Flexible couplings – Things you need to know on the subject of sizing and deciding on.

Why a flexible coupling? A flexible coupling is present to transmit power (torque) from one shaft to another; to compensate for minor levels of misalignment; and, in certain cases, to provide protective functions such as for example vibration dampening or performing as a “fuse” in the case of torque overloads. Therefore, industrial power transmission often demands flexible rather than rigid couplings.

When the time involves specify replacements for flexible couplings, it’s human nature to take the easy path and simply find something similar, if not really similar, to the coupling that failed, maybe applying a few oversized fudge factors to be conservative. All 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 normally worthwhile actually if it just verifies the previous style. But, it might cause you to something completely different that will are better and go longer. A different coupling style may also extend the life of bearings, bushings, and seals, stopping fretted spline shafts, minimizing sound and vibration, and reducing long-term maintenance costs.

Sizing and selection
The rich variety of available flexible Scroll Vacuum Pump couplings provides an array of performance tradeoffs. When selecting among them, withstand the temptation to overstate provider factors. Coupling assistance factors are designed to compensate for the variation of torque loads typical of different driven systems and to give reasonable service existence of the coupling. If chosen as well conservatively, they are able to misguide selection, increase coupling costs to unneeded levels, and actually invite damage somewhere else in the machine. Remember that properly selected couplings usually should break before something more costly will if the machine is definitely overloaded, improperly managed, or in some way drifts out of spec.

Determining the right type of flexible coupling begins with profiling the application the following:

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

• True torque requirements of the driven part of the machine, rather than the rated horsepower of the primary mover – note the range of adjustable torque resulting from 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 required suit between shaft and bore

• Shaft-to-shaft misalignment – take note amount of angular offset (where shafts are not parallel) and quantity of parallel offset (length between shaft centers if the shafts are parallel but not axially aligned); also be aware whether driving and driven models are or could be sharing the same base-plate

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

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

But even after these basic technical information are identified, other selection criteria should be considered: Is simple assembly or installation a concern? Will maintenance problems such as for example lubrication or periodic inspection end up being acceptable? Will be the components field-replaceable, or will the whole coupling need to be replaced in case of failing? How inherently well-balanced may be the coupling style for the speeds of a specific application? Will there be backlash or free play between your parts of the coupling? Can the equipment tolerate much reactionary load imposed by the coupling because of misalignment? Remember that every flexible coupling design provides 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 principal organizations, metallic and elastomeric. Metallic types make use of loosely installed parts that roll or slide against one another or, alternatively, nonmoving parts that bend to consider up misalignment. Elastomeric types, however, gain flexibility from resilient, nonmoving, rubber or plastic material components transmitting torque between metallic hubs.

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

• Torsional stiffness, meaning hardly any “twist” happens 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 existence of certain natural oils or chemicals

• Electric motor get, 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 additional 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 fat/lower cost, in terms of torque capacity relative to maximum bore capacity

• Quieter operation

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

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

Standards
For the most part, industry-wide standards do not exist for the common design and configuration of flexible couplings. The exception to this may be the American Gear Manufacturers Assn. standards relevant in THE UNITED STATES for flangedtype equipment couplings and the bolt circle for mating both halves of the couplings. The American Petroleum Institute has requirements for both regular refinery assistance and particular purpose couplings. But other than that, industry specifications on versatile couplings are limited by features such as bores/keyways and matches, stability, 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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