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Coupling-
“Coupling is a mechanical device which is used to join two shafts“.
Couplings are used to join the prime movers like engines, motors etc with other mechanisms or machines. Couplings are also used to join one machine to another machine.
Couplings can join two shafts in a straight axis. In different axis and axis with different angles Couplings are also used to prevent the overloading of machine parts and shocks from one shaft to other shafts.
Here, Shafts are normally available at 7 metres in length but greater lengths can be obtained by coupling smaller or broken shafts.
Types of couplings-
There are two main coupling and its types and that is-
- Rigid couplings
- Flexible couplings
Here is a detailed guide to mechanical coupling types–
Rigid couplings-
Rigid couplings are used to join two shafts in a complete alignment. These couplings have no outside force or torque acting on them. They are not flexible and subjected to negligible forces.
Types of rigid couplings-
Some Types of rigid couplings are:
- Sleeve or muff coupling
- Split muff or compression coupling
- Flange coupling
I. Sleeve or muff coupling-
It is also called a box or collar coupling. It is the simplest type of rigid coupling which is used to join shafts of smaller sizes.
This coupling consists of a hollow cylinder, known as a sleeve or muff which is mounted on two shafts to be joined.
The inner diameter of the hollow cylinder is made equal to the diameter of the shaft. The shaft is connected to the hollow cylinder by means of a jib head key.
One half of this cylindrical sleeve or muff is connected to the end of one shaft and the other half is connected to the end of another shaft.
The two shafts and muff have slots cut on them to accommodate a key which has a taper of 1: 100.
For making a muff coupling assembly two shafts of equal diameter should be assembled in a straight line. Besides, the shafts should have key seats of equal size to fix the key.
The muff to be used in muff coupling is generally made of cast iron whereas the key is made of mild steel.
The assembly of muff coupling consists of a drive shaft and a driven shaft. The driver shaft is connected to the motor or engine and transmits power to the driven shaft.
The power transmission from the driving shaft to the driven shaft depends upon the ultimate stress that the Fey can bear. Failure of key causes failure of the coupling.
Shafts of different diameters can also be joined by muff coupling. In this case, two halves of the muff are drilled according to the size of the shafts to be joined in them.
The shafts of the corresponding diameter are fixed in the muffs. Two separate keys are fixed in shafts and muffs either in a straight line or opposite to each other.
Types of Muff Coupling-
Some Other Types of Muff Couplings are-
- Half-lap Muff Coupling
- Taper pin-type muff coupling
- Set-screw muff coupling
A. Half-lap Muff Coupling:
A half-lap muff coupling consists of two shafts whose ends are cut to overlap each other. A key is fixed between the muff and shafts to make the joint firm.
The overlap provided on the shafts has a taper of 1: 12. In this type of coupling power is transmitted by the lapped shafts and the key acts as a joining element between the muff and shafts.
The taper given in the lap prevents shafts to be separated while pulled in tension. Being a good coupling its use is limited due to the high cost of manufacture.
B. Taper pin type Coupling:
This type of muff coupling joins two shafts of equal size by means of a muff, which are drilled to accommodate a taper pin.
In this coupling taper pins fixed across the shafts and muffs in place of keys, transmit power.
C. Set-screw muff coupling-
a set-screw muff coupling. In this coupling, the set screw is used to fix the shafts with a muff. It is used to transmit low power from shaft-1 to shaft-2.
Advantages and Limitations of Sleeve or muff coupling-
Here are some advantages and disadvantages of muff or sleeve coupling–
Advantages-
- The main advantage of this coupling is that there is no projecting part on the outer surface, so the working of the coupling is safe.
- It is easy to open the coupling. It can be done by removing the key and shifting the whole muff on the shafts to separate them.
Limitations-
- This coupling cannot transmit much power and torque. In such cases, the failure of the key may cause damage to the coupling.
II. Split-muff or Compression Coupling-
In this coupling, the muff is made in two similar parts. The shafts are fixed between these parts by means of studs, nuts and feather keys.
For a strong grip between shafts and muff two, four or six bolts may be used.
