Vibration and Forces - Vibration damper and detuner

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Index

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1. Engine vibration and critical speed
2. Force and Moments in slow speed diesel engine
3. Vibration damper and detuner
4. Static and dynamic loading and balancing in engine
5. Primary and secondary forces and couple
6. Torsional vibration
7. Axial vibration details
8. Purifier vibration reasons
9. Important element in ship vibration

10. Engine Dynamics

Vibration damper and detuner

Main engine 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 from 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.

Vibration Dampers:- these are the devices which absorb part of vibrational energy.

• The fitting of a tuning wheel or vibration damper will change the frequency of vibration of engine shaft.
• The vibration damper is commonly fitted at the free end of the crankshaft of a medium speed and other engines where the torsional vibration is to be reduced from its peak amplitude.
• The damper is provided with sufficient mass so that it can also act as a flywheel. 

1. Lubricating oil damper

The damper consists of two concentric circular masses, assembled of a pack of leaf springs at their joining faces. The spring space is filled up with lubricating oil through connected grooves, The driving member is rigidly bolted with the shaft and is a part of the vibrating shaft. But the driven mass will tend to oppose the vibration of the driver. The upper concentric mass moves normally and evenly with the engine rotating mass, when there is a vibration, the resiliently fitted upper mass will follow the fixed mass with a phase displacement. The spring pack will be compressed and oil will flow from one space to another through connected grooves. The inertia lag of the upper mass will restrict the amplitude of vibration to enter the critical range.

2. Silicone Damper
There is disadvantage in using lubricating oil in a spring and oil dashpot type viscous damper. The vibration energy absorbed in the damping process will heat up oil and oil loses viscosity. The damper efficiency drops. Further development has led to the introduction of silicone damper.
These dampers are self-contained, hermetically sealed units. The damper comprises of a light flywheel casing which is rigidly connected at the end of the vibrating shaft. A loosely fitted floating mass (inertia ring) is inserted within the flywheel rim casing.

The space between the floating mass and the casing around it is filled with silicone fluid. Silicone has higher viscosity than lubricating oil having the property of little change in viscosity with rise of temperature.

The vibration of the casing attached to engine shaft will be opposed by the inertia of inertia ring and the viscous drag effect of silicone with which both the masses have surface contact.

The viscous shear force between both masses will absorb most of the vibration energy. The inertia ring, by its inertia will try to maintain a uniform motion. The viscous fluid which absorbs the vibration energy will of course be heated up, but when the damper is situated outside the engine casing in open air the heat will be dissipated away by air circulation.

3. Rubber damper uses the elasticity of rubber to absorb part of the vibrational energy.

Detuners:-

They are frequency control devices used to change the frequency of the system.

1. top bracing:- it supports the engine, increases the stiffness and raise the natural frequency beyond the operating range.

2. flexible couplings:- those couplings sectionalise the system, the flexible element absorbs part of the vibrational energy and hence decrease its amplitude.

3. hydraulic oil filled mechanical detuners:- here the oil gets pass to and fro past the springs, causing detaining as well as damping.