Part II
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Eugene I. Rivin
2n
of legs, with "n
"legs ("n
" might be odd) loaded when hubs arc rotating in the forward direction and the othern
legs loaded during the reverse rotation. Deformation of each leg is independent. The radial (compensation) stiffness of the coupling withZ
= 4 iskt
is stiffness of one leg in compression (tangential direction),kr
is stiffness in shear (radial direction), and a is angle of rotation of the coupling. Equation 15 shows that the total radial forceF
fluctuates both in magnitude and in direction during one revolution.Z
&#; 6,F
is constant and is directed along the misalignment vector.kr / kt
varies with changing rubber durometerH
, and for typical spider proportions,kr / kt
= 0.26-0.3 for mediumH
= 40-50, andkr / kt
= 0.4 for hard rubber spiders,H
= 70-75.kt
;Reff
= ~0.75Rex
= 0.75(Dex
/2). The ratios between torsional and compensation stiffness values are as follows:A
allows one to select a coupling design better suited to a specific application. If the main purpose is to reduce misalignment-caused loading of the connected shafts and their bearings for a given value of torsional stiffness, then the least value ofA
is the best, together with a large external radius. If the main purpose is to modify the dynamic characteristics of the transmission, then minimization ofktor
is important.ktor
, radial stiffnesskrad
, external diameterDex
, and design indexA
.Tr
= 7 N-m has the number of legsZ
= 4 while larger sizes haveZ
= 6 or 8. This explains differences inA
(A
= 1.96, close to theoretical 1.8, forZ
= 4;A
= 0.98-1.28, close to theoretical 1.15-1.25, forZ
= 6 or 8).A
are quite consistent for a given type of coupling. Some variations can be explained by differences in design proportions and rubber blends between the sizes.Tr
(due to the use of multiple cylindrical elements with the same relative compression in each space between protruding blades, and due to high allowable compression of the rubber cylinders (Ref. 6), thus allowing use of smaller diameters and, consequently, many sets of cylinders around the circumference). Test results for such coupling forTr
= 350 Nm are shown asTr
, but to the same coupling at different transmitted torques.Part I of this article appeared in the October issue, It provided an overview and general classifications of power transmission couplings, along with selection and performance criteria for rigid couplings and misalignment-compensating couplings. Part II continues the discussion with selection and performance criteria for torsionally flexible and combination-purpose couplings.Torsionally flexible couplings usually have high torsional compliance (as compared with the torsional compliance of shafts and other transmission components) in order to enhance their influence on transmission dynamics. Figure 9a shows an example of a purely torsionally flexible coupling with an elastomeric flexible element having low stiffness in the torsional direction (shear of rubber ring) and high stiffness in the misalignment-compensation directions (compression of rubber ring). Figure 9b shows a torsionally flexible coupling with a metal flexible element (Bibby-style coupling). The flexible element is a spring steel band wrapped around judiciously shaped teeth on each hub and deforming between the teeth. The deformations become more restrained with increasing transmitted torque; thus the coupling has a strongly nonlinear torsional stiffness characteristic of the hardening type. The lowest stiffness is at zero torque (Fig 9c) increasing towards the rated torque (Fig 9d), becoming very high at the allowed peak torque (Fig. ge) and approaching a rigid condition at an overload/shock torque (Fig 9f). Since some misalignment-compensating ability is desirable for many applications, use of purely torsionally flexible couplings, with combination-purpose couplings being used as torsionally flexible couplings with more or less compensating ability.For torsionally flexible and combination- purpose couplings, torsional stiffness is usually an indicator of payload capacity In such cases, the basic design criterion can be formulated as a ratio between the stiffness in the basic misalignment direction and the torsional stiffness. In the following analysis, only radial misalignment is considered. Since couplings are often used as the cheapest connectors between shafts, and since end users often do not have full understanding of what is important for their applications, it is of interest to analyze what design parameters are important for various applications.Torsional flexibility is introduced into transmission systems when there is a danger of developing resonance conditions and/or transient dynamic overloads. Their influence on transmission dynamics can be due to one or more of the following factors: torsional compliance, damping or nonlinearity of load-deflection