This article takes an in depth look at Shaft Couplings.
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Shaft couplings are components designed to connect two shafts, facilitating the transfer of power from a driveshaft to a driven shaft while accommodating misalignments and mounting inaccuracies. These misalignments can occur due to temperature variations and the gradual loss of positioning precision. Shaft couplings offer mechanical flexibility, enabling smooth rotation between the shafts and minimizing impact, wear, vibration, noise, and the 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 couplings, which will be discussed in detail in the succeeding chapters.
In the design of flexible shafts for connected machinery, engineers must balance trade-offs between acceptable offset for fatigue life and the maximum rotational speed before encountering issues such as the first bending frequency or whirling, which can be detrimental. Most designs are engineered to operate below these critical frequencies. However, some systems operate above the first bending frequency by employing snubbers to control radial excursions and mitigate potential damage.
Typically, the first axial bending frequency is encountered before other issues. When operating speeds exceed this frequency, the risk of damage is significantly reduced.
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 lengthy, single-piece shaft, which can be costly and challenging to transport, assemble, and maintain, and may lead to inaccuracies. In the event of a failure in a one-piece shaft, the entire unit must be replaced. Consequently, using two coupled shafts with couplings is often a more practical and cost-effective solution.
Shaft couplings enable the transmission of power between shafts of different diameters and are essential when the shafts from two separate pieces of equipment are manufactured independently.
Achieving precise alignment and positioning of drive and driven shafts is challenging and time-consuming. Even when shafts are manufactured to the same specifications, machining errors can still impact alignment and positioning accuracy.
Shaft misalignments negatively affect the power transmission system. These misalignments can result from thermal expansion, as well as vibrations, movements, or impacts during operation. They reduce efficiency due to power losses and generate unwanted forces that cause vibration and noise. Misalignment also accelerates wear and increases the risk of mechanical failure due to the additional stress it creates. To address these issues, shaft couplings are essential for absorbing mounting and positioning errors.
Shaft misalignments can manifest in various forms, often in combination. Parallel or radial misalignment occurs when the shafts’ centerlines are parallel but offset by a small angle (e.g., 0.5 degrees on one end and -0.5 degrees on the other) with a spacer tube between them. Angular misalignment happens when the shafts’ centerlines are not parallel and intersect at an angle, which can occur horizontally or vertically. Axial misalignment is characterized by the ends of the shafts being displaced along 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. Flexible couplings can provide electrical isolation when sensitive electronic components are being driven in a high voltage environment.
If an external impact affects the system, the shaft coupling helps prevent the transmission of this impact to the equipment. This protection is crucial, as such impacts can potentially damage the equipment.
Shaft couplings help prevent the transfer of heat from the power source to the driven shaft. By doing so, they mitigate the effects of thermal expansion, which can cause surrounding components to shift from their proper positions and degrade their accuracy.
The Thermal Expansion Coefficient (CTE) influences changes in the length of a driveline as temperatures fluctuate. These changes must be managed by the axial compliance of the couplings. Imperfect installation, temperature variations in machinery, or suspension travel can lead to parallel offsets. As heat causes thermal expansion in various components, having a coupling with adequate axial compliance becomes crucial.
For larger misalignments, Hooke joints and gear couplings can be used, though they do not maintain constant velocity and may involve issues such as galling, wear, lubrication needs, and increased mass. Constant velocity joints, like diaphragm couplings, are designed to handle less rotation at each end while offering better bending stiffness and improved conditions for higher speeds before reaching resonance.
If a design using constant velocity diaphragm couplings is subjected to rotations of two degrees or more at each end, high transmitted torque can cause a trigonometric component of this torque to convert into bending stresses on the flexible elements. This situation is unstable and often results in failure. To mitigate this, reducing bending rotation at each end may require a longer spacer tube to accommodate the same parallel offset and achieve a lower first bending frequency, which presents a significant design challenge.
Rigid shaft couplings are designed to maintain zero or minimal misalignments and are used in applications where precise alignment is crucial. If the driven shaft lacks bearing support, the load is transferred to the drive shaft, making a rigid shaft coupling essential for providing additional support to the assembly. However, rigid couplings generally offer limited shock absorption and vibration dampening capabilities.
