Miscellaneous Machine Components
In the previous chapters, we have discussed different types of machine components that are frequently used in a mechanism or a machine. Similar kinds of components with minor variations have been grouped together. In this chapter, we intend to explain the working principles of some selective components of steam and I.C. engines owing to their marvellous design and extensive usage in different industrial applications. Since some of the items are fairly complicated, solid model or pictorial views of the objects are also presented for a better understanding of the component.
Since a bearing is an important item for any engineering object and is an essential component for any mechanism or machine, it is discussed at length.
Bearings provide support for rotating shafts. Generally, two bearings, one at each end, are sufficient to support a shaft. However, for a long shaft mounted with a number of machine elements (such as gears, pulleys, wheels, and so on), intermediate supports or bearings are usually provided. Bearings are classified into two main categories—sliding bearings and antifriction bearings.
Sliding bearings may be further classified into journal bearings and thrust bearings. In a journal bearing, the bearing pressure is perpendicular to the axis of the shaft. The journal is the part of the shaft that comes in contact with the bearing. Journal bearings may be of various kinds—solid bearings, bush bearings, pedestal bearings, hanger bearings and so on.
FIG. 16.1 Sliding solid bearing
The simplest kind of sliding bearing is formed by drilling a hole in a supporting piece to receive the shaft (Fig. 16.1). In this bearing, the rotating shaft has a sliding contact with the bearing which is kept stationary. Since the friction between the mating parts is very high, an oil hole is provided at the top to lubricate the bearing. This form of bearing (known as solid bearing) has no provision for adjustment of wear. Hence they are rarely used in practice.
FIG. 16.2 Bush bearing
Bush bearings A bush bearing is a small modification of a solid bearing. A bush is pressed inside the bore of the bearing (made of cast iron). Its rotation is prevented by means of a grub screw or a dowel pin, as shown in Fig. 16.2. The bush is made of a softer material such as brass, bronze or gun metal. It undergoes wear and tear and can be replaced at regular intervals.
Pedestal bearings or Plummer block This type of bearing consists of i) a cast iron pedestal, ii) gun metal, or brass bush split into two halves called “brasses”, and iii) a cast iron cap and two mild steel bolts. The detailed drawing of a pedestal bearing is shown in Fig. 16.3. The rotation of the bush inside the bearing housing is arrested by a snug at the bottom of the lower brass. The cap is tightened on the pedestal block by means of bolts and nuts. The detailed part drawings of another plummer block with slightly different dimensions are also shown in Fig. 16.4.
Thrust bearings are used to support a load that acts along the axis of a shaft. The simplest form of this type of bearing is a collar thrust bearing (Fig. 16.5) consisting of one or more collars integral with the shaft. The axial load coming onto the shaft is actually withstood by the collars that are pressed against the bearing.
Rolling Contact Bearings
The term rolling contact bearing or antifriction bearing is employed to describe that class of bearing in which the main load is transferred through elements in rolling contact instead of sliding contact. In this type of a bearing, rolling friction is negligible in comparison to the sliding friction encountered in the bearings mentioned earlier. Therefore, the term “antifriction” is used throughout the industry though it is not correct.
The design of antifriction bearings is quite complicated since the designers must consider different factors, such as fatigue loading, friction, heat, corrosion resistance, kinematic problems, material properties, lubrication, assembly, bearing life, cost, and others to arrive at an optimum solution. There are different types of standardised antifriction bearings. Some of them are illustrated in Fig. 16.6. A particular type of bearing may be subdivided into different categories (say, light, medium, heavy) depending on the load and other conditions they are subjected to.
FIG. 16.3 Plummer block
FIG. 16.5 Thrust bearing
FIG. 16.6 Different types of antifriction bearings
Irrespective of the types, bearings consist of four major parts—an inner race, outer race, balls or rollers, and cage or separator.
The nomenclature of a rolling contact bearing is described in Fig. 16.7. In low-priced bearings, the separator is sometimes omitted although it has a vital role in separating the rolling elements to avoid rubbing contact between them.
Depending on the direction of loads coming onto the bearings, antifriction bearings can be classified as radial bearings and thrustbearings.