characteristics.If a resonance condition occurs before installation (or change) of the coupling, then shifting of natural frequency due to use of a high torsional-compliance coupling can eliminate resonance; thus dynamic loads and torsional vibrations will be substantially reduced. However, in many transmissions (e.g., vehicle transmissions), frequencies of the disturbances acting on the system and natural frequencies (especially in variable speed transmissions) may vary widely. In such instances, a simple shift of the natural frequencies of the drive may lead to a resonance occurring at other working conditions, but the probability of its occurrence is not lessened. A reduction in the natural frequency of a drive, for example, is advisable for the drive of a milling machine only at the highest spindle speeds and may be harmful introduced in the low-speed stages.A shift of natural frequencies of the drive may be beneficial in transmissions with narrow variations in working conditions. If: however, a drive is operated in the pre-resonance region, an increase in torsional compliance would lead to increased amplitudes of torsional vibrations, and thus to a nonuniform rotation. In some cases excessive torsional compliance may lead to a dynamic instability of the transmission and create intensive self-excited torsional vibrations.An important feature of multispeed or variable-speed transmissions is the changing of effective torsional compliances of components with changing output speeds due to changing reduction coefficients, although the physical condition of the components does not change (Ref 1). As a result, the role of the coupling as a compliant member can dramatically change depending on configuration of the drive. While compliance of a coupling of any reasonable size installed in the high-speed part of the system (close to the driving motor) would not have any noticeable effect at low output rpm, compliance of a coupling installed in the low-speed part of the system (close to the working organ, such as a wheel of the vehicle or a cutter of a mining combine) would be very effective, but the coupling size and cost might become excessive due to high torques transmitted to the spindle at low rpm.When the damping of a system is increased without changing its torsional stiffness, the amplitude of torsional vibrations is reduced at the resonance and in the near-resonance zones. Increased damping is especially advisable when there is a wide frequency-spectrum of disturbances acting on a drive; e.g., for the drives of universal milling machines. The effect of increased damping in a torsionally flexible coupling of a milling machine transmission is illustrated in Figure 10 (natural frequencies f= 10 Hz, f= 20 Hz). Figure 10a shows the resonance for an OEM coupling (flexible element made from neoprene rubber, log decrement δ &#; 0.4). After this element was made from a butyl rubber (same compliance, but δ &#; 1.5), the peak torque amplitude was reduced by ~ 1.8 times, the clearance opening (source of intensive noise) was eliminated, and oscillations with fn2 excited by the second harmonic of the excitation force, became visible (Fig. 11b). A common misconception about using high-damping elastomers for coupling elements is their alleged high heat generation at resonance. Due to vibratory torque amplitude reduction with high-damping couplings, the heat generation at the resonance iswhen the high-damping coupling is used (Ref. 1). The influence of a flexible element on the total energy dissipation in a transmission increases with increasing of its damping capacity, of the torque amplitude in the element and ofits compliance. For maximum efficiency, the flexible element of a coupling must therefore have as high an internal energy dissipation as possible; it must also possess maximum permissible compliance, and must be located in the part ofthe system where the intensity of vibrations is the greatest.A nonlinear dynamic system may automatically detune away from resonance at a fixed-frequency excitation. For example, when damping is low, a relative change of the stiffness by a factor of 1.3 reduces the resonance amplitude by ~ 1.7 times, but a relative change of stiffness by a factor of 2 reduces the resonance amplitude by ~ 1.85 times. Thus, nonlinear torsionally flexible couplings can be very effective in transmissions where high-intensity torsional vibrations may develop and where the coupling compliance constitutes a major portion of the overall compliance.Vehicles usually have variable speed transmissions. The same is often true for production machines. In order to keep the coupling size small, it is usually installed close to the driving rnotor/engine, where it rotates with a relatively high speed and transmits a relatively small torque. At the lower speeds of an output member, the installed power is not fully utilized and the absolute values of