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NAVY ENGINEERING BULLETIN MARCH 2003 27 RAN Condition Monitoring – Vibration Analysis WOMT PETER DERBY FLEET CONDITION ASSESSMENT UNIT In the previous edition of the Bulletin the Fleet Condition Assessment Unit addressed the RAN Spectrographic Oil Analysis. This article aims to give the reader an insight into Vibration Analysis Understanding the Fundamentals of Vibration Analysis Vibration Analysis is well proven as a powerful tool for assessing the condition of rotating machinery. It is known as the most useful of any parameter used for condition monitoring, such as temperature, current or lube condition. Fundamental to using this tool effectively however, is gaining an understanding of the basic machinery faults that can be diagnosed and recognising typical failure characteristics. Introduction What is vibration? It is the movement of a body about a reference position. What causes vibration? Vibration is caused by an excitation force such as: Driving force acting on a machine, reaction from the load on the machine, additional stress caused by a problem such as: • Unbalance • Misalignment • Looseness • Anti-Friction Bearing Defects To analyse a problem, the analyst tries to relate the vibration to the excitation force. The following will concentrate on the accurate identification of the above mentioned faults through vibration analysis. Unbalance Simple Unbalance (Imbalance) occurs when the center of mass of a rotating object differs from the center of rotation (static unbalance). More general unbalance occurs when the rotation axis is not coincident with the principal axis of inertia of the rotating body – often called dynamic unbalance. Causes of Unbalance include: • Improper assembly of parts • Material build up of fan blades and impellers • Broken or missing rotor parts Characteristics of unbalance include: • High amplitude radial peaks at 1 x shaft RPM • Very low axial vibration levels at 1 x shaft RPM (except for overhung rotors) • Any harmonics of shaft RPM (integer multiples of shaft RPM) are very low in amplitude. • If harmonics of shaft RPM have significant amplitude, other faults should be suspected. Misalignment Misalignment occurs when the centerlines of two shafts are offset or meet at an angle. Different categories of this defect include: • Offset misalignment • Angular misalignment • Bearing misalignment Vibration caused by angular misalignment: • Shaft centerlines meet at an angle • Often strong component at 1 x RPM • May cause dominant vibration in any plane • Therefore it is important to take 2 radial readings per bearing plus one axial reading per shaft Vibration caused by offset misalignment • Shaft centerlines parallel but offset
28 NAVY ENGINEERING BULLETIN MARCH 2003 • Strong radial component at 2 x run speed • Almost always combined with angular misalignment Characteristics of misalignment include: • High axial vibration at 1,2,3 x shaft RPM • High radial vibration at 1,2,3 x shaft RPM • Higher orders of shaft RPM (>4 x) are generally low in amplitude (depending on type of coupling and the stiffness of the structure) Looseness There are two main types of looseness, structural looseness and rotating element looseness. Structural looseness: • Faulty or eroded base mounts • Cracked or split casings • Improperly torqued bearing caps • Bearing supports Rotating looseness: • Impellers • Fan blades • Bearings • Couplings Characteristics of looseness include: • Spectra show large number of orders • Vibration is often directional – horizontal and vertical amplitudes may differ significantly • “Half – harmonics” and “one – third harmonics” may occur in some cases. Anti – Frictional Bearing Defects Causes of premature bearing failure include: • Improper techniques for mounting bearings on shaft and in housing during rebuild • Excessive or inadequate lubrication • Incorrect application of bearing shortens life • Excessive vibration causes increased stress Characteristics of bearing defects include: • Harmonics of non-synchronous peaks visible (multiples of bearing frequencies) • Sidebands may become apparent (shaft frequency for inner race fault and cage frequency for rolling element fault) Calculating Defect Frequencies: • Require physical dimensions and shaft speed (pitch and ball/roller diameters, number of balls/rollers etc) • Or need manufactures bearing data Journal Bearing Defects High vibration in journal, sleeve or plain bearings can be caused by: • Excessive internal clearances • Improper loading of the bearing • Improper lubrication Excessive clearance: • Even small excitation force, eg. slight imbalance can cause significant vibration • Looseness in bearing results in many harmonics of shaft frequency in the spectrum • Presence of low amplitude shaft orders in the spectrum not automatically cause for alarm Incorrect loading: • Excessive loading can shorten bearing life • Too little load can cause oil whirl; • Under normal conditions, shaft rides on oil film • Due to friction and leakage, oil circulates in the bearing clearance from 38 to 49% of RPM • Oil whirl shows in this frequency range • If bearing load is too low, the force exerted by the oil on the shaft may dominate, causing shaft whirl about the centerline, possibly contacting the journal Improper lubrication: • May give high frequency, nonsynchronous vibration sometimes known as “dirtwhirl” In summary, it is true to say that vibration analysis can help ship’s staff and support agencies. Useful results can be achieved from analysing most shipboard machinery and also give operational benefits by reducing radiated and internal ship’s noise. Ship’s staff are encouraged to ensure that accurate and reliable condition assessments are obtained by consistent and proper practices. Vibration testing of rotating machinery must be conducted uniformly so measured data stays comparable regardless of when, where or by whom the measurement were taken. Need More Information For further information regarding Vibration Analysis please contact CPOMT(E) Glen Collins, I/C VA cell (02) 9359 2441 or e-mail glen.collins@defence.gov.au
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