Why a flexible coupling? A flexible coupling exists to transmit power (torque) from one shaft to some other; to pay for minor levels of misalignment; and, using cases, to provide protective functions such as for example vibration dampening or acting as a “fuse” in the case of torque overloads. For these reasons, commercial power transmission frequently calls for flexible instead of rigid couplings.
When enough time involves specify replacements for flexible couplings, it’s human nature to take the easy path and simply find something similar, if not identical, to the coupling that failed, maybe applying a few oversized fudge factors to be conservative. Too often, however, this practice invites a do it again failure or pricey 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 look for the right type of coupling can be worthwhile even if it only verifies the prior style. But, it could lead you to something totally different that will work better and last longer. A different coupling design may also prolong the life span of bearings, bushings, and seals, stopping fretted spline shafts, minimizing sound and vibration, and cutting long-term maintenance costs.
Sizing and selection
The rich selection of available flexible couplings provides a wide range of performance tradeoffs. When selecting among them, resist the temptation to overstate service factors. Coupling services factors are intended to compensate for the variation of torque loads normal of different powered systems and to provide for reasonable service existence of the coupling. If chosen too conservatively, they can misguide selection, raise coupling costs to unneeded levels, and even invite damage elsewhere in the machine. Remember that correctly selected couplings usually should break before something more costly will if the system is certainly overloaded, improperly operated, or in some way drifts out of spec.
Determining the proper type of flexible coupling starts with profiling the application as follows:
• Prime mover type – electric electric motor, diesel engine, other
• Real torque requirements of the driven aspect of the machine, instead of the rated hp of the primary mover – notice the number of adjustable torque caused by cyclical or erratic loading, “worst-case” startup loading, and the amount of start-stopreversing activity common during regular operation
• Vibration, both linear and torsional
• Shaft sizes, keyway sizes, and the required suit between shaft and bore
• Shaft-to-shaft misalignment – note amount of angular offset (where shafts aren’t parallel) and quantity of parallel offset (length between shaft centers if the shafts are parallel but not axially aligned); also notice whether traveling and driven devices are or could possibly be sharing the same base-plate
• Axial (in/out) shaft movement, BE distance (between ends of generating and driven shafts), and any other space-related limitations.
• Ambient conditions – mainly heat range and chemical or oil exposure
But actually after these basic technical information are identified, other selection criteria is highly recommended: Is simple assembly or installation a concern? Will maintenance issues such as for example lubrication or periodic inspection be acceptable? Are the components field-replaceable, or will the entire coupling need to be replaced in the event of failing? How inherently well-balanced may be the coupling style for the speeds of a particular application? Is there backlash or free play between the components of the coupling? Can the gear tolerate much reactionary load imposed by the coupling due to misalignment? Remember that every flexible coupling style offers strengths and weaknesses and associated tradeoffs. The main element is to get the design suitable to the application and Worm Gearbox budget.
Application specifics
Originally, flexible couplings divide into two main organizations, metallic and elastomeric. Metallic types use loosely installed parts that roll or slide against one another or, alternatively, non-moving 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 suitable to applications that want or permit:
• Torsional stiffness, meaning hardly any “twist” takes place between hubs, in some instances offering positive displacement of the driven shaft for each incremental movement of the traveling shaft
• Operation in relatively high ambient temps and/or presence of certain natural oils or chemicals
• Electric motor get, seeing that metallics generally aren’t recommended for gas/diesel engine drive
• Relatively constant, low-inertia loads (metallic couplings are generally not recommended for generating reciprocal pumps, compressors, and additional pulsating machinery)
Elastomeric types are suitable to applications that require or permit:
• Torsional softness (enables “twist” between hubs so that it absorbs shock and vibration and will better tolerate engine get and pulsating or fairly high-inertia loads)
• Greater radial softness (allows even more angular misalignment between shafts, puts less reactionary or side load on bearings and bushings)
• Lighter fat/lower cost, in terms of torque capacity relative to maximum bore capacity
• Quieter operation
Thoroughly review the suggested application profile with the coupling vendor, getting not merely their recommendations, but also the reason why behind them.
Failure modes
The wrong applications for every type are those characterized by the circumstances that a lot of readily shorten their lifestyle. In metallic couplings, premature failure of the torque-transmitting component most often results from steel fatigue, usually due to flexing caused by excessive shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting component frequently results from excessive heat, from either ambient temperatures or hysteresis (inner buildup in the elastomer), or from deterioration because of contact with certain oils or chemicals.
Standards
Generally, industry-wide standards usually do not exist for the normal design and configuration of flexible couplings. The exception to this may be the American Gear Producers Assn. standards relevant in THE UNITED STATES for flangedtype equipment couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute provides criteria for both standard refinery program and particular purpose couplings. But other than 
that, industry specs on versatile couplings are limited by features such as bores/keyways and matches, balance, lubrication, and parameters for ratings.
Information for this content was supplied by Mark McCullough, director, advertising & program engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.