Jul. 01, 2024
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.
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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.
&#;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.
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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:
This is the third installment in a weekly 3-part series on flexible vs rigid couplings running through December. Here, Ruland&#;s Bobby Watkins shares his insight into the best options for different coupling applications. This excerpt was taken from Watkins&#; presentation during the Design World Webinar, Flexible vs Rigid Couplings.
A little bit about Ruland Manufacturing. We have manufacture all types of couplings: Rigid couplings, servo insert couplings, bellows, metal bellows, metal disc, oldham types and beams, so we offer the full range of types of motion control couplings.
The types of misalignment earlier were covered pretty well, and again, talking about rigids in particular at this point, they&#;re probably the most misapplied, maybe the most misunderstood, of all the couplings. They&#;re not going to accommodate any of these types of misalignment, but they do have a purpose and they&#;re very, very effective when used in the right circumstance.
Again, why is misalignment a concern? If you&#;ve got misalignment and your coupling is not addressing that misalignment, you&#;re going to have problems, either with broken couplings or bearing failures, and again, if you&#;ve got misalignment present in your system, no doubt, you need a flexible coupling to solve those issues for you. Backlash, windup, and bearing load. Some of the types of motion control couplings have windup. Some of the types of couplings like the servo insert couplings and the beam couplings, have some windup, some vibration absorption capabilities, shock absorption capabilities, and these are important features to have in certain applications where you don&#;t necessarily want or need high torsional stiffness, but you need some vibration dampening. The bellows type, disc type, those are high torsional stiffness types, and of course, the rigids are probably the ultimate torsionally stiff coupling. Let&#;s see.
This was talked about earlier, if you use the wrong coupling in an application, you&#;re going to have bearing load issues and no more so than with the rigid. If you put a rigid in an application where there is misalignment, bad things are going to happen and probably very quickly. You&#;re going to need bearings, destroy components, and bearing load consideration is very, very important as part of the design. Managing misalignment, again, this is just showing a few different types. The following image is exaggerated, by the way. It would be nice if bellows couplings could really forgive that much in angular, but they can&#;t.
It makes a good marketing picture, I guess. Again, selection of the correct coupling is critical. Address the application misalignment. This is a huge issue that we see in the field. Many, many types of couplings, bellows couplings in particular, fail if you install them either compressed or extended. That&#;s a very important thing to address. We see quite a bit of problems with customers on installation issues.
Rigid couplings. The benefits. Best torque transmission capabilities, call that highest torque transmission capabilities, size for size. Almost zero windup, the most precise coupling available. It&#;s basically like welding your shafts together. Suitable for shaft support in push-pull applications, relatively low cost. The drawbacks, as we&#;ve talked about. It has no misalignment capabilities, no allowance for thermal expansion. Even if you have your shafts perfectly aligned, but you&#;ve got a high-speed stepper or servo motor cranking up at 10,000 RPM, you&#;re going to get some thermal expansion of that motor shaft. The rigid coupling is not going to accommodate it, it&#;s going to push that load right back on the bearing, and you&#;re going to cook your servo or stepper motor bearing. You&#;re going to cook the motor. Something to keep in mind is that even if you&#;ve got perfectly aligned shafts, you&#;ve got to watch out for that thermal expansion of motor shaft. We tell customers to be careful once you get over about 5,000 RPM. You start getting over 5,000 RPM, you&#;re probably going to have thermal expansion as an issue at that point.
One of the realities you have to deal with is mounting rigid couplings. If you don&#;t have a design in place to make your alignment easy, they can be very difficult to align. You&#;re shimming, you&#;re using lasers&#; I&#;ll give you an example on an actuator. A linear actuator, a ball screw actuator, if you use clearance holes on your motor bracket flange, what you do is you bolt the motor up to the flange, slide the coupling on, and tighten down the coupling first. The coupling will pull the motor into alignment for you. You use the rigid coupling at assembly to pull your motor and your ball screw shaft into alignment with each other and then bolt down the motor last. If you build in the correct clearances it works extremely well. We sell a tremendous amount of rigid couplings in actuator applications where there&#;s a motor bracket.
If you don&#;t have a motor bracket, you can set up your bracketry in such a way where you&#;ve got some clearance and forgiveness built in. You put the rigid coupling on, let it pull things around it into alignment, then go tighten your components down. This way of doing it works very, very well. However, if you&#;ve got high RPM applications, this isn&#;t going to help you because you&#;re still not accounting for thermal expansion. One of the other things about rigid couplings that we run into, being a manufacturer, is we&#;ll have customers call us and they&#;ll say, &#;You know, I&#;ve tried five different types of couplings. I&#;m breaking them all. I just want to put a rigid coupling in there, and the problem&#;s going to go away.&#; This is a common call that we do get, and it&#;s typically because the customer has a bad alignment issue and he&#;s breaking flexible couplings because he&#;s asking more than they can give. We try to protect customers from themselves sometimes because there&#;s a rush sometimes to put a rigid in there as a fix-all for problems with flexible couplings. If you build in a way to do your assembly, you can use the coupling to do the alignment for you. It becomes very simple. Again, very accurate, high torque, and it can be a great coupling if used correctly in the right application.
The Oldham couplings are wonderful. They&#;re a little compromise, if you will. They&#;re not as torsionally stiff as a bellow or a disc, but they are stiffer than a jaw coupling. They&#;re a good compromise. They work in a tremendous amount of applications where you don&#;t quite need the high torsional stiffness of a bellows. The beauty of the Oldham is it&#;s very, very forgiving, especially in parallel. They&#;re becoming very, very popular, in stepper and servo drive systems. They&#;re stiff enough, they&#;re cost effective, they&#;re easy to install, they&#;re very, very forgiving. The beam type coupling, is typically for encoder applications and very light duty applications. They have a lot of windup to them, which if you ask too much of a beam coupling, all that windup&#;s going to give you some positioning error, but again, they work very well for transducers and encoders and applications where there&#;s not a lot of torque.
There&#;s been a lot of talk about the bellows as well&#; high torsional stiffness, very, very popular. You can get the high speed, high torsional stiffness out of this that you cannot get out of a rigid. You can get well over 5,000 rpm. Then, of course, the curve-jaw coupling, or the servo insert on a jaw coupling. It&#;s going to give you that vibration absorption, that dampening, where in a lot of applications, if you&#;ve got a very aggressive move profile, you&#;ve got shock loads. If you use the rigid or a bellows, which don&#;t have any ability to absorb shock load, it can either destroy couplings or beat up bearings and so forth. A servo jaw coupling gives you that vibration and shock load absorption. The disc coupling has similar qualities to the bellows coupling. High torsional stiffness, high RPM ability, and it can forgive a little more in angular than the bellows couplings do, but not quite as popular. The bellows couplings seem to be much more popular, especially here in North America.
Product selection. Again, multiple solutions exist when selecting a coupling. Hopefully I covered everything.
Ruland
www.ruland.com
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