Question:Question: State applications of polymers and other non metallic materials on board ship

Due date: Due date: 31st December 2008 (before 1700 hrs)

Assessment:Assessment: 5%

Vibration is the motion of a particle or a body or a system of connected bodies displaced from a position of equilibrium.

Most vibrations are undesirable in machines and structures because they produce:

• increased stresses

• energy losses

• cause added wear

• increase bearing loads

• induce fatigue

• create passenger discomfort in vehicles

• absorb energy from the system

Forced vibration is when an alternating force or motion is applied to a mechanical system. Examples of this type of vibration include a shaking washing machining due to an imbalance, transportation vibration or the vibration of a building during an earthquake.

In forced vibration the frequency of the vibration is the frequency of the force or motion applied, with order of magnitude being dependent on the actual mechanical system.

Free vibration occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely. Examples of this type of vibration are pulling a child back on a swing and then letting go or hitting a tuning fork and letting it ring. The mechanical system will then vibrate at one or more of its "natural frequencies" and damp down to zero.

Components in a vibrating system have three properties of interest. They are:

• mass (weight)• elasticity (springiness) • damping (dissipation)

Most physical objects have all three properties, but in many cases one or two of those properties are relatively insignificant and can be ignored

For example, the damping of a block of steel, or in some cases, the mass of a spring).

The property of mass (weight) causes an object to resist acceleration. It also enables an object to store energy, in the form of velocity (kinetic) or height (potential).

The property of elasticity enables an object to store energy in the form of deflection. A common example is a spring, but any piece of metal has the property of elasticity. That is, if you apply two equal and opposite forces to opposite sides of it, it will deflect. Sometimes that deflection can be seen; sometimes it is so small that it can't be measured with a micrometer. The size of the deflection depends on the size of the applied force and the dimensions and properties of the piece of metal. The amount of deflection caused by a specific force determines the "spring rate" of the metal piece. Note that all metals (in the solid state) have some amount of elasticity.

The property of damping enables an object to DISSIPATE energy, usually by conversion of kinetic (motion) energy into heat energy. The misnamed automotive device known as a "shock absorber" is a common example of a damper. If you push on the ends of a fully extended "shock absorber" (so as to collapse it) the rod moves into the body at a velocity related to how hard you are pushing. Double the force and the velocity doubles. When the "shock" is fully collapsed, and you release your hand pressure, nothing happens (except maybe you drop it). The rod does not spring back out. The energy (defined as a force applied over a distance) which you expended to collapse the damper has been converted into heat which is dissipated through the walls of the shock absorber.

The resonant frequency, ωn of an object (or system) is the frequency at which the system will vibrate if it is excited by a single pulse. As an example, consider a diving board. When a diver bounces on the end of the board and commences a dive, the board will continue to vibrate up and down after the diver has left it. The frequency at which the board vibrates is it’s resonant frequency, also known as it’s natural frequency. Another example is a tuning fork. When struck, a tuning fork "rings" at it’s resonant frequency. The legs of the fork have been carefully manufactured so as to locate their resonant frequency at exactly the acoustic frequency at which the fork should ring.

m

kn where "k" is the appropriate

elasticity value and "m" is the appropriate mass value.

A waveform is a pictorial representation of a vibration.

Example:

• Machines of some kind are used in nearly every aspect of our daily lives

• How many times have you touched a machine to see if it was "running right"? With experience, you have developed a "feel" for what is normal and what is abnormal in terms of machinery vibration.

• Even the most inexperienced driver knows that something is wrong when the steering wheel vibrates or the engine shakes. In other words, it's natural to associate the condition of a machine with its level of vibration.

VIBRATION AS AN INDICATOR OF MACHINERY CONDITION

• Of course, it's natural for machines to vibrate. Even machines in the best of operating condition will have some vibration because of small, minor defects. Therefore, each machine will have a level of vibration that may be regarded as normal or inherent. However, when machinery vibration increases or becomes excessive, some mechanical trouble is usually the reason. Vibration does not increase or become excessive for no reason at all. Something causes it - unbalance, misalignment, worn gears or bearings, looseness, etc.

WHAT IS VIBRATION?Vibration can be defined as simply the cyclic or oscillating motion of a machine or machine component from its position of rest.

