Why a flexible coupling? A flexible coupling is present to transmit power (torque) in one shaft to another; to pay for minor amounts of misalignment; and, using cases, to supply protective features such as for example vibration dampening or performing as a “fuse” regarding torque overloads. For these reasons, commercial power transmission frequently demands flexible rather than rigid couplings.

When the time involves specify replacements for flexible couplings, it’s human nature to take the simple path and simply find something similar, if not really identical, 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 begin with the assumption that the prior coupling Rotary Piston Vacuum Pump failed since it was the wrong type for that application. Taking period to look for the right type of coupling is certainly worthwhile actually if it only verifies the previous design. But, it might lead you to something totally different that will work better and go longer. A different coupling design may also expand the life span 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 choosing among them, withstand the temptation to overstate program factors. Coupling services factors are intended to compensate for the variation of torque loads typical of different powered systems and to provide for reasonable service existence of the coupling. If chosen as well conservatively, they can misguide selection, raise coupling costs to unnecessary levels, and also invite damage elsewhere in the machine. Remember that correctly selected couplings usually should break before something more costly does if the machine is overloaded, improperly managed, or somehow drifts out of spec.

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

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

• Genuine torque requirements of the driven aspect of the system, rather than the rated hp 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 normal operation

• Vibration, both linear and torsional

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

• Shaft-to-shaft misalignment – be aware amount of angular offset (where shafts are not parallel) and quantity of parallel offset (range between shaft centers if the shafts are parallel however, not axially aligned); also take note whether driving and driven units are or could 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 substance or oil exposure

But even after these basic technical information are identified, additional selection criteria should be considered: Is simple assembly or installation a concern? Will maintenance issues such as for example lubrication or periodic inspection end up being acceptable? Are the components field-replaceable, or does the entire coupling need to be changed in the event of failing? How inherently well-balanced may be the coupling style for the speeds of a particular application? Is there backlash or free of charge play between the parts of the coupling? Can the equipment tolerate very much reactionary load imposed by the coupling due to misalignment? Understand that every flexible coupling style provides strengths and weaknesses and linked tradeoffs. The main element is to find the design best suited to the application and budget.

Application specifics
Originally, flexible couplings divide into two principal groups, 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, however, gain flexibility from resilient, non-moving, rubber or plastic components transmitting torque between metallic hubs.

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Metallic types are suitable to applications that want or permit:

• Torsional stiffness, meaning hardly any “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 get, while metallics generally are not recommended for gas/diesel engine drive

• Relatively continuous, low-inertia loads (metallic couplings are generally not recommended for generating reciprocal pumps, compressors, and various other pulsating machinery)

Elastomeric types are suitable to applications that want or permit:

• Torsional softness (enables “twist” between hubs so that it absorbs shock and vibration and may better tolerate engine travel and pulsating or relatively high-inertia loads)

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

• Lighter fat/lower cost, with regards 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 seen as a the circumstances that a lot of readily shorten their life. In metallic couplings, premature failing of the torque-transmitting element most often results from metal fatigue, usually due to flexing due to extreme shaft misalignment or erratic, pulsating, or high-inertia loads. In elastomeric couplings, breakdown of the torque-transmitting element most often results from extreme warmth, from either ambient temperatures or hysteresis (internal buildup in the elastomer), or from deterioration because of contact with certain oils or chemicals.

Standards
Generally, industry-wide standards do not can be found for the normal design and configuration of flexible couplings. The exception to the 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 provides specifications for both regular refinery provider and particular purpose couplings. But besides that, industry specifications on versatile couplings are limited to features such as bores/keyways and fits, stability, lubrication, and parameters for ratings.

Information for 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.