Flexible couplings – Things you should know on the subject of sizing and selecting.

Why a flexible coupling? A flexible coupling is present to transmit power (torque) from one shaft to some other; 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” regarding torque overloads. For these reasons, industrial 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 easy 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, nevertheless, this practice invites a do it again failure or expensive 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 period to look for the right kind of coupling can be worthwhile also if it just verifies the previous style. But, it could lead you to something completely different that will are better and go longer. A different coupling design may also prolong the life of bearings, bushings, and seals, preventing fretted spline shafts, minimizing sound and vibration, and slicing long-term maintenance costs.

Sizing and selection
The rich variety of available flexible couplings provides an array of performance tradeoffs. When choosing among them, resist the temptation to overstate support factors. Coupling provider factors are designed to compensate for the variation of torque loads normal of different powered systems and to provide for reasonable service existence of the coupling. If chosen too conservatively, they can misguide selection, raise coupling costs to unneeded levels, and also invite damage elsewhere in the machine. Remember that correctly selected couplings generally should break before something more expensive does if the system is overloaded, improperly operated, or somehow drifts out of spec.

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

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

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

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

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

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

But actually after these fundamental technical details are identified, other selection criteria is highly recommended: Is simple assembly or installation a concern? Will maintenance problems such as for example lubrication or periodic inspection be acceptable? Will be the elements field-replaceable, or will the whole coupling have to be replaced in the event of a failure? How inherently well-balanced is the coupling design for the speeds of a specific application? Is there backlash or free of charge play between your elements of the coupling? Can the gear tolerate much reactionary load imposed by the coupling due to misalignment? Understand that every flexible coupling style provides strengths and weaknesses and associated tradeoffs. The key is to get the design suitable to your application and budget.

Application specifics
Primarily, flexible couplings divide into two principal groupings, metallic and elastomeric. Metallic types make use of loosely fitted parts that roll or slide against each other or, alternatively, non-moving parts that bend to take up misalignment. Elastomeric types, however, gain flexibility from resilient, non-moving, rubber or plastic components 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” happens between hubs, in some cases offering positive displacement of the driven shaft for each incremental motion of the generating shaft

• Operation in relatively high ambient temps and/or existence of certain oils or chemicals

• Electric motor travel, while metallics generally aren’t recommended for gas/diesel engine drive

• Relatively continuous, low-inertia loads (metallic couplings are generally not recommended for driving reciprocal pumps, compressors, and additional pulsating machinery)

Elastomeric types are suitable to applications that want or permit:

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

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

• Lighter excess weight/lower cost, in conditions of 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 reasons behind them.

Failure modes
The incorrect applications for every type are those characterized by the conditions that a lot of readily shorten their life. In metallic couplings, premature failing of the torque-transmitting component most often results from metal fatigue, usually because of flexing due to extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, break down of the torque-transmitting element most often results from extreme temperature, from either ambient temperature ranges or hysteresis (inner buildup in the elastomer), or from deterioration because of connection with certain oils or chemicals.

For the most part, industry-wide standards usually do not exist for the common design and configuration of flexible couplings. The exception to this may be the American Gear Producers Assn. standards relevant in THE UNITED STATES for flangedtype gear couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute has criteria for both regular refinery program and Cycloidal Gear Reducer special purpose couplings. But besides that, industry specs on versatile couplings are limited by features such as for example bores/keyways and matches, balance, lubrication, and parameters for ratings.

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