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This article takes an in depth look at shaft couplings.
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Shaft couplings are devices that connect two shafts to transmit power from the driveshaft to the driven shaft while absorbing some degrees of misalignments and mounting errors between the two shafts. Misalignments exist between two shafts as a result of the changes in temperature and deterioration of positioning accuracy over time. Shaft couplings provide mechanical flexibility to allow smooth rotation between the shafts and reduce impact, wear, vibration, noise, and risk of equipment failure.
Shaft couplings are commonly utilized in mechanisms requiring transmission of power involving equipment like motors, generators, pumps, compressors, turbines, engines, and machines. There is a wide array of shaft coupling types for a variety of operating conditions. The main classifications of shaft couplings are rigid shaft couplings and flexible shaft couplings, which will be discussed in detail in the succeeding chapters.
Power transmission between two shafts is the primary function of shaft couplings. Power is transferred from the driveshaft to the driven shaft through a shaft coupling connected between them. The driveshaft is rotated by a power source (maybe electrical or mechanical). The shaft coupling then facilitates the rotation of the driven shaft.
Shaft couplings eliminate the need for a long, one-piece shaft. The use of a one-piece shaft is expensive and it is difficult to transport, assemble, and maintain, and it can cause inaccuracies. If a one-piece shaft fails, its entirety must be replaced. This makes the use of two-coupled shafts a more practical move.
Shaft couplings make power transmission between two shafts that differ in diameter possible. They are also used if the shafts of two different pieces of equipment are manufactured separately.
Aligning and positioning the drive and driven shafts with high precision is difficult and takes a considerable amount of time. Even though both shafts have the same specifications, there are still machining errors that can affect the alignment and shaft positioning accuracy.
The presence of shaft misalignments has negative effects on the power transmission system. It is caused by thermal expansion or deterioration of alignment and positioning caused by vibrations, movement, or bumps during motion. It significantly reduces the efficiency caused by power losses. Unwanted forces are exerted on the surrounding parts as the shafts rotate; this causes vibration and noise. It also increases wear and the risk of mechanical failure due to the induced stress of the misalignment. Therefore, a shaft coupling must be used to absorb such mounting and positioning errors and misalignments.
Shaft misalignments occur in different forms and usually exist in combinations. Parallel or radial misalignment exists when the centerlines of the shafts are parallel but are offset from each other. In an angular misalignment, the centerlines of the shafts are not parallel; they intersect at an angle. Both forms may occur horizontally or vertically. Lastly, in an axial misalignment, the adjacent ends of the two shafts are displaced away from each other in the axial direction.
Shaft couplings protect the nearby components in several instances. They dampen vibration, which affects the accuracy of other components (e.g., ball screws, actuators). They reduce the effect of shock loads (or torque changes) from one shaft to another.
If an external impact acts on the system, the shaft coupling will prevent transmission of the impact to the equipment. This is important as the impact can damage the equipment.
Shaft couplings prevent the transfer of heat originating from the power source to the driven shaft. The thermal expansion causes the surrounding components to shift from their correct position, causing degradation of their accuracy.
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Rigid shaft couplings are designed for shafts to have zero or very minimal misalignments and are used when the precision alignment must be maintained. If the driven shaft is not bearing supported, its load is carried by the drive shaft; hence, a rigid shaft coupling is necessary to provide additional support for the assembly. However, rigid couplings have poor shock absorption and vibration dampening.
The types of rigid shaft couplings are as follows:
Sleeve coupling is the simplest type of shaft coupling, and it is used when transmitting light to medium torques. It is composed of a thick and hollow cylindrical tube called a sleeve or muff whose inner diameter is the same as the shaft. The sleeve transmits the torque across the shafts. It is attached to each end of the shaft. Keys are fitted between an individual shaft and its hub, the part wherein the shaft is fitted, to ensure the sleeve and the shafts do not slip. Lastly, the sleeve coupling has threaded inserts to prevent the shafts from moving in a longitudinal direction when bolts are inserted into them.
In split muff coupling, the sleeves or muffs consist of two or three semi-cylinders that are fastened together by bolts and nuts. Each shaft is also keyed to its hub. Split muff couplings are easy to install and maintain, and they can be dismantled without changing the positions of the shafts. The three-piece construction variant of split muff couplings maintains the original position of one shaft while adjustments are made on the other.
Generally, split muff couplings are used for medium to heavy-duty loads with moderate rotational speeds. The other names of split muff coupling are clamped coupling and compression coupling.
A flange coupling is a type of shaft coupling that consists of two separate flanges that are fastened together by bolts and nuts. Similar to sleeve and split muff couplings, each shaft is keyed to its flange hub. To bring both shafts in the same centerline and maintain their alignment, one of the flanges in a flange coupling has a projected section while the other flange has a matching recessed portion.
