Shaft
A shaft is a rotating machine element which is used to transmit power from one place to other place. Carbon steels of grade 40C8, 45C8, 50C4, 50C12 are normally used as shaft materials.
Material properties
It should have good heat treatment properties.
It should have high strength
It should have low notch sensitivity factor.
It should have good machinability.
It should have high wear resistance.
Types of shaft
Transmission shaft:
These shafts transmit power between the source and machines absorbing power. The counter shafts, line shafts, overhead shafts all shafts are transmission shafts.
Machine shafts:
These shafts from an integral part of the machine itself.
Design of shafts
The shaft may be designed on the basis of
1. Strength
2. Rigidity and stiffness
In designing shaft on the basis of strength the following cases may be consider
1. Shaft subjected to fluctuating loads
2. Shafts subjected to twisting moment only
3. Shaft subjected to combined twisting moment and bending moment
4. Shaft subjected to bending moment only
Shaft Couplings
A shaft coupling is a device for fastening together the ends of two shafts, so that the rotary motion of one causes rotary motion of the other.
1. Rigid coupling
It is used to connect two shafts which are perfectly aligned. Since these couplings cannot absorb any misalignment the shafts to be connected by a rigid coupling must have good lateral and angular alignment. The types are
Flange coupling
A flange coupling is a type of coupling device 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.
One of the most simple and common forms of coupling is the flange coupling. It consists of two flanged sleeves or hubs, each of which is keyed to the end of one of the two shafts to be connected. The sleeves are held together and prevented from rotating relative to each other by bolts through the flanges.
A flange coupling usually applied to a coupling having two separate cast iron flanges. Each flange is mounted on the shaft and keyed to it. The faces are turned up at right angle to the axis of the shaft. One of the flange has a projected portion and the other flange has a corresponding recess. This helps to bring the shaft into line and to maintain alignment. The two flanges are coupled together by means of bolt and nuts. This male/female coupling method also creates a stable connection that is resistant to shifting, keeping the flange coupling sturdily in place.
Muff coupling
Muff coupling One of the simple type of rigid coupling is a sleeve coupling which consists of a cylindrical sleeve keyed to the shafts to be connected It is made of cast iron. It consists of a hollow cylinder whose inner diameter is that same as that of the shaft. It is fitted over the ends of two shafts by means of a key. The power transmitted from one shaft two other shafts by means of a key and a sleeve.
Outer diameter of sleeve D=2d+13mm
Length of sleeve L=3.5d
d- diameter of shaft
Split muff (or) compression coupling
The coupling consists of two cones which are placed on the shafts to be coupled and a sleeve that
fits over the cones. Three bolts are used to draw the cones towards each other and thus wedge them firmly between the shafts and the outer sleeve. The usual proportions for these couplings in terms of shaft diameter d are approximately as follows:
2. Flexible coupling
It is used to connect two shafts having lateral and angular misalignments. The types are
Universal coupling
These joints are capable of handling relatively large angular misalignment and they are widely used in agricultural machinery, machine tools and automobiles. There are many forms of these couplings, available commercially but they essentially consist of two forks keyed or screwed to the shaft. There is a center piece through which pass two pins with mutually perpendicular axes and they connect the two fork ends such that a large angular misalignment can be accommodated.
Old-ham coupling
These couplings can accommodate both lateral and angular misalignment to some extent. An Oldham coupling consists of two flanges with slots on the faces and the flanges are keyed or screwed to the shafts. A cylindrical piece, called the disc, has a narrow rectangular raised portion running across each face but at right angle to each other. The disc is placed between the flanges such that the raised portions fit into the slots in the flanges. The disc may be made of flexible materials and this absorbs some misalignment.
Bushed pin type coupling
One of the most commonly used flexible coupling is a bushed pin type flexible flange coupling in which torque is transmitted from one flange to the other through a flexible bush put around the bolt. These are used when excessive misalignment is not expected such as a coupling between a motor and a generator or a pump mounted on a common base plate.
Flexible Couplings
Flexible couplings are the most common mechanical means of compensating for unavoidable errors in alignment of shafts and shafting. When correctly applied, they are highly efficient for joining lengths of shafting without causing loss of power from bearing friction due to misalignment, and for use in direct motor drives for all kinds of machinery. Flexible couplings are not intended to be used for connecting a driven haft and a driving shaft that are purposely placed in different planes or at an angle but are intended simply to overcome slight unavoidable errors in alignment that develop in service.
Manage to compensate for totally 4 shaft misalignments:
Angular
Offset
Combination (angular + offset)
Axial movement
There is a wide variety of flexible coupling designs; most of them consist essentially of two flanged members or hubs, fastened to the shafts and connected by some yielding arrangement. Balance is an important factor in coupling selection or design.
Design of coupling
Shaft couplings are used in machinery for several purposes
1. To reduce the transmission of shock loads from one shaft to another.
2. To provide for connection of shaft of units those are manufactured separately.
3. To introduce protection against over loads.
4. To provide for misalignment of the shaft or to introduce mechanical flexibility.
Requirement of a good shaft coupling
1. It should hold the shaft in perfect alignment.
2. It should transmit the full power from one shaft to the other shaft without losses.
3. It should be easy to connect or disconnect.
4. It should have no projecting parts.