Why a flexible coupling? A flexible coupling is present to transmit power (torque) in one shaft to another; to compensate for minor levels of misalignment; and, using cases, to supply protective functions such as for example vibration dampening or acting as a “fuse” in the case of torque overloads. Therefore, commercial power transmission often demands flexible instead of rigid couplings.
When the time involves specify replacements for flexible couplings, it’s human nature to take the simple path and find something similar, if not really identical, to the coupling 
that failed, maybe applying a few oversized fudge factors to be conservative. All too often, however, this practice invites a repeat failure or pricey system damage.
The wiser approach is to start with the assumption that the prior coupling failed because it was the incorrect type for that application. Taking period to look for the right kind of coupling is definitely worthwhile also if it just verifies the previous style. But, it might lead you to something totally different that will work better and go longer. A different coupling style may also extend the life of bearings, bushings, and seals, preventing fretted spline shafts, minimizing sound and vibration, and trimming long-term maintenance costs.
Sizing and selection
The rich selection of available flexible couplings provides an array of performance tradeoffs. When choosing among them, withstand the temptation to overstate services factors. Coupling assistance factors are intended to compensate for the variation of torque loads usual of different motivated systems and also to give reasonable service lifestyle of the coupling. If chosen as well conservatively, they can misguide selection, increase coupling costs to unnecessary levels, and also invite damage elsewhere in the machine. Remember that correctly selected couplings generally should break before something more costly does if the machine is definitely overloaded, improperly operated, or in some way drifts out of spec.
Determining the proper type of flexible coupling begins with profiling the application form as follows:
• Prime mover type – electrical electric motor, diesel engine, other
• Genuine torque requirements of the driven part of the machine, rather than the rated hp of the primary mover – take note 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
• Cycloidal Gearbox Shaft-to-shaft misalignment – note degree of angular offset (where shafts are not parallel) and quantity of parallel offset (range between shaft centers if the shafts are parallel but not axially aligned); also notice whether traveling and driven products are or could possibly be sharing the same base-plate
• Axial (in/out) shaft movement, BE range (between ends of generating and driven shafts), and any other space-related restrictions.
• Ambient conditions – mainly temp range and chemical substance or oil exposure
But even after these simple technical information are identified, additional selection criteria is highly recommended: Is ease of assembly or installation a consideration? Will maintenance issues such as lubrication or periodic inspection end up being acceptable? Will be the components field-replaceable, or will the whole 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? Will there be backlash or free play between the components of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling due to misalignment? Remember that every flexible coupling style provides strengths and weaknesses and associated tradeoffs. The main element is to get the design best suited to the application and budget.
Application specifics
Primarily, flexible couplings divide into two primary groupings, metallic and elastomeric. Metallic types use loosely installed parts that roll or slide against one another or, alternatively, non-moving parts that bend to consider up misalignment. Elastomeric types, on the other hand, gain flexibility from resilient, non-moving, rubber or plastic components transmitting torque between metallic hubs.
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Metallic types are suitable to applications that want or permit:
• Torsional stiffness, meaning very little “twist” takes place between hubs, in some cases providing positive displacement of the driven shaft for every incremental movement of the traveling shaft
• Operation in relatively high ambient temperatures and/or existence of certain natural oils or chemicals
• Electric motor drive, while metallics generally aren’t suggested for gas/diesel engine drive
• Relatively continuous, low-inertia loads (metallic couplings are generally not 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 it absorbs shock and vibration and can better tolerate engine get and pulsating or relatively high-inertia loads)
• Greater radial softness (allows more angular misalignment between shafts, puts less reactionary or part load on bearings and bushings)
• Lighter pounds/lower cost, with regards to torque capacity in accordance with maximum bore capacity
• Quieter operation
Thoroughly review the suggested application profile with the coupling vendor, getting not only their recommendations, yet also the reason why behind them.
Failure modes
The wrong applications for each type are those seen as a the conditions that most readily shorten their lifestyle. In metallic couplings, premature failing of the torque-transmitting element frequently results from metal fatigue, usually due to flexing due to extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, break down of the torque-transmitting element frequently results from excessive high temperature, from either ambient temperatures or hysteresis (inner buildup in the elastomer), or from deterioration due to contact with certain oils or chemicals.
Standards
Generally, industry-wide standards do not can be found for the normal design and configuration of flexible couplings. The exception to the may be the American Gear Producers Assn. standards applicable in North America 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 support and particular purpose couplings. But besides that, industry specs on versatile couplings are limited to features such as bores/keyways and fits, stability, lubrication, and parameters for ratings.
Information because of this content was provided by Tag McCullough, director, marketing & software engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.