Flexible couplings - Things you need to know on the subject of sizing and selecting.

Why a flexible coupling? A flexible coupling exists to transmit power (torque) from one shaft to some other; to compensate for minor levels of misalignment; and, in certain cases, to provide protective functions such as vibration dampening or acting as a “fuse” in the case of torque overloads. Therefore, industrial power transmission often calls for flexible instead of 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 similar, to the coupling that failed, maybe applying a few oversized fudge factors to be conservative. Too often, nevertheless, 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 incorrect type for that application. Taking time to look for the right kind of coupling is usually worthwhile even if it just verifies the prior style. But, it could cause you to something completely different that will are better and last longer. A different coupling style may also extend the life of bearings, bushings, and seals, avoiding fretted spline shafts, minimizing sound and vibration, and slicing long-term maintenance costs.

Sizing and selection
The rich selection of available flexible couplings provides a wide variety of performance tradeoffs. When selecting among them, resist the temptation to overstate support factors. Coupling provider factors are designed to compensate for the variation of torque loads normal of different driven systems and also to give reasonable service lifestyle of the coupling. If chosen too conservatively, they are able to misguide selection, increase coupling costs to unneeded levels, and even invite damage somewhere else in the system. Remember that properly selected couplings generally should break before something more expensive will if the system can be overloaded, improperly operated, or in some way drifts out of spec.

Determining the proper kind of flexible coupling begins with profiling the application form as follows:

• Primary mover type - electric motor, diesel engine, other

• Real torque requirements of the driven side of the machine, instead of the rated horsepower of the prime mover - note the number of adjustable torque resulting from 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 desired fit between shaft and bore

• Shaft-to-shaft misalignment - notice degree of angular offset (where shafts aren't parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel but not axially aligned); also note whether traveling and driven products are or could possibly be sharing the same base-plate

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

• Ambient conditions - mainly heat range range and chemical or oil exposure

But also after these fundamental technical details are identified, various other selection criteria is highly recommended: Is ease of assembly or installation a factor? Will maintenance problems such as lubrication or periodic inspection be acceptable? Will be the elements field-replaceable, or will the whole coupling have to be replaced in case of a failure? How inherently well-balanced is the coupling design for the speeds of a specific application? Will there be backlash or free play between your Water-lubricated Air Compressor 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 get 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 use loosely installed parts that roll or slide against each other or, alternatively, nonmoving parts that bend to take up misalignment. Elastomeric types, on the other hand, gain versatility from resilient, nonmoving, rubber or plastic material elements 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 each incremental motion of the generating shaft

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

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

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

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

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

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

• Lighter excess weight/lower cost, in conditions of 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 reasons behind them.

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

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 applicable in North America for flangedtype gear couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute has standards for both standard refinery services and unique purpose couplings. But other than that, industry specs on flexible couplings are limited by features such as for example bores/keyways and suits, balance, lubrication, and parameters for ratings.

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

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