In this coupling, the two halves of the muff grip the shafts and power are transmitted due to friction between them, so it is also called split-up muff coupling.
The shafts are clamped between the split muff by nuts and bolts, so it is called clamp coupling.
Generally, split muff coupling can transmit low and medium twisting moments. If a key is fixed in between the shaft and muff, the coupling can transmit high twisting foments also. It is mainly used for line shafts.
Advantages and Limitations of Split-muff or Compression Coupling–
Here are some advantages and disadvantages of Split-muff or Compression Coupling–
Advantages-
- This coupling can be assembled and disassembled easily.
- For fixing the coupling the shafts are not required to be removed from the bearings on which they are supported.
- The coupling can be used in any position of shafts.
- Ribs increase the strength of coupling.
- Bolt heads, nuts etc. are covered by ribs of coupling, so it runs safely.
Limitations-
- High power and twisting moments can not be transmitted. So a key has to be fixed by making a slot in the shaft.
III. Flange Coupling-
It is a standard form of coupling, generally used to join two shafts. For example, a motor coupled to a generator or pump.
It is a rigid type of coupling which consists of two flanges made of cast iron or steel. Both flanges have the same size and are connected to the shafts by keys at their ends.
The two flanges are together joined by nuts and bolts. The number of nuts and bolts maybe three, four or six depending upon the power to be transmitted.
Holes are made in the flanges to accommodate the nuts and bolts at equal intervals. The bolts should pass both the flanges and tighten by nuts in the perfect fitting.
A gasket should be used between the flanges to make a smooth running without shock. For the alignment of shafts, one flange contains a circular projection and the other contains a corresponding circular slot.
The length of the projection may be up to 10 mm. Such an arrangement is called a spigot and socket centring.
In some cases, a length of about 10 mm is projected out of the flange to the other flange in which clearance is provided to accommodate the projecting shaft.
Perfect alignment of shafts can be obtained by such an arrangement. This coupling provides accurate, rigid and strong joints between shafts and the alignment of the shaft also maintains.
Types of Flange Coupling-
There are three types of flange couplings:
- Rigid or solid flange coupling.
- Unprotected flange coupling.
- Protected flange coupling.
A. Rigid or solid flange coupling-
This is known as a forged or marine type of coupling. In this coupling, the flange is an integral part of the shaft i.e. flange is made at the end of the shaft by forging.
Solid coupling which consists of flanges joined by headless tapered bolts. The number of bolts maybe four to twelve.
This type of coupling is used in shafts of marine propellers, vertical hydraulic turbines, steam turbo-generators etc.
Advantages and Limitations Rigid or solid Flange Couplings-
Here are some advantages and disadvantages of Rigid or solid flange Coupling–
Advantages-
- Coupling is compact and concise.
- It is light, strong and reliable.
- It is used to transmit high power.
- Long shafts can be obtained.
Limitations-
- High manufacturing cost.
- Complicated manufacturing process.
B. Unprotected flange coupling-
A coupling with bolt heads and nuts projecting out of flanges is called unprotected flange coupling. It is unprotected because the projecting heads of nuts and bolts can cause harm to materials, machines and workers.
C. Protected flange coupling-
This type of flange coupling is provided with a protecting flange which covers the projecting nuts and bolts for their protection.
Advantages and Limitations of Flange Coupling–
Here are some advantages and disadvantages of Flange Coupling–
- An accurate, rigid and strong joint is obtained.
- This joint can bear shocks and vibrations for its smooth running.
- Shafts are aligned perfectly by this coupling.
Limitations-
- The shafts which are joined by this coupling can accommodate machine elements such as the pulley, gear etc, only in split form.
Flexible Couplings-
Flexible couplings are used to join such shafts which are not in a straight line axis, and also have a small amount of angular displacement.
These shafts are misaligned and bear loads, vibrations, shocks etc. and transfer them from one shaft to another shaft.
Misalignment of shafts is caused due to incorrect fitting, load on shafts, wear of bearings, change in temperature etc.
A flexible coupling is suitable in these situations and is used to transmit high and medium power.
