Ms chapter 6


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Ms chapter 6

  1. 1. Question:State applications of polymers and other non metallic materials on board shipDue date: 31st December 2008 (before 1700 hrs)Assessment: 5%
  2. 2. 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
  3. 3. Free vibration occurs when a mechanical system is set off with aninitial input and then allowed to vibrate freely. Examples of this typeof vibration are pulling a child back on a swing and then letting go orhitting a tuning fork and letting it ring. The mechanical system willthen vibrate at one or more of its "natural frequencies" and dampdown to zero.Forced vibration is when an alternating force or motion is appliedto a mechanical system. Examples of this type of vibration include ashaking washing machining due to an imbalance, transportationvibration or the vibration of a building during an earthquake.In forced vibration the frequency of the vibration is the frequency ofthe force or motion applied, with order of magnitude beingdependent on the actual mechanical system.
  4. 4. 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 ignoredFor example, the damping of a block of steel, or in some cases, the mass of a spring).
  5. 5. 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 theform of deflection. A common example is a spring, but any piece ofmetal has the property of elasticity. That is, if you apply two equaland opposite forces to opposite sides of it, it will deflect. Sometimesthat deflection can be seen; sometimes it is so small that it cant bemeasured with a micrometer. The size of the deflection depends onthe size of the applied force and the dimensions and properties of thepiece of metal. The amount of deflection caused by a specific forcedetermines the "spring rate" of the metal piece. Note that all metals(in the solid state) have some amount of elasticity.
  6. 6. 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 acommon example of a damper. If you push on the ends of a fullyextended "shock absorber" (so as to collapse it) the rod moves into thebody at a velocity related to how hard you are pushing. Double theforce and the velocity doubles. When the "shock" is fully collapsed,and you release your hand pressure, nothing happens (except maybeyou drop it). The rod does not spring back out. The energy (defined asa force applied over a distance) which you expended to collapse thedamper has been converted into heat which is dissipated through thewalls of the shock absorber.
  7. 7. The resonant frequency, ωn of an object (or system) is the frequency atwhich the system will vibrate if it is excited by a single pulse. As anexample, consider a diving board. When a diver bounces on the end ofthe board and commences a dive, the board will continue to vibrate upand down after the diver has left it. The frequency at which the boardvibrates is it’s resonant frequency, also known as it’s natural frequency.Another example is a tuning fork. When struck, a tuning fork "rings" atit’s resonant frequency. The legs of the fork have been carefullymanufactured so as to locate their resonant frequency at exactly theacoustic frequency at which the fork should ring. k ωn = where "k" is the appropriate elasticity value and "m" is the m appropriate mass value.
  8. 8. A waveform is a pictorial representation of a vibration. Example:
  9. 9. VIBRATION AS AN INDICATOR OF MACHINERYCONDITION• 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, its natural to associate the condition of a machine with its level of vibration.• Of course, its 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.
  10. 10. • 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 IS VIBRATION? Vibration can be defined as simply the cyclic or oscillating motion of a machine or machine component from its position of rest.
  11. 11. WHAT CAUSES VIBRATION?Forces generated within the machine cause vibration. These forces may:3.Change in direction with time, such as the force generated by a rotatingunbalance.2. Change in amplitude or intensity with time, such as the unbalancedmagnetic forces generated in an induction motor due to unequal air gapbetween the motor armature and stator (field).3. Result in friction between rotating and stationary machine components inmuch the same way that friction from a rosined bow causes a violin string tovibrate.4. Cause impacts, such as gear tooth contacts or the impacts generated by therolling elements of a bearing passing over flaws in the bearing raceways.5. Cause randomly generated forces such as flow turbulence in fluid-handlingdevices such as fans, blowers and pumps; or combustion turbulence in gasturbines or boilers.
