Here is a list of machine elements.
1. Power Transmission Shafts:
Shaft is a machine element, which is used to transmit power from one member to another member. When a tangential force is acting on the shaft, a turning moment is set up. This makes the shaft to rotate for transmitting power from one shaft to another. Pulleys, gears etc. are mounted on the shafts through keys.
Generally the cross-section of the shafts will be circular (i.e., cylindrical in length) also they are solid throughout their length. However square or rectangular shafts are also available depending upon specific applications.
ADVERTISEMENTS:
They are broadly classified into two types:
(i) Power Transmission Shafts:
As the name implies, they transmit power from one member to another member.
(ii) Machine Shafts:
ADVERTISEMENTS:
As the name suggests, they are the part of machine only. For example crank shaft.
Generally for regular shafts carbon steels are used. However for higher strengths Ni, Ni-Cr steels are used.
ADVERTISEMENTS:
Whenever power is to be transmitted from one member to another member, shafts are used.
Figure 43.1 shows a flywheel or gear mounted shaft. Shaft is supported in the bearing at A and B.
2. Axle:
A non-rotating member similar to that of a shaft is called as an axle. It simply supports a rotating body.
ADVERTISEMENTS:
One might have seen the pulley mounted on the axle on the wells, where pulley is rotating, when the rope is pulled and the axle is stationary.
Key is a member used to prevent relative rotational motion between shaft and flange or pulley. It is a piece of metal, rectangular in cross-section. The slots on outer surface of shaft and inner surface of the bore are cut along the length. When these slots are matched they give a cross section same as that of the key. The key is then inserted in this hole which prevents the rotational motion of flange or pulley on the shaft.
Keys:
ADVERTISEMENTS:
Various types of keys are used in practice as shown in Fig. 43.2. Key is metallic piece inserted mating shaft and hole to prevent their relative motion. Pulleys, gears, fly wheels etc. are secured to the shaft by means of keys.
Key Way:
Keys are inserted in the key ways cut in the mating parts i.e., shaft and hub (or gears, pulleys, fly wheels etc.).
Main types of keys are:
(a) Sunk Keys:
Here the keys are inserted in the key ways such that they are half in the shaft and remaining half in the hub portion.
Various types under the sunk key category are:
(i) Parallel Key:
The key has uniform rectangular or square cross section throughout its entire length. Such keys are used for securing pulleys and gears.
(ii) Taper Key:
This is similar to the parallel sunk key excepting that it has taper in its thickness along the length. The taper may be 1 in 100. The width is uniform and only the thickness has the taper as shown.
If the key has thickness, width and length of say 10, 20 and 100 mm at one end its thickness at the other end will be 9 mm while all other dimensions will remain same if it has a taper of 1 in 100. Due to taper insertion and removal is easy and it also prevents relative circular and axial moment.
(iii) Gib Head Key:
It is taper sunk key with head at one end. The head makes it easy to remove the key. Such keys are used where the shaft and hub are required to remove frequently.
(iv) Feather Key:
It is a special type of parallel sunk key and is secured to the shaft by screws as shown. The key allows transmission of turning moment as well as axial force. It is generally used in drilling machines and clutches.
(v) Woodruff Key:
It has semicircular cross section which partly sits in similar key way in the shaft portion. It can be used to assembled to any shaft or hub with taper.
(vi) Round Key:
It has a circular cross section and fits into the circular key way in the shaft and hub. There are chances of slip under heavy loads. Therefore such keys are used for light loads.
(b) Saddle Keys:
Saddle keys are attached to the hub portion and just sit over the shaft. There is key way only in one mating portion generally the hub. These are used for light loads.
Basically there are two types:
(i) Flat Saddle:
The key is flat like parallel key which sits on flattened surface of shaft. There is key way in the hub. (Fig. 43.2f)
(ii) Hollow Saddle:
The lower side of the key is hollow i.e., it has the same contour as that of the shaft. The holding force is the friction only. (Fig. 43.2g)
3. Coupling and Their Types:
The arrangement used to connect two rotating shafts with each other is known as coupling.
The simplest type of coupling is muff coupling. This is used for two co-axial shafts. The ‘muff’, which is a hollow cylindrical piece of metal is keyed to both shafts and thus transmits the motion and power.
The muff may be manufactured in one piece or in two halves. In the latter case, coupling is known as split muff coupling.
