Flexible couplings – Things you need to know about sizing and deciding on.

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, using cases, to provide protective features such as for example vibration dampening or acting as a “fuse” in the case of torque overloads. Therefore, industrial power transmission frequently demands flexible rather than rigid couplings.

When the time comes to specify replacements for flexible couplings, it’s human nature to take the simple path and simply find something similar, if not really identical, to the coupling that failed, probably applying a few oversized fudge factors to be conservative. Too often, nevertheless, this practice invites a repeat failure or pricey system damage.

The wiser approach is to start with the assumption that the prior coupling failed because it was the wrong type for that application. Taking time to determine the right type of coupling can be worthwhile also if it only verifies the previous design. But, it might cause you to something completely different that will work better and last longer. A different coupling design may also extend the life span of bearings, bushings, and seals, stopping fretted spline shafts, minimizing sound and vibration, and cutting long-term maintenance costs.

Sizing and selection
The rich variety of available flexible couplings provides a wide range of performance tradeoffs. When choosing among them, resist the temptation to overstate program factors. Coupling provider factors are intended to compensate for the variation of torque loads standard of different motivated systems and also to provide for reasonable service life of the coupling. If chosen too conservatively, they are able to misguide selection, raise coupling costs to needless levels, and actually invite damage elsewhere in the system. Remember that correctly selected couplings usually should break before something more expensive will if the system is definitely overloaded, improperly operated, or somehow drifts out of spec.

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

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

• True torque requirements of the driven side of the system, rather than the rated horsepower of the primary mover – notice the range of adjustable torque resulting from cyclical or erratic loading, “worst-case” startup loading, and the quantity 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 – note degree of angular offset (where shafts aren’t parallel) and quantity of parallel offset (length between shaft centers if the shafts are parallel but not axially aligned); also be aware whether traveling and driven products are or could be sharing the same base-plate

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

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

But actually after these basic technical information are identified, other selection criteria is highly recommended: Is ease of assembly or installation a factor? Will maintenance issues such as lubrication or periodic inspection become acceptable? Are the components field-replaceable, or does the whole coupling need to be replaced in the event of failing? How inherently well-balanced is the coupling design for the speeds of a specific application? Will there be backlash or free play between the components of the coupling? Can the gear tolerate much reactionary load imposed by the coupling because of misalignment? Remember that every flexible coupling design has strengths and weaknesses and connected tradeoffs. The key is to find the design suitable to your application and budget.

Application specifics
In the beginning, flexible couplings divide into two main organizations, metallic and elastomeric. Metallic types make use of loosely installed parts that roll or slide against each other or, alternatively, nonmoving parts that bend to consider 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” takes place between hubs, in some cases providing positive displacement of the driven shaft for every incremental movement of the driving shaft

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

• Electric motor travel, seeing that metallics generally are not recommended for gas/diesel engine drive

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

Elastomeric types are suitable to applications that want or permit:

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

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

• Lighter fat/lower cost, when it comes 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, but also the reasons behind them.

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

Generally, industry-wide Scroll Air Compressors Standards usually do not exist for the normal design and configuration of flexible couplings. The exception to this may be the American Gear Producers Assn. standards applicable in North America for flangedtype equipment couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute has requirements for both standard refinery program and particular purpose couplings. But besides that, industry specs on flexible couplings are limited to features such as for example bores/keyways and matches, balance, lubrication, and parameters for ratings.

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


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