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.
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.
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.
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.
Continue on page 2
Some couplings are designed to operate on special high-speed machines that run continuously. These versions generally come in larger sizes or with special features to make them suitable for specific applications.
Such machines tend to be high powered, high speed, and expensive. They are driven by synchronous motors, gas turbines or steam turbines at speeds ranging from 3,600 to 20,000 rpm. Capacities can exceed 1,000 hp. The high cost of this equipment usually precludes having spare units.
For example, a pump on a spared, redundant system in an ammonia processing plant is a general-purpose unit. On the other hand, a 67,000-hp boiler feed pump in a base station and a compressor drive train in the same ammonia plant are special-purpose machines. The same is true for most gas-turbine generators on peaking or cogeneration systems. There are no spares for this type of equipment, making them essential to continuous, trouble-free plant operation.
Although high powered, such machines are sensitive to eccentric loads or moments that would seem insignificant to general-purpose machinery. As a result, they need closer alignment, generally to within 0.0005 in./in. of separation between flex points. Therefore a coupling with 18-in. separation requires aligning to within 0.009 in.
Coupling failures are more critical in special high-speed equipment. To illustrate, a general-purpose coupling usually fails in the flexible element, which is easily replaced. But when a special-purpose coupling fails, it's best to quickly replace the entire unit to minimize downtime, and inspect it for damage later. Even if the flexible element appears to be the only damaged part, other internal components may be damaged (bent or sprung), which would cause vibration if they were reapplied in high-speed service. Therefore, it may be necessary to send the entire unit back to the manufacturer for refurbishing.
The first couplings for special-purpose machines evolved from generalpurpose gear types. They typically have heat-treated components and may have lapped or coated teeth for increased life. Most of them require continuous lubrication to carry away the heat generated at high speeds. Also, most are at least 18-in. long, making them easy to remove for bearing replacement or other repairs without disturbing the driving or driven machines. Some exceed 36 in. long, which helps to reduce angular misalignment between shafts. Operating speeds range up to 10,000 rpm.
Most elastomeric element couplings for special-purpose applications are of the compression type. They generally find use on the low-speed (between motor and gearbox) side of special-purpose drive trains. The elastomeric element helps in damping vibration or tuning a system to avoid torsional critical speeds (synchronous motor and variable-speed drive applications).
Metallic-element couplings include custom disc and diaphragm versions. Both types have no internal radial clearance, giving them good balance characteristics for high-speed operation. They come in two versions: marine and reduced moment. In the marine version, the flexible element fits in the center of the coupling between the ends of the connected shafts. The reduced-moment version has flexible elements that fit over the shaft at each end of the coupling. This reduces overhung load and bending loads on the equipment bearings.
Disc couplings are often used for reduced- moment applications, such as on compressors. They accommodate up to ½-in. axial shaft displacement. Diaphragm couplings transmit torque between inner and outer sleeves bolted to a flexible diaphragm. Common uses include marine versions for gas turbine drives where axial expansion of shafts due to heat may exceed 1 in. Both disc and diaphragm couplings are suitable for applications over 500 hp and speeds up to 20,000 rpm.
To lube or not
There is a trend today to replace lubricated couplings, which require periodic maintenance, to non-lubricated ones that need less maintenance. Before making the switch, ask yourself "should the coupling design be changed at all"? Consider the total cost, the effect of different coupling characteristics on the drive system, and training on how to install and maintain the new coupling.
• Total Cost. Include not only the coupling cost but the total cost of a changeover. Do you need to perform a new system analysis? Do you need a new guard? Do you need to retrain maintenance personnel?
The cost of a non-lubricated coupling may be 30 to 100% higher than a lubricated one. Also the change-over requirements (new guard, system analysis, and re-training) may double that cost. On the other hand, savings in maintenance and downtime usually outweigh the extra up-front cost. Lubricated couplings have an estimated service life of about 3 to 5 yr whereas the estimated life of non-lubricated couplings is 5 to 8 yr.
• Effect of coupling characteristics. A non-lubricated coupling may have better balance, but it may also be heavier and torsionally softer than a gear coupling. This can move lateral or torsional critical speeds into the operating speed range.
• Training. Switching to a different type of coupling usually requires installation or maintenance training to avoid problems. A disc coupling, for example, is more sensitive to axial setup than a gear coupling.
Continue on next page
Do's and Don'ts
Couplings often fail soon after they're installed, replaced, or taken apart and reassembled on the equipment. The reason: someone installed it incorrectly.
Here are some tips to ensure a successful installation:
• Give the coupling manufacturer as much information as possible to aid in the selection.
• Make sure that maintenance people are trained to install the type being used.
• Use the bolts supplied with the coupling because they are usually of a special design. If one is lost, buy a spare set from your coupling supplier.
• Tighten the nuts with a torque wrench.
• Align the equipment to limits specified by the manufacturer.
• Apply the right lubricant, if required. On the flip side, avoid doing these things:
• The coupling won't fit on the shaft. Get the torch and heat the heck out of the hub, or open up the bore until it slides on the shaft.
• The keys are too tight. Grind them until they fall in place.
• I lost one of the bolts. Get one that fits from the maintenance department.
• These bolts are snug enough.
• This grease works good in our bearings, so put it in the coupling.
• We can buy another type of coupling at 30% less cost than what we're using now.
Jon R. Mancuso is Engineering Manager of Kop-Flex Inc., Emerson Power Transmission Corp., Baltimore, Md.
Some information for this article is taken from Jon Mancuso's book "Couplings and Joints" published by Marcel Dekker and a Short Course presented at the 27th Turbomachinery Symposium on Sept. 21, 1998.