  12. 12. Some of the most common machinery problems that cause vibration include:2.Misalignment of couplings, bearings and gears2. Unbalance of rotating components3. Looseness4. Deterioration of rolling-element bearings5. Gear wear6. Rubbing7. Aerodynamic/hydraulic problems in fans, blowers and pumps8. Electrical problems (unbalance magnetic forces) in motors9. Resonance10. Eccentricity of rotating components such as "V" belt pulleys or gears
  13. 13. VIBRATION AND MACHINE LIFEQuestion: "Why worry about a machines vibration?"Once a machine is started and brought into service, it will not runindefinitely. In time, the machine will fail due to the wear and ultimatefailure of one or more of its critical components. And, the mostcommon component failure leading to total machine failure is that ofthe machine bearings, since it is through the bearings that all machineforces are transmitted.Answer :1. Increased dynamic forces (loads) reduce machine life.2. Amplitudes of machinery vibration are directly proportional tothe amount of dynamic forces (loads) generated.3. Logically then, the lower the amount of generated dynamicforces, the lower the levels of machinery vibration and the longerthe machine will perform before failure.
  14. 14. When the condition of a machine deteriorates, one of two (and possiblyboth) things will generally happen:3.The dynamic forces generated by the machine will increase inintensity, causing an increase in machine vibration. Wear, corrosion or a build-up of deposits on the rotor mayincrease unbalance forces. Settling of the foundation may increasemisalignment forces or cause distortion, piping strains, etc.2. The physical integrity (stiffness) of the machine will bereduced, causing an increase in machine vibration. Loosening or stretching of mounting bolts, a broken weld, acrack in the foundation, deterioration of the grouting, increased bearingclearance through wear or a rotor loose on its shaft will result inreduced stiffness to control even normal dynamic forces.
  15. 15. VIBRATION AS A PREDICTIVE MAINTENANCE TOOLThere are many machinery parameters that can be measured andtrended 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 temperatureIn addition, machinery performance characteristics such as flow ratesand pressures can also be monitored to detect problems. In the case ofmachine tools, the inability to produce a quality product in terms ofsurface finish or dimensional tolerances is usually an indication ofproblems. All of these techniques have value and merit.
  16. 16. A vibration predictive maintenance program consists of three logicalsteps:1. DETECTIONmeasuring and trending vibration levels at marked locations on eachmachine 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 vibrationmonitoring systems. The objective is to reveal significant increases in amachines vibration level to warn of developing problems.
  17. 17. 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.3. CorrectionOnce problems have been detected and identified, requiredcorrections can be scheduled for a convenient time. Of course, in themeantime, any special requirements for repair personnel (includingoutside repair facilities), replacement parts and tools can be arrangedin advance to insure that machine downtime is kept to an absoluteminimum.
  18. 18. CHARACTERISTICS OF VIBRATIONVibration is simply defined as "the cyclic or oscillating motion of amachine or machine component from its position of rest or itsneutral position.“Whenever vibration occurs, there are actually four (4) forcesinvolved that determine the characteristics of the vibration. Theseforces 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 bythe 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.
  19. 19. The characteristics needed to define the vibration include:2.Frequency The amount of time required to complete one full cycle of thevibration is called the period of the vibration.5.Displacement The total distance traveled by the vibrating part from one extreme limit of travel to the other extreme limit of travel. This distance is also called the "peak-to-peak displacement".9.Velocity The time required to achieve fatigue failure is determined by bothhow 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 oftime, it is a simple matter to calculate the speed or velocity required.Thus, a measure of vibration velocity is a direct measure of fatigue.
  20. 20. 1. Acceleration Acceleration is the rate of change of velocity.4. 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".
  21. 21. 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.
  22. 22. Stresses in Ships These may be divided into two classes:2. Structural – affecting the general structure and shape of the ship.3. 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 Stresses Hogging and Sagging; Racking; effect of water pressure; and drydocking. Principal Local Stresses Panting; Pounding; effect of local weights and vibration.
  23. 23. 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. Figure 2 Figure 3• 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.
  24. 24. Racking• 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. Effect 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 4 Figure 5
  25. 25. 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 6
  26. 26. Pounding When a ship is pitching, her bows often lift clear of the water and then come down heavily, as shown in Fig. 7. Figure 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.”
  27. 27. 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. Drydocking 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, Figure 8 it can always be remedied by the use of bilge blocks.