Why a flexible coupling? A flexible Oil less Air Compressor coupling exists to transmit power (torque) from one shaft to some other; to compensate for minor levels of misalignment; and, in certain cases, to supply protective functions such as vibration dampening or acting as a “fuse” in the case of torque overloads. Therefore, industrial power transmission frequently calls for flexible rather than 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 similar, to the coupling that failed, maybe 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 because it was the incorrect type for that application. Taking time to look for the right type of coupling can be worthwhile even if it only verifies the previous style. But, it might cause you to something totally 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 sound and vibration, and reducing 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 support factors are designed to compensate for the variation of torque loads typical of different motivated systems and also to provide for reasonable service existence of the coupling. If chosen as well conservatively, they are able to misguide selection, increase coupling costs to unneeded levels, and even invite damage elsewhere in the machine. Remember that properly selected couplings usually should break before something more expensive will if the machine is definitely overloaded, improperly managed, or somehow drifts out of spec.

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

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

• True torque requirements of the driven part of the machine, rather than the rated horsepower of the primary mover – note the range of adjustable torque resulting from 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 desired suit between shaft and bore

• Shaft-to-shaft misalignment – notice amount 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 take note whether driving and driven systems are or could possibly be sharing the same base-plate

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

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

But also after these basic technical information are identified, other selection criteria should be considered: Is ease of assembly or installation a consideration? Will maintenance problems such as for example lubrication or periodic inspection end up being acceptable? Will be the components field-replaceable, or does the whole coupling need to be replaced in the event of a failure? How inherently well-balanced may be the coupling style for the speeds of a specific application? Will there be backlash or free play between your parts of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling because of misalignment? Remember 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
In the beginning, flexible couplings divide into two principal groups, metallic and elastomeric. Metallic types use loosely installed parts that roll or slide against each other or, alternatively, nonmoving parts that bend to take up misalignment. Elastomeric types, however, gain flexibility from resilient, nonmoving, rubber or plastic material components transmitting torque between metallic hubs.

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Metallic types are suitable to applications that require or permit:

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

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

• Electric motor get, as metallics generally are not suggested for gas/diesel engine drive

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

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

• Torsional softness (enables “twist” between hubs so that it absorbs shock and vibration and will better tolerate engine get and pulsating or fairly high-inertia loads)

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

• Lighter fat/lower cost, in terms of 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 incorrect applications for each type are those characterized by the conditions that a lot of readily shorten their life. In metallic couplings, premature failure 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 heat, from either ambient temps or hysteresis (internal 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 may be the American Gear Manufacturers Assn. standards applicable in THE UNITED STATES for flangedtype equipment couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute has standards for both standard refinery assistance and particular purpose couplings. But other than that, industry specs on flexible couplings are limited by features such as bores/keyways and suits, stability, lubrication, and parameters for ratings.

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