Commonly used general-purpose couplings include (from top) gear, grid, elastomer (compression), and disc types.

Production operations and processes that must run without interruption often depend on the reliability of special-purpose machines, such as large pumps, compressors, and gas-turbine-driven generators that operate at high speeds and deliver high loads. These same manufacturing plants also contain generalpurpose equipment, where reliability is less critical.

Choosing a flexible shaft coupling for either machine type may seem intimidating, considering that there are over 100 varieties. However, the process is pretty straightforward for general applications. Simply choose a design that meets the basic requirements such as angular and parallel shaft misalignment, operation in various environments (chemicals and heat), vibration and shock load isolation, and easy disassembly for repair.

Some types of general-purpose couplings run on high-speed, highload machinery as well. But they nearly always need higher capacity and special features to suit more stringent requirements. For this reason, it's probably easiest to narrow down the list of candidates by a process of elimination. Choose only those couplings for further review that meet the required high speed and high load capacity, as well as any special needs unique to the application. Then select one from the paired-down list that meets the basic application requirements as well.

Basic versions

From a functional standpoint, flexible couplings come in three basic types: mechanical, elastomeric, and metallic element. Of these, only mechanical element types need lubrication.

Mechanical-element versions generally obtain their flexibility through rolling or sliding of mating parts. Elastomeric couplings operate by deforming one or more resilient elements. Metallic- element types work by deflecting thin metallic discs or diaphragms.

These three basic types serve both general and special-purpose applications. However, they usually have added features for the latter use.

General purpose

Most couplings operate at low speeds on equipment that can shut down without disrupting major plant operations. Maximum speed varies with the driving motor, and ranges from 1,800 to 3,600 rpm depending on shaft size. These couplings generally come in basic designs that transmit torque from one shaft to another while accommodating misalignment and axial motion between the ends of the shafts.

Examples of special-purpose couplings include (top, left to right) gear and elastomer (compression), plus (bottom, left to right) disc, and diaphragm types. The long gear, disc, and diaphragm couplings can be removed for repairs without moving connected driving and driven equipment. The disc version is a reduced-moment type with flexible elements (at the ends) that go over the connected shafts. The diaphragm coupling is a marine style that bolts to the flanges of connected equipment (gas-turbine generator). The elastomeric coupling is a closecoupled, heavy-duty version that goes between a motor and pinion of a compressor drive train.

In most cases, the connected equipment needs only moderate alignment -- usually to within 0.001 in./in. of separation between flex points (flexible elements in the couplings). Therefore, a coupling with 10-in. separation should be aligned to within 0.010 in. The flexible elements in these couplings, often considered throw-away-parts, are easy to inspect and replace.

The most common mechanical-element couplings for general-purpose use include the gear and grid types. Geared versions consist of two hubs with external teeth that engage internal teeth on a one or two-piece sleeve. The meshing teeth transmit torque between the connected shafts, and the spaces between teeth accommodate shaft misalignment. These couplings offer high torque capacity.

Grid couplings have two hubs with grooves rather than teeth. A flexible steel grid rides in the grooves to transmit load, and it flexes to allow misalignment.

Elastomeric-element couplings include shear and compression types. They work by either stretching or compressing a resilient member (rubber or plastic) between two rotating hubs on the connected shafts. Shear types are suitable for low-torque applications, generally under 100 hp, compression types generally for applications over 100 hp. Depending on the specific material, elastomeric elements resist some types of corrosion but are usually limited to temperatures below 200°F.

Metallic-element couplings include two versions, disc and diaphragm. In a disc coupling, alternating bolts on a common circle attach to the driving and driven shafts. These bolts transmit torque through the flexible disc in the form of tension forces. For applications over 100 hp, disc couplings are one of the most common types applied in general- purpose machines. However, diaphragm couplings are normally limited to special-purpose applications because they're more expensive than other types.

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