Ken Gralow
Gray Electric Co.
Schenectady, New York
Technical Education Committee Member
Shaft couplings are devices that connect two rotating shafts together. They efficiently transfer motion and power from the drive unit to the driven unit without adversely impacting either piece of rotating equipment. Under ideal conditions, both shafts should function as a continuous unit.
The design of a flexible coupling is to accommodate small amounts of shaft misalignment. Coupling manufacturers have designed their couplings to withstand the forces resulting from excessive shaft misalignment. Unfortunately, shaft alignment tolerances have sometimes been governed by the coupling manufacturers’design speecifications. These are maximum values that are dimensionally possible for a specific coupling. The coupling misalignment tolerances reported by coupling manufacturers apply ONLY to the coupling.
Life of the coupling
A common industry error is to assume that this misalignment capability extends to the driver/driven equipment. It should be noted, however, that the tolerances offered by coupling manufacturers are to ensure the life of the coupling, not the life of the drive and driven units. Misunderstanding of the coupling manufacturers’ literature perpetuates the old practice of straight edge or feeler gauge alignment.
The general thought process is: “We purchase good couplings. The coupling manufacturers state that their coupling can withstand a reasonable amount of misalignment.” This thought process is totally wrong! One coupling manufacturer states in its coupling catalog: “Although couplings can operate satisfactorily at the misalignment listed in the catalog, both the coupling life and machine bearing wear can be greatly improved if the machines are aligned better than the maximum value that the coupling can accommodate. It is recommended that the working misalignment should not exceed 20% of the catalog values.”
Types of misalignment
Shaft alignment is the positioning of the rotational centers of two or more shafts such that they are collinear when the driver and driven machines are under normal operating conditions. Deviation from this collinear relationship constitutes misalignment (Figure 1). There are two components of misalignment: offset misalignment and angular misalignment. Offset misalignment, sometimes referred to as parallel misalignment, is the distance between the shaft centers of rotation measured at the plane of power transmission. This is typically measured at the shaft centerline. The typical units for this measurement are mils (where 1 mil = 0.001 in. or .025 mm).
Angular misalignment, sometimes referred to as gap misalignment, is the difference in the slope of one shaft, usually the driver, as compared to the slope of the shaft of the other machine, usually the driven machine. The units for this measurement are comparable to the measurement of the slope of a roof (i.e., rise/run). In this case, the rise is measured in mils and the run (distance along the shaft) is measured in inches. The units for angular misalignment are mils/1 in. These two separate alignment conditions exist along two planes of potential misalignment, the horizontal plane (side to side) and the vertical plane (up and down), so there are actually four alignment parameters to be measured and corrected. They are horizontal angularity, horizontal offset, vertical angularity and vertical offset.
A large number of premature machine failures can be traced back to faulty alignment. Shafts/coupling misalignment is responsible for over half of all excessive vibration in machines. The misalignment of two rotating shafts generates forces that produce great stresses on both the rotating and stationary components of the driver and driven units.
Even though the coupling has been designed to tolerate the large stresses generated as a result of misalignment and will most likely not fail; the bearings and seals of misaligned machines cannot tolerate these same stresses and will fail prematurely.
The small internal clearances of bearings and seals cannot tolerate these forces that can be equated to constant hammering. Excessive misalignment of a value greater than 2 mils (.05mm) offset for a 2-pole machine under normal operating conditions can generate severe forces that are applied directly to the machine bearings and cause excessive fatigue and wear of the shaft seals. Angular misalignment can further decrease bearing life by inducing additional bearing load in the axial direction. Extreme cases of shaft misalignment can cause excessive bending stresses to the shaft resulting in shaft failure from rotational bending.
Figure 2 illustrates that exponentially high gains are obtained from small adjustments going from what is alignment tolerances, and then considered to be an “acceptable or good” alignment to a precision alignment. One of the most important factors for extending machine life is precision alignment. The calculated life expectancy of ball and roller bearings (L10) is the rating of fatigue life for a specific bearing. Analysis of bearing life is related to forces applied to the bearings and has determined that as the force applied to a given bearing increases the life expectancy decreases by the cube of that increase. If the amount of force on a bearing is increased by a factor of 3 as a result of misalignment, the life expectancy amounts of the machine's bearing will decrease by a factor of 27 (3 x 3 x 3 = 27).