torque (and of amplitudes of torsional vibrations) transmitted by the highspeed shaft are small. In vehicles, the installed power is not fully utilized most of the time. Thus, an important advantage of couplings with nonlinear load-deflection characteristics is feasibility of making a resonably small coupling with low torsional stiffness and high rated torque. An overwhelming majority of power transmission systems are loaded with less than 0.5 Tr for 80-90% of the total "up" time. A nonlinear coupling with a hardening load-deflection characteristic such as one in Figure 14 provides low torsional stiffness for most of the time, but since its stiffness at the rated torque is much higher, its size can be relatively small.A huge variety of combination purpose couplings is commercially available. Unfortunately, selection of a coupling type for a specific application is often based not on an assessment of performance characteristics of various couplings, but on the coupling cost or other non-technical considerations. As a result, bearings of the shafts connected by the coupling may need to be more frequently replaced than when an optimized coupling is used; the device might be noisier than it would be with a coupling type optimal for the given application, etc.Figure 11 shows some popular designs of combination-purpose couplings.Combination-purpose couplings do not have a compensating member. As a result, compensation of misalignment is accomplished, at least partially, by the same mode(s) of deformation of the flexible element as used for transmitting the payload. To better understand the behavior of combination-purpose couplings, an analysis of the compensating performance of a typical coupling with a spider-like flexible element is helpful. The coupling in Figure 12 shows a schematic of the jaw coupling in Figure 11a. It consists of hubs 1 and 2 connected with a rubber spider 3 having an even number Z =of legs, with ""legs ("" might be odd) loaded when hubs arc rotating in the forward direction and the otherlegs loaded during the reverse rotation. Deformation of each leg is independent. The radial (compensation) stiffness of the coupling with= 4 iswhere F is radial force caused by the radial misalignment e and acting on the connected shafts,is stiffness of one leg in compression (tangential direction),is stiffness in shear (radial direction), and a is angle of rotation of the coupling. Equation 15 shows that the total radial forcefluctuates both in magnitude and in direction during one revolution.For a coupling with&#; 6,oris constant and is directed along the misalignment vector.The ratiovaries with changing rubber durometer, and for typical spider proportions,= 0.26-0.3 for medium= 40-50, and= 0.4 for hard rubber spiders,= 70-75.A spring/tubular spider coupling modification is shown in Figure 8 (Ref 4). In this design, each leg of the spider can be represented by a coil spring loaded radially by the transmitted torque. For the tightly coiled extension springThe torsional stiffness ofboth spider coupling designs iswhere the effective radius= ~0.75= 0.75(/2). The ratios between torsional and compensation stiffness values are as follows:for the spring spider coupling per Figure 8, Z = 6,In general, the ratio of radial (compensating) stiffness and torsional stiffness of a combination-purpose flexible coupling can be represented aswhere the "Coupling Design Index"allows one to select a coupling design better suited to a specific application. If the main purpose is to reduce misalignment-caused loading of the connected shafts and their bearings for a given value of torsional stiffness, then the least value ofis the best, together with a large external radius. If the main purpose is to modify the dynamic characteristics of the transmission, then minimization ofis important.The bulk of designs of torsionally flexible or combination-purpose couplings employ elastomeric (rubber) Hexible elements. Couplings with metal springs possess the advantages of being more durable and of having characteristics less dependent on frequency and amplitude of torsional vibrations. However, they may have a larger number of parts and higher cost, especially for smaller sizes. As a result, couplings with metal flexible elements, as of now, have found their main applications in large transmissions, usually for rated torques 1,000 N-m and up. Use of the modified spider coupling in Figure 8 may change this situation.Couplings with elastomeric flexible elements can be classified in two subgroups:Couplings in which the flexible element contacts each hub along a continuous surface (shear couplings as in Figure 9a, toroidal shell couplings, couplings with a solid rubber discicone, etc.). Usually, torque transmission in these couplings is accommodated by shear deformation of rubber;Couplings in which the flexible element consists of several independent or interconnected sections (rubber