Why a flexible coupling? A flexible coupling exists to transmit power (torque) in one shaft to another; to pay for minor levels of misalignment; and, using cases, to supply protective features such as for example vibration dampening or acting as a “fuse” regarding torque overloads. Therefore, commercial power transmission often calls for 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 simply find something similar, if not really similar, 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 since it was the incorrect type for that application. Taking time to look for the right kind of coupling is definitely worthwhile even if it only verifies the previous design. But, it might cause you to something completely different that will work better and go longer. A different coupling style may also prolong the life of bearings, bushings, and seals, preventing fretted spline shafts, minimizing noise and vibration, and cutting long-term maintenance costs.
Sizing and selection
The rich selection of available flexible couplings provides a wide selection of performance tradeoffs. When choosing among them, resist the temptation to overstate service factors. Coupling service factors are designed to compensate for the variation of torque loads typical of different driven systems and to give reasonable service life of the coupling. If chosen too conservatively, they can misguide selection, increase coupling costs to unnecessary levels, and also invite damage somewhere else in the system. Remember that correctly selected couplings usually should break before something more costly does if the machine is overloaded, improperly operated, or somehow drifts out of spec.
Determining the right type of flexible coupling begins with profiling the application the following:
• Primary mover type – electrical electric motor, diesel engine, other
• Real torque requirements of the driven part of the machine, instead of the rated hp of the prime mover – note the number of adjustable torque resulting from cyclical or erratic loading, “worst-case” startup loading, and the quantity 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 – notice amount of angular offset (where shafts aren’t parallel) and quantity of parallel offset (range between shaft centers if the shafts are parallel but not axially aligned); also note whether driving and driven systems are or could be sharing the same base-plate
• Axial (in/out) shaft movement, BE range (between ends of traveling and driven shafts), and any other space-related limitations.
• Ambient conditions – generally temperature range and chemical or oil exposure
But also after these fundamental technical details are identified, various other selection criteria should be considered: Is ease of assembly or installation a account? Will maintenance Vacuum Pump problems such as lubrication or periodic inspection become acceptable? Are the elements field-replaceable, or will the whole coupling have to be changed in case of failing? How inherently well-balanced may be the coupling design for the speeds of a specific application? Is there backlash or free of charge play between the elements of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling because of misalignment? Understand that every flexible coupling style provides strengths and weaknesses and associated tradeoffs. The key is to get the design suitable to the application and budget.
Application specifics
Initially, flexible couplings divide into two main organizations, metallic and elastomeric. Metallic types make use of loosely installed parts that roll or slide against one another or, alternatively, nonmoving parts that bend to take up misalignment. Elastomeric types, on the other hand, gain flexibility from resilient, nonmoving, rubber or plastic material components transmitting torque between metallic hubs.
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Metallic types are suitable to applications that require or permit:
• Torsional stiffness, meaning very little “twist” happens between hubs, in some instances providing positive displacement of the driven shaft for each incremental motion of the traveling shaft
• Operation in fairly high ambient temps and/or existence of certain natural oils or chemicals
• Electric motor drive, as metallics generally aren’t recommended for gas/diesel engine drive
• Relatively continuous, low-inertia loads (metallic couplings aren’t recommended for traveling reciprocal pumps, compressors, and various other pulsating machinery)
Elastomeric types are suitable to applications that want or permit:
• Torsional softness (enables “twist” between hubs so it absorbs shock and vibration and may better tolerate engine get and pulsating or relatively high-inertia loads)
• Greater radial softness (allows more angular misalignment between shafts, puts much less reactionary or side load on bearings and bushings)
• Lighter fat/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 merely their recommendations, yet also the reason why behind them.
Failure modes
The wrong applications for every type are those seen as a the conditions that a lot of readily 
shorten their life. In metallic couplings, premature failure of the torque-transmitting element most often results from steel 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 component most often results from excessive warmth, from either ambient temperature ranges or hysteresis (internal buildup in the elastomer), or from deterioration due to connection with certain oils or chemicals.
Standards
Generally, industry-wide standards usually 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 relevant 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 regular refinery program and special purpose couplings. But besides that, industry specifications on flexible couplings are limited to features such as for example bores/keyways and fits, balance, lubrication, and parameters for ratings.
Information because of this article was provided by Tag McCullough, director, advertising & program engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.