Alignment bracket assembly integrity check and sag determination

Methods for verifying the mechanical integrity of an alignment bracket, particularly a multi-configurable alignment bracket, used for aligning co-rotatable in-line machine shafts, and for determining the amount of alignment bracket "sag" to be accounted for when subsequently employing readings taken with the alignment bracket. The invention employs a previously-established relationship between sag and deflection of the alignment bracket under the influence of a known force to determine sag and to verify mechanical integrity of the alignment bracket in situ, avoiding the need for the usual prealignment sag determination on a separate straight pipe or mandrel. In the practice of the method, the alignment bracket is set up in a position for use, on the actual shafts being aligned. The extension bar of the alignment bracket is positioned above the centerlines of the shafts, and a known force is applied by hanging a known weight at a predetermined point on the alignment bracket. This force results in a deflection of the extension bar, which is measured. From the measured deflection, a sag value is determined based on a previously-determined relationship. To verify integrity of the alignment bracket, measured deflection is preliminarily compared to a range of acceptable deflection values for the particular alignment bracket dimensional configuration.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the art of aligning co-rotatable 
in-line machine shafts which are coupled together for operation by means 
of a shaft coupling. More particularly, the invention relates to methods 
for evaluating a multi-configurable alignment fixture or bracket to verify 
mechanical integrity of the alignment bracket, and to determine the amount 
of "sag" to be accounted for when subsequently employing readings taken 
with the alignment bracket. 
As is well known, whenever two rotating machine shafts are coupled 
together, such as the shaft of an electric motor and the shaft of a pump, 
it is important that the shafts be aligned within predetermined 
tolerances. Such shafts, when in perfect alignment, have their extended 
center lines (axes of rotation) coinciding along a straight line. 
Misalignment can lead to vibration, excessive wear, and ultimate 
destruction of couplings, bearings, seals, gears and other components. 
A number of shaft alignment methods are known, which generally have in 
common the use of suitable alignment fixtures, also termed alignment 
brackets. The alignment brackets are employed to measure particular 
relative displacements (also termed offsets) as the shafts are rotated 
together through one revolution, taking readings at various angular 
positions. Traditionally the shafts are stopped at the 0.degree., 
90.degree., 180.degree. and 270.degree. angular positions to take 
readings. However, as disclosed in commonly-assigned related application 
Ser. No. 07/892,587 filed concurrently herewith by Kenneth R. Piety and 
Daniel L. Nower and entitled "Shaft Alignment Data Acquisition", the 
entire disclosure of which is hereby expressly incorporated by reference, 
readings may be taken at a number of angular positions other than 
0.degree., 90.degree., 180.degree. and 270.degree., and in an automated 
system data may be collected as the shafts are turned smoothly in their 
normal direction of rotation, such that no counter-rotation is allowed. 
Each relative displacement is measured between a point referenced to one 
of the shafts by means of the alignment bracket and a point on the other 
shaft. Dial indicators are often employed, these dial indicators having a 
plunger which moves a hand on the face of the dial indicator. 
The readings are then used to calculate machine moves which will bring the 
shafts into alignment. The 0.degree., 90.degree., 180.degree. and 
270.degree. angular positions at which readings are conventionally taken 
lie in geometric planes in which either of the machines, for example the 
motor, may be moved for purposes of alignment. In particular, the mounting 
bolts of the machine may be loosened, and the machine may be either moved 
in a horizontal plane, moved in a vertical plane by placing or removing 
shims under one or more of the feet of the machine, or both. There are 
well developed calculation methods and procedures known in the art for 
determining what machine moves to make to achieve an aligned condition 
based on measurement of relative displacement (offset) data at the 
0.degree., 90.degree., 180.degree. and 270.degree. positions mentioned. 
An alignment bracket typically has a base firmly clamped or otherwise 
affixed to one shaft, and an extension bar or arm extends laterally from 
the base in a direction generally parallel to the shafts across the 
coupling over to a reference point adjacent a point on the periphery of 
the other shaft. A device for measuring displacement, such as a dial 
indicator, is positioned so as to measure relative displacement in a 
radial direction (offset) from the reference point to the point on the 
periphery of the other shaft as the shafts are rotated together while 
stopping at the 0.degree., 90.degree., 180.degree. and 270.degree. angular 
positions to take and record readings. The position of the alignment 
bracket is then reversed so as to be fixedly referenced to the other 
shaft, establishing a reference point adjacent a point on the periphery of 
the one shaft, and the procedure is repeated. Alternatively, a pair of 
alignment brackets may be employed for simultaneous readings. 
