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

When enough time comes to specify replacements for flexible couplings, it’s human nature to take the easy path and find something similar, if not really 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 prior coupling failed because it was the incorrect type for that application. Taking period to look for the right type of coupling is usually worthwhile even if it only verifies the prior style. But, it could lead you to something totally different that will work better and go longer. A different coupling design 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 selection of performance tradeoffs. When selecting among them, withstand the temptation to overstate program factors. Coupling support factors are designed to compensate for the variation of torque loads common of different motivated systems and to give reasonable service life 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 costly does if the system is certainly overloaded, improperly managed, or in some way drifts out of spec.

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

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

• True torque requirements of the driven part of the machine, instead of the rated hp of the primary mover – note the range of variable torque caused by 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 match between shaft and bore

• Shaft-to-shaft misalignment – take note degree of angular offset (where shafts aren’t parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel however, not axially aligned); also notice whether traveling and driven systems are or could 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 temperature range and chemical substance or oil exposure

But also after these basic technical information are identified, various other selection criteria is highly recommended: Is ease of assembly or installation a concern? Will maintenance problems such as lubrication or periodic inspection become acceptable? Are the components field-replaceable, or does the whole coupling need to be changed in case of failing? How inherently well-balanced is the coupling design for the speeds of a specific application? Will there be backlash or free of charge play between the parts of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling due to misalignment? Understand that every flexible coupling style has strengths and weaknesses and associated tradeoffs. The key is to get the design suitable to the application and budget.

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

Keep on page 2

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

• Torsional stiffness, meaning hardly any “twist” happens between hubs, in some instances offering positive displacement of the driven shaft for every incremental motion of the driving shaft

• Operation in fairly high ambient temps and/or presence of certain oils or chemicals

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

• Relatively constant, low-inertia loads (metallic couplings aren’t recommended for driving reciprocal pumps, compressors, and various other pulsating machinery)

flexible jaw couplingElastomeric types are suitable 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 relatively high-inertia loads)

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

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

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

Standards
Generally, industry-wide standards usually do not exist for the normal design and configuration of flexible couplings. The exception to this is the American Gear Manufacturers Assn. standards relevant in North America for flangedtype gear couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute has requirements for both regular refinery provider and particular purpose couplings. But other than that, industry specs on flexible couplings are limited by features such as for example bores/keyways and suits, stability, lubrication, and parameters for ratings.

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