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P&P May 07 - p21.qxp 12/07/2007 08:59 Page 21

maintenance matters

MICHAEL D’ANGELO

isengineered coupling productsmanager,Kop-Flex,a business unit ofEmerson Power Transmission,based in Baltimore,MD

Five easy tips to optimize coupling performance - 15937 Five easy tips to optimize coupling performance

MAINTENANCE MATTERS THERE ARE TWO TYPES OF COUPLINGS AVAILABLE on themarket: sliding and deforming. Sliding couplings eventuallywear due to parts that slide across each other during misalign-ment, such as gear, grid and jaw types, with several requiring lubrication to perform properly. Deforming couplings, such as disc, diaphragm and elastomercouplings, are marketed as “low maintenance” as they don’trequire lubrication and are only limited by material or shelf life, with many offering infinite life if maintained properly. Despite recent advancements in coupling design and man-ufacturing, nearly all couplings eventually show wear if notproperly installed and maintained. To help couplings perform better and last longer, there are some easy tips to follow. Bolted coupling connections are designed so a certain num-ber of bolts always carry the torque load. This is why mosttorque-carrying bolted connections consist of body-fitted orcoupling bolts. Never substitute a regular bolt for a coupling bolt. Use the right lubricant: Lubricant is the lifeblood of lubri-cated couplings. Seventy-five percent of premature couplingfailure is attributed to lubrication issues: old lubricant, leakinglubricant, insufficient lubricant or incorrect lubricant. In addi- tion, maintenance personnel don’t always realize that cou- plings require a different lubricant than bearings.Bearings use grease classified by the National LubricatingGrease Institute (NLGI) as Grade 2. Couplings require a grease equivalent to NLGI Grade 1 to withstand centrifugal forces creat- ed by rotation. An NLGI Grade 1 grease incorporates thickenerswith a density closer to oil, manufactured to resist centrifugalseparation. Using bearing grease in a coupling will lead to pre- mature failure. Keep track of installation and maintenance instructions: Practice preventive maintenance: Regardless of couplingtype, perform periodic checks according to installation andmaintenance instructions. It’s important to note that couplings can be checked without shutting down an application or dis- mantling the components.With lubricated couplings, thermography can be incorpo-rated as a predictive maintenance tool. By knowing the typicaloperating temperature of a coupling, it is possible to identify variances and lubricate the coupling prior to failure. If the tem- perature subsides, lubricate the coupling. If the temperatureremains elevated after lubrication, thoroughly inspect the cou-pling during the next scheduled down day for damage. Inspect general disc couplings dynamically using a strobelight. By adjusting the flash, disc pack links can be visuallyinspected to look for buckling, corrosion or cracks that are usu-ally only seen when the coupling is still. Regardless of coupling type, proper installation and mainte-nance is imperative for safety and maximum life. A good start is reading and understanding coupling installation and mainte- nance instructions. Far too often, this documentation is mis-placed by the time the coupling reaches the installation site. Know alignment limits: Coupling alignment, consisting of acombination of shaft offset, angle and axial movement, is important to know prior to installation. Equipment alignment is typically tighter than coupling alignment. Minimal axialmovement does not affect lubricated couplings.This is not the case with deforming couplings. Axial growthor movement from a neutral position creates constant stress in the flex elements that decrease the ability to transmit torqueand handle angular misalignment. Prior to installation, famil-iarize yourself with misalignment limits and recommended running angles for all couplings by checking the installation instructions, not product catalogs or marketing literature. Don’t over- or under-tighten bolts: Unlike other powertransmission components, couplings often include bolted con- nections that transmit torque. A torque-transmitting coupling bolt is designed to optimize its clamping of the mating partswhile still carrying shear stresses induced by torque. When abolt is over-torqued, it is less able to carry shear stresses, lead- ing to failure. In an under-torqued bolt, lack of clamping can loosen the connection. A loose bolt will slide relative to its mat-ing parts and fret. Over time, the loose connection will causefatigue failure.

Optimize coupling performance

Carefully following installation and maintenance guidelines that address misalignment issues, bolt tightening torques andcoupling lubrication is critical to ensure coupling perfor-mance. In addition, regular inspection and maintenance can prevent extensive and costly downtime.
P & P P & P May 2007www.risiinfo.com

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