From the geometry just described, it will be appreciated that the reference 
point on the alignment bracket attached to the one shaft rotates about the 
projected centerline (axis of rotation) of the one shaft to define a 
circle centered on that projected centerline, and vice versa for the other 
shaft, and that the distance and direction of the distance between the two 
shaft centerlines as projected can be determined at any transverse plane 
along the shaft axes. From the thus measured distances and directions of 
the distances between the two shaft centerlines as projected in two 
transverse planes, both offset misalignment and angular misalignment 
components may be calculated. 
When readings are taken with the extension bar in an angular position above 
the shafts or below the shafts, the readings are affected by the amount 
the brackets sag under the force of gravity due to their weight. To obtain 
accurate values for misalignment correction, the sag must be subtracted 
from the measurement before any calculations are made to align the shaft 
center lines. Bracket sag primarily results from beam deflection of the 
extension bar, but there can be additional contributions to sag, such as 
flexibility in the alignment bracket base or in the fastening devices 
employed to attach the base to the shafts to be aligned. Thus even 
laser-based alignment brackets which employ beams of light rather than an 
extension bar are subject to sag to some extent. 
It is relevant to note there are a great many specific alignment bracket 
dimensional configurations which may be achieved, even with a given 
alignment bracket. Thus, the extension bar may be adjusted to different 
lengths to suit the particular alignment configuration, and one or more 
spacer blocks may be employed to space the extension bar in a radial 
direction away from the shafts in order to provide clearance around a 
particular coupling. The invention is particularly applicable to such 
multi-configurable alignment brackets. 
In a typical prior art method for measuring sag, the alignment bracket or 
brackets are set up on a straight segment of pipe deemed to be 
sufficiently rigid, or on a suitable mandrel, exactly as the bracket is to 
be later set up during the shaft alignment process. After the brackets are 
set up, the pipe or mandrel is rotated such that the extension bar of the 
alignment bracket is above the pipe or mandrel, that is, lying in a 
vertical plane defined generally by the pipe centerline, which is defined 
as the 0.degree. shaft angular position. Depending upon the particular 
type of dial indicator employed, the dial indicator is either zeroed or an 
absolute reading is taken. Then, the pipe or mandrel is rotated such that 
the alignment bracket extension bar is below the pipe or mandrel in the 
180.degree. angular position, and another reading is taken. Bracket sag is 
determined from the difference between the two readings. 
This prior art method suffers a number of disadvantages. Inherently, error 
can be induced when the alignment brackets are moved from the straight 
pipe to the actual shafts being aligned. No matter how careful the user 
is, bolts and clamps tends to be tightened in a different order, and to a 
different torque. In addition, the brackets may inadvertently be set up at 
a shorter or longer distance. All of these factors affect the amount of 
bracket sag. 
Another disadvantage is that it is possible to mount the alignment brackets 
in a manner such that the data measured is inaccurate, particularly 
brackets that are multi-configuration brackets. This can be caused by not 
tightening all clamps properly, by damaged brackets, or a defective dial 
indicator. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method for 
determining the sag of an alignment bracket employed in the alignment of a 
pair of co-rotatable in-line shafts. 
It is another object of the invention to provide such a method which 
provides accuracy equal to or greater than previous methods. 
It is another object of the invention to provide such a method for 
determining sag which requires less time than previous methods. 
It is yet another object of the invention to provide a method for checking 
the mechanical integrity of an alignment bracket as positioned for use. 
The invention provides a method for determining the sag of an alignment 
bracket in situ, bypassing the need for setting the alignment bracket up 
on a straight pipe prior to aligning. The method thus determines sag as 
the brackets are set up on the machines themselves, for greater accuracy. 
Moreover, the method provides an integrity check of the entire bracket 
assembly before any alignment data is taken. 
In overview, the invention employs a previously-established relationship 
between sag and deflection of the alignment bracket under the influence of 
a known forcing function, which typically is simply a known test weight. 
The relationship can be derived from theoretical beam deflection 
equations, or can simply be measured and recorded for a variety of common 
alignment bracket configurations. 
