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06/09/2010
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Maximising pump/drive motor efficiencies relies on selecting the correct drive coupling, according to David Brooksbank, director of marketing at Altra Industrial Motion.
 "Companies that upgrade their pumping systems, or install new systems, both with the objective of improving efficiency, commit themselves to substantial investment," he says, "not only in terms of the pumps themselves, but also with new energy efficient motors to drive them."
Such investments can easily be wasted if the pump/motor combination is not integrated in a way that not only maximises the pump and motor efficiencies, but also protects the combined equipment from wear and tear, warns Brooksbank.
He also points out that achieving the optimum pump/motor interface would not be a problem if driving and driven shafts were perfectly aligned, if machines didn't vibrate, and if the shafts never actually moved.
"They don't meet any of these criteria, however; which is why, in the real world of power transmission, flexible couplings are required that can accommodate the flaws and dynamics inherent in most systems," he explains.
However, when it comes to selecting couplings, pump users should beware of the trap that more is better. "Oversizing a flexible coupling can result in a reduction in required flexibility or misalignment compensation; and a coupling that is too large can also put additional stresses on the pumps and motors being coupled," advises Brooksbank.
On the other hand, a flexible coupling with too much misalignment capacity may be too soft or too compliant, which, in turn, can cause vibration or an unbalanced condition in rotation.
Brooksbank encourages plant engineers to consider four basic conditions of shaft misalignment, or movement. First, parallel misalignment, which occurs when the two shafts do not share the same axis of rotation. Second, angular misalignment, which applies when shafts are neither coaxial nor parallel.
Third, end float – a condition that occurs when either or both shafts display axial movement, moving in and out. Fourth, torsional flexibility in planes perpendicular to the shaft axis – caused by shock or vibration.
"To be considered flexible, a coupling must handle parallel and angular misalignment," insists Brooksbank. "In addition, couplings with four-way flexibility accommodate both end float and torsional movement."
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Author
Brian Tinham
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