For ball bearings: L10 = (C/P)3 x 106
- L10 is the number of revolutions in which 10% of a large population of identical bearings will fail under the same load conditions.
- C is the basic dynamic load rating, the load that will give a life in millions in revolutions, which can be found in bearing catalogs.
- P is the dynamic equivalent load applied to the bearing.
Again, looking at Figure 2, the exponential increase in machine life can clearly be seen as the graph is followed from 40 mils/in. offset to 2 mils/in. offset.
Over the past twenty years there has been a great deal of research devoted to determining the proper alignment values for rotating machines. Today, shaft alignment tolerances are based on shaft rpm rather than coupling manufacturers’ specifications. Even though there are no specific tolerance standards published by ISO or ANSI, generally accepted alignment tolerances are shown in Table 1.
These tolerances are key to establishing a standard of comparison by which all alignments can be judged. They create a level of comparison and accountability for an alignment that is done properly. Using Table 1 as a guide for acceptable alignment tolerances, and then comparing that to published misalignment tolerances for an elastometric coupling of .062"/in. offset, we can see that even if we use the previous mentioned recommendation that misalignment should not exceed 20% of this value for a machine running at 1800 rpm with an acceptable offset of .004/in. we can see that .065"/in. x 20% = .013", or 13 mils/in. This number falls far outside the acceptable range in Table1.
Reduces bearing life
A study performed by the University of Tennessee found even small amounts of misalignment significantly reduce bearing life. This study showed that a driver and driven unit that were misaligned by an offset of only 10% of the coupling manufacturer's allowable offset had corresponding 10% reduction in inboard bearing life. The study further showed that if the misalignment offset was 70% of the coupling manufacturer's allowable offset, there was a corresponding 50% reduction in the inboard bearing life. Table 2 is a summary of the University of Tennessee’s findings.
Using these two examples, we can clearly see that the coupling misalignment tolerances recorded by the coupling manufacturers apply only to the coupling. These are the maximum values that are dimensionally possible for a specific coupling. Even though the mechanics of a coupling may tolerate severe misalignment, the impact that the extreme misalignment has on both the driver and the driven unit will severely impact the bearings and shafts as well as cause excessive vibration.
particularly susceptible to the damage of misalignment is the mechanical shaft seal. Even slight misalignment allows contamination to enter the seal, leading to premature wear. A mechanical shaft seal should run trouble free until the carbon face has worn away. Inspection of seals that have been removed from leaking pumps has shown that in 85% of the cases, carbon face wear was minimal. The seals failed prematurely as a result of seal movement caused by pump and motor misalignment. Another machine component In conclusion, the case for precision alignment is obviously apparent. A correctly aligned machine can result in lower operating costs through the reduction of downtime and repair frequency. Precision alignment increases the life of bearings, seals, shaft couplings and gear trains.
References
- Maintenance Technology. “Understanding Shaft Alignment: Basics.” By VibrAlign, Inc. http://www.mtonline.com/articles/ 1202_shaftalign.cfm
- Maintenance Technology. “Study Shows Shaft Misalignment Reduces Bearing Life.” By J. Wesley Hines, Stephen Jesse, and Andrew Edmondson, The University of Tennessee, and Dan Nower, Computational Systems, Inc. http:// www.mt-online.com/articles/0499ma.cfj?pf=1
- Machinery Vibration: Alignment. “Specifying Shaft Alignment.” By Victor Wowk, P.E., Machine Dynamics, Inc. 2000. McGraw-Hill Book Co.
- Solutions. Vol 2. No.4. Update International, Inc. Denver, CO. November 1990. “Why Precision Alignment?” By Greg R. Buscarello, Operations Manager, Update International, Inc.
- Shaft Alignment Handbook. By John Piotrowski. Cincinnati, OH. Marcel Dekker, Inc. New York and Basel.
- Mechanical Engineering Publications Limited. London, 1991. “Couplings and Shaft Alignment.” By Michael Neale, Paul Needham, and Roger Horrell,
- Vibration Institute Proceedings, “Alignment of Rotating Machinery,” Houston, Texas, 1991.
- Machinery Alignment Handbook, Vibralign, 1994.
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