Why a flexible coupling? A flexible coupling exists to transmit power (torque) from one shaft to some other; to pay for minor amounts 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. Therefore, industrial 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 easy path and simply 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 do it again failure or costly 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 time to look for the right kind of coupling is usually worthwhile actually if it only verifies the prior style. But, it might lead you to something completely different that will work better and go longer. A different coupling design may also lengthen the life of bearings, bushings, and seals, stopping fretted spline shafts, minimizing sound 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 selecting among them, resist the temptation to overstate assistance factors. Coupling provider factors are designed to compensate for the variation of torque loads regular of different motivated systems and to provide for reasonable service existence of the coupling. If chosen too conservatively, they can misguide selection, raise coupling costs to needless levels, and also invite damage somewhere else in the machine. Remember that properly selected couplings usually should break before something more expensive does if the machine is certainly overloaded, improperly operated, or in some way drifts out of spec.
Determining the proper type of flexible coupling begins with profiling the application form the following:
• Prime mover type – electric electric motor, diesel engine, other
• Real torque requirements of the driven side of the machine, rather than the rated hp of the prime mover – take note the range of adjustable torque caused by cyclical or erratic loading, “worst-case” startup loading, and the quantity of start-stopreversing activity common during normal operation
• Vibration, both linear and torsional
• Shaft sizes, keyway sizes, and the required fit between shaft and bore
• Shaft-to-shaft misalignment – be aware degree of angular offset (where shafts are not parallel) and amount of parallel offset (range between shaft centers if the shafts are parallel however, not axially aligned); also be aware whether driving and driven units are or could be posting the same base-plate
• Axial (in/out) shaft movement, End up being distance (between ends of driving and driven shafts), and any other space-related restrictions.
• Ambient conditions – mainly temp range and chemical substance or oil exposure
But actually after these simple technical information are identified, various other selection criteria is highly recommended: Is simple assembly or installation a account? Will maintenance problems such as for example lubrication or periodic inspection be acceptable? Will be the components field-replaceable, or does the whole coupling need to be changed in the event of failing? How inherently well-balanced is the coupling design for the speeds of a particular application? Will there be backlash or free play between the elements of the coupling? Can the equipment tolerate 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 best suited to the application and budget.
Application specifics
Initially, flexible couplings divide into two major groupings, metallic and elastomeric. Metallic types use 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 versatility from resilient, non-moving, rubber or plastic 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 cases offering positive 
displacement of the driven shaft for each incremental motion of the driving shaft
• Operation in fairly high ambient temperatures and/or presence of certain natural oils or chemicals
• Electric motor get, seeing that metallics generally are not recommended for gas/diesel engine drive
• Relatively constant, low-inertia loads (metallic couplings are generally not recommended for driving 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 will better tolerate engine drive 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 weight/lower cost, when it comes to torque capacity relative to maximum bore capacity
• Quieter operation
Thoroughly review the suggested application profile with the coupling vendor, getting not only their recommendations, but also the reason why behind them.
Failure modes
The wrong applications for each type are those characterized by the circumstances that most readily shorten their life. In metallic couplings, premature failure of the torque-transmitting component most often results from steel fatigue, usually because of Reciprocating Vacuum Pump flexing caused by extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting component most often results from extreme warmth, from either ambient temperatures or hysteresis (internal buildup in the elastomer), or from deterioration due to contact with certain natural oils or chemicals.
Standards
For the most part, industry-wide standards usually do not can be found for the normal design and configuration of flexible couplings. The exception to this is the American Gear Manufacturers Assn. standards relevant in THE UNITED STATES for flangedtype equipment couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute has specifications for both regular refinery support and particular purpose couplings. But other than that, industry specs on versatile couplings are limited to features such as for example bores/keyways and suits, stability, lubrication, and parameters for ratings.
Information because of this article was provided by Mark McCullough, director, advertising & application engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.