FIG. 16.7 Nomenclature of a ball bearing
Radial bearings This type of bearings are extensively used in the industry to withstand normal loads coming on to shafts. On the basis of the type of rolling elements being used, they may be further subdivided into ball bearings, roller bearings and taper bearings. Fig. 16.8 shows a sectional view of an assembled, self-aligned radial ball bearing mounted on a shaft. The inner race is shrunk fit onto the shaft by a locknut and a grub screw (not shown). The outer race is snugly fitted to the casing. The balls are retained in the cage made of sheet-steel pressed to shape. Sometimes the entire bearing is enclosed by two end-covers screwed to the housing by a number of screws as shown in the figure. Felt washers are also used to protect against contamination of dirt, fumes, and others. Nowadays, felt washers are being replaced by a seal of neoprene rubber. Grease or mineral oil is used as a lubricant, depending on the application.
FIG. 16.8 Self-aligned radial ball bearing
Thrust bearings These bearings are employed to support axial loads (Fig. 16.6). In general, balls are used as rolling elements although for heavy loads, rollers are also used. Frequently, it may so happen that the shaft is subjected to both axial and radial loads. In such a situation, a taper roller bearing is very useful as it can carry both these loads or any combination of the two. Fig. 16.9a exhibits a typical taper roller bearing with its outer race removed and placed at the top to have a clearer view of the inside of the bearing. The nomenclature of a taper roller bearing is somewhat different from that of a ball or straight roller bearings. The inner ring is referred to as cone and the outer ring as cup. Fig. 16.9b illustrates the application of taper roller bearings to support shafts carrying the gears of a gear box. Owing to the use of bevel gears, both axial and radial loads result and hence taper roller bearings are being used. Please note the fixation arrangement of the gears using collars, washer, and so on.
FIG. 16.9a Taper roller bearing
FIG. 16.9b Taper roller bearing arranged in a gear box
Selection of Bearings
Radial bearings are generally manufactured in three series—light, medium, and heavy—depending on the size of the shaft and the loads which the bearing is subjected to. The bearings may be made of double rows also so as to carry heavier radial or thrust load. Bearing manufacturers have formed their own numbering system to designate different bearings, based on the AFBMA standard which has established standard boundary dimensions, such as, bearing bore, outer diameter, width, fillet size on the shaft and housing shoulders, and so on. The entire plan is made very flexible so that different combinations of inside diameters, outside diameters and widths can be chosen to suit a particular application. In general, the bearings are identified by a two-digit number known as the dimension-series code. The first number in the code stands for the width series and second number for the outside diameter series. The detailed dimensions of the bearings are obtained from the table of the relevant dimension series.
Mounting of Antifriction Bearings
The methods of mounting of bearings on a shaft vary widely with the design of shaft, type of bearing, and nature of service required.
Different bearing manufacturers’ handbooks provide elaborate mounting details. They are mounted usually with rotating ring (inner or outer), a press fit, and stationary ring, a push fit. For that, ball bearings are heated in warm oil at about 750 °C to 800 °C and driven in position by light hammering with a soft metal hammer or mallet. Only the inner race must be hammered. The process is not so simple when the bearing has the added function of positioning or axially locating the shaft. The inner ring is backed up against the shaft-shoulder and is held in position by nuts threaded onto the shaft. Variation may be possible in the mounting arrangement, for example, the function of the shaft-shoulder may also be performed by the retaining ring, the hub of gear, or pulley or spacing rings.
The function of an engine, whether a steam engine or internal combustion (I.C.) engine, is to convert heat energy of the steam or fuel into useful mechanical power. For an overall idea of the operational principle of a steam engine, the essential components of the engine are represented by a line diagram in Fig. 16.10. The names of the individual parts are labelled in the diagram. Piston 2 is fixed to piston rod 3 and reciprocates in cylinder 1 actuated by the steam being admitted through ports 4 and 5 placed at the two ends of the cylinder. The admission and exhaust of steam are controlled by valves (not shown in the figure). The other end of the piston is connected to connecting rod 9 through crosshead 6 which moves along a straight line through guide 7. The other end of the connecting rod is attached to wheel 12. Thus, reciprocating motion of the piston 2 is converted to rotary motion of crankshaft 8. This arrangement is known as the slider crank mechanism. The crank is keyed to the crankshaft (not shown) on which the eccentric is also mounted. The eccentric gives motion to a slide valve that controls the flow of steam to and from the cylinder.