disk and finger sleeve couplings as in Figure 11c, spider couplings as in Figures 11a and 11b, couplings with rubber blocks, etc.). Usually, torque transmission in these couplings is accommodated largely by compression or "squeeze" of rubber; thus they are usually smaller for a given rated torque.Comparative evaluation of the commercially available couplings based on available manufacturer-supplied data on flexible couplings is presented in Figure 13. Plots in 'Figures 13a-d give data on torsional stiffness, radial stiffness, external diameter, and design indexThe "modified spider" coupling in Figure 11b is different from the conventional spider coupling shown schematically in Figure 11a by four features: its legs are tapered, instead of uniform width, and made thicker even in the smallest cross section, at the expense of reduced thickness of protrusions on the hubs; lips on the edges provide additional space for bulging of the rubber when the legs are compressed; and the spider is made of a very soft rubber. These features substantially reduce stiffness values while retaining the small size characteristic of the spider couplings.Data for "toroid shell" couplings in Figure 13 are for the coupling as shown in Figure 10d.The "spider coupling" for= 7 N-m has the number of legs= 4 while larger sizes have= 6 or 8. This explains differences in= 1.96, close to theoretical 1.8, for= 4;= 0.98-1.28, close to theoretical 1.15-1.25, for= 6 or 8).Values ofare quite consistent for a given type of coupling. Some variations can be explained by differences in design proportions and rubber blends between the sizes.Plots in Figure 13 help to select a coupling type best suited for a particular application, but do not address issues of damping and nonlinearity. Damping can be easily modified by proper selection of the elastomer. High damping is beneficial for transmission dynamics, and may even reduce thermal exposure of the coupling.A coupling with a hardening nonlinear characteristic may have high torsional compliance for the most frequently used sub-rated (fractional) loading in a relatively small coupling. Accordingly, the misalignment-compensating properties of a highly nonlinear coupling would be superior at fractionalloads. The coupling in Figure 14 (Ref. 5) employs radially compressed rubber cylinders for torque transmission in one direction (117) and for the opposite direction (118), between hubs (111 and 113) attached to the connected shafts. This design combines the desirable nonlinearity with a significantly smaller size for a given(due to the use of multiple cylindrical elements with the same relative compression in each space between protruding blades, and due to high allowable compression of the rubber cylinders (Ref. 6), thus allowing use of smaller diameters and, consequently, many sets of cylinders around the circumference). Test results for such coupling for= 350 Nm are shown asin Figure 13; in this case the data does not refer to different, but to the same coupling at different transmitted torques.1. Rivin, E.I., Marcel Dekker, Inc., NY.2. "Torsionally Rigid Misalignment Compensating Coupling," U.S. Patent 5,595,540.3. "Universal Cardan Joint with Elastomeric Bearings,"U.S. Patent 6,926,611.4. "Spider Coupling," lJ.S. Patent 6,733,393.5. "Torsional Connection with ,Radially Spaced Multiple :Flexible Elements," U.S. Patent 5,630,758.6. Rivin, E.I. "Shaped Elastomeric Components for Vibration Control Devices,", Vol. 33, No. 7, pp. 18-23.This article has been reproduced with the permission of Power Transmission Engineering.
A coupling is a mechanical element part that connects two shafts together to accurately transmit the power from the drive side to the driven side while absorbing the mounting error, misalignment, etc. of the two shafts.
Coupling in the machine industry is interpreted as &#;a part that connects two shafts together&#;, and is generally called &#;coupling&#;, &#;shaft coupling&#; or &#;joint&#;. Let&#;s discuss in detail what is Coupling and their types.
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both.
In a more general context, a coupling can also be a mechanical device that serves to connect the ends of adjacent parts or objects. Couplings do not normally allow disconnection of shafts during operation, however, there are torque-limiting couplings that can slip or disconnect when some torque limit is exceeded.
The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. Selection, installation, and maintenance of couplings can lead to reduced maintenance time and maintenance costs.
The role of a coupling (shaft fitting)
A shaft coupling is a mechanical component that connects the driveshaft and driven shaft of a motor, etc., in order to transmit power. Shaft couplings introduce mechanical flexibility, providing tolerance for shaft misalignment. The former is called a coupling and the latter is called a shaft coupling.
