In mechanical system coupling can be defined as a kind of linkage between two rotating shafts which joins driving and driven shafts together. The joint between two shafts may be permanent or temporary. In simple words we can say that coupling is used to join input and output shafts in any power transmission system such as in machine tools gear box shaft is connected to the input engine shaft by means of coupling; engine shaft with pump or compressor shafts etc. The function of the coupling is almost same as the clutches but clutches are temporary joint while the coupling joints are permanent connection.
The basic purpose of the coupling is to join two shafts permanently. The shafts in the power transmission have not collinear connections always; they may be collinear, intersecting axes and parallel with little eccentricity. That’s why different type of couplings is used in mechanical power transmission. According to the requirement and functions different types of coupling are used. The basic features of the different types of coupling are almost same which are given as follow:
- Power Transmission
- Join misaligned shafts
- Reduce shocks and vibration
- Easily to assemble and disengage
Shafts can be joined in three ways because generally in three different ways shafts are aligned, these are when both the shafts are parallel or we can say when some eccentricity is present between them, collinear shafts and last one is intersecting shafts (having some angular deflection). All these type of joints require different kind of coupling which is describing below.
Types of Couplings:
1. Oldham’s Coupling:
Oldham’s coupling is used to connect the two parallel shafts when some eccentricity is present between two rotating shafts. In this the two misaligned shafts have disc shape flanges with rectangular slot at the middle. Both the flanges have slot cut which is at right angle to each other. In between two shaft a circular disc assembled having tongue on both side at right angle to fit between the slots of flanges. When shafts rotate then the tongue of the flanges slides into the recess of the flanges of the shafts. The middle disc rotates about its centre but both the misaligned shafts rotate their own axis by means of sliding tongue into the grooves which results the centre of the middle disc traces a circular orbit. A spring is required to remove the backlash of this running coupling. The maximum sliding velocity of the both the tongue in the slots will be the peripheral velocity for the middle disc at midpoint along the circular orbit.
For better understanding watch the following video.
2. Hook’s Coupling:
This is also known as hook’s joint, universal joint or coupling. It is called universal joint because it can able to joint two shafts having intersecting axes. It is used to joint connect two non-parallel and intersecting shafts which has some angular misalignment. The input and output shafts are connected using hook’s joint. The input driving shaft rotates at a uniform angular speed while the driven shaft rotates with varying angular speed i.e. both shafts have different angular speed. Both the shafts rotate in fixed bearing and have a fork at the end. Each fork has four ends and sides which are connected by centre piece. The centre joint generally spherical in shape but can be cross and square according to the requirement. At the centre piece the arms of forks are join at right angle. The fork connection between two shafts provides the motion to the intersecting axes shafts. The main application of this coupling in automobiles where it is used in power transmission from gear box to rear axle.
3. Double Hook’s coupling:
As we studied in hook’s coupling the input and output speeds are not uniform but in efficient power transmission we always require uniform speed at outlet that’s why double hook joint comes into play. As name implies a double hook joint has two hook’s joint or universal joint which are connected by means of an intermediate shaft. For obtaining uniform angular speed or constant velocity ratio the input and output shafts always make equal angle with the intermediate shaft and the forks used in the intermediate shafts should rotate in the same plane because if the angular misalignment between input-output and the intermediate shafts is equal then the input and output shafts will always remain in correct angular alignment. So by using double hook’s coupling we can obtain uniform angular velocity at driven shaft when both the shafts have intersecting axes.