There are three types of flange couplings. Unprotected-type flange couplings have the fastening bolts and nuts protruded outside the flange disc. Protected-type flange couplings, on the other hand, have the fastening bolts and nuts enclosed and protected inside the perimeter of the flanges. Lastly, in marine-type flange couplings, the flanges are held together by tapered headless bolts.
Flange couplings are used in pressurized piping systems, in transmitting heavy loads, and for shafts with large diameters.
In spline couplings, the sleeve has internal teeth that match the external teeth of the spline shaft. The matching teeth prevent the shafts from slipping and being misaligned instead of using a fitted key. The load is equally distributed along the circumference.
Spline couplings can accommodate radial and slight angular misalignments and can resist overloading. They are suitable for high rotational speeds.
Flexible shaft couplings have flexible elements that permit the inevitable misalignment and axial displacement of the shafts brought by temperature changes and positioning deterioration. This aspect makes them more advantageous than rigid shaft couplings that require manual alignment once the alignment of the shaft has been displaced.
Only slight misalignments are tolerated with flexible shaft couplings. It should be corrected if excessive misalignments occur to prevent wear and breakage. Unlike rigid couplings, they are capable of absorbing shock and dampening vibrations.
The classification of the different kinds of flexible shaft couplings are as follows:
Mechanically flexible couplings obtain their flexibility from loose-fitting or rolling or sliding components. These couplings often require regular lubrication.
A gear coupling is a modification of the flange coupling. In these couplings, the flange and its hub are separate parts. The hubs have protruding gear teeth on their external diameter that match the internal gear teeth of the flange. When acting together, the toothed flange and hub have a 1:1 gear ratio. Because of this, a gear coupling can transmit high torque; this increases with increasing gear teeth size at high speeds.
Roller chain couplings have radial sprocket hubs that are connected by a double-strand roller chain. The mesh of the sprocket teeth and the chain transmit the torque, and the clearances compensate for the parallel, axial, and angular misalignments.
Roller chain couplings have simpler and more compact construction and are less expensive than gear couplings but transmit less power. They are used for transmitting low to medium torque at moderate speeds. Nylon chains are also available as an alternative to metal chains to eliminate the need for lubrication.
Grid couplings consist of two radially slotted hubs that are meshed with a serpentine strip of spring steel. The spring steel gives the coupling torsional flexibility that can withstand a wide range of torque, speed, and misalignments. As the load increases, the contact of the spring steel to the hub teeth is shortened. The spring steel flexes to accommodate misalignments. Grid couplings are effective in absorbing shock and dampening vibration. To maintain its functionality and efficiency, the grid coupling must be regularly lubricated and secured tightly to retain lubrication and prevent contamination.
Elastomeric flexible couplings gain their flexibility from the compression and shear of a resilient polymer (e.g., plastic, rubber, elastomeric material). They absorb shock and vibration better than other types of flexible couplings. However, the melting temperature of the elastomeric element limits its operating temperature.
Tyre couplings have a thick rubber, polyurethane, or polyether element that connects the two hubs. The element transmits the torque under shear. Tyre couplings can accommodate high degrees of misalignment and reduce the transmission of shock and vibration. They can transmit a wide range of torque at moderate to high speeds.
Jaw couplings consist of an elastomeric spider insert that fits between two intermeshing jaws of the coupling hub. The spider experiences compression as torque is transmitted across the shafts. The torsional stiffness and torque capacity vary with the number, shape, and width of the jaws. Jaw couplings can transmit torque effectively while dampening vibrations and adjusting misalignments. They have greater vibration resistance and can be used in motion control applications. They are resistant to dirt, moisture, and oil and do not need additional lubrication.
Oldham‘s couplings consist of a center plastic disc in which two projected rectangular sections called tongue on either side lie perpendicularly to each other and another two metal discs (made from aluminum or steel) with matching grooves fastened to the flange. The center disc is sandwiched between the two metal discs and its tongue is free to slide in the grooves of the flanges to compensate for the misalignments. The assembly creates a slight offset in the parallel centers of the shaft.
Oldham‘s couplings have a compact size and are used for applications requiring zero backlashes. They have high torque capacity and can accommodate high lateral misalignments. In case of torque overload, the center disc will break first; this avoids torque transmission and potential damage to other machine components.
Bushed coupling (or bush pin-type coupling) is another modification of the flange coupling. Instead of bolts, pins with rubber bushing are used to fasten the flanges of the bushed coupling. The rubber bushing provides flexibility to the coupling and also helps in absorbing shock and vibration. It allows a higher level of misalignment. However, bushed couplings are difficult to assemble and dismantle. Bushed couplings are used in medium-duty applications in motors and machines.
Metallic element couplings obtain their flexibility from the flexing of thin metallic discs or diaphragms.