The steam engine has ceased to exist owing to its low efficiency. Still, we shall discuss some of the items for their ingenuous design and usage in other machinery, may be in a modified form.
FIG. 16.10 Essential components of a steam engine
Stuffing boxes are necessary to prevent leakage of working fluid between a sliding or turning pair of machine elements, for example, where the piston rod reciprocates through the cylinder cover of a steam engine or where a rotating shaft passes through a vessel containing fluid under high pressure.
The piston rod of a steam engine must move freely, yet there should not be any leakage of steam through the space between the rod and the cover. A gland and stuffing box arrangement serves both these purposes quite efficiently. Fig. 16.11 illustrates various parts of a stuffing box in assembled condition.
FIG. 16.11 Sectional drawing of stuffing box
The solid model of the stuffing box is presented in Fig. 16.12. Usually, the stuffing box is cast as an integral part of either the cylinder end or the cylinder cover. It is produced either from grey cast iron or cast alloys of iron. The internal machining of the stuffing box part is done by drilling and subsequent boring operation to fit the neck bush at one end and gland at the other. The bush must have a tight fit to prevent any rotation with respect to the cover or piston. Sometimes the gland is lined with a brass bush. Both the bushes can be replaced easily whenever they are worn out due to friction. The annular space created between the piston and the cover is filled with suitable packing materials compressed by a gland through studs and nuts, as shown in Fig. 16.11. The end of the bushes are bevelled at an angle of 15°–30° so that the packing is tightly pressed against the piston rod when downward forces are applied on the packing by the gland.
The materials used for packing are asbestos fibre, leather, natural or synthetic rubber, cork, and so on, depending on the type of fluid being handled. The soft packing is usually impregnated with a lubricating substance. An oil cup is provided on the top of the gland to lubricate the rod as shown in Fig. 16.12. The flange of the gland is oval in shape when two studs are used. In case of a large stuffing box, depending on the number of studs to be used, the shape may be triangular (three studs), square (four studs) or even circular (four or more studs).
FIG. 16.12 Solid model of stuffing box
As mentioned earlier, a connecting rod connects the crosshead of an engine to a crank. It is subjected to both axial and transverse forces, the former due to the piston load and the longitudinal inertia, the latter due to the transverse component of the inertia of the rod itself. Rods for high-speed engines are made of I-section for greater resistance to bending in the plane of ocillation and to reduce weight. In engines running at moderate speeds, the rods are generally circular. The two ends of the connecting rod are referred to as the “big” and “small” ends and have a variety of designs depending on the type of bearings being used at the crank and crosshead.
Locomotive connecting rods are steel forged of rectangular or I-section, usually about six times the crank length. The rod is usually of uniform width and thickness but the depth decreases towards the small ends. Fig. 16.13 shows one of the views of the connecting rod indicating overall dimensions.
FIG. 16.13 Locomotive connecting rod
The isometric view of the small end and wedge are illustrated in Fig. 16.14. The reader can get a clear idea of the arrangement of the bearings at the two ends.
The bearings (also known as brasses), split into halves, are secured to the ends of the rods by steel straps (Fig. 16.14). The adjustment for wear due to the friction between the brass and the crankpin or crosshead pin are provided by the tapering of the cotter and wedge. The small end's brasses are made of gun metal; those for the large end are of gun metal as well but with white metal pads. The cotters bear on the end of the rod and on the wedge. They do not touch the straps. They are prevented from slackening by set-screws. A projection is made on the strap with provision to fit the lubricating cup to facilitate lubrication at the pin.
FIG. 16.14 Small end of locomotive connecting rod
The drawing of the large-end bearing is more or less similar to that of the small end. The brasses are made thicker at the two sides since friction and wear will be greater at the sides than at the top and bottom. The big-end brasses are provided with flanges to prevent any axial movement