Why a flexible coupling? A flexible coupling exists to transmit power (torque) from one shaft to another; to compensate for minor amounts of misalignment; and, in certain cases, to provide protective functions such as vibration dampening or acting as a “fuse” in the case of torque overloads. For these reasons, industrial power transmission often calls for flexible rather than rigid couplings.

When the time comes to 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 repeat failure or costly system damage.

The wiser approach is to start with the assumption that the previous coupling failed because it was the wrong type for that application. Taking time to determine the right type of coupling is worthwhile even if it only verifies the previous design. But, it might lead you to something totally different that will work better and last longer. A different coupling design may also extend 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 variety of available flexible couplings provides a wide range of performance tradeoffs. When selecting among them, resist the temptation to overstate service factors. Coupling service factors are intended to compensate for the variation of torque loads typical of different driven systems and to provide for reasonable service life of the coupling. If chosen too conservatively, they can misguide selection, raise coupling costs to unnecessary levels, and even invite damage elsewhere in the system. Remember that properly selected couplings usually should break before something more expensive does if the system is overloaded, improperly operated, or somehow drifts out of spec.

Determining the right type of flexible coupling starts with profiling the application as follows:

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

• Real torque requirements of the driven side of the system, rather than the rated horsepower of the prime mover – note the range of variable torque resulting from 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 desired fit between shaft and bore

• Shaft-to-shaft misalignment – note degree of angular offset (where shafts are not parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel but not axially aligned); also note whether driving and driven units are or could be sharing the same base-plate

• Axial (in/out) shaft movement, BE distance (between ends of driving and driven shafts), and any other space-related limitations.

• Ambient conditions – mainly temperature range and chemical or oil exposure

But even after these basic technical details are identified, other selection criteria should be considered: Is ease of assembly or installation a consideration? Will maintenance issues such as lubrication or periodic inspection be acceptable? Are the elements field-replaceable, or does the entire coupling have to be replaced in the event of a failure? How inherently well-balanced is the coupling design for the speeds of a particular application? Is there backlash or free play between the components of the coupling? Can the equipment tolerate much reactionary load imposed by the coupling due to misalignment? Remember that every flexible coupling design has strengths and weaknesses and associated tradeoffs. The key is to find the design best suited to your application and budget.

Application specifics

Initially, flexible couplings divide into two primary groups, metallic and elastomeric. Metallic types use loosely fitted parts that roll or slide against each other or, alternatively, non-moving parts that bend to take up misalignment. Elastomeric types, on the other hand, gain flexibility from resilient, non-moving, rubber or plastic elements transmitting torque between metallic hubs.

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

• Torsional stiffness, meaning very little “twist” occurs between hubs, in some cases providing positive displacement of the driven shaft for each incremental movement of the driving shaft

• Operation in relatively high ambient temperatures and/or presence of certain oils or chemicals

• Electric motor drive, as 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 other pulsating machinery)

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

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

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

• Lighter weight/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 only their recommendations, but also the reasons behind them.

Failure modes

The wrong applications for each type are those characterized by the conditions that most readily shorten their life. In metallic couplings, premature failure of the torque-transmitting element most often results from metal 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 element most often results from excessive heat, from either ambient temperatures or hysteresis (internal buildup in the elastomer), or from deterioration due to contact with certain oils or chemicals.

Standards

For the most part, industry-wide standards do not exist for the common design and configuration of flexible couplings. The exception to this is the American Gear Manufacturers Assn. standards applicable in North America for flangedtype gear couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute has standards for both standard refinery service and special purpose couplings. But other than that, industry specifications on flexible couplings are limited to features such as bores/keyways and fits, balance, lubrication, and parameters for ratings.

Information for this article was provided by Mark McCullough, director, marketing & application engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.

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