• When a machine fails or breaks down, the consequences can range from annoyance to financial disaster, or personal injury and possible lose of life

• For this reason, the early detection, identification and correction of machinery problems is paramount to anyone involved in the maintenance of industrial machinery to insure continued, safe and productive operation

WHAT CAUSES VIBRATION?Forces generated within the machine cause vibration. These forces may:1.Change in direction with time, such as the force generated by a rotating unbalance.

2. Change in amplitude or intensity with time, such as the unbalanced magnetic forces generated in an induction motor due to unequal air gap between the motor armature and stator (field).

3. Result in friction between rotating and stationary machine components in much the same way that friction from a rosined bow causes a violin string to vibrate.

4. Cause impacts, such as gear tooth contacts or the impacts generated by the rolling elements of a bearing passing over flaws in the bearing raceways.

5. Cause randomly generated forces such as flow turbulence in fluid-handling devices such as fans, blowers and pumps; or combustion turbulence in gas turbines or boilers.

Some of the most common machinery problems that cause vibration include:1.Misalignment of couplings, bearings and gears

2. Unbalance of rotating components

3. Looseness

4. Deterioration of rolling-element bearings

5. Gear wear

6. Rubbing

7. Aerodynamic/hydraulic problems in fans, blowers and pumps

8. Electrical problems (unbalance magnetic forces) in motors

9. Resonance

10. Eccentricity of rotating components such as "V" belt pulleys or gears

VIBRATION AND MACHINE LIFE

Question: "Why worry about a machine's vibration?"

Once a machine is started and brought into service, it will not run indefinitely. In time, the machine will fail due to the wear and ultimate failure of one or more of its critical components. And, the most common component failure leading to total machine failure is that of the machine bearings, since it is through the bearings that all machine forces are transmitted.

Answer : 1. Increased dynamic forces (loads) reduce machine life.2. Amplitudes of machinery vibration are directly proportional to the amount of dynamic forces (loads) generated. 3. Logically then, the lower the amount of generated dynamic forces, the lower the levels of machinery vibration and the longer the machine will perform before failure.

When the condition of a machine deteriorates, one of two (and possibly both) things will generally happen:

1.The dynamic forces generated by the machine will increase in intensity, causing an increase in machine vibration.

Wear, corrosion or a build-up of deposits on the rotor may increase unbalance forces. Settling of the foundation may increase misalignment forces or cause distortion, piping strains, etc.

2. The physical integrity (stiffness) of the machine will be reduced, causing an increase in machine vibration.

Loosening or stretching of mounting bolts, a broken weld, a crack in the foundation, deterioration of the grouting, increased bearing clearance through wear or a rotor loose on its shaft will result in reduced stiffness to control even normal dynamic forces.

VIBRATION AS A PREDICTIVE MAINTENANCE TOOLThere are many machinery parameters that can be measured and trended to detect the onset of problems. Some of these include:

1. Machinery vibration2. Lube oil analysis including wear particle analysis3. Ultrasonic (thickness) testing4. Motor current analysis5. Infrared thermography6. Bearing temperature

In addition, machinery performance characteristics such as flow rates and pressures can also be monitored to detect problems. In the case of machine tools, the inability to produce a quality product in terms ofsurface finish or dimensional tolerances is usually an indication of problems. All of these techniques have value and merit.

A vibration predictive maintenance program consists of three logical steps:

1. DETECTION measuring and trending vibration levels at marked locations on each machine included in the program on a regularly scheduled basis. Typically, machines are checked on a monthly basis.

However, more critical machines may be checked more frequently or, perhaps, continually with permanently installed on-line vibration monitoring systems. The objective is to reveal significant increases in a machine's vibration level to warn of developing problems.

3. CorrectionOnce problems have been detected and identified, required corrections can be scheduled for a convenient time. Of course, in the meantime, any special requirements for repair personnel (including outside repair facilities), replacement parts and tools can be arranged in advance to insure that machine downtime is kept to an absolute minimum.

2. ANALYSISOnce machinery problems have been detected by manual or on- line

monitoring, the obvious next step is to identify the specific problem(s) for scheduled correction. This is the purpose of vibration analysis – to pinpoint specific machinery problems by revealing their unique vibration characteristics.

