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 instead of rigid couplings.

When the time involves specify replacements for flexible couplings, it’s human nature to take the simple path and find something similar, if not really similar, to the coupling that failed, probably 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 start with the assumption that the previous coupling failed since it was the wrong type for that application. Taking period to determine the right type of coupling is normally worthwhile Air Compressor actually if it just verifies the previous style. But, it could lead you to something completely different that will work better and last longer. A different coupling design may also lengthen the life of bearings, bushings, and seals, stopping fretted spline shafts, minimizing noise 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 to give reasonable service lifestyle of the coupling. If chosen as well conservatively, they are able to misguide selection, increase coupling costs to needless levels, and actually invite damage somewhere else in the system. Remember that correctly selected couplings generally should break before something more costly will if the machine is certainly overloaded, improperly operated, or in some way drifts out of spec.

Determining the right type of flexible coupling begins with profiling the application form as follows:

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

• Actual torque requirements of the driven aspect of the machine, instead of the rated horsepower of the primary mover – notice the range of adjustable torque caused by 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 fit between shaft and bore

• Shaft-to-shaft misalignment – take note amount of angular offset (where shafts aren’t parallel) and amount of parallel offset (distance between shaft centers if the shafts are parallel however, not axially aligned); also be aware whether traveling and driven models are or could be sharing the same base-plate

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

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

But even after these fundamental technical information are identified, additional selection criteria should be considered: Is simple assembly or installation a thought? Will maintenance issues such as lubrication or periodic inspection be acceptable? Are the elements field-replaceable, or will the whole coupling have to be changed in case of failing? How inherently well-balanced is the coupling design for the speeds of a particular application? Is there backlash or free of charge play between your elements 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 connected tradeoffs. The key is to find the design best suited to the application and budget.

Application specifics
Originally, flexible couplings divide into two main organizations, metallic and elastomeric. Metallic types make use of loosely fitted parts that roll or slide against one another or, alternatively, non-moving parts that bend to consider up misalignment. Elastomeric types, on the other hand, gain versatility from resilient, non-moving, rubber or plastic elements transmitting torque between metallic hubs.

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

• Torsional stiffness, meaning very little “twist” happens between hubs, in some instances offering positive displacement of the driven shaft for every incremental movement of the traveling shaft

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

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

• Relatively constant, low-inertia loads (metallic couplings aren’t recommended for generating reciprocal pumps, compressors, and various 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 may better tolerate engine drive and pulsating or relatively high-inertia loads)

• Greater radial softness (allows more angular misalignment between shafts, puts much less reactionary or side 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 only their recommendations, but also the reason why behind them.

Failure modes
The wrong applications for every type are those seen as a 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, breakdown of the torque-transmitting component frequently results from excessive temperature, from either ambient temperature ranges or hysteresis (inner 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 normal design and configuration of flexible couplings. The exception to this may be the American Gear Producers Assn. standards relevant in THE UNITED STATES for flangedtype equipment couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute offers requirements for both regular refinery provider and special purpose couplings. But other than that, industry specifications on versatile couplings are limited by features such as for example bores/keyways and matches, balance, lubrication, and parameters for ratings.

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