Why a flexible coupling? A flexible coupling is present to transmit power (torque) from one shaft to another; to pay for minor levels 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 torque overloads. Therefore, commercial power transmission often 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 similar, to the coupling that failed, probably applying a few oversized fudge factors to be conservative. All too often, however, this practice invites a do it again failure or expensive 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 determine the right type of coupling is certainly worthwhile also if it only verifies the prior design. But, it could lead you to something completely different that will are better and go longer. A different coupling style may also lengthen the life span of bearings, bushings, and seals, stopping fretted spline shafts, minimizing noise and vibration, and reducing 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 assistance factors. Coupling assistance factors are intended to compensate for the variation of torque loads typical of different driven systems and also to provide for reasonable service lifestyle of the coupling. If chosen too conservatively, they are able to misguide selection, increase coupling costs to unnecessary levels, and actually invite damage elsewhere in the machine. Remember that correctly selected couplings generally should break before something more costly will if the machine is definitely overloaded, improperly managed, or somehow drifts out of spec.
Determining the proper type of flexible coupling begins with profiling the application form the following:
• Primary mover type – electric electric motor, diesel engine, other
• Actual torque requirements of the driven side of the machine, instead of the rated horsepower of the primary mover – take note the range of variable 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 desired suit Tractor Gear Reducer between shaft and bore
• Shaft-to-shaft misalignment – notice amount of angular offset (where shafts aren’t parallel) and amount of parallel offset (length between shaft centers if the shafts are parallel however, not axially aligned); also take note whether driving and driven systems are or could possibly be posting the same base-plate
• Axial (in/out) shaft movement, BE distance (between ends of traveling and driven shafts), and any other space-related restrictions.
• Ambient conditions – primarily temp range and chemical or oil exposure
But actually after these fundamental technical information are identified, additional selection criteria should be considered: Is simple assembly or installation a account? Will maintenance problems such as lubrication or periodic inspection become acceptable? Will be the elements field-replaceable, or will the whole coupling have to be changed in the event of a failure? How inherently well-balanced is 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 gear tolerate much reactionary load imposed by the coupling due to misalignment? Remember that every flexible coupling style offers strengths and weaknesses and linked tradeoffs. The main element is to find the design suitable to your application and budget.
Application specifics
In the beginning, flexible couplings divide into two principal groups, metallic and elastomeric. Metallic types use loosely installed parts that roll or slide against one another or, alternatively, non-moving parts that bend to consider up misalignment. Elastomeric types, however, gain flexibility from resilient, nonmoving, rubber or plastic material elements 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 each incremental movement of the driving shaft
• Operation in fairly high ambient temperatures and/or existence of certain natural oils or chemicals
• Electric motor travel, while metallics generally aren’t recommended for gas/diesel engine drive
• Relatively continuous, low-inertia loads (metallic couplings are generally not recommended for traveling reciprocal pumps, compressors, and other pulsating machinery)
Elastomeric types are suitable to applications that want or permit:
• Torsional softness (enables “twist” between hubs so it absorbs shock and vibration and may better tolerate engine get and pulsating or fairly high-inertia loads)
• Greater radial softness (allows even more angular misalignment between shafts, puts less reactionary or part load on bearings and bushings)
• Lighter excess weight/lower cost, in terms of torque capacity in accordance with maximum bore capacity
• Quieter operation
Thoroughly review the suggested application profile with the coupling vendor, getting not merely their recommendations, but also the reasons behind them.
Failure modes
The incorrect applications for each type are those characterized by the conditions that most readily shorten their existence. In metallic couplings, premature failure of the torque-transmitting component most often results from steel fatigue, usually because of flexing caused by excessive shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, break down of the torque-transmitting element most often results from extreme temperature, from either ambient temperatures or hysteresis (internal buildup in the elastomer), or from deterioration because of connection with certain natural oils or chemicals.
Standards
Generally, industry-wide standards usually do not exist for the normal design and configuration of flexible couplings. The exception to this may be the American Gear Manufacturers Assn. standards relevant in North America for flangedtype gear couplings and the bolt circle for mating the two halves of the couplings. The American Petroleum Institute provides requirements for both regular refinery services and particular purpose couplings. But besides that, industry specs on versatile 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 provided by Tag McCullough, director, advertising & software engineering, Lovejoy, Inc., Downers Grove, Ill., and excerpted from The Coupling Handbook by Lovejoy Inc.