Types of coupling - A variety of shaft couplings — Shaft couplings are available in different sizes and material finishes

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A shaft coupling is a mechanical device used to connect rotating shafts and absorb misalignments between them. Couplings can be rigid or flexible depending on the alignment accuracies of the system and torque requirements. Shaft couplings are used for power and torque transmission between two rotating shafts such as on motors and pumps, compressors, and generators.

Types of Couplings and Their Applications

The different styles and types of shaft couplings are summarized below.

Beam Couplings

The beam coupling consists of single or multiple helical cuts in the coupling body which typically can accommodate parallel misalignments up to 0.025 inch and angular misalignments up to 7 degrees. They are used primarily for motion control applications where torques are typically below 100 inch-lbs. Zero backlash designs available ensure positioning accuracy between driving and driven shafts.

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Bellows Couplings

Bellows couplings are also suited to motion control applications. They consist of multiple convolutions of metal which provide high torsional stiffness which is important to positioning applications. Torsional stiffness reduces the level of angular and parallel misalignment they can accommodate compared with beam couplings, although torque transmission capabilities are about similar.

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Chain Couplings

Chain Couplings are suited to power transmission applications and are used to transmit power up into the hundreds of horsepower range. Angular and parallel misalignment allowances are typically 2 degrees and 0.015 inches, respectively. Typical chain couplings use special chain sprockets and double wide roller chains whose clearances permit the design to operate as a flexible coupling.

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Jaw Couplings

Curved and Straight Jaw couplings are used for both motion control and light power transmission applications and consist of pairs of multi-jawed hubs that mate with elastomeric spiders. The design allows for backlash-free torque transmission. Accommodation for parallel misalignment usually approaches 0.01 inch and angular misalignment about 1 degree. Elastomeric spiders give these couplings some damping capacity and of often the spiders are available in different durometers to lend specific properties to the individual couplings. These couplings often operate without lubricant and can transmit torque up into the 1000 inch-lb. range.

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Diaphragm Couplings

Diaphragm couplings are generally used for high power transmission such as found in turbomachinery. Typically they employ one or more flexible metal convoluted discs which transmit power to an inner spacer shaft then back out through another diaphragm stack to the driven machinery. A principal advantage over gear type couplings is their lack of lubrication requirements. Diaphragm couplings are capable of high torque transmission and high-speed operation.

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Disc Couplings

Disc couplings use single or multiple discs and single or double stages which bolt to the shaft hubs. They are used for power transmission and rely on the flexibility of their thin metal discs to transmit torque and accommodate angular misalignment. They are not especially good at managing parallel misalignment. They are capable of transmitting high torques and are often used to couple high horsepower motors, gas turbines, etc. to loads.

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Gear Couplings

Gear couplings also transmit high torques. They have misalignment capabilities generally about 0.01-0.02 inch in parallel and 2 degrees in angular. Gear couplings are often used in pairs with spacer shafts to span the distance between the driving and driven equipment. They generally require lubricant although some designs intended for lighter duty use lubricant free nylons or other polymers for the center sleeve.

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Grid Couplings

Grid couplings employ spring-like connecting elements that weave between slots machined in the coupling hubs. They are capable of high torque transmission with an added bonus of shock absorption and torsional vibration dampening. They operate without lubricant. They are appropriate for power transmission and capable of handling parallel misalignment up to 0.30 inch and angular misalignment of about ¼ degree.

Oldham Couplings

Oldham couplings handle high degrees of parallel misalignment owing to their sliding element design. Use of an elastomer center element instead of metal is popular in modern versions. Some manufacturers claim an ability to tolerate up to 5-degree angular misalignment through the use of cylindrical, rather than rectangular, sliders.

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Schmidt Couplings

Schmidt couplings are designed specifically to operate on shafts that are offset. They aren’t flexible couplings in the strictest sense, which are designed to accommodate slight misalignment in shafts that are theoretically parallel and square to one another. Schmidt couplings are used in papermaking, printing, and similar machines and function more like a 1:1 gearbox in a more compact space.

