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December 28, 2016

Alignment Considerations of Machines with Stiff Elastomeric Couplings

By Stan Riddle

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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.vibradev.wpengine.com.