Flexible couplings – Things you need to know about sizing and selecting.

Why a flexible coupling? A flexible coupling exists to transmit power (torque) from one shaft to some other; to compensate for minor levels of misalignment; and, using cases, to provide protective features such as vibration dampening or acting as a “fuse” regarding torque overloads. For these reasons, commercial power transmission often demands flexible instead of rigid couplings.

When enough time comes to specify replacements for flexible couplings, it’s human nature to take the simple path and find something similar, if not really similar, to the coupling that failed, maybe applying a few oversized fudge Die Casting factors to be conservative. Too often, however, this practice invites a repeat failure or expensive system damage.

The wiser approach is to begin with the assumption that the previous coupling failed because it was the incorrect type for that application. Taking time to determine the right type of coupling is definitely worthwhile actually if it only verifies the prior design. But, it could cause you to something totally different that will work better and go longer. A different coupling design may also lengthen the life span of bearings, bushings, and seals, avoiding fretted spline shafts, minimizing noise and vibration, and slicing long-term maintenance costs.

Sizing and selection
The rich variety of available flexible couplings provides a wide range of performance tradeoffs. When choosing among them, withstand the temptation to overstate provider factors. Coupling services factors are designed to compensate for the variation of torque loads standard of different motivated systems and to give reasonable service life of the coupling. If chosen too conservatively, they can misguide selection, increase coupling costs to unneeded levels, and also invite damage somewhere else in the machine. Remember that correctly selected couplings generally should break before something more costly does if the machine is certainly overloaded, improperly managed, or in some way drifts out of spec.

Determining the right type of flexible coupling begins with profiling the application form the following:

• Primary mover type – electric motor, diesel engine, other

• True torque requirements of the driven part of the machine, instead of the rated horsepower of the primary mover – notice the range of adjustable torque caused by cyclical or erratic loading, “worst-case” startup loading, and the amount of start-stopreversing activity common during normal operation

• Vibration, both linear and torsional

• Shaft sizes, keyway sizes, and the required match between shaft and bore

• Shaft-to-shaft misalignment – take note degree of angular offset (where shafts aren’t parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel however, not axially aligned); also take note whether traveling and driven systems are or could possibly be sharing the same base-plate

• Axial (in/out) shaft movement, End up being length (between ends of traveling and driven shafts), and any other space-related restrictions.

• Ambient conditions – mainly heat range and chemical substance or oil exposure

But even after these basic technical information are identified, other selection criteria is highly recommended: Is ease of assembly or installation a thought? Will maintenance problems such as for example lubrication or periodic inspection become acceptable? Are the components field-replaceable, or will the entire coupling need to be replaced in the event of a failure? How inherently well-balanced is the coupling style for the speeds of a specific application? Is there backlash or free play between the elements of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling due to misalignment? Understand that every flexible coupling style offers strengths and weaknesses and connected tradeoffs. The key is to get the design suitable to the application and budget.

Application specifics
Originally, flexible couplings divide into two principal organizations, metallic and elastomeric. Metallic types use loosely installed parts that roll or slide against each other or, alternatively, nonmoving parts that bend to take up misalignment. Elastomeric types, however, gain versatility from resilient, nonmoving, rubber or plastic material elements transmitting torque between metallic hubs.

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Metallic types are best suited to applications that require or permit:

• Torsional stiffness, meaning hardly any “twist” takes place between hubs, in some cases offering positive displacement of the driven shaft for every incremental movement of the driving shaft

• Operation in fairly high ambient temperature ranges and/or existence of certain oils or chemicals

• Electric motor get, as metallics generally aren’t suggested for gas/diesel engine drive

• Relatively constant, low-inertia loads (metallic couplings aren’t recommended for traveling reciprocal pumps, compressors, and additional pulsating machinery)

Elastomeric types are best suited to applications that want or permit:

• Torsional softness (enables “twist” between hubs so that it absorbs shock and vibration and can better tolerate engine drive and pulsating or relatively high-inertia loads)

• Greater radial softness (allows even more angular misalignment between shafts, puts less reactionary or part load on bearings and bushings)

• Lighter excess weight/lower cost, when it comes to torque capacity in accordance with maximum bore capacity

• Quieter operation

Thoroughly review the suggested application profile with the coupling vendor, getting not merely their recommendations, but also the reasons behind them.

Failure modes
The incorrect applications for every type are those seen as a the conditions that a lot of readily shorten their lifestyle. In metallic couplings, premature failure of the torque-transmitting component frequently results from metallic fatigue, usually because of flexing caused by excessive shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting element frequently results from extreme high temperature, from either ambient temps or hysteresis (inner buildup in the elastomer), or from deterioration because of contact with certain oils or chemicals.

For the most part, industry-wide standards do not exist for the normal design and configuration of flexible couplings. The exception to the is the American Gear Producers Assn. standards applicable in North America for flangedtype equipment couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute offers specifications for both regular refinery support and particular purpose couplings. But besides that, industry specifications on versatile couplings are limited by features such as bores/keyways and matches, balance, lubrication, and parameters for ratings.

Information because of this content was provided by Tag McCullough, director, marketing & application engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.


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