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, using cases, to supply protective features such as for example vibration dampening or performing as a “fuse” regarding torque overloads. Therefore, commercial power transmission often calls for flexible instead of rigid couplings.

When enough 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. All too often, nevertheless, this practice invites a repeat failure or costly system damage.

The wiser approach is to start with the assumption that the prior coupling failed since it was the incorrect type for that application. Taking period to look for the right type of coupling can be worthwhile also if it only 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, stopping fretted spline shafts, minimizing sound and vibration, and reducing long-term maintenance costs.

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

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

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

• Real torque requirements of the driven part of the machine, rather than 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 normal operation

• Vibration, both linear and torsional

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

• Shaft-to-shaft misalignment – take note degree of angular offset (where shafts aren’t parallel) and quantity of parallel offset (length between shaft centers if the shafts are parallel however, not axially aligned); also be aware whether Dry Screw Vacuum Pumps generating and driven models are or could possibly be sharing the same base-plate

• Axial (in/out) shaft movement, BE distance (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 actually after these basic technical details are identified, various other selection criteria is highly recommended: Is ease of assembly or installation a thought? Will maintenance problems such as for example lubrication or periodic inspection become acceptable? Will be the components field-replaceable, or will the entire coupling have to be changed in case of a failure? How inherently well-balanced is the coupling style for the speeds of a particular application? Is there backlash or free of charge play between your elements of the coupling? Can the gear tolerate much reactionary load imposed by the coupling because of misalignment? Understand that every flexible coupling style provides strengths and weaknesses and linked tradeoffs. The key is to get the design best suited to your application and budget.

Application specifics
Originally, flexible couplings divide into two major organizations, metallic and elastomeric. Metallic types use loosely fitted parts that roll or slide against each other or, alternatively, non-moving parts that bend to take up misalignment. Elastomeric types, on the other hand, gain versatility from resilient, non-moving, rubber or plastic material 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” occurs between hubs, in some instances offering positive displacement of the driven shaft for every incremental movement of the driving shaft

• Operation in relatively high ambient temps and/or presence of certain 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 aren’t recommended for traveling reciprocal pumps, compressors, and various other pulsating machinery)

Elastomeric types are suitable to applications that require or permit:

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

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

• Lighter pounds/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 only their recommendations, yet also the reason why behind them.

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

For the most part, industry-wide standards usually do not exist for the common design and configuration of flexible couplings. The exception to the may be the American Gear Producers Assn. standards applicable in THE UNITED STATES for flangedtype gear couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute provides requirements for both standard refinery provider and unique purpose couplings. But besides that, industry specs on versatile couplings are limited to features such as for example bores/keyways and fits, 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.