CHARACTERISTICS OF VIBRATIONVibration is simply defined as "the cyclic or oscillating motion of a machine or machine component from its position of rest or its 'neutral' position.“Whenever vibration occurs, there are actually four (4) forces involved that determine the characteristics of the vibration. These forces are:1. The exciting force, such as unbalance or misalignment.

2. The mass of the vibrating system, denoted by the symbol (M).

3. The stiffness of the vibrating system, denoted by the symbol (K).

4. The damping characteristics of the vibrating system, denoted by the symbol (C).

The exciting force is trying to cause vibration, whereas the stiffness, mass and damping forces are trying to oppose the exciting force and

control or minimize the vibration.

The characteristics needed to define the vibration include:1.Frequency

The amount of time required to complete one full cycle of the vibration is called the period of the vibration.

2.DisplacementThe total distance traveled by the vibrating part from one extremelimit of travel to the other extreme limit of travel. This distance is also called the "peak-to-peak displacement".

3.VelocityThe time required to achieve fatigue failure is determined by both

how far an object is deflected (displacement) and the rate of deflection (frequency). If it is known how far one must travel in a given period of time, it is a simple matter to calculate the speed or velocity required. Thus, a measure of vibration velocity is a direct measure of fatigue.

4. Acceleration Acceleration is the rate of change of velocity.

5. Phase

With regards to machinery vibration, is often defined as "the position of a vibrating part at a given instant with reference to a fixed point or another vibrating part". Another definition of phase is: "that part of a vibration cycle where one part or object has moved relative to another part".

Vibration in Ship

• Vibration from engines, propellers, etc., tends to cause strains in the after part of the ship.

• It is resisted by special stiffening of the cellular double bottom under engine spaces and by local stiffening in the region of the stern and after peak.

Stresses in Ships

These may be divided into two classes:

1. Structural – affecting the general structure and shape of the ship.

2. Local – affecting certain localities only.A ship must be built strongly enough to resist these stresses,

otherwise they may cause strains.It is, therefore, important that we should understand the principal

ones and how they caused and resisted.

Principal Structural StressesPrincipal Structural StressesHogging and Sagging; Racking; effect of water pressure; and

drydocking.Principal Local StressesPrincipal Local Stresses

Panting; Pounding; effect of local weights and vibration.

Hogging and Sagging

• These are longitudinal bending stresses, which may occur when a ship is in a seaway, or which may be caused in loading her.

• Figure 2 shows how a ship may be hogged and Figure 3 how she may be sagged by the action of waves.

• When she is being loaded, too much weight in the ends may cause her to hog, or if too much weight is placed amidships, she may sag.

Figure 2Figure 2

Figure 3Figure 3

• Figure 4 shows how a ship may be “racked” by wave action, or by rolling in a seaway.

• The stress comes mainly on the corners of the ship, that is, on the tank side brackets and beam knees, which must be made strong enough to resist it.

• Transverse bulkheads provide very great resistance to this stress.

Racking

Figure 4Figure 4

Effect of Water PressureEffect of Water Pressure

Water pressure tends to push-in the sides and bottom of the ship. It is resisted by bulkheads and by all transverse members (Fig. 5).

Figure 5Figure 5

Panting

• Panting is an in and out motion of the plating in the bows of a ship and is caused by unequal water pressure as the bow passes through successive waves.

• Fig. 6 illustrates how it is caused.

• It is greatest in fine bowed ships.

• For the means adopted to resist it,

see “Peak Tanks.”

Figure 6Figure 6

PoundingPounding

When a ship is pitching, her bows often lift clear of the water and then come down heavily, as shown in Fig. 7.

This is known as “pounding” and occurs most in full-bowed ships. It causes damage to connections and riveting in the three strakes

of plating next to the keel and in the general girder-work of the inner bottom just abaft the collision bulkhead.

For the strengthening to resist pounding see “Cellular Double Bottoms.”

Figure 7 Figure 7

Local Weights• Local strengthening is introduced to resist stresses set up local

weights in a ship, such as engines.

• This is also done where cargoes imposing extraordinary local stresses are expected to be carried.

DrydockingDrydocking

It can be seen from Fig. 8 that a ship, when in drydock and supported by the keel blocks, will have a tendency to sag at the bilges.

In modern ships of normal size, the cellular double bottom is strong enough to resist this stress without any further strengthening.

It is worth noting that if sagging does occur, it can always be remedied by the use of bilge blocks.

Figure 8Figure 8