As a result, this coupling flexibility can reduce uneven wear on the bearing, equipment vibration, and other mechanical troubles due to misalignment.
Flexible Shaft Couplings can help prevent these issues by transmitting torque while compensating for parallel, angular, and axial misalignment between drive components. When installed correctly, flexible shaft couplings can also reduce vibration, minimize noise, and protect driveshaft components.
Shaft couplings are used for power and torque transmission between two rotating shafts such as motors and pumps, compressors, and generators. Shaft couplings are available in a small type mainly for FA (factory automation) and a large casting type used for large power transmissions such as in wind and hydraulic power machinery.
Different types of shaft Couplings are:
MORE: What is Fluid Coupling?
The following types of couplings and how they work:
As the name suggests, a rigid coupling permits little to no relative movement between the shafts. Engineers prefer rigid couplings when precise alignment is necessary.
Any shaft coupling that can restrict any undesired shaft movement is known as a rigid coupling, and thus, it is an umbrella term that includes different specific couplings. Some examples of this type of shaft coupling are sleeve, compression, and flange coupling.
Once a rigid coupling is used to connect two equipment shafts, they act as a single shaft. Rigid couplings find use in vertical applications, such as vertical pumps.
They are also used to transmit torque in high-torque applications such as large turbines. They cannot employ flexible couplings, and hence, more and more turbines now use rigid couplings between turbine cylinders. This arrangement ensures that the turbine shaft acts as a continuous rotor.
Any shaft coupling that can permit some degree of relative motion between the constituent shafts and provide vibration isolation is known as a flexible coupling. If shafts were aligned all the time perfectly and the machines did not move or vibrate during operation, there would be no need for a flexible coupling.
Unfortunately, this is not how machines operate in reality, and designers have to deal with all the above issues in machine design. For example, CNC machining lathes have high accuracy and speed requirements in order to perform high-speed processing operations. Flexible couplings can improve performance and accuracy by reducing the vibration and compensating for misalignment.
These couplings can reduce the amount of wear and tear on the machines by the flaws and dynamics that are a part of almost every system. As an added bonus they&#;re generally rather easy to install and have a long working life.
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&#;Flexible coupling&#; is also an umbrella term and houses many specific couplings under its name. These couplings form the majority of the types of couplings in use today. Some popular examples of flexible couplings are gear coupling, universal joint and Oldham coupling.
A Sleeve coupling is a basic type of coupling. This consists of a pipe whose bore is finished to the required tolerance based on the shaft size. Based on the usage of the coupling a keyway is made in the bore in order to transmit the torque by means of the key. Two threaded holes are provided in order to lock the coupling in position.
Sleeve couplings are also known as Box Couplings. In this case, shaft ends are coupled together and abutted against each other which are enveloped by muff or sleeve. A gib head sunk keys hold the two shafts and sleeve together
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.
The split muff coupling is also called compression coupling or clamp coupling. It is a rigid type of coupling. In this coupling, the sleeve is made of two halves. The halves of the muff are made of cast iron. The two halves of the sleeve are clamped together by means of mild steel studs or bolts and nuts.
The split muff coupling is also called compression coupling or clamp coupling. It is a rigid type of coupling. In this coupling, the sleeve is made of two halves.
The halves of the muff are made of cast iron. One-half of the muff is fixed from below and the other half is placed from above. The two halves of the sleeve are clamped together by means of mild steel studs or bolts and nuts.
The number of bolts can be four or eight. They are always in multiples of four. The bolts are placed in recesses formed in the sleeve halves.
The advantage of this coupling is that the position of the shafts need not be changed for assembling or disassembling of the coupling. This coupling may be used for heavy-duty and moderate speeds.
Flange Coupling is a driving coupling between rotating shafts that consists of flanges one of which is fixed at the end of each shaft, the two Flanges being bolted together with a ring of bolts to complete the drive.
