Why a flexible coupling? A flexible coupling is present to transmit power (torque) from one shaft to some other; to pay for minor amounts of misalignment; and, using cases, to supply protective features such as vibration dampening or acting as a “fuse” regarding torque overloads. For these reasons, industrial power transmission frequently calls for 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 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 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 period to determine the right type of coupling is definitely worthwhile also if it just verifies the previous design. But, it could lead you to something totally different that will are better and last longer. A different coupling style may also prolong the life span of bearings, bushings, and seals, stopping fretted spline shafts, minimizing sound and vibration, and slicing long-term maintenance costs.
Sizing and selection
The rich selection of available flexible couplings provides an array of performance tradeoffs. When selecting among them, withstand the temptation to overstate program factors. Coupling service factors are designed to compensate for the variation of torque loads usual of different motivated systems and to give reasonable service existence of the coupling. If chosen too conservatively, they can misguide selection, increase coupling costs to unnecessary levels, and even invite damage somewhere else in the system. Remember that properly selected couplings generally should break before something more costly will if the machine is usually overloaded, improperly operated, or in some way drifts out of spec.
Determining the right kind of flexible coupling starts with profiling the application form the following:
• Prime mover type – electrical electric motor, diesel engine, other
• True torque requirements of the driven side of the system, instead of the rated horsepower of the prime mover – take note the range 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 suit between shaft and bore
• Shaft-to-shaft misalignment – note amount of angular offset (where shafts are not parallel) and quantity of parallel offset (distance between shaft centers if the shafts are parallel but not axially aligned); also take note whether generating and driven products are or could be sharing the same base-plate
• Axial (in/out) shaft movement, BE distance (between ends of driving and driven shafts), and any other space-related restrictions.
• Ambient Taper Pulleys conditions – generally temperature range and chemical substance or oil exposure
But even after these simple technical details are identified, other selection criteria should be considered: Is ease of assembly or installation a thought? Will maintenance issues such as for example lubrication or periodic inspection become acceptable? Are the elements field-replaceable, or will the entire 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? Is there backlash or free of charge play between your components of the coupling? Can the gear tolerate much reactionary load imposed by the coupling because of misalignment? Remember that every flexible coupling style has strengths and weaknesses and associated tradeoffs. The key is to find the design best suited to the application and budget.
In the beginning, flexible couplings divide into two principal groupings, 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 consider up misalignment. Elastomeric types, however, gain versatility from resilient, non-moving, rubber or plastic components 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” takes place between hubs, in some instances providing positive displacement of the driven shaft for every incremental motion of the traveling shaft
• Operation in fairly high ambient temperatures and/or presence of certain oils or chemicals
• Electric motor get, as metallics generally are not recommended for gas/diesel engine drive
• Relatively continuous, low-inertia loads (metallic couplings aren’t recommended for traveling reciprocal pumps, compressors, and various other pulsating machinery)
Elastomeric types are best suited to applications that require or permit:
• Torsional softness (enables “twist” between hubs so it absorbs shock and vibration and may better tolerate engine get and pulsating or relatively 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 only their recommendations, yet also the reasons behind them.
The wrong applications for every type are those characterized by the circumstances that most readily shorten their existence. In metallic couplings, premature failing of the torque-transmitting element frequently results from metallic fatigue, usually because of flexing due to 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 temps or hysteresis (inner buildup in the elastomer), or from deterioration due to connection with certain oils or chemicals.
For the most part, industry-wide standards do not can be found for the normal design and configuration of flexible couplings. The exception to the is the American Gear Manufacturers 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 provides criteria for both regular refinery services and unique purpose couplings. But besides that, industry specifications on flexible couplings are limited to features such as for example bores/keyways and matches, stability, lubrication, and parameters for ratings.
Information because of this article was supplied by Mark McCullough, director, marketing & program engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.