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With the ever-increasing demand in turbomachinery applications of couplings that connect various pieces of equipment, greater performance is required of them. Traditionally, gear couplings were used in these applications but required constant lubrication to extend their life. They would eventually wear out and need replacement. For many years, new turbomachinery applications have been using metallic flexible element couplings which are designed for infinite life and have much lower maintenance requirements.
Metallic flexible element couplings are of two basic types: disc and diaphragm. Each of these can be further separated into different styles – disc and diaphragm couplings. Both of these can be used for a wide variety of applications, including axial compressors, centrifugal compressors, diesel engines, induction motors, synchronous motors, expanders, gas turbines, steam turbines, generators, pumps, blowers, and more.
Power and speed ratings
These couplings have been used for powers ratings of 150,000+ hp and speeds ratings of 40,000+ RPM, depending on the size of the coupling. Operating temperatures from -40ºF to 300ºF (-40ºC to 150ºC) are common. Higher temperatures can also be accommodated by making allowances in the material strength. Both types of couplings can also be customized with additional features such as a redundant drive that engages in the event of flexible element failure, a torque monitoring system or electrical insulation.
Disc couplings accommodate misalignment by using a flexible disc which is connected by alternating bolts to opposing flanges. The thickness of the discs and distance between bolts determines the amount of flexibility. Disc couplings can be circular, scalloped or straight-sided, depending on the shape of the flexible discs.
Diaphragm couplings accommodate misalignment by allowing movement of their outer diameter relative to inner diameter, or vice versa. The diaphragm is plate shaped with a contoured profile machined into one or both sides. The amount of flexibility is determined by the thickness of the contoured profile and the difference between outer and inner diameters. Diaphragm couplings can have straight or wavy flexible profiles, can be continuous or contain holes, can have single or multiple diaphragms, and can be made as a bolted or welded assembly.
Both disc and diaphragm type couplings can be arranged in a marine style or reduced moment configuration. In the marine style configuration, the flexible elements are attached outboard of the shaft end, allowing one size coupling to accommodate a large range of shaft sizes. In the reduced moment configuration, the flexible elements are moved over the shaft end, as close to the shaft bearings as possible, to minimize loading on the bearings.
More in Turbomachinery International magazine Nov. /Dec. 2011.