This type of coupling is meant to bring two tube ends together in a flush, sealed manner. This two-piece coupling unit consists of a keyed receiving side for the flanged end to be fastened to, so it may be married to the opposing tube end, which also has a flanged end.
Each flange has either a male or female coupler opening so that when the two ends are brought together, they are aligned without causing resistance or drag in the material being passed through them. This male or female coupling method also creates a stable connection that is resistant to shifting, keeping the flange coupling sturdily in place.
Flange couplings are typically used in pressurized piping systems where two pipe or tubing ends have to come together. The connecting methods for flange couplings are usually very strong because of either the pressure of the material or the sometimes-hazardous nature of materials passed through many industrial piping systems.
High thread count nut and bolt connections are used to secure the flange couplings in place. These nuts and bolts are usually made from tempered steel or alloys to provide enduring strength and the ability to be tightened to the utmost level to ensure the piping system doesn&#;t leak at any flanged junction. Most flange couplings utilize four, six, or up to 12 bolt assemblies.
Gear couplings are designed to transmit torque between two shafts that are not collinear. They typically consist of two flexible joints one fixed to each shaft which are connected by a spindle, or third shaft.
The gear coupling connects the drive motor to the gearbox in hoist mechanisms, but it can also connect the gearbox directly to smaller wire rope drums using a flanged half.
In terms of their design, gear couplings transmit torque via hubs with crowned gear teeth that are in permanent mesh with the straight gear teeth of the sleeves a design that provides the highest torque transmission for the smallest size.
They also run at high speeds, conform to the AGMA bolting pattern and compensate for angular, radial, and axial shaft misalignment.
Oldham couplings are a three-piece assembly comprised of two lightweight aluminum or corrosion-resistant stainless-steel hubs and a center disk.
The tenons on the hubs mate to the slots in the disk with a slight press fit, allowing the coupling to operate with zero backlashes. Oldham couplings are commonly used in servo-driven systems that require precise motion control and low inertia, balanced design.
The Oldham coupling is a form of flexible coupling designed for applications that must be free from backlash. They are also increasingly being used as a replacement for straight jaw couplings. The Oldham coupling consists of three discs.
Two of the discs are connected to either side of the drive, while the third, made from one of several different plastics, is sandwiched in between with a tongue and groove design.
The tongue and groove on one side is perpendicular to the tongue and the groove on the other. Springs are often used to reduce the coupling&#;s backlash.
During operation, the center disk slides on the tongues, or tenons, of each hub (which are orientated 90° apart) to transmit torque. While the couplings accommodate a small amount of angular and axial misalignment, they are especially useful in applications with parallel misalignment.
The Oldham coupling features several other advantages including their compact size and potential for electrical isolation through the plastic center disk. The couplings may also act as a sort of fuse for a machine.
If torque limits are exceeded the center disc of the coupling will break apart first, preventing torque transmission and potential damage to more costly machine components.
A universal or hook coupling is used to connect two shafts whose axes intersect at a small angle. The bending of the two shafts may be constant, but in actual practice, it changes when the momentum is transferred from one shaft to another.
The main application of universal or hook coupling is found in transmission from the gearbox to automobiles&#; differential or back axle.
In such a case, we use a coupling of two hooks, connecting the gearbox at one end and the differential at the other end at each end of the propeller shaft. The coupling of a hook is also used to transmit electricity to the various spindles of several drilling machines. It is used as a knee joint in a milling machine.
A diaphragm coupling consists of one or more metallic membranes which are attached at the outside diameter of a drive flange and transfer torque radially through the diaphragm to an inside diameter attachment. The other type of metallic membrane coupling is disk coupling.
Diaphragm couplings utilize a single or a series of plates or diaphragms for flexible members. It transmits torque from the outside diameter of a flexible plate to the inside diameter, across the spool or spacer piece, and then from the inside to the outside diameter.
A jaw coupling is a type of general-purpose power transmission coupling that also can be used in motion control (servo) applications. It is designed to transmit torque (by connecting two shafts) while damping system vibrations and accommodating misalignment, which protects other components from damage.