Disc couplings contain single or multiple flexible steel discs bolted between flanges. The discs come in square, circular, octagonal, and scalloped shapes. In double disked couplings, a central member is used to connect the hubs. The discs are flexed and may be deflected as the coupling accommodates misalignments while providing torsional rigidity. Since there are no sliding parts, lubrication is not required. Disc couplings are used in motor generators, blowers, fans, compressors, and pumps.
Diaphragm couplings use convoluted flexible plates called diaphragms. The diaphragms are bolted to each coupling hub and are connected to an intermediate member called the spool. Power is transmitted in the coupling from the drive shaft diaphragm to the spool and finally to the driven shaft diaphragm. The diaphragms are deformed as they accommodate misalignments. Since there are no sliding parts, lubrication is not required. Diaphragm couplings are typically used in high power transmission systems such as turbomachinery and industrial machinery requiring high torque and high speed.
Schmidt couplings consist of three linked discs that can accommodate large parallel misalignments. The constant velocities of the drive and driven shafts and torque transmission remain unaffected while compensating for the parallel misalignment. Schmidt couplings are commonly used in roller-equipped machines found in papermaking, printing, etc.
Miscellaneous couplings obtain their flexibility from a combination of the mechanisms above.
Beam couplings consist of a single piece of material that is made flexible by slight uncoiling of the helical cuts along its length. The uncoiling of helical cuts accommodates misalignments and motion of the coupled shafts. Its single-piece construction eliminates the backlash resulting from couplings made from multiple parts. Unlike couplings with polymeric elements, beam couplings are suitable at elevated temperatures.
Bellow couplings consist of two hubs connected by a thin, slightly flexible corrugated metallic section. They have high torsional stiffness that accurately transmits torque and motion. They can tolerate limited amounts of misalignments and have zero to minimal backlash. They also perform well in hot environments due to the absence of polymeric elements.
The other kinds of flexible shaft couplings are the following:
Universal coupling, also known as the universal joint or Hooke‘s Joint, provides a full range of motion and accommodates the largest misalignments. It is used to transmit torque and motion between shafts that are not parallel but intersecting at an angle. A universal joint is composed of two yokes, one for each shaft; the yokes are connected by a cross-shaped member called the spider. The two shaft yokes are at right angles with each other.
The major disadvantage of universal joints is their oscillating velocity output. Since universal joints deal with large amounts of misalignments, the driven shaft‘s rotational velocity oscillates even though the drive shaft rotates at a constant velocity. Larger misalignments result in larger oscillations in the velocity of the driven shaft; a straighter junction will have a lower oscillating velocity. However, this can be corrected by using multiple universal joints. Universal couplings are typically used in industrial machinery and as a component in a vehicle‘s drive train.
Fluid couplings, or hydrodynamic couplings, consist of an impeller attached to the driveshaft, which acts as a pump and a runner attached to the driven shaft,which acts as a turbine. There is no mechanical contact between the impeller and the runner. The torque is transmitted hydraulically by a fluid rather than mechanically. Energy is absorbed in the impeller; this accelerates the fluid to the runner. After striking the runner, the fluid decelerates and returns to the impeller for circulation. Fluid velocity is faster on the outer diameter of the impeller and runner.
Backlash is an unwanted motion from connected parts that are not precisely aligned or fitted. Though couplings allow small amounts of backlash, it should be limited within the system‘s threshold. Excessive backlash causes a high level of misalignment, wear, mechanical stress, and even breakage. A backlash-free coupling is critical for motion control systems. In power transmission systems, backlash equates to power loss.
Backlash can be minimized by replacing worn-out, loosely-fitted, and defective components. Check the coupling inserts, gear teeth, splines, rubber bushing, springs, bolts, and other parts for wear or loose-fitting. The hub and the key must have no unnecessary clearances with the shaft. Backlash can also be controlled by keeping the torque at a consistent speed and direction. Generally, flexible shaft couplings have more backlash than rigid couplings.
All shaft couplings have windup. Shaft coupling windup or torsional deflection occurs when the torque of one shaft is greater than that of the other, causing difference in angular displacement. This causes loss of motion, which leads to inaccurate positioning in motion control systems.
The factors to consider in selecting a shaft coupling are the torque transmission rating (horsepower), maximum speed (rpm), misalignments, stiffness, inertia, capability to absorb shock and dampen vibration, shaft mounting, environmental conditions, applications, cost, and more. These factors must be studied before acquiring a shaft coupling in order to prolong its service life, preserve its efficiency, and prevent failure.
Once the shaft coupling has been used, measures to minimize or eliminate backlash and windup must be taken. Excessive misalignments must be corrected. Lastly, shaft couplings must be lubricated regularly, and their internals must be free from contamination and dirt.
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