the forces applied by the connecting rod to the bottom end, varies in magnitude with both the changes in cylinder pressure and the angle of thrust applied by the connecting rod during the power stroke. The compression stroke, which acts as a resistance to turning, further compounds this variation torque. If the shaft is not adequately dimensioned, then fatigue failure through cyclic torsional stressing could occur.
Torsional vibration indicates a situation where an applied turning moment causes the shaft to 'wind up' (twisting along its length) and then unwind again as the stiffness of the shaft re-asserts itself over the applied torque. There is a limit beyond which any shaft (and indeed any component) subject to a cyclic stress will fatigue and fail. For this reason, torsional stressing beyond the designed value should be avoided.

Transverse Vibration

• Guide force moments are caused by the transverse reaction forces acting on the crossheads due to the connecting rod/crankshaft mechanism. 
• These moments may excite engine vibrations, moving the engine top athwartships and causing a rocking (excited by H moment) or twisting (excited by X-moment) movement of the engine. Guide force moments are harmless except when resonance vibrations occur in the engine/double bottom system. 
• As this system is very difficult to calculate with the necessary accuracy engine makers strongly recommends as standard that top bracing is installed between the engine’s upper platform brackets and the casing side for its two-stroke engines. 
• The top bracing comprises stiff connections (links) either with friction plates which allow adjustment to the loading conditions of the ship or, alternatively, a hydraulic top bracing. 
• With both types of top bracing the above-mentioned natural frequency will increase to a level where resonance will occur above the normal engine speed.

Resonance

• It is known that any elastic system has a natural vibration which will have some frequency. If the system is subjected to forced vibration and the frequency of the rhythmically applied forces is very near to or the same as the natural frequency of the elastic system, resonance occurs. 
• When the frequency of the applied force approaches the natural frequency of the elastic system the amplitude of the vibration increases at an alarming rate. The amplitude of the vibration is equivalent to strain. As stress is proportional to strain, stress also increases at an alarming rate. 
• Operation of machinery where a resonant condition occurs is extremely dangerous and early failure must be expected of the part operating under resonant conditions. It is for this reason that resonant conditions are of great importance. 

 

The role of vibration dampers

• Vibration dampers are often fitted to the main engine to dampen torsional and axial vibrations of the shaft line. 
• Several types of torsional vibration dampers operating on different principles are available. Most of them have a fixed wheel bolted to the crankshaft and a floating wheel connected to fixed wheel that is turned with it. 
• In one type the two wheels are connected by means of spirally arranged springs. When there is torsional vibration the fixed wheel tends to turn a little faster or slower bending the spring. The absorption of energy by the springs dampens the vibrations. 
• In another type brake linings are provided between the two wheels which are held together by spring loaded bolts. During normal rotation the two wheels will turn together without any slip. In case there is torsional vibration, the fixed wheel will move a little faster or slower relative to the free wheel and in the process, there will be some rubbing of the friction linings providing the damping. 
• In yet another variant inertia masses are placed in compartments formed between the two wheels. The clearance space around the inertia mass in each compartment is filled with silicone fluid which has a high viscosity. Due to the vibratory movement of the fixed wheel fluid is squeezed past from one side to the other providing damping of the vibratory forces. 
• In MAN B&W engines an axial vibration damper is provided at the forward end to counteract large axial vibrations. The damper consists of an integral collar on the foremost journal that is enclosed in cover on both sides which are filled with oil under pressure tram the main engine bearing oil system. Fore and aft movement of the collar due to axial vibration is damped by the restriction to oil flow from one side to the other on account of the small passages.