Categories: Shaft Alignment,Machinery Maintenance,Other Topics

Alignment Considerations of Machines with Stiff Elastomeric Couplings

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By on December 28, 2016

These types of couplings are quite common in industry, and there are various companies producing similar types of couplings.  They are capable of transmitting large amounts of torque in a reasonably small space.  They also do not require lubrication, and are quite easy to install, in most circumstances.  These attributes make them quite desirable in some industrial applications.

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The elements themselves are quite tolerant of small amounts of misalignment.  But the shafts, bearings, and seals may not be able to tolerate the same degree of misalignment.

A few years ago, I checked the alignment on a 250hp pump, driven by an electric motor, and coupled with this style coupling.  The mechanics in this facility were replacing the mechanical seal every few months, and didn’t know why.  I checked their alignment, and found it to be out of tolerance only slightly, which did not explain the reason for so many seal failures.

We removed the coupling element, and found that misalignment was obvious-just by sight.  We laser aligned the machines with a Fixturlaser NXA, while the coupling element was removed.  Then we re-installed the element.  The seal failures were eliminated.

Why where the seals failing? Per the installation instructions for one manufactures 6″ diameter coupling, the max allowable misalignment of 0.031” offset and 0.037/1” angularity will not cause a premature fatigue failure of the flexible element in normal use.

A 0.037” /1″ (37.o mils/1″) angularity is almost 53 times the max. allowable angular SHAFT alignment tolerance of 0.0007″/1″ (0.7mil/1″) for 1800 RPM machines and 74 times the 0.0005″/1″ (0.5mil/1″) angular tolerance for machines operating at 3600 RPM. Also, the offset “coupling” tolerance is 7.75 to 15 times the allowable “shaft” alignment tolerances for 1800 and 3600 RPM.

While the coupling element can take it, the bearings and seals simply cannot tolerate that degree of misalignment.  To be fair, the coupling installation instructions also states that “care should be taken to achieve the best alignment.”  Remember, flexible couplings are not infinitely flexible.  In all cases, make sure the alignment quality is good enough for all machine components – not just the coupling.

If you have questions about alignment quality, please feel free to contact us at www.vibralign.com.

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

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