Why a flexible coupling? A flexible coupling is present to transmit power (torque) in one shaft to some other; to Shaft Collar compensate for minor amounts of misalignment; and, using cases, to supply protective features such as vibration dampening or performing as a “fuse” regarding torque overloads. Therefore, industrial power transmission frequently demands flexible rather than rigid couplings.

When enough time involves specify replacements for flexible couplings, it’s human nature to take the easy path and simply find something similar, if not identical, 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 start with the assumption that the previous coupling failed because it was the wrong type for that application. Taking time to look for the right type of coupling is normally worthwhile actually if it just verifies the previous style. But, it could lead you to something completely different that will are better and last longer. A different coupling design may also extend the life span of bearings, bushings, and seals, avoiding fretted spline shafts, minimizing sound and vibration, and cutting long-term maintenance costs.

Sizing and selection
The rich variety of available flexible couplings provides an array of performance tradeoffs. When selecting among them, resist the temptation to overstate support factors. Coupling program factors are intended to compensate for the variation of torque loads standard of different driven systems and also to give reasonable service life of the coupling. If chosen too conservatively, they are able to misguide selection, raise coupling costs to needless levels, and even invite damage elsewhere in the machine. Remember that correctly selected couplings usually should break before something more expensive does if the system is normally overloaded, improperly operated, or somehow drifts out of spec.

Determining the proper type of flexible coupling starts with profiling the application form the following:

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

• True torque requirements of the driven part of the system, instead of the rated horsepower of the prime mover – notice the number of adjustable torque caused by 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 fit between shaft and bore

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

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

• Ambient conditions – generally heat range and chemical substance or oil exposure

But actually after these fundamental technical details are identified, additional selection criteria is highly recommended: Is simple assembly or installation a factor? Will maintenance problems such as lubrication or periodic inspection end up being acceptable? Are the elements field-replaceable, or does the entire coupling need to be changed in the event of a failure? How inherently well-balanced is the coupling style for the speeds of a specific application? Is there backlash or free play between your components of the coupling? Can the equipment tolerate much reactionary load imposed by the coupling due to misalignment? Understand that every flexible coupling design has strengths and weaknesses and associated tradeoffs. The key is to get the design suitable to the application and budget.

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

Continue on page 2

Metallic types are suitable to applications that want or permit:

• Torsional stiffness, meaning very little “twist” occurs between hubs, in some instances providing positive displacement of the driven shaft for every incremental motion of the driving shaft

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

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

• Relatively continuous, 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 it absorbs shock and vibration and may better tolerate engine travel and pulsating or fairly 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, with regards to torque capacity relative to maximum bore capacity

• Quieter operation

Thoroughly review the suggested application profile with the coupling vendor, getting not merely their recommendations, but also the reason why behind them.

Failure modes
The wrong applications for each type are those seen as a the circumstances that a lot of readily shorten their life. In metallic couplings, premature failure of the torque-transmitting element most often results from metal fatigue, usually because of flexing caused by excessive shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting component frequently results from excessive heat, from either ambient temperatures or hysteresis (inner buildup in the elastomer), or from deterioration due to contact with certain natural oils or chemicals.

Generally, industry-wide standards 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 applicable in North America 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 provider and unique purpose couplings. But besides that, industry specifications on versatile couplings are limited to features such as bores/keyways and fits, balance, lubrication, and parameters for ratings.

Information for this article was provided by Tag McCullough, director, marketing & software engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.