More particularly, the methods of the invention begin with the step of 
setting up the alignment bracket in a position for use, on the actual 
shafts being aligned. The base of the alignment bracket is attached to one 
shaft, and the extension bar extends over to the other shaft. The bracket 
and shafts are positioned such that the extension bar lies in a vertical 
plane defined generally by the shaft centerlines, for example, above the 
shafts in the 0.degree. angular position. It will be appreciated, however, 
that the same results can be obtained by positioning the alignment bracket 
with the extension bar at the 180.degree. position. 
Next, a known force is applied to the alignment bracket, such as by simply 
hanging a known weight on the alignment bracket at a predetermined point 
on the alignment bracket, preferably at a point near the end of the 
extension bar for maximum leverage. This force results in a relative 
displacement between a reference point typically at the end of the 
extension bar and the other shaft. This deflection is then measured, 
preferably employing the same indicator which is to be subsequently used 
during the actual alignment procedure. 
A sag value for the alignment bracket is then determined from the measured 
deflection based on a previously-determined relationship for the 
particular dimensional configuration of the alignment bracket. The 
previously-determined relationship can be recorded in a simple lookup 
table, or may be embodied in either a theoretically-calculated or 
empirically derived equation. When a lookup table embodiment is employed, 
the lookup table has entries for a plurality of different alignment 
bracket mounting configurations. 
In an integrity check aspect of the invention, the measured deflection is 
compared to a range of acceptable deflection values for the particular 
alignment bracket dimensional configuration. If the deflection does not 
fall within the range, the user is thus advised to check the bracket set 
up. 
In a combined method, first the measured deflection is compared to a range 
of acceptable deflection values. Then, in the event the measured 
deflection is within the range of acceptable deflection values, the sag 
value is determined from the measured deflection based on the 
previously-determined relationship. 
BRIEF DESCRIPTION OF THE DRAWING 
While the novel features of the invention are set forth with particularity 
in the appended claims, the invention, both as to organization and 
content, will be better understood and appreciated, along with other 
objects and features thereof, from the following detailed description 
taken in conjunction with the drawings, in which:

DETAILED DESCRIPTION 
Referring to the single drawing FIGURE, first and second in-line shafts 10 
and 12 are coupled to each other by means of a coupling 14. The shafts 10 
and 12 are connected to respective machines (not shown) such as a motor 
driving a pump through the shafts and coupling 14. After the degree of 
misalignment is measured, one of the machines is moved after loosening its 
mounting bolts, moving the machines in a horizontal plane, and/or 
inserting or removing shims under one or more machine feet, all as 
necessary, to bring the shafts 10 and 12 into an aligned condition, as is 
well known. 
Also shown in the FIGURE, in highly diagrammatic form, is an alignment 
bracket or fixture, generally designated 16. It will be appreciated that 
the FIGURE is intended to concisely illustrate th principles and practice 
of the invention with reference to typical alignment bracket geometry. 
Further details of alignment bracket construction are disclosed in the 
above-incorporated application Ser. No. 07/892,587. 
The alignment bracket 16, includes a base 18 attachable to one of the 
shafts, in this example the shaft 10, by means of a suitable clamping 
device 20, represented as an adjustable strap 20. The strap 20 is 
representative of any one of a number of clamp-like mechanical 
attachments, typically including chains, swing links, and various forms of 
tightening nut arrangements to accommodate various shaft sizes. The 
alignment bracket 16 also includes an extension bar 22 which extends from 
the base 18 in a lateral direction along the shafts 10 and 12 to a 
reference point 24 adjacent the other one of the shafts, in this example 
adjacent the shaft 12. The particular extension bar 22 depicted comprises 
two separately-adjustable segments, a relatively-sturdy tubular segment 26 
adjustably attached to the base 18, and a relatively shorter tip element 
segment 28 adjustably attached to the tubular segment 26, the distal end 
of the tip element segment 28 defining the reference point 24. 
Located at the reference point 24 is a dial indicator 30 including a 
plunger 32. To reduce the weight on the end of the extension bar 22 (which 
weight would further increase "sag"), the dial indicator 30 is attached to 
a base-like fixture element 34 firmly affixed to the shaft 12 by means of 
another clamping device 36 like the clamping device 20. The plunger 32 of 
the dial indicator engages the tip element segment 28. The dial indicator 
30 may alternatively be attached to the tip element segment 28, with the 
plunger engaging the fixture element 34. Although a dial indicator 30 is 
illustrated, any suitable measuring device may be employed. In the case of 
an automated system, a suitable transducer is employed which produces data 
signals. 
