Shaft alignment is a critical step in the installation of a rotating machine, in a new installation or a repaired machine. Skipping or rushing this step may decrease operating efficiency and shorten the life of the machine. The procedure for aligning two rotating machines requires measuring their relative shaft positions and adjusting one or both machine housings, usually by shimming the feet. Until recently, however, the precision with which the shafts had to be aligned was an open question. This changed with the publication of the American National Standards Institute/Acoustical Society of America (ANSI/ASA) standard 2.75-17. Here is a summary of what it covers and how it will benefit users involved in shaft machine alignment.
The need for a standard
Before going any further, it is important to recognize that the ANSI/ASA S2.75-17 standard represents a big step forward. Previously, there was no industry-wide standard for defining shaft alignment tolerances and best practices; thus, the task fell to machine manufacturers and organizations focused on industry-specific applications.
For example, the American Petroleum Institute (API) 670 has provided shaft alignment tolerances for certain hydrodynamic pumps used in the petrochemical industry. Guidance on shaft alignment tolerances and best practices has been developed by various industry mechanical engineering experts and shaft alignment instrument suppliers. Although the more popular ones use similar methodologies and curves to illustrate tighter tolerances for higher speed machines, they vary widely in terms of allowable residual misalignment.
Purpose and scope
In December 2013, the Vibration Institute and the ASA launched a joint effort to create an industry-wide universal shaft alignment standard. This effort culminated in 2017 with the publication of “ANSI/ASA S2.75-2017: Shaft Alignment Methodology, Parts 1 and 2. Part 1: General Principles, Methods, Practices and Tolerances”. This standard deals with shaft alignment of the most common machine configuration: a horizontal machine with a driving and driven element, each with two bearings (one set of four bearings) and a flexible coupling between the shafts. “Part 2: Vocabulary” defines the terms used in Part 1. Part 3, which is scheduled for publication in 2022, will deal with shaft alignment of vertical machines.
In addition to guidance on shaft alignment tolerances, ANSI/ASA S2.75-2017 provides methodology for manual and laser measurements. It also establishes alignment quality levels, outlines best practices for corrective movements, and addresses basic mounting and foundation issues. In addition, the document includes several informative appendices, including:
- alignment principles
- machine motion calculation formulas
- identification and correction of pipe deformation
- running offline methods (OLTR)
- laser detection systems
- graphical alignment modeling
- repeatability
- machine alignment and installation checklist
Tolerances
Among the fundamental concerns that ANSI/ASA S2.75-2017 addresses are the acceptable tolerances of relative shaft position (shaft alignment). It also prescribes tolerances for other critical factors such as base flatness and level, shaft runout, coupling runout, soft foot and movement of OLTR machines.
Additionally, the standard specifies an allowance for pipe and conduit deformation, which “shall not be sufficient to cause changes in shaft alignment of a magnitude greater than 50 micrometers (2 mils; mil = 1/1000 of an inch) measured vertically or horizontally”. at the hitch. The included Appendix C provides a methodology for identifying and correcting this condition when aligning pump shafts.
Alignment Principles
It is important to note that the ANSI/ASA S2.75-2017 standard provides a comprehensive approach to the shaft alignment process, including a flowchart that shows key steps and decision points.
Of the two common methods of assessing shaft-to-shaft alignment (see upper rotator image), one uses the offset and angularity between shaft axes to indicate alignment. The other evaluates the offset at each of the two mating faces relative to the distance between them, producing a pair of angles described in mils of offset/inches of separation (mils/in or µm/mm).
ANSI/ASA S2.75-2017 refers to the flexible element between mating hubs as the mating mechanical link (CML). The angularity between the CML and each hub that occurs at a point called the flex plane accommodates shaft-to-shaft misalignment. Because these two bending plane angles represent the work done by the flexible coupling more accurately than the offset and angularity values, ANSI/ASA S2.75-2017 uses this method to establish the tolerances of alignment.
Another advantage of this method is that it reduces the required tolerance at the two bending planes from two values (an offset and an angle) to a single angle, making it easier to achieve.
Alignment Quality Levels
ANSI/ASA S2.75-2017 provides alignment quality levels in units of mils/in (µm/mm) based on machine operating speed and flex plane angles, directly related to the ratio of offset to flex plane level vs. flex plane separation. Tolerances are provided in tables and graphically on an alignment grade chart (Image 1). They can also be calculated by the formula in equation 1.
The table highlights three levels of alignment: AL4.5 = minimal; AL2.2 = acceptable; and AL1.2 = excellent. A machine builder, service provider or end user can choose any level of alignment depending on the construction of the machine and the operating conditions, independent of the operating speed. For example, a pump manufacturer who builds heavy-duty machines for tough service might specify AL2.0 for their machines, while a machine tool maker wanting exceptionally smooth operation might specify AL1.0. A manufacturing facility may specify AL1.2 for newly installed machines, but allow AL2.2 when boundary conditions (eg bolt or base related) restrict machine movement.
For example, a coupling alignment offset measurement (reverse dial indicator or laser system) of 0.004 inches on a flex plane and a 2 inch flex plane separation would yield a ratio of 4 mils/2 inches = 2 mils/ thumb (Picture 4) . The Degrees of Alignment Chart (Image 3) shows that at 1800 rotations per minute (RPM), a flex plane angle of 2 mils/in is greater than AL2.2 and less than AL4.5. To improve this alignment on AL1.2, both flex plane angles should be less than 0.72 mils/in, with an actual measured offset of less than 1.44 mils at each flex plane. These values can be calculated from the previously mentioned formula. Note: Both angles of the bending plane must be within tolerance, so evaluate the larger of the two.
Many alignment technicians are familiar with the tolerance tables provided by the various alignment tool vendors. Usually these tables give shaft centerline and angularity offset values for common machine speeds when the coupling hub separation is less than 4 inches and offset values at the coupling hub when the separation is greater than 4 inches. This method represents a compromise between concerns about the forces misalignment places on couplings and the desire to have tolerances in the format that was popular when coupling alignment was done only with straight edges and feeler gauges .
For convenience, ANSI/ASA S2.75-2017 provides offset and angularity format tables with values that meet its minimum AL4.5, acceptable AL2.2, and excellent AL1.2 tolerances. Following these values will ensure compliance with the corresponding tolerances in the standard, but geometric differences between the tolerance formats may result in closer alignment than necessary. While not bad for the machine, it may take some extra time and effort.
Make machine moves
ANSI/ASA S2.75-2017 is not a training manual, but it does provide information and guidelines for moving machine cases, a step in the alignment process that can be frustrated by issues such as foot slack and the base or bolt. conditions. For example, it mentions screw jacks and related techniques for adjusting machine position in a controlled manner and discusses the importance of positioning axial spacing (mating gap). Although limited in scope, this information will be useful to alignment technicians experiencing these issues.
The lack of a comprehensive shaft alignment standard has been a barrier to creating effective training and work procedures. ANSI/ASA S2.75-2017 marks a new day for end users, instrument suppliers, and consultants involved in machine shaft alignment. With a comprehensive standard, produced with input from a wide range of technical machinery experts, working procedures and technical specifications can agree, and shaft alignment technicians won’t have to rely on a patchwork of best practices and sometimes erroneous rules of thumb.
Part 1 of ANSI/ASA S2.75-2017 covers alignment of common four-bearing assemblies with flexible couplings, and Part 3, soon to be released, will cover vertical machines that often have solid couplings or cardan shaft drives. Although additional parts may be available, Parts 1 and 3 together will encompass the main segment of common industrial machinery.
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