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Why a flexible coupling? A flexible coupling exists to transmit power (torque) in one shaft to some other; to pay for minor levels of misalignment; and, in certain cases, to provide protective features such as for example vibration dampening or acting as a “fuse” regarding torque overloads. Therefore, industrial power transmission frequently demands flexible rather than rigid couplings.

When the time comes to specify replacements for flexible couplings, it’s human nature to take the simple path and simply find something similar, if not identical, to the coupling that failed, probably applying a few oversized fudge factors to be conservative. Too often, however, this practice invites a do it again failure or expensive system damage.

The wiser approach is to start with the assumption that the previous coupling failed since it was the wrong 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 could cause you to something completely different that will work better and last longer. A different coupling style may also prolong the life of bearings, bushings, and seals, stopping fretted spline shafts, minimizing noise and vibration, and slicing long-term maintenance costs.

Sizing and selection
The rich variety of available flexible couplings provides a wide variety of performance tradeoffs. When choosing among them, withstand the temptation to overstate services factors. Coupling services factors are designed to compensate for the variation of torque loads normal of different driven systems and also to give reasonable service life of the coupling. If chosen too conservatively, they can misguide selection, increase coupling costs to unneeded levels, and also invite damage elsewhere in the system. Remember that correctly selected couplings usually should break before something more costly does if the machine is normally overloaded, improperly operated, or somehow drifts out of spec.

Determining the right kind of flexible coupling begins with profiling the application form the following:

• Primary mover type – electrical engine, diesel engine, other

• Genuine torque requirements of the driven side of the machine, rather than the rated hp of the primary mover – be aware the number of variable 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 match between shaft and bore

• Shaft-to-shaft misalignment – note amount of angular offset (where shafts aren’t parallel) and quantity of parallel offset (distance between shaft centers if the shafts are parallel however, not axially aligned); also be aware whether traveling and driven models are or could possibly be sharing the same base-plate

• Axial (in/out) shaft movement, End up being length (between ends of traveling and driven shafts), and any other space-related restrictions.

• Ambient conditions – primarily temp range and chemical or oil exposure

But also after these simple technical information are identified, other selection criteria is highly recommended: Is ease of assembly or installation a factor? Will maintenance issues such as lubrication or periodic inspection be acceptable? Will be the components field-replaceable, or will the entire coupling have to be changed in case of failing? How inherently well-balanced is the coupling design for the speeds of a specific application? Is there backlash or free of charge 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 design provides strengths and weaknesses and linked tradeoffs. The main element is to find the design suitable to the application and budget.

Application specifics
Originally, flexible couplings divide into two primary groups, metallic and elastomeric. Metallic types make use of loosely fitted parts that roll or slide against one another or, alternatively, nonmoving parts that bend to take up misalignment. Elastomeric types, however, gain versatility from resilient, non-moving, rubber or plastic material elements transmitting torque between metallic hubs.

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

• Torsional stiffness, meaning very little “twist” takes place between hubs, in some instances providing positive displacement of the driven shaft for every incremental motion of the Rotary Vane Vacuum Pumps driving shaft

• Operation in fairly high ambient temperatures and/or existence of certain natural oils or chemicals

• Electric motor travel, seeing that metallics generally are not suggested for gas/diesel engine drive

• Relatively constant, low-inertia loads (metallic couplings are generally not recommended for generating reciprocal pumps, compressors, and other pulsating machinery)

Elastomeric types are suitable to applications that require or permit:

• Torsional softness (allows “twist” between hubs so that it absorbs shock and vibration and will better tolerate engine drive and pulsating or fairly 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, when it comes 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 incorrect applications for every type are those characterized by the conditions that most readily shorten their existence. In metallic couplings, premature failure of the torque-transmitting component most often results from metal fatigue, usually because of flexing caused by extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting element most often results from excessive temperature, 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 do not exist for the normal design and configuration of flexible couplings. The exception to the is the American Gear Manufacturers Assn. standards relevant in THE UNITED STATES for flangedtype gear couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute provides requirements for both regular refinery services and particular purpose couplings. But other than that, industry specs on flexible couplings are limited to features such as bores/keyways and suits, balance, lubrication, and parameters for ratings.

Information because of this article was supplied by Tag McCullough, director, advertising & program engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.