There are several types of rigid shaft couplings, including:
A sleeve coupling is the simplest type of shaft coupling, designed for transmitting light to medium torques. It consists of a thick, hollow cylindrical tube known as a sleeve or muff, which has an inner diameter that matches the shaft diameter. The sleeve transmits torque between the shafts and is secured to each end of the shafts. Keys are used between each shaft and its hub to prevent slippage of the sleeve. Additionally, threaded inserts in the sleeve coupling help keep the shafts from moving longitudinally when bolts are installed.
A split muff coupling features sleeves or muffs made up of two or three semi-cylindrical pieces that are secured together with bolts and nuts. Each shaft is keyed to its hub to ensure proper alignment. Split muff couplings are valued for their ease of installation and maintenance, as they can be disassembled without disturbing the alignment of the shafts. In the three-piece version of the split muff coupling, one shaft's position remains unchanged while adjustments are made to the other.
Typically, split muff couplings are used for medium to heavy-duty applications with moderate rotational speeds. They are also known as clamped couplings or compression couplings.
A flange coupling is a shaft coupling that consists of two separate flanges joined together with bolts and nuts. Like sleeve and split muff couplings, each shaft is keyed to its respective flange hub. To align both shafts along the same centerline and maintain proper alignment, one flange features a projecting section while the other has a matching recessed portion.
There are three main types of flange couplings: Unprotected flange couplings have their fastening bolts and nuts exposed outside the flange disc. Protected flange couplings have the bolts and nuts enclosed within the perimeter of the flanges for added protection. Marine flange couplings use tapered headless bolts to secure the flanges together.
Flange couplings are commonly used in pressurized piping systems, for transmitting heavy loads, and for shafts with large diameters.
Spline couplings feature a sleeve with internal teeth that align with the external teeth of the spline shaft. This tooth engagement prevents slippage and misalignment of the shafts, eliminating the need for a fitted key. The load is distributed evenly around the circumference.
Spline couplings can handle radial and slight angular misalignments and are resistant to overloading. They are well-suited for applications involving high rotational speeds.
Flexible couplings are designed with elements that accommodate inevitable misalignments and axial displacements of the shafts due to temperature fluctuations and alignment deterioration. This flexibility provides an advantage over rigid shaft couplings, which require manual adjustment if the shaft alignment is compromised.
Flexible couplings can tolerate only slight misalignments. Excessive misalignments should be corrected to prevent wear and potential breakage. Unlike rigid couplings, flexible couplings can absorb shocks and dampen vibrations.
The various types of flexible couplings are classified as follows:
Mechanically flexible couplings achieve their flexibility through components that fit loosely or through rolling or sliding actions. These couplings typically require regular lubrication to maintain optimal performance.
A gear coupling is an advanced version of the flange coupling. In this design, the flange and hub are distinct components. The hubs feature protruding gear teeth on their external diameter that mesh with the internal gear teeth of the flange. This configuration results in a 1:1 gear ratio between the toothed flange and hub. Gear couplings are capable of transmitting high torque, with the torque capacity increasing as the size of the gear teeth grows, particularly at high speeds.
Roller chain couplings feature radial sprocket hubs connected by a double-strand roller chain. The sprocket teeth engage with the chain to transmit torque, while the clearances accommodate parallel, axial, and angular misalignments.
These couplings offer a simpler and more compact design compared to gear couplings and are generally more cost-effective. However, they transmit less power and are suited for applications involving low to medium torque at moderate speeds. Nylon chains are available as an alternative to metal chains, providing a lubrication-free option.
Grid couplings feature two radially slotted hubs meshed with a serpentine strip of spring steel. This spring steel imparts torsional flexibility to the coupling, allowing it to handle a wide range of torque, speed, and misalignments. As the load increases, the contact between the spring steel and the hub teeth decreases, with the spring steel flexing to accommodate misalignments. Grid couplings are effective at absorbing shocks and dampening vibrations. To ensure optimal performance and prevent contamination, the coupling must be regularly lubricated and securely fastened.
Elastomeric flexible couplings achieve flexibility through the compression and shear of a resilient polymer, such as plastic, rubber, or elastomeric materials. These couplings are particularly effective at absorbing shocks and vibrations compared to other types of flexible couplings. However, their operating temperature is limited by the melting point of the elastomeric material.