Types of Flexible Couplings-
Some prominent flexible couplings are listed and described below:
- Bushed-pin Type Coupling
- Universal Coupling
- Oldham Coupling
I. Bushed-pin Type Coupling-
This type of coupling consists of a driving pin to transmit motion and twisting moment from one shaft to another shaft.
These pins are rigidly fixed by nuts on one of the flanges, whereas loosely fixed in other flanges having corresponding holes.
In this flange, the empty space around the pin is filled with a compressible and flexible material like leather, rubber washer, etc.
This washer or bush protects the pin from vibrations and shocks during power transmission from one shaft to another shaft.
This coupling functions like a rigid coupling but it contains pins instead of bolts. The joint of this coupling is not like a socket and spigot type, instead, it has a clearance of about 5 mm.
This facilitates the coupling to run with lateral misalignment up to 0.5 mm and angular alignment up to 1.5°. It can also bear some axial misalignment.
This coupling is used for high and medium power transmission. It is suitable for driver and driven equipment which are installed on the same base and do not carry an excess of misalignment.
For example- an electric motor connected to a compressor, pump or gearbox.
II. Universal Coupling-
It is a coupling of robust parts, also known as Hooke’s joint. It is used to connect two axes that are not in alignment.
The axis of shafts may be such that they cut each other at a point with an angular misalignment of 5° to 30. It can work equally good if during operation the misalignment keeps on changing the position of shafts.
This coupling consists of two shafts fitted with a fork at their ends by keys, centre block with two arms at right angles is fixed with the fork by means of two pins.
Both the pins have collars at each end and are fixed by split pins or lock pins. The fork and centre block is made of cast iron or mild steel (MS).
These couplings require lubrication.
They are used in automobiles such as buses, trucks (for tranny motion from the gearbox to the differential), joint of the knee in milling machines and many types of machines and agriculture implements for transmitting power.
III. Oldham Coupling-
This coupling was designed by OLDHAM and so named by his name.
This coupling is used to join such shafts which have parallel axis but not in linear positions ie. they are in some lateral alignment This is also suitable to be used in a situation when the distance between the axis of two shafts changes during the operation
Oldham coupling in dismantling form consists of two flanges A and B which have rectangular grooves cut across the diameter. Each flange is fixed to the shaft by means of keys.
In between the flanges, a circular disc is used which has projecting rectangular parts on both faces.
The rectangular projections on the disc are fixed in the corresponding slots made on flanges, which cross each other at 90° when assembled.
During the operation, tongues slide in slots of flanges. At any instant speed of the driver shaft and driven shaft are the same.
The plate and flanges of this coupling are made of cast iron or steel. They can transmit power up to a speed of 150 rpm.
If the disc of coupling is made of fibre or contact surfaces lined with leather lining, it can be used as flexible coupling.
Purpose of coupling-
The purpose of using couplings are:
- To increase the length of shafts.
- To join broken shafts.
- To prevent overload.
- To join two machines to obtain the required work. For example, joining a pump to a motor to increase the pressure energy of water, joining an electric generator to the engine to get electric power.
- To provide mechanical flexibility between shafts of two units.
Desirable properties of a coupling-
The desirable properties of a coupling connecting two shafts are listed below:
- Couplings should be assembled and disassembled easily.
- The twisting moment acting on the shaft should be transmitted through the coupling.
- Complete alignment of shafts should be maintained by the coupling.
- Projecting parts in a coupling should be avoided. If a part like a nut-bolt, key etc projects from the coupling, it should be covered by cover plates or a suitable flange.
- Coupling should be able to reduce shocks and vibrations of shafts at the time of power transmission.
Applications of Coupling-
Here are some general applications or uses of coupling-
- For transmission of Power from one to another shaft
- For connecting shafts between each other
- Induce Flexibility while power transmission
- For Overloads Protections
- For Shock Reduction
Wrapping Up-
This is it for Coupling. I sincerely hope you enjoyed your time here and learned something. If so, spread the useful information to your friends, coworkers, or those in need.
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Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.