4. Rigid Coupling:
Rigid coupling is used where the axes of both the shafts are collinear. Rigid couplings are simple and less costly, but the major drawback of this type of coupling is that they cannot tolerate any kind of small misalignment between the axis of the shafts. Different types of coupling come under this category some are them are as follow:
- Muff coupling
- Clamp coupling
- Flange coupling
a.) Muff coupling:
This coupling also known as sleeve coupling because it has sleeve or a kind of hollow cylinder. The driven and driving shafts are joined in the sleeve by means of a key. Generally sunk key is used in muff coupling which makes it simplest coupling among all. The force transformation takes place from driving shaft to key then from key to sleeve and then to the driven shaft by means of key. Shearing force between the key and shaft is responsible for the power transmission. Key should be the weakest part in any kind of coupling. Muff coupling used for small diameter shafts. The power transmission shafts should not have diameter more than 70-80mm. This is the type of rigid coupling so always proper alignment is required otherwise joint will fail. It can’t absorb any kind of jerk and vibrations so that the operation should be free from any vibration.
b.) Clamp coupling:
Clamp coupling is the modified form of the muff coupling and also known as split muff coupling i.e. it has sleeve which is divided into the two half that’s why it is known as split muff coupling. The two-split portion of the sleeve are clamped together using nuts and bolts. Both the shafts tightened together into the clamps, 4 or 8 nuts are used to tight the shaft into the split muffs. There is always some clearance in between the both muffs at centre line for the tightening space of the nuts. The torque is transmitted from one shaft to another by means of friction between the sleeve and the shafts periphery. A key is also placed in between the sleeve and shaft for the proper connection which is necessary for the torque transmission. Both friction and shear force between key and shaft are responsible for the torque transmission. The major difference between the clamp coupling and muff coupling is the way of torque transmission. In muff coupling torque is transferred by means of shear forces of the key where as in clamp coupling torque is transmitted by the two ways one is by means of friction between the sleeve and shaft and other by shear resistance between key and shafts. This is also a type of rigid coupling so that it can’t tolerate any jerk and vibration, but this is easy in construction and making connection between two shafts is easy. This coupling is not suitable for high speed applications. Its applications in line shaft power transmissions.
c.) Flange coupling:
As name implies flange coupling has flanges for the joining of both the shafts. It is another type of rigid coupling so all the conditions are also applicable here like proper alignment of both the shafts and connection should not have any kind of vibrations and jerks. Here we use two flanges one is for input shaft and the other is for the output shaft. The shafts are joining with the flanges by means of keys and both the flanges are connected together by using nut and bolt connections. Key is responsible for the power transmission in this coupling. The flanges have spigot and recesses at the centre for the proper connection. The proper alignment is always required for the good connection. This is used for high speed applications because it has proper and tight connection by means of flanges and nut-bolts. It is widely used in power transmission because easy to assemble and simple in construction.
5. Flexible coupling:
This is another category of coupling. As name suggest it has flexible connection between the input and the output shaft because it is very difficult to make a proper aligned connection between the two mating shafts and there are various reasons for the misalignment between the two shafts such as deflection of shafts, thermal expansions etc. so to remove all such causes flexible coupling comes into the play. Flexible coupling able to tolerate small eccentricity between the two shafts and in flexible couplings some kind of flexible element know as bush are used as extra part which helps in reduction of vibrations and jerks so which make it to use in little vibration conditions. We can say that flexible couplings are the best for the power transmission between two shafts. The cost and the complex construction is the main disadvantage of this coupling.
Bush-Pin flexible coupling is one of the most famous type of flexible coupling. In this bushes are used for absorbing the shocks and vibrations. The bushes are made up of the flexible elements like rubber. The construction of bush-pin coupling is almost same as the flange coupling as we discussed above the only extra part is bushes. These bushes are mounting in between the flanges and the shafts. Bushes are not only able compensate the small eccentricity between the two mating shafts as well as provide a tight connection between flange and shaft due to this reason Bush-Pin coupling used for high torque transmissions. 0.1 to 0.5 mm eccentric distance can be easily compensated by using flexible bushes. The angular misalignments are also tolerated by using this coupling. Here also the shear forces between the key and shaft is responsible for the torque transmission. The extra part leads to increase its cost and complexity of constructions which is the only drawback of such coupling.
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