It will be appreciated that the distance between the reference point 24 and 
the shaft 12 varies in a radial direction as the shafts 10 and 12 are 
rotated together. The resultant relative displacement as a function of 
angular position is measured and indicated by the dial indicator 30 or 
other measuring device. 
Alignment brackets such as the alignment bracket 16, may be assembled in a 
variety of dimensional configurations to suit various specific alignment 
situations. In particular, the overall length of the extension bar 22, 
indicated as dimension "a", is adjustable by means of suitable sliding 
clamping arrangements (not shown), for example to adapt to couplings 14 of 
various axial extents. Likewise, the radial distance of the extension bar 
22 from the shafts 10 and 12, indicated as dimension "c", is adjustable, 
for example to clear couplings 14 of various diameters. Typically, 
adjustment of dimension "c" is accomplished by installing one or more 
spacers (not shown) as part of the base element 18. 
In the method of the invention, the alignment bracket 16 is set up in a 
position for use, with the extension bar 22 lying in a vertical plane 
defined generally by the centerlines of the shafts 10 and 12. In the 
FIGURE, the extension bar 20 is positioned above the shafts 10 and 12. 
A known force is then applied to the alignment bracket 16, such as by 
hanging a known weight 40 from the extension bar 22 at a predetermined 
point 42, spaced a distance "b" from the base element 18. In this example, 
the predetermined point 42 is, for convenience, located at the end of the 
tubular segment 26 where the tip element segment 28 is attached. The 
resultant deflection of the alignment bracket 16, including beam 
deflection of the extension bar 22 and any flexing in the base 18 and its 
mounting 20, are measured at the reference point 24, by means of the dial 
indicator 30. Thus, "before" and "after" dial indicator readings are taken 
prior to and after application of the known weight 40, and subtracted. 
Alternatively, the dial indicator 30 may be "zeroed" prior to application 
of the known weight 40, and simply read directly thereafter. 
Although the invention is illustrated and described herein with emphasis on 
a manual method, it will be appreciated that this description is for 
clarity only, and that the invention may readily be implemented in an 
automated system employing computer-based data acquisition techniques. 
Also, while the invention is illustrated with reference to an alignment 
bracket having an extension bar 22, it will be appreciated that beam 
deflection of the extension bar 22 is not the only cause of sag, and even 
optically-based alignment brackets are subject to sag, and in addition are 
subject to the possibility of an insecure attachment. 
In general, the mechanical integrity of the bracket is verified and the 
value of sag for the particular dimensional configuration of the alignment 
bracket is determined from the measured deflection based on a 
previously-determined relationship, which conveniently may be recorded in 
a TABLE such as appears hereinbelow by way of example. For each expected 
dimensional configuration, the TABLE includes an entry for average 
measured deflection, an entry for the acceptable interval of variation, 
and average bracket sag. For each bracket configuration, when measured 
deflection is within the acceptable interval of variation, there is a 
generally linear relationship between actual measured deflection and 
actual bracket sag, which can be determined and recorded for subsequent 
use. 
In an overview of use, first the measured deflection is compared against 
the acceptable interval of variation to establish the mechanical integrity 
of the bracket set up. Then the sag of the alignment bracket 16 for the 
specific dimensional configuration is determined by using the measured 
deflection value, along with dimensions "a", "b" and "c" to predict the 
corresponding sag value based on the TABLE, utilizing the 
previously-determined linear relationship. This value of sag is the value 
which will be employed in subsequent calculations to determine machine 
moves to bring the shafts 10 and 12 into alignment. 
By way of example, reproduced below is an excerpt from a sag lookup table. 
TABLE 
______________________________________ 
Average Acceptable 
Measured Variation in 
Average 
Dimension (inches) 
Deflection 
Deflection Bracket 
a b c (mils) (mils) Sag (mils) 
______________________________________ 
10.5 9 5 1.5 .+-.0.5 1.5 
10.5 9 6 2.0 .+-.0.5 1.5 
10.5 9 7 2.5 .+-.0.5 2 
10.5 9 8 3 .+-.0.5 2.5 
. . . . . . 
. . . . . . 
. . . . . . 
12 9 5 3 .+-.1.0 2.5 
12 9 6 3.5 .+-.1.0 2.5 
12 9 7 3.5 .+-.1.0 3.0 
12 9 8 4 .+-.1.0 3.5 
. . . . . . 
