The present invention relates generally to the art of aligning, employing a suitable alignment fixture, machine sets including first and second rotatable machine shafts which are coupled together for operation by means of a shaft coupling. More particularly, the invention relates to a method for determining whether alignment is at least acceptable, and to an alignment analyzer including a display for graphically displaying the amount of misalignment in a meaningful manner. Although disclosed herein in the context of aligning co-rotatable in-line machine shafts, the invention is applicable to a wide variety of other configurations of both co-rotatable and independently rotatable shafts, including without limitation parallel shafts, shafts coupled by right angle gear boxes, vertical machines, and machine trains of three or more components.
The invention herein is related to the invention of Daniel L. Nower, Willie T. King and Kenneth R. Piety, U.S. application Ser. No. 08/072,397, filed Jun. 3, 1993, concurrently herewith, entitled "CENTRALIZED ALIGNMENT MANAGEMENT SYSTEM", the entire disclosure of which is hereby expressly incorporated by reference.
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 methods are known for checking the alignment of machine sets, and for performing an alignment job if required. These methods generally have in common the use of suitable alignment fixtures, also termed alignment brackets. The alignment fixtures are employed to measure particular relative displacements (also termed offsets) as the shafts are rotated together through one revolution, while stopping for example at 0.degree., 90.degree., 180.degree. and 270.degree. rotational positions to take offset readings. Each relative displacement is measured between a point referenced to one of the shafts by means of the alignment fixture and a point on the other shaft. Dial indicators are often employed, these dial indicators having a plunger which either moves a hand on the face of an analog dial indicator, or causes an indication on the display of a digital indicator.
Based on the measured offsets, the amount of misalignment can be calculated, and a determination made regarding whether the alignment is within tolerances. If the alignment is not within tolerances, then the measured offsets are used to calculate machine moves which will tend to bring the shafts into alignment, which machine moves are then accomplished. The 0.degree., 90.degree., 180.degree. and 270.degree. rotational 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 are loosened and the machine is either moved in a horizontal plane; or the machine is 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 a plurality of shaft angular positions.
After a machine move has been accomplished, the alignment fixture is again employed to measure offset data, and it is again determined whether the alignment is within tolerances. Further machine moves are calculated and made as necessary. Thus the process is an iterative one. There are several reasons for the iterative nature of the process. For example, misalignments in the horizontal and vertical planes are separately considered, but they are interactive. Thus, machine moves resulting in an improvement in alignment in one plane may adversely affect alignment in the other plane.
Various machine set alignment approaches are described in greater detail in Piety et al U.S. application Ser. No. 07/892,587, filed Jun. 3, 1992, now U.S. Pat. No. 5,263,261, entitled "SHAFT ALIGNMENT DATA ACQUISITION", the entire disclosure of which is hereby expressly incorporated by reference. Alignment approaches include the reverse indicator method wherein a suitable alignment fixture or bracket is employed to measure a pair of relative displacements (offsets) in a radial direction at a plurality of shaft angular positions, and the face-and-rim method. The "rim" part of the face-and-rim method is measurement of a relative displacement (offset) in a radial direction (the same as one of the offsets measured in the practice of the reverse indicator method), and the "face" part of the face-and-rim method is measurement of a relative displacement in an axial direction. Traditionally, offset measurements are made at the 0.degree., 90.degree., 180.degree. and 270.degree. rotational positions, as this facilitates calculation of machine moves required to bring the machine sets and thus the shafts into alignment. However, and as is disclosed in the above-incorporated U.S. application Ser. No. 07/892,587, now U.S. Pat. No. 5,263,261, offset data may be collected at a plurality of arbitrary measurement positions, with a minimum of three measurement positions.
The determination of whether alignment is or is not acceptable implies the ability to express an "amount" of misalignment, preferably in a manner amenable to straightforward comparison to predefined tolerances. There are a number of prior art approaches to expressing the "amount" of misalignment, and for accordingly determining whether alignment is within tolerances, or whether machine moves are required to correct excessive misalignment.
One such approach is known as Total Indicator Runout. A problem, however, with this particular approach is that the "amount" of misalignment expressed is dependent upon the alignment fixture set up and on the size of the machines being aligned. Thus, particular misalignment tolerance limits are nearly impossible to predefine.
Another approach is to express centerline offsets in terms of mils per inch, which is a way of expressing the slope of a line between flexure planes, where the line is angled to connect the points where the centerlines of the two shafts intersect respective flexure planes. (It is said that every coupling has two flexure planes.) This approach to expressing the "amount" of misalignment is employed in the "Bulletin No. 5" supplement dated May 22, 1985 to "Alignment Manual for Horizontal Flexibly-Coupled Rotating Machines", 3rd edition, by Malcolm G. Murray, Jr. One disadvantage is that it is often difficult to determine the exact location of each flexure plane. These exact locations are needed in order to calculate the slope between the flexure planes.
The problem of how to express the "amount" of misalignment and, more particularly, how to define acceptable alignments within predetermined tolerances is discussed in detail in Zatazelo U.S. Pat. No. 4,586,264, entitled "METHODS FOR MEASURING ALIGNMENT OF COUPLED SHAFTS." The approach there described is to express offset misalignment and angle misalignment (angularity) separately at the coupling center. ("Angularity" in this context, while typically specified in units of mils per inch, is entirely different from the mils per inch used to specify centerline offset between flexure planes, as briefly described just above.) Thus, the offset and angularity approach recognizes that there are two relevant misalignment components. Either or both may be present in a given situation. In the case of offset misalignment, shaft centerlines may be parallel, but they do not intersect. Angular misalignment occurs when shafts intersect at an angle. Angular misalignment is manifested as a difference in distance between coupling hub faces across a diameter of the coupling hub faces.
However, specifying offset misalignment and angle misalignment still does not express a single "amount" of misalignment. With prior art approaches, angularity and offset misalignment values are separately compared to respective tolerance limits for the two types of misalignment.