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CHAP CHAPTE TERR-1 1 INTRODUCTION Shaft Alignment 1. The propulsion shafting alignment is a process, which consists of two parts: • The desig design n and and analy analysis sis • The alignment procedure and measurements
The terminology and requirements for the shaft alignment will vary depending on the machinery application, the propulsion system’s size, as well as on the perception of the alignment process itself. Propulsion shafting is a system of revolving rods that transmit power and motion from the main drive to the propeller. The shafting is supported by an appropriate number of bearings. Propulsion shaft alignment is a static condition observed at the bearings supporting the propulsion shafts. In order for the propulsion shafting alignment to be properly defined, the t he following minimum set of parameters (whichever may be applicable need to be confirmed as acceptable: •
!earing vertical offset
• !earing re reactio tions • "isali salign gnm ment ent angl angles es • #ran #ran$s $sha haft ft%%s web web defl deflec ecti tion ons s • &ear mi misalignme nment • 'haf 'haftt and and bear bearin ings gs%% str stren engt gth h • #oup #oupli ling ng bolt bolts% s% stre streng ngth th
The alignment is considered to be satisfactory when it is possible to control the above parameters, and maintain them within the required limits under all operating conditions of the vessel.
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Shaft alignment and its importane Reasons for shaft alignment
2. As the warship’s displacement increases, and consequently, the installed power of the main drive increase, the propulsion shafting alignments are increasingly more sensitive to disturbances affecting vertical offset of the bearings. These disturbances primarily result from hull deflections and temperature change. The shaft alignment problem can be summarized as follows: • igh sensitivity of the shaft alignment to small disturbances in the bearing vertical position • !isparity between highly fle"ible hull girder structure and the rigid propulsion shafting • Temperature variations in different regions, thus varying various clearances • #roblems in maintaining the desired accuracy of the shaft alignment analysis • $nconsistency and inaccuracies in conducting the alignment procedure Prop!lsion Shaft Alignment . Propeller shaft alignment is different from any other $ind of conventional alignment as all shaft bearings (Plummer bloc$, stern tube and brac$et bearings may not be installed in a straight line. 'haft alignment condition of a propulsion shafting may be defined as an arrangement of shaft bearings with offsets relative to a reference line, which under proper operating conditions ensures an optimal load distribution on the bearings. ) shaft is a rotating part used to transmit power, motion, or analogical information. It often carries rotating machine elements (gears, pulleys, cams, etc., which assist in the transmission. ) shaft is a member of a fundamental mechanical pair: the *wheel and a+le.* 'haft alignment is the process to align two or more shafts with each other to within a tolerated margin. It is an absolute reuirement for machinery before the machinery is put in service. )lignment is the ad-ustment of an ob-ect in relation with other ob-ects, or a static orientation of some ob-ect or set of ob-ects in relation to others. ptimal shaft alignment should ensure the following conditions, ptimal load distribution on
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the shaft bearings such that all shaft line bearings are positively loaded and load ta$en by any one of the bearings does not e+ceed a specified value depending on the load carrying capability of the bearing.
Present "ethodolog# for Alignment /. 0uring installation the shaft including propeller shaft, intermediate shaft and cran$shaft are decoupled from each other and laid down on supports. Then, necessary ad-ustment of the height of each support, including possible temporary supports, is made to ensure that the calculated &)P2 and ')&2 between the mating flanges are reali3ed. That is to say, although the appropriate bearing offset can be determined by calculations, it is e+tremely difficult to chec$ the offset during installation. Therefore the gaps and sags are used as an indication of the bearing offsets actually reali3ed. 4hen shafts cannot be stably laid down alone, temporary supports or additional e+ternal forces are provided by -ac$s may be added as long as they are ta$en into account in the calculations.
5igure 1
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5igure 6 'pecifically, the propeller shaft is laid down first, and then its flange is ta$en as references to ad-ust the height of each support, including possible temporary supports, for the intermediate shaft to ensure that the calculated &)P2 and ')&2 between the mating flanges are reali3ed. )fter the intermediate shaft has laid down, its forward flanges become new reference for ad-usting the position of main engine by raising, lowering or tilting the engine to ensure that the calculated &)P2 and ')&2 between the mating flanges are reali3ed.
Defining Shaft "isalignment 7. ) 0efinition: 'haft misalignment is the deviation of relative shaft position from a colinear a+is of rotation measured at the points of power transmission when euipment is running at normal operating conditions. 5or a fle+ible coupling to accept both parallel and angular misalignment there must be at least two points where the coupling can fle+ to accommodate the misalignment condition. There are three factors that influence alignment in rotating machinery a The speed of the drive train b The ma+imum deviation at either fle+ing point or point of power transmission c The distance between the fle+ing points or points of power transmission.
