Machinery Alignment introduction

Machinery Alignment  introduction                                                                                                         Machinery Alignment  introduction

Machinery Alignment  introduction

For most rotating machines used in the process industries, the trend is toward higher speeds, higher horsepowers per machine, and less sparing.

The first of these factors increases the need for precise balancing and alignment. This is necessary to minimize vibration and premature wear of bearings, couplings, and shaft seals.
Any kind of alignment, even straightedge alignment, is better than no
alignment at all.
It can give
greatly improved bearing and seal life, lower vibration, and better overall
reliability.


Alignment Tolerances
Before doing an alignment job, we must have tolerances to work toward. Otherwise, we will not know when to stop.
Tolerances must be established before alignment, in order to know when to stop. Various tolerance bases exist. One authority recommends 1/2-mil maximum centerline offset per in. of coupling length, for hot running misalignment.
A number of manufacturers have graphs which recommend tolerances based on coupling span and speed. A common tolerance in terms of face-and-rim measurements is 0.003-in, allowable face gap difference and centerline offset. This ignores the resulting accuracy variation due to face diameter and spacer length differences, but works adequately for many machines.
A better guideline is illustrated in Figure 1, which shows an upper, absolute misalignment limit, and a lower, “don’t exceed for good longterm operation limit.

FIG 1

Figure 1 can be applied to determine allowable misalignment for machinery equipped with non lubricated metal disc and diaphragm couplings, up to perhaps 10,000 rpm. If the machinery is furnished with gear type couplings, Figure 5-2 should be used up to 3,600 rpm only. At speeds
higher than 3,600 rpm, gear couplings will tolerate with impunity only those shaft misalignments which limit the sliding velocity of engaging gear teeth to less than perhaps 120 in. per minute. For gear couplings, this velocity can be approximated by

 V = (𝜋DN) tan 𝛼

where
D = gear pitch diameter, in.
N = revolutions per minute
2tana = total indicator reading obtained at hub outside diameter,
divided by distance between indicator planes on driver and
driven equipment couplings.

Choosing an Alignment Measurement Setup

Many such setups are possible, generally falling into three broad categories: face-andrim,
reverse-indicator, and face-face-distance.
1-Reverse-Indicator Method

FIG 2

ADVANTAGES
1. Accuracy is not affected by axial movement of shafts in sleeve bearings.
2. Both shafts turn together, either coupled or with match marks, so coupling eccentricity and surface irregularities do not reduce accuracy of alignment readings.
3. Face alignment, if desired, can be derived quite easily without direct measurement.
4. Rim measurements are easy to calibrate for bracket sag. Face sag, by contrast, is considerably more complex to measure.
5. Geometric accuracy is usually better with reverse-indicator method in process plants, where most couplings have spacers.
6. With suitable clamp-on jigs, the reverse-indicator method can be used quite easily for measuring without disconnecting the coupling or removing its spacer. This saves time, and for gear couplings,
reduces the chance for lubricant contamination.
7. For the more complex alignment situations, where thermal growth and/or multi-element trains are involved, reverse-indicator can be used quite readily to draw graphical plots showing alignment conditions and moves. It is also useful for calculating optimum moves of two or more machine elements, when physical limits do not allow full correction to be made by moving a single element.
8. When used with jigs and posts, single-axis leveling is sufficient for ball-bearing machines, and two-axis leveling will suffice for sleevebearing machines.
9. For long spans, adjustable clamp-on jigs are available for reverseindicator application, without requiring coupling spacer removal.Face-and-rim jigs for long spans, by contrast, are usually nonadjustable custom brackets requiring spacer removal to permit face mounting.
10. With the reverse-indicator setup, we mount only one indicator per bracket, thus reducing sag as compared to face-and-rim, which mounts two indicators per bracket. (Face-and-rim can do it with
one per bracket if we use two brackets, or if we remount indicators and rotate a second time, but this is more trouble.)

2- Face-and-Rim Method

FIG3

1. It can be used on large, heavy machines whose shafts cannot be turned.
2. It has better geometric accuracy than reverse-indicator, for large diameter couplings with short spans.
3. It is easier to apply on short-span and small machines than is reverse indicator, and will often give better accuracy.

3- Face-Face-Distance Method

FIG 4

1. It is usable on long spans, such as cooling tower drives, without elaborate long-span brackets or consideration of bracket sag.
2. It is the basis for thermal growth measurement in the Indikon proximity probe system, and again is unaffected by long axial spans.
3. It is sometimes a convenient method for use with diaphragm couplings such as Lucas Aerospace (Utica, New York), allowing mounting of indicator holders on spacer tube, with indicator contact points on diaphragm covers.

4- Laser-Optic Alignment



Types of Misalignment

There are two basic types of misalignment: parallel (or offset) and angular. Both types can be found in the vertical and horizontal planes. Typically, a combination of offset and angular misalignment is found in both directions, as shown in Figure5. To achieve our goal, we must correct both types of misalignment in each direction.

FIG 5

ALIGNMENT HINTS

Initial Alignment Check It is necessary to first obtain a complete set of indicator readings with the

machines at ambient temperature

FIG 6

The following is the procedure to be followed for obtaining these readings.

--The indicator bar either must be free of sags or compensated for in the readings.

-- Check the coupling for concentricity. If not concentric, replace the coupling.

--Zero the dial at top of the coupling.

-- Record the readings at 90-degree increments taken clockwise as indicated in Figure 6.

-- For any reading on a shaft, the algebraic sum of the left and right (9 and 3 o’clock) must equal the top and bottom (12 and 6 o’clock). The calculations below are for the example illustrated in Figure 6,

in which shafts A and B are out of alignment as illustrated by

the difference in the sums of the (L + R) readings for shafts A and B and the difference in the sums of the (T+ B) readings for A and B.

Shaft A:                                                   Shaft B:

L1 + R1 = +12+ ( + 24) = + 36          L2+ R2 = -26 + ( - 22) = -48

T1 +B1 =0 +( +36) = + 36                 T2+ B2 = 0 + ( - 48) = -48

Note, however, that this difference, which represents the amount of misalignment at the coupling, is not the amount of correction needed to be performed at the machine feet. This must be determined by using rise-and-run concepts.

-- The dial indicator should start at mid range and not exceed the total range. In other words, do not peg the indicator. If misalignment exceeds the indicator span, it will be necessary to roughly align the

machine before proceeding.

NORMAL ALIGNMENT TOOLS 


1- DIAL INDICATORS
2- V-brackets
3-dial indicator brackets/stands with landing pads
4-short rods with reversible dial clamps
5- medium rods  with reversible dial clamps
6- long rods  with reversible dial clamps

7- Tape measure & tightening tool