These types of coupling are composed of three parts: two metallic hubs and an elastomer insert called an element, but commonly referred to as a &#;spider&#;. The three-part press fit together with a jaw from each hub fitted alternately with the lobes of the spider. Jaw coupling torque is transmitted through the elastomer lobes in compression.
A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque between two shafts while allowing for angular misalignment, parallel offset, and even axial motion, of one shaft relative to the other.
A beam coupling consists of a single piece of material made flexible by the removal of material in a helical pattern along its length.
As with all couplings, the purpose of a beam coupling is to transmit torque between two shafts, but unlike a rigid coupling, a beam coupling can accommodate angular misalignment, parallel offset, and even axial motion, of one shaft relative to the other.
The beam coupling also differs from other coupling types in that its one-piece construction prevents the backlash usually encountered by couplings made of multiple parts.
Beam couplings can be found in a variety of materials including titanium and acetal with stainless steel and aluminum being the two most common. The light weight of an aluminum beam coupling means they are suited for applications where a high level of responsiveness is needed.
Stainless steel, on the other hand, while providing greater strength and torsional stiffness, has a greater mass and thus does not have the same level of responsiveness.
A fluid coupling is a special type that uses hydraulic fluid to transmit torque from one shaft to another.
The shaft coupling consists of an impeller connected to the driving shaft and a runner connected to the driven shaft. The whole setup is fixed in a housing, also known as a shell.
When the driving shaft rotates, the impeller accelerates the fluid, which then comes into contact with the runner blades. The fluid then transfers its mechanical energy to the runner and exits the blades at a low velocity.
A fluid coupling is used in automobile transmission, marine propulsion, locomotive and some industrial applications with constant cyclic loading.
A disc coupling, by definition, transmits torque from a driving to a driven bolt or shaft tangentially on a common bolt circle. Torque is transmitted between the bolts through a series of thin, stainless steel discs assembled in a pack. Misalignment is accomplished by deforming the material between the bolts.
This type of coupling is a high-performance motion control coupling designed to be the torque transmitting element (by connecting two shafts together) while accommodating shaft misalignment. It is designed to be flexible while remaining torsionally strong under high torque loads. Typically, disc couplings can handle speeds up to 10,000 r/min.
There are two different styles of disc coupling:
Torsion ally stiff and still flexible, disc couplings are a great solution for high-speed applications. The downside is that they are more delicate than the average coupling and can be damaged if misused. Special care should be taken to ensure that misalignment is within the ratings of the coupling.
Bush couplings are mainly used as flexible links in applications where reliable link transfer is required under severe operating conditions. A bush coupling consists of two hubs that can be made of different materials and are fitted with pins where rubber bushes are attached.
These types of coupling are flexible couplings that are reliable and for this reason, they are widely applied to hoisting applications.
The coupling bolts are known as pins. Rubber or leather bushes are used on top of pins. Also, there is a variation in the construction of two parts of the coupling.
There is a 5 mm clearance remaining between the faces of the two halves of the coupling. And there is no rigid connection between them, and the drive is through compressed rubber or leather bushes.
Bellows couplings are one form of flexible coupling with twin coupling ends called hubs capping a precision-engineered corrugated tube that serves as the coupling body.
Bellows couplings are known for their exceptional torsional rigidity to accurately transmit velocity, angular position, and torque. Their slight flexibility (at the corrugated bellows) serves to address limited amounts of axial, angular, and parallel misalignment between the shafts or other components being joined.
Bellows couplings are typically made from a stainless-steel tube that is hydroformed (or in some cases welded) to create deep corrugations. Such hydroformed bellows begin as a sheet of stainless steel or other metal.
This sheet is drawn into a tube which is then pressurized from within against a ribbed die to form a corrugated shape. Then the end hubs are welded or bonded in some manner to this coupling bellows.
Shaft couplings are used in machinery for many purposes, the most common of which are the following:
A good shaft coupling should have the following requirements:
Coupling maintenance requires a regularly scheduled inspection of each coupling. It consists of:
Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include:
The only way to improve coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. Some external signs that indicate potential coupling failure include:
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