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T#pes of misalignment a Parallel b )ngular $ #ombination
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O%&eti'e of alignment %. The ob&ective of accurate shaft alignment is to increase the operating life of the machine. To achieve this the machine components that are most li'ely to suffer failure must be operated within their design specifications. Those most li'ely to fail are the bearings, seals, coupling and shafts ( and
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alignment has a significant influence on the life of each these, but particularly on the bearings. a )ccurately aligned machinery will achieve: b 8educed a+ial and radial forces on the bearings to ensure longer bearing life, c 9liminate the possibility of shaft failure from cyclic fatigue, d "inimi3e the amount of wear on coupling components, e "inimi3e the amount of shaft bending from the point of power transmission in the coupling to the coupling end bearing. f "aintain proper internal rotor clearances, g 8educe power consumption. h ower vibration levels on bearing housings, machine casings and rotors. !ut note that there is instance where slight amounts of misalignment have resulted in reduced vibration levels. There is a case for some caution about relating vibration amplitude to misalignment.
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CHAPTER-( Shaft Alignment Design and Re'ie) *eneral ;. ) shaft alignment designer has to ensure, and the reviewer has to verify, that the strength of the designed parts (bearings, shafts, coupling bolts, couplings is sufficient to prevent the stress e+erted by the acting loads to damage the same. In particular, the alignment design should satisfy the following: a !earing condition: • )cceptable reaction load • 9ven load distribution throughout the bearing b 'haft strength c 'atisfactory cran$shaft deflections d )cceptable gear contact condition e 'atisfactory coupling bolts strength f )cceptable clutches and fle+ible coupling misalignment tolerances.
Re'ie) 's+ Design `
). Analytical models do not always represent the propulsion systems accurately and may not always provide sufficient information to ensure an *error free+ alignment procedure. The review process serves to verify soundness of an e"isting design, and it has to thoroughly follow the alignment criteria and guidelines, The design process is more comple" than the review itself. $t requires e"perienced personnel and is a time consuming effort with a goal of defining a satisfactory set of parameters to comply with all alignment criteria The design process, if conducted properly, should essentially optimize propulsion shafting for the given parameters
Re'ie) -. verall, the plan review during and after construction is conducted to verify to itself and its committees that a vessel,
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structure, item of material, equipment or machinery is in compliance with the /ules, 0uides, standards or other applicable criteria. 1ngineers need to confirm that all information required for review is received: a) haft alignment model b) cope of submitted calculation c) /esults of analysis d) haft alignment procedure )fter the review is completed, the reviewer needs to document the result of this review. The review of the submitted shaft alignment analysis and procedure is to be conducted by inspecting the results of the alignment analysis and by conducing chec$ analysis shaft alignment software.
Shaft Alignment "odel 1<. !y e+ercising sound -udgment, the engineer should verify that the submitted discrete model represents the actual propulsion system with sufficient accuracy: The engineer should verify that the line shaft model and reduction gear model correspond to the respective design drawings. The diesel engine euivalent model shall be evaluated by confirming that the engine euivalent model complies with engine design particulars (engine type, diameters, location of the timing gear, etc.. !earing offsets shall be verified to include hot and cold conditions.
Sope of Cal!lation 11. ) goal of the shafting alignment calculation is to provide data to the ship production personnel in order to ensure satisfactory alignment under all operating conditions of the vessel (from ballast to full=load. )ccordingly, the submitted calculations shall be conducted and verified for: a 0ry doc$ condition b 4aterborne vessel, hot and cold engine or gear bo+. )s the alignment procedure starts in the dry doc$ (positioning of the
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bearings, slope boring, etc., the calculation needs to provide sufficient information to the production personnel for the dry doc$ procedures. It may be beneficial to conduct most of the alignment procedures (sag and gap, and bearing reaction load verification in the dry doc$ -ust before launching of the vessel, as one can ta$e advantage of the fact that the alignment analysis can be uite accurately confirmed for the dry=doc$ condition, as the alignment is not influenced by hull deflections which are difficult to predict. nce the vessel is launched, it is also important to evaluate the alignment%s sensitivity to hull deflections.
Res!lts ,erifiation 16. The verification shall include, but not be limited to the following: a Influence coefficient matri+ b !earing reactions c 0eflection curvature d 'tern tube bearing slope boring reuirements e )ngular inclination at the main gear wheel f 'hear forces and bending moments g )llowable loads on all bearings .
ators onsidered for shaft alignment 1. Influence #oefficient "atri+
a3 The influence coefficient matri" tabulates a relationship among relative reactions in bearings and the unit offset change at each particular bearing. b3 The influence coefficient matri" can be used to evaluate shafting sensitivity to possible disturbances in the bearing offset and assess changes in the bearing reactions. c) The influence coefficient matri" can be used to assess hull deflection influence on the propulsion shafting. The problem is that the influence coefficient matri" provides information on sensitivity of the shafting, but it gives no indication of the supporting hull structure behavior. d) The larger the influence coefficient number, the more sensitive a
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particular bearing will be to the offset change at the respective bearing4support. 1/. !earing reactions
a3 atisfactory bearing reactions are one of the primary criteria for alignment acceptance. $t is difficult to establish an acceptability margin, as the factors influencing reaction load are very difficult to predict accurately. b3 Alignment is acceptable as long as the bearing reactions are always positive 5under all operating4loading conditions3 and no bearing is unloaded. Any positive static load is therefore acceptable. 17. 0eflection #urve 8elative misalignment between the bearing and the shaft may be evaluated from information defined by deflection curvature. 0eflection curvature defines the angle of the shaft inclination at each node of the system. The angle is measured from the theoretical 3ero alignment line.
5igure /
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1>. 'hear 5orces and !ending "oments
hear forces and bending moments on the shaft should be within acceptable limits, in association with other stresses in the shaft. 6orces and moments on propulsion machinery are to be within the limits specified by the equipment manufacturers.
1;. 'lope !oring?!earing Inclination
lope boring or bearing inclination is adopted as a marine industry practice to prevent e"cessive edge loading of the tail shaft bearing. 1@. 'hear 5orces and !ending "oments
hear forces and bending moments on the shaft should be within acceptable limits, in association with other stresses in the shaft. 6orces and moments on propulsion machinery are to be within the limits specified by the equipment manufacturers.$n addition, some diesel engine manufacturers require bending moments and shear forces at the main engine after flange to be within the required boundaries in order to protect the engine from eventual harmful misalignments .
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CHAPTER-. Shaft alignment proed!re
1A. The shaft alignment procedure is not e+pected to start before the vessel stern bloc$s are fully welded and all of the heavy stern structure is in place. nly then should the reference line for positioning the shafts, bearings, main engine and gear bo+ be established. This is not always the case, however. 'ome yards do start the procedure much earlier, even during bloc$ stage, or without a fully welded stern area of the vessel, or?and with no superstructure in place. The propulsion shafting alignment procedure can be summari3ed in the following activities: a 'ighting through (bore sighting b !earing slope boring or bearing inclination c 9ngine bedplate pre sagging d 'ag and &ap e 8eactions measurements f !earing=shaft misalignment evaluation g 'haft eccentricity (run out verification.
Shaft sighting 6<. The process of establishing the reference line to carryout alignment is often called sighting through or bore sighting. The procedure is conducted by • • •
ptical instruments aser Piano wire
a 'ighting through procedure is commonly conducted as follows: b Telescope, laser or piano wire is normally positioned in front of the after stern tube bearing. c 8eference line is defined so as to match the centerline of the after stern tubes bearing.
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d Target points are then defined at the location of the intermediate shaft bearings, gearbo+ flange or main engine flange. e Target points are offset for values corresponding to the prescribed bearing offsets for the dry doc$ condition. f 'haft line bearings and gearbo+ or main engine are then positioned into place.
5igure 7 g 'lope boring angles are mar$ed. If bearing inclination is conducted instead of slope boring, the inclination angle is applied to the '?T bearing and bearing is fi+ed in place inclined. 61. 5actors considered for minimi3ing disturbances for bore sighting
a) Temperature of the vessel’s structure must be stable and as even as possible. 6or that reason, bore sighting is normally conducted in early morning hours before the sunrise. b) At this point of the vessel construction, the ma&or welding wor' should be completed on the stern bloc' of the vessel. This is to prevent eventual structural deformation, which may result from e"cessive welding. c) eavy structural parts and equipment shall be installed on the
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vessel 5superstructure, main engine, etc.3. Piano )ire method
22. #iano wire application in a *sighting through+ procedure of establishing a centerline of the shafting. The wire enters the aft 4T bearing from the stern and is pulled straight to the main engine flange. #rescribed bearing offset is now applied by measuring the vertical distance from the piano wire to the location of the particular intermediate shaft.
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#ositions of the bearings and a slope boring angle are defined using a piano wire as a reference. b) 7hen applying the prescribed displacement and slope, the theoretical data must be corrected for piano wire sagging. c3 7hen the piano method is used, one needs to apply the correction for the piano wire sagging. a)
Optial and laser methods 6. The optical method, also known as bore sighting, is mostly
used to check and correct the alignment of waterborne bearings before installing the main propulsion shafting. Although optical measurements do not provide direct bearing reactions, they are important for establishing the builder’s bearing offsets and they
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indicate how the outboard bearing struts are bored prior to the bearing installation. The method is typically used to determine two separate alignment parameters: the location of the bearing supports relative to a datum (i.e., a line representing the straightline alignment through all the bearing centers) and the localized alignment of the bearing commonly referred to as cant and skew. The optical method uses the line-of-slight relationship of the bearing bores to the shaft axis of rotation to establish an optical reference line and to determine the location of the shaft bearings relative to that line.
Shaft alignment methods There are several proven methods for assessing the alignment of main propulsion shafting. The most common procedures include (/+ The h#dra!li &a0
The hydraulic jack method is a common technique used to measure the reactions of the line shaft bearings. Bac$=up method is a direct way to chec$ bearing reactions. 0ue to its simplicity, it is the most widely applied method in the industry. "easurements are conducted by hydraulic -ac$s, which are placed in close pro+imity to the bearing which reaction is to be measured. It is strongly recommended to use hydraulic -ac$s in combination with the load cell, as the measurement accuracy will significantly improve.
5igure ; Theoretical Bac$ up process
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6>. Cydraulic -ac$ should be located as close to the bearing as possible. The foundation on which the -ac$ is placed should be sufficiently stiff. Bac$=up measurement may also be used for the shaft run out verification. Cowever, the -ac$=up method is not very suitable for it since the shaft rotation can be applied only in steps, one angle of rotation at the time. The load redistribution problem may also be related to the turning gear loc$=up. The turning gear not only moves the shaft hori3ontally, but also loc$s some portion of the reaction at the contact point between gears. 6;. )dvantages of the -ac$=up method a It uses simple measuring euipment such as hydraulic -ac$ and the dial gauge. b )ccuracy is significantly improved in combination with load cell measurement. c It is the only method that provides reaction load directly. 6@. 0isadvantages of the -ac$=up method a It reuires the same preparation time for each repeated measurement. b "easurement results in wide hysteresis if load cell is not used. c Installation inaccuracies due to "isalignment of the hydraulic -ac$"isalignment of the dial gauge d Though it directly records the load, -ac$=up method does not measure bearing reaction directly, as the -ac$ is lifting ne+t to the bearing location. This reuires correction factors to be applied, which introduce some error as well. 6A. Strain ga!ge The strain gauge method is a more analytical techniue developed to measure the inboard or outboard shaft bearing reactions using strain gages mounted at predetermined locations along the shaft. The strain gauge method requires a combination of computation and strain measurements. If a shaft line rests on a number of bearings, a theoretical distribution of bending stress may be calculated. If the bending stresses, determined from the measured strains at an appropriate number of stations, deviate from the theoretical, this is taken to be caused by an alignment that differs from the theoretical straight-line case. Both the horizontal and the vertical direction may be controlled.
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<. )dvantages a oads on normally inaccessible bearings can sometimes be determined. b 8eadings can easily be ta$en after the gauges are fitted. c The effects of oil film formation and propeller thrust may be studied. 1. 0isadvantages a The method reuires the s$illed fitting and operation of strain gauges and suitable data acuisition and analysis software b Time is reuired for calculations after ta$ing the strain readings. .(+ *ap and sag
The gap and sag method is used to determine the initial alignment settings. The ag and 0ap procedure is commonly applied as an alignment verification method prior to the shafting assembly. The ag and 0ap should not be regarded as an acceptable method of confirming the final alignment condition, but rather as a cursory chec' of the preassembly condition of the shafting. This is because of the relative inaccuracy and inconsistency of the ag and 0ap measurement itself, as well as the difficulties in 'nowing which condition is actually being measured. The accuracy of the method is a problem because it is often conducted using filler gauges. 88. #rocedures followed before carrying out gap and sag method a) 1ngine and reduction gear are installed. b) Temporary supports are installed. c) hafts are placed inside the vessel and propeller is mounted. d) #ropeller shaft is in contact with a bottom shell at the foremost stern tube. 34. Theoretical background &ap is defined as the difference in distance between the top or bottom edges of the unconnected flange pair. &ap at each flange is
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calculated from the angular inclination of the shaft (at flange location and the flange diameter. Total gap is obtained by linear summation of the gaps at both flanges. The theory behind the procedure is the same beam theory applied in shaft analysis of the whole assembled system, and the calculation is conducted as follows: )lignment is defined and calculated for the assembled system. Position and offset of the temporary bearings are defined. )ssembled system is detached at flanges and each shaft is analy3ed separately, displacements and slope at the each end of the shaft (flange connection are calculated.
5igure @
ag is now calculated by ta'ing the bending displacement at each flange location and subtracting the same from the deflection of the mating flange.
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