. . . . . . 
. . . . . . 
16.25 15 5 9.0 .+-.1.5 7.5 
16.25 15 6 11.5 .+-.1.5 9.5 
16.25 15 7 13.5 .+-.1.5 11.5 
16.25 15 8 15.0 .+-.1.5 13.0 
. . . . . . 
. . . . . . 
. . . . . . 
18.125 15 5 10.5 .+-.2.0 10.0 
18.125 15 6 12.5 .+-.2.0 11.5 
18.125 15 7 14.0 .+-.2.0 12.5 
18.125 15 8 15.5 .+-.2.0 13.5 
______________________________________ 
The foregoing table is a simplified version wherein, for each bracket 
configuration, as defined by dimensions "a", "b" and "c", there is an 
average measured deflection in mils (column four), an acceptable interval 
of variation in mils (column five) and an average bracket sag value 
(column six), also in mils, to be used in subsequent calculations. In this 
particular table, each configuration has only one measured deflection 
value. However, in practice, it will be appreciated that, for each 
configuration as defined by the dimensions "a", "b" and "c", actual 
measured deflection may have a particular value which is close to, but not 
exactly the same as, the measured deflection entry per the table. Such a 
difference, for example, may result in slight variations in the dimensions 
"a", "b" and "c" dependent upon the particular bracket setup, and other 
inevitable tolerance variations. In such cases, a particular sag value is 
determined from the measured deflection by taking the table value and 
adjusting by a correction factor, which is determined in any suitable 
manner, such as simple linear interpolation. 
Another feature of the above table is that, for a given bracket 
configuration as specified by dimensions "a", "b" and "c", the measured 
deflection entry in column four may be viewed as an expected value. Actual 
measured deflection may vary from the table value, such as within a range 
of .+-.1.0 mil, or other previously-determined range, which is termed a 
range of acceptable values. The range may also be expressed as a 
percentage. In the event the actually measured deflection falls outside 
this range, the user knows that there is a problem with alignment bracket 
integrity, which problem must be corrected before proceeding. 
The following two examples illustrate the practice of the invention. 
In a first example, the bracket 16 is set up as indicated in the FIGURE, 
with a dimension "a" of 18.125 inches, a dimension "b" of 15 inches, and a 
dimension "c" of 5 inches. The known weight 40 was hung at the point 42, 
and the resultant deflection measured to be 10.5 mils. The measured 
deflection in this case agrees with the deflection value in the table, and 
a sag value of 10.0 mils is used, without further adjustment, in 
subsequent alignment calculations in the manner well known in the art. Had 
the measured deflection been 10.6 mils, well within .+-.2.0 mils of 10.5 
mils, then a slightly different value for sag would be employed, dependent 
upon a previously-established relationship for the particular 
configuration which would be a value slightly less than 10.1 mils. 
In any event, with this agreement between the measured deflection value and 
the table, the integrity of the bracket 16 setup is indicated as 
satisfactory, and the value of sag to be used for subsequent machine move 
calculations is 10.0 mils. 
In a second example, the bracket 16 is set up with a dimension "a" of 12.0 
inches, a dimension "b" of 9 inches, and a dimension "c" of 8 inches. The 
deflection was measured to be 8.5 mils. From the table, the measured 
deflection should have been 4 mils. The measured deflection in this case 
does not agree with the deflection value in the table, and is well outside 
of a range of acceptable deflection values. The user is accordingly 
advised that the bracket 16 is not set up properly. In this particular 
example, upon inspection, a clamp for the extension bar 22 was found to be 
loose. After tightening, a deflection of 4.5 mils was measured, which is 
within the range of acceptable values for this particular configuration 
(in this case, the range of acceptable values .+-.1.0 mil). The value of 
sag for machine move calculations is accordingly 3.5 mils, plus a further 
correction factor based on the actual measured deflection. 
In view of the foregoing, it will be appreciated that the present invention 
saves time by avoiding the need for a prealignment sag determination on a 
separate pipe or mandrel, eliminates error induced by remounting an 
alignment bracket on machine shafts after determining sag on a separate 
pipe or mandrel, and provides an integrity check of the alignment bracket 
as mounted to ensure the acquisition of accurate alignment data. 
While specific embodiments of the invention have been illustrated and 
described herein, it is realized that numerous modifications and changes 
will occur to those skilled in the art. It is therefore to be understood 
that the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.