Tyre couplings use a thick rubber, polyurethane, or polyether element to connect two hubs. This element transmits torque through shear forces. Tyre couplings are capable of handling significant misalignments and effectively reduce shock and vibration transmission. They are suitable for transmitting a broad range of torque at moderate to high speeds.
Jaw couplings feature an elastomeric spider insert that fits between two interlocking jaws on the coupling hubs. As torque is transmitted between the shafts, the spider undergoes compression. The torsional stiffness and torque capacity of jaw couplings depend on the number, shape, and width of the jaws. These couplings effectively transmit torque while dampening vibrations and accommodating misalignments. They offer excellent vibration resistance and are suitable for motion control applications. Additionally, jaw couplings are resistant to dirt, moisture, and oil, and do not require additional lubrication.
Oldham's couplings consist of a central plastic disc with two rectangular projections, known as tongues, positioned perpendicularly to each other. This disc is sandwiched between two metal discs, typically made of aluminum or steel, each featuring matching grooves. The tongues on the central disc slide within these grooves to accommodate misalignments, resulting in a slight offset between the parallel centers of the shafts.
Oldham's couplings are compact and designed for applications that require zero backlash. They offer high torque capacity and can handle significant lateral misalignments. In the event of a torque overload, the central disc is engineered to fail first, protecting other components from potential damage by interrupting torque transmission.
A bushed coupling (or bush pin-type coupling) is a variation of the flange coupling. Instead of using bolts, this type employs pins with rubber bushings to secure the flanges. The rubber bushings introduce flexibility, helping to absorb shocks and vibrations while accommodating higher levels of misalignment. However, bushed couplings can be challenging to assemble and disassemble. They are commonly used in medium-duty applications for motors and machinery.
Metallic element couplings achieve flexibility through the bending of thin metallic discs or diaphragms.
Disc couplings feature one or more flexible steel discs mounted between flanges. These discs can be square, circular, octagonal, or scalloped in shape. In double-disc couplings, a central component connects the hubs, allowing the discs to flex and accommodate misalignments while maintaining torsional rigidity. Since disc couplings have no sliding parts, they do not require lubrication. They are commonly used in applications such as motor generators, blowers, fans, compressors, and pumps.
Diaphragm couplings utilize flexible, convoluted plates known as diaphragms. These diaphragms are attached to each coupling hub and connected through an intermediate component called the spool. Power is transmitted from the drive shaft diaphragm to the spool and then to the driven shaft diaphragm. The diaphragms flex to accommodate misalignments. Because there are no sliding parts, lubrication is unnecessary. Diaphragm couplings are commonly employed in high-power transmission systems, such as turbomachinery and industrial machinery, where high torque and high speed are required.
Schmidt couplings feature three interconnected discs designed to handle significant parallel misalignments. They maintain constant velocities and effective torque transmission between the drive and driven shafts while compensating for parallel misalignment. Schmidt couplings are commonly used in roller-equipped machinery, such as those found in papermaking and printing industries.
Miscellaneous couplings derive their flexibility from a blend of various mechanisms discussed above.
Beam style couplings are available in both single and multiple beam configurations. Single beam couplings reduce bearing loads caused by angular misalignments. They are crafted from a single piece of material, which is made flexible through helical cuts running along their length, allowing for slight uncoiling.
In single beam couplings, the helical cuts accommodate shaft misalignments and movements. Their single-piece design eliminates backlash issues common in multi-part couplings. Both single and multi-beam couplings can be used in applications involving elevated temperatures.
Multi-beam couplings offer enhanced torsional stiffness, leading to better system response. They feature two or three overlapping beams, which address issues of low torsional rigidity. Multi-beam designs allow for shorter beams without compromising the coupling's ability to handle misalignments.
Bellows couplings feature two hubs connected by a thin, flexible, corrugated metallic section. These couplings offer high torsional stiffness, ensuring precise torque and motion transmission. They can handle a limited degree of misalignment and exhibit zero to minimal backlash. Additionally, bellows couplings perform effectively in high-temperature environments due to the lack of polymeric components.