Flanged Coupling:
Flange couplings are used to connect two co-axial shafts.
Flange is a circular disc with a bore at centre. A keyway is cut on the bore along its length. Bore diameter is slightly greater than shaft diameter. Numbers of holes are drilled on the flange. One flange each is keyed to the shaft, while the flanges are connected to each other with bolts.
The types are:
(1) Unprotected flange coupling –
(2) Protected type flange coupling. A projection is provided on the outer surface of each flange. This projection prevents any part of nut, bolt when broken, from moving away, because of centrifugal force and hence prevents the hazards.
If the load is fluctuating simple flange coupling will transfer these fluctuation to the prime mover. To avoid this, rubber bushes are put in the holes of one of the shaft. These bushes act as shock absorbers.
This is used to connect two parallel shafts with small distance between them.
Universal Coupling or Hooke’s Joint:
This type of coupling is used to connect two shafts having interesting axes. This type is widely used in transmitting power from crank shaft to the rear wheel axle in heavy automobiles.
In this type a fork is fixed to end of each shaft. Two forks are connected together with a cross head and pins.
The disadvantage is that we get non-uniform angular velocity at output. This is overcome by using two couplings in line.
4. Bearings and Their Types:
Bearing, as its name suggests, bears the load of shaft without offering any resistance to its rotational motion.
Bearings are used to support the shaft and to transfer the load to the ground. But if the shaft is supported on a rough surface, large amount of power will be lost in overcoming the friction. Hence in ‘bearing’ by means of proper ‘lubrication’, the direct contact between shaft and supporting surface is prevented. Thus the frictional losses are reduced by very large extent.
Normally two types of bearings are used:
(i) Journal Bearing:
In this type the Journal bearing or the ‘plummer block’ is lined with special material to reduce the friction. In this, the shaft theoretically makes line contact. Hence this type has high load bearing capacity.
Journal Bearings Applications:
Used for shafts for general purpose machinery with medium loads and medium speeds. For example, compressor shafts overhang transmission shafts, shafts used with group drives in floor mills.
(ii) Rolling Contact Bearings (Fig. 43.10):
In this type two concentric rings, known as races are used with balls or rollers in the annular space between them. The inner race is fixed to the shaft while outer race is held stationary in plummer block. The balls or rollers transfer the shaft load to outer race. This type is known as rolling contact bearing. This gives large reduction in frictional losses, but have relatively low load carrying capacity. Hence these are also known as antifriction bearings.
Rolling Contact Bearing Applications:
These are used in automobile wheels, or high speed machinery. We can design and select appropriate bearing for a given axial load, radial load and an expected life of bearing. These bearings are safe, as they make a squeaky noise before failing.
There are two types of thrust bearings viz.- foot step and collar thrust.
Foot step bearing is used to take the axial load (acting along its length) of the shaft.
It comprises of three parts. Two parts are identical circular discs having a groove along their diameters on one face. These discs are mounted on the shafts with keys. The third part is central disc. It is also circular disc of same size but it has two projections along its diameter but at right angle to each other on its either side. When these parts are assembled, transfer the rotating motion of one shaft to the other by means of sliding of the projection on central disc in the grooves in the end discs.
5. Flywheel:
Flywheel stores energy when more energy is being produced and gives back the same energy when less energy is being produced.
Or in the other words, “it is an energy reservoir, it stores energy when more energy is produced and gives back when less energy is produced, so that the crank shaft has uniform speed”. If no fly wheel is used then the working of the engine will be jerky.
In case of 4-stroke cycle engines there are four strokes suction, compression, power generation and exhaust. And power is produced only during the power stroke and no power is produced during suction, compression and exhaust strokes.
The energy produced during the power stroke will be stored in the flywheel in the form of inertia and is given back during other strokes the crack shaft so as to have the uniform speed.
As shown in Fig. 43.12(b) a wheel is mounted on the shaft and which is being rotated by hand. If the hand is removed wheel will continue rotating for some time depending upon energy imparted to it.
Flywheel Construction and Types:
Flywheels are made out of castings. Depending upon the diameter and speed they are classed into 3-types.
(i) Smaller flywheel with web and without arms for sizes upto 550-650 mm diameter.
(ii) Medium sized flywheel for sizes upto 2.25 m to 2.50 m dia. and arms provided as shown.
(iii) For sizes above 2.5 m diameter and for higher speeds split type of CI fly wheels are used as shown.