Clamping Couplings

Rigid one- and two-piece Clamping couplings lack accommodation for shaft misalignment and are popular for slow or intermittent shafting arrangements where alignment is not of concern. They are the simplest form of shaft coupling and apart from their lack of misalignment tolerance, are inexpensive zero-backlash devices.

Other Couplings

Coupling designs are many only the principal ones are detailed above. Other coupling types include meshing tooth, or Hines, designs, pin and bush couplings, and spline couplings.

Coupling Applications and Industries

Couplings correct for an inability to produce or maintain perfect alignment in coupled machines. Some machines dispense with the need for couplings by running close coupled, meaning that the bearings of the motor support an extended shaft upon which the rotating component of the driven equipment – a pump impeller, for instance – mounts. Where this is practical to do it is done to dispense with the alignment problem. Often, though, the machines require their own bearings and as a result a need to connect their independent shafts. Two misalignments characteristics that can be expected include parallel (or offset) and angular.

For some couplings, backlash is an important concern. Motion control applications where the position of the driven equipment is precisely tied to the position of a servo- or stepper motor rely on zero-backlash couplings to assure that no slop exists in the system. Backlash is a lesser concern for most power transmission applications—pumps/motors for instance—where efficient torque transmission is the primary objective. Here, misalignment can lead to higher energy use, accelerated bearing wear, excessive vibration, etc.

Both the beam and the bellows couplings offer zero backlash and are frequently used for transferring the relatively small torques of motion control.

For power transmission, generally, all-metal couplings such as the gear and disc designs are capable of transmitting higher torques than those that use elastomeric elements such as jaw couplings.

Although couplings are designed to accommodate misalignment, they aren’t substitutes for aligning machines during installation.

Shaft Coupling Considerations

In selecting couplings a designer first needs to consider motion control varieties or power transmission types. Most motion control applications transmit comparatively low torques. Power transmission couplings, in contrast, are designed to carry moderate to high torques. This decision will narrow coupling choice somewhat. Torque transmission along with maximum permissible parallel and angular misalignment values are the dominant considerations. Most couplings will publish these values and using them to refine the search should make picking a coupling style easier. Maximum RPM is another critical attribute. Maximum axial misalignment may be a consideration as well. Zero backlash is an important consideration where feedback is used as in a motion control system.

Some power transmission couplings are designed to operate without lubricant, which can be a plus where maintenance is a concern or difficult to perform. Lubricated couplings often require covers to keep the grease in. Many couplings, including chain, gear, Oldham, etc., are available either as lubricated metal-on-metal varieties and as metal and plastic hybrids where usually the coupling element is made of nylon or another plastic to eliminate the lubrication requirements. There is a reduction in torque capacity in these unlubricated forms compared to the more conventional designs.

Important Attributes

Coupling Style

Most of the common styles have been described above.

Maximum RPM

Most couplings have a limit on their maximum rotational speed. Couplings for high-speed turbines, compressors, boiler feed pumps, etc. usually require balanced designs and/or balanced bolts/nuts to permit disassembly and reassembly without increasing vibration during operation. High-speed couplings can also exhibit windage effects in their guards, which can lead to cooling concerns.

Max Transmitted Horsepower or Torque

Couplings are often rated by their maximum torque capacity, a measurable quantity. Power is a function of torque times rpm, so when these values are stated it is usually at a specified rpm (5HP @ 100 rpm, for instance). Torque values are the more commonly cited of the two.

Max Angular Misalignment

One of the shaft misalignment types, angular misalignment capacity is usually stated in degrees and represents the maximum angular offset the coupled shafts exhibit.

Max Parallel Misalignment

Parallel misalignment capacity is usually given in linear units of inches or millimeters and represents the maximum parallel offset the coupled shafts exhibit.

Max Axial Motion

Sometimes called axial misalignment, this attribute specifies the maximum permissible growth between the coupled shafts, given generally in inches or millimeters, and can be caused by thermal effects.

Types of Shaft Couplings – A Thomas Buying Guide