Other types of flexible couplings include:
The universal coupling, also known as the universal joint or Hooke’s Joint, offers a wide range of motion and accommodates significant misalignments. It is used to transmit torque and motion between shafts that intersect at an angle rather than being parallel. A universal joint consists of two yokes, each attached to one of the shafts, connected by a cross-shaped component called the spider. The yokes are oriented at right angles to 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, also known as hydrodynamic couplings, consist of an impeller mounted on the driveshaft and a runner attached to the driven shaft. The impeller functions as a pump, while the runner acts as a turbine. There is no direct mechanical contact between the impeller and the runner. Torque is transmitted hydraulically through a fluid rather than mechanically. Energy is transferred to the fluid by the impeller, which then accelerates it towards the runner. Upon impacting the runner, the fluid slows down and returns to the impeller for recirculation. The fluid velocity is higher at the outer edges of both the impeller and the runner.
Shaft couplings are crucial for all sizes and types of shafts used in power transmission. Regardless of the mechanism's size, misalignment can occur and must be addressed to alleviate stress on shafts and bearings. In factory automation (FA), shaft couplings are vital for ensuring smooth and efficient operation. Lightweight couplings are particularly important in motion control applications for robotics and medical instruments.
Jaw couplings are well-suited for light power transmission tasks. They feature jaws that encircle a hub and engage with an elastomeric insert known as a spider. The driven jaws align with the driving jaws and exert force towards them, ensuring effective power transfer.
Backlash refers to the undesired movement between connected components that are not perfectly aligned or fitted. While couplings can tolerate minor amounts of backlash, it should be kept within the system’s tolerance levels. Excessive backlash can lead to significant misalignment, increased wear, mechanical stress, and potential component failure. In motion control systems, minimizing backlash is crucial, as it directly impacts performance and precision. In power transmission systems, excessive backlash translates into power losses.
To reduce backlash, replace any worn, loosely-fitted, or defective components. Inspect coupling inserts, gear teeth, splines, rubber bushings, springs, bolts, and other parts for signs of wear or looseness. Ensure that there are no unnecessary clearances between the hub and the shaft. Additionally, controlling backlash involves maintaining a consistent torque speed and direction. Generally, flexible couplings exhibit more backlash compared to rigid couplings.
All shaft couplings experience windup, also known as torsional deflection. This phenomenon occurs when there is a discrepancy in torque between the connected shafts, leading to varying degrees of angular displacement. As a result, motion can be lost, leading to inaccuracies in positioning within motion control systems.
When selecting a shaft coupling, it's essential to consider several factors including the torque transmission capacity (in horsepower), maximum operational speed (rpm), the degree of permissible misalignment, stiffness, inertia, shock absorption, vibration damping, shaft mounting requirements, environmental conditions, specific applications, and cost. Thoroughly evaluating these factors ensures the coupling's longevity, maintains its efficiency, and reduces the risk of failure.
After installation, it is crucial to address issues such as backlash and windup. Correct any excessive misalignments and ensure regular lubrication of the coupling. Additionally, keep the coupling’s internal components clean and free from contaminants to ensure optimal performance.
Flexible shafting is essential in a wide range of applications, from kitchen food mixers to 600 MW steam turbines. As the torque increases, so does the diameter and wall thickness of the shaft, which in turn enhances its bending rigidity. However, any imposed motion, typically a combination of axial and parallel offsets, introduces bending stress that can quickly lead to fatigue failure. Mitigating this factor is crucial to ensuring the longevity and reliability of the shafting system.
Traditional arrangements of constant velocity, fatigue-tolerant couplings at each end of a spacing tube involve eight flanges and four sets of bolted connections between the driving and driven machines. This high part count can lead to imbalances and lower resonant speeds due to the increased rotating mass. Such assemblies often require balancing to address tolerances at the mating surfaces.
By employing ‘virtual hinges’ at each end of the spacing tube, and using a 100% carbon fiber construction with sophisticated robotic placement of the carbon fiber in hyperbolic diaphragms, it is possible to halve the number of flanges and bolts, and reduce the mass by up to 80% compared to conventional solutions. This approach eliminates the need for balancing and allows for higher operational speeds given the same length and power requirements.
Advanced composites used in spacing tubes can be designed to have close to zero Coefficient of Thermal Expansion (CTE). Although these materials offer better performance and lower costs in volume, they involve higher upfront tooling costs for different geometries. Prototype solutions employing this method are already being used in aircraft propulsion and offshore seawater pumping. Mid-volume applications may find the upfront costs justifiable and are currently under testing.
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