Alignment Tolerances

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"Flexible" Couplings - False sense of security?


Far too many people in industry think that couplings take care of misalignment and in general refer to them as "flexible" couplings. Not so! The word "flexible" along with the word coupling just gives uniformed users a false sense of security and into thinking everything will be all right no matter how much misalignment exists!

If flexible couplings took care of misalignment then there should be no related vibration problems - right? If that were true then why do the bearings & seals fail when these couplings are taking such good care of the misalignment? Because the couplings do not take care of the misalignment - they are not designed to handle excessive misalignment! Straight edge "alignment" is not close enough - period! Good (precision) alignment, by whatever method, takes care of misalignment.

Yes, some couplings do better than others on particular applications because most coupling manufacturers are using better materials and increasingly better designs. Yes, there are many good couplings manufactured. BUT, they still do not protect the bearings and seals in the absense of proper "tolerance" alignment! When couplings wear or fail many look for a different style or different manufacturers coupling - treating only the symptom not the root cause - which is generally misalignment or in some cases lubrication.

The primary purpose of the coupling is to transmit the power of the driver to the driven machine - period. Look at the word - coupling. Couplings stiffen under torque and do not flex the way they do in our hands or during assembly. This stiffness causes shaft deflection eating up the precious little bearing clearance we have causing more bearing load and shortening the bearing life. Also, this stiffness subjected to misalignment - causes the vibration which is a symptom! The more misalignment the higher the vibration!


“A flexible coupling is not designed to compensate for excessive misalignment. Its purpose is to permit slight movement of pump or driver shafts or any driven equipment while transmitting power. Excess misalignment can cause short coupling life due to sliding and working action of coupling connectors. Modern machinery operates at ever increasing speeds; even minor alignment errors lead to high vibration on bearing loads resulting in machinery damage and production downtime.”

Courtesy ATR Sales Inc.
manufacturer of Atra-Flex® Couplings


Believe what you want and I'm sure some of you will - but there is no substitute for good maintenance practices and precision alignment is an extremely important practice. By the way, Atra-Flex® Couplings, do a very good job in many troublesome applications.

Have a Great Day!

Norm Voelzow • Voelzow & Company, Inc,

© Copyright 2003 Voelzow & Company, Inc. Wingate, North Carolina

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Quoted text © Copyright 2003 ATR Sales Inc. Santa Ana, California

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Eaton Airflex® VC Clutch/Brake

If you need a great clutch or brake coupling, or a clutch/brake package, be sure to visit the Eaton Airflex® website. Having worked for Eaton over 16 years, I can personally vouch for these products.


PS: Click on the VC Clutch or Eaton logo for more information from the Clutch/Brake Pro's!

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