factor of alignment-p

8/16/2019 Factor of Alignment-P

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LESSON

1LECTURE

FACTOR AFFECTING THE ALIGNMENT

SUB-OBJECTIVE

At the end of this lesson the trainee will be able to demonstrate an understandingthe factor affecting on alignment.

1.0 INTRODUCTION

Many types of plant equipment including pump fans belt drives, gear boxeselectric motors, turbines and even generators are the driven equipment or prime over. The driven and prime over equipment must connected to each

other by some device (coupling) with correct alignment.

orrect alignment is defined as a perfectly straight line running through thecenter line of the two shaft and that the faces of the two half couplings areexactly at right angle to this line.

2.0 VALUE OF PROPER ALIGNMENT

!hy is correct alignment so important to equipment operation" Thecondition of misalignment of shafts causes the ma#ority of machinecomponent failures. This is due primarily to accepting tolerances that shouldnot be accepted and also accepting apparently good readings which, in

reality, are false.

Misalignment, regardless of the magnitude will result, eventually in un$necessary mechanical problems which will require time and money tocorrect. The most affected parts of a unit that suffers from misalignment are%

&. oupling'. earing *eals+. *haft

2.1 EFFECT ON COUPLING

The most affected part of a unit that suffers from misalignment is thecoupling. egardless of the type employed on a unit, either rigid or flexible,the coupling does not compensate for gross permanent misalignment.*ome people are of the opinion that since the coupling is termed -flexible itrequires less accurate alignment. This is not so. This type of couplingprovides allowances only for unintentional, unexpected, but ever presentshort periods of misalignment created by the inherent characteristics of theunit/s operation. 0t is because these flexible couplings are designed toaccommodate these forces that they do not fail as readily as bearings or seals, which are not designed for any great amount of misalignment.

2.2 EFFECT ON BEARING, SEALS AND SHAFTS

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*tresses that accompany misalignment also have a severe effect uponbearings, both antifriction and plain, thereby reducing their life. 9roper alignment cannot extend the natural life of an antifriction bearing.Misalignment can certainly reduce their natural life. !hen the life of abearing is determined, it is done without misalignment forces being present.

2.3 SHAFT AND OTHER PARTS

Most mechanical seals are designed to function properly only when minimumshaft deflection is encountered, thus, mechanical seals fail due to shaftdeflection created by misalignment.

6nli;e other mechanical problems which begin as a minor deficiency andgrow into something quite noticeable and ma#or, misalignment is as severethe first revolution as it is when the machine finally fails. This is the casewhen a machine does not shift due to misalignment forces. 0t is through thisminor deficiency that the ma#or failure can stem. 0t is true that stresses from

misalignment are in direct proportion to the speed of the unit, with theamount of initial misalignment remaining constant. *peed of unit shouldgenerally dictate the tolerances allowed for alignment. 5peratingcharacteristics will also govern initial and operating alignment tolerances.The following discussion of shaft alignment is dependent on ;eeping thesethoughts of mind.

3.0 FACTOR AFFECTING ON ALIGNMENT

9rior to discussing the particular procedure to be employed on a given unit,there are several factors that may mechanics either don/t understand or failto consider..

3.1 PIPING STRAIN

9ractically all manufactures assembled units, both driver and driven on acommon base are factory alignment. This factory alignment only serves thefactory purpose to determine if and how the unit can be alignment within itsmechanical limits.


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been placed in the lines during shut down, final alignment not be performeduntil these have been removed.

3ote% 5n a installed unit, when pump casing or piping removed for maintenance purpose and at the time of reinstallation the piping strain canagain activate.

3.2 DOWEL PINS AND PUMP CASING JOINTS

5n some unit dowel pins are provided for exact pump casing #oint, becauseit is very critical to the proper operation of the pump.

Manufacturers of these pumps require that this #oint be evenly loaded toinsure proper operation of that unit. Most pump designs allow a space atthis #oint. 1uring the assembly of the pump, the mechanic equali=es thisspace using feeler gages. >& $ .>>+). This will insure proper support of that bearing without placing anundue strain on the casing.

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Fi. 1-1. P!"# B$%&i' S!##(&).

3.* SOFT FOOTING

All driver support feet must be on the same plane. This condition isextremely important and should be one of the first problem areas to bechec;ed. 1rivers with four or more feet are the only ones to possible createthis problem. The trade name of that problem is ;nown as -*oft


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Fi. 1-2. S(+) F(()i'.

>+) mils when one foot is loosened and the others are tight.

3. MAGNETIC CENTER

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An electric motor is said to run in its magnetic center. This means that therotor is pulled into operating position by the magnetic force whenever themotor is running.


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The second concept of hot alignment is that of ;nowing where the unit willgo. 0f the facts are not ;nown as to the unit/s movement, it is easy to secondguess the units movement, if the facts, or calculations, are correct as towhere the unit will move, the unit will align itself. 0f it does not, at least it willmove in the desired direction. 5ne apparent advantage of this concept isthat it is possible not be forced into an additional shim changed based uponhot readings. There is one limitation. 0f cold alignment is drastically off, as inthe case with a steam turbine driving a cold service pump, putting the unit onstream should be done slowly and cautiously to allow warm up andpositioning of the shafts.

3early all units are aligned cold with allowances made for expected thermalgrowth. egardless of whether these allowances have been made or not, ahot chec; should be performed. This chec; will confirm the hot position of the shaft

*.0 ALIGNMENT TOLERANCES

9erfect alignment is the desired ob#ection but in the practical field and inmany cases the achievement of (>.>>) alignment reading is quite difficult andtime consuming #ob. *o the alignments for a unit can accept with sometolerances. emember a stoc; set of alignment tolerances which aresuitable for all of industry #ust simply does not exist. As ;ey a goodalignment tolerances for a given unit is one which permit the unit to runwithout creating forces great enough to causes the components to failprematurely. According to that view forces generated by misalignment aredirectly related to the speed of the shafts, it is logical to use speed as thegoverning agent to establish alignment tolerances. 4conomics is the other factor for establishing the acceptable tolerances. 9M elow

Mils on 51+ Mils on > 9M Above

' Mils (T0) on 51

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& Mil (T0) on


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.0 MISALIGNMENT DETECTION

0t should be noted that misalignment can be detected while the machine is inoperation.


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ut more commonly by parallel or, better yet 51. *haft center lines do notintersect to correct for this condition movement is made for one half the T0of 51 indicator.

Fi. 1-*. OD Di4#6%;$"$').

.2 ANGULAR OR FACE MISALIGNMENT

This type of misalignment is represent by


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.3 MISALIGNMENT B< COMBINATION OF ANGULAR = PARALLEL

That condition involves a combination of these two condition as shown in


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Fi. 1-. A6i'"$') R$;(&8 S>$$).

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factor of alignment-p

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