Why a flexible coupling? A flexible coupling exists to transmit power (torque) in one shaft to another; to compensate for minor levels of misalignment; and, in certain cases, to provide protective functions such as vibration dampening or performing as a “fuse” regarding torque overloads. For these reasons, commercial power transmission frequently calls for flexible instead of rigid couplings.

When the time involves 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 costly system damage.

The wiser approach is to begin with the assumption that the prior coupling failed since it was the incorrect type for that application. Taking period to determine the right kind of coupling can be worthwhile also if it just verifies the prior design. But, it could lead you to something completely different that will are better and go longer. A different coupling style may also extend the life of bearings, bushings, and seals, avoiding fretted spline shafts, minimizing noise and vibration, and trimming 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, withstand the temptation to overstate provider factors. Coupling provider factors are intended to compensate for the variation of torque loads standard of different powered systems and to give reasonable service life of the coupling. If chosen as well conservatively, they are able to misguide selection, raise coupling costs to unnecessary levels, and actually invite damage elsewhere in the system. Remember that correctly selected couplings generally should break before something more expensive does if the system is normally overloaded, improperly operated, or in some way drifts out of spec.

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

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

• Actual torque requirements of the driven side of the machine, instead of the rated hp of the prime mover – be aware the number of adjustable 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 suit between shaft and bore

• Shaft-to-shaft misalignment – be aware degree of angular offset (where shafts are not parallel) and quantity of parallel offset (length between shaft centers if the shafts are parallel but not axially aligned); also be aware whether generating and driven units 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 – mainly heat range and chemical or oil exposure

But even after these basic technical details are identified, additional selection criteria is highly recommended: Is simple assembly or installation a thought? Will maintenance issues such as lubrication or periodic inspection be acceptable? Will be the components field-replaceable, or will the whole coupling need to be changed in case of failing? How inherently well-balanced is the coupling style for the speeds of a particular application? Is there backlash or free of charge play between your components of the coupling? Can the gear tolerate much reactionary load imposed by the coupling because of misalignment? Remember that every flexible coupling design provides strengths and weaknesses and connected tradeoffs. The main element is to get the design suitable 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 each other or, alternatively, nonmoving parts that bend to consider up misalignment. Elastomeric types, however, gain versatility from resilient, non-moving, rubber or plastic material components transmitting torque between metallic hubs.

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

• Torsional stiffness, meaning hardly any “twist” occurs between hubs, in some cases providing positive displacement of the driven shaft for every incremental motion of the generating shaft

• Operation in fairly high ambient temperature ranges and/or existence of certain Gearbox natural oils or chemicals

• Electric motor get, as metallics generally aren’t suggested for gas/diesel engine drive

• Relatively continuous, low-inertia loads (metallic couplings aren’t recommended for generating reciprocal pumps, compressors, and additional pulsating machinery)

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

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

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

• Lighter weight/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 reasons behind them.

Failure modes
The wrong applications for each type are those characterized by the conditions that a lot of readily shorten their existence. In metallic couplings, premature failing of the torque-transmitting element frequently results from steel fatigue, usually because of flexing due to extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting element most often results from excessive warmth, from either ambient temps or hysteresis (inner buildup in the elastomer), or from deterioration because of contact with certain oils or chemicals.

Standards
For the most part, industry-wide standards usually do not can be found for the common 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 standards for both standard refinery service and particular purpose couplings. But besides that, industry specs on flexible 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 Tag McCullough, director, advertising & application engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.