Flexible couplings – Things you should know on the subject of sizing and selecting.

Why a flexible coupling? A flexible coupling is present to transmit power (torque) in one shaft to another; to compensate for minor amounts of misalignment; and, in certain cases, to provide protective functions such as for example vibration dampening or acting as a “fuse” in the case of China Vacuum Pump torque overloads. Therefore, commercial power transmission often demands flexible instead of rigid couplings.

When the time involves specify replacements for flexible couplings, it’s human nature to take the easy path and 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 repeat failure or expensive system damage.

The wiser approach is to begin with the assumption that the previous coupling failed since it was the incorrect type for that application. Taking period to look for the right type of coupling is usually worthwhile also if it just verifies the prior design. But, it could cause you to something completely different that will work better and go longer. A different coupling style may also extend the life span 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 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 typical of different powered systems and to provide for reasonable service existence of the coupling. If chosen as well conservatively, they can misguide selection, increase coupling costs to unneeded levels, and actually invite damage elsewhere in the machine. Remember that correctly selected couplings usually should break before something more costly will if the machine can be overloaded, improperly operated, or somehow drifts out of spec.

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

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

• True torque requirements of the driven part of the system, rather than the rated hp of the primary 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 normal operation

• Vibration, both linear and torsional

• Shaft sizes, keyway sizes, and the desired match between shaft and bore

• Shaft-to-shaft misalignment – note degree of angular offset (where shafts are not parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel however, not axially aligned); also note whether traveling and driven systems are or could 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 restrictions.

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

But actually after these fundamental technical details are identified, additional selection criteria should be considered: Is simple assembly or installation a account? Will maintenance problems such as for example lubrication or periodic inspection be acceptable? Are the elements field-replaceable, or will the whole coupling need to be replaced in case of failing? How inherently well-balanced may be the coupling design for the speeds of a specific application? Will there be backlash or free of charge play between the elements of the coupling? Can the equipment tolerate much reactionary load imposed by the coupling because of misalignment? Remember that every flexible coupling style offers strengths and weaknesses and connected tradeoffs. The key is to get the design suitable to the application and budget.

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

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Metallic types are suitable 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 generating shaft

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

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

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

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

• Torsional softness (allows “twist” between hubs so that it absorbs shock and vibration and may better tolerate engine drive 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 weight/lower cost, when it comes 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 incorrect applications for each type are those characterized by the circumstances that most readily shorten their existence. In metallic couplings, premature failure of the torque-transmitting component frequently results from metallic 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 element most often results from excessive warmth, from either ambient temps or hysteresis (internal buildup in the elastomer), or from deterioration because of connection with certain oils or chemicals.

Standards
For the most part, industry-wide standards do not can be found for the common design and configuration of flexible couplings. The exception to the may be the American Gear Producers Assn. standards applicable in North America for flangedtype equipment couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute offers standards for both standard refinery service and special purpose couplings. But other than that, industry specs on flexible couplings are limited to features such as for example bores/keyways and fits, balance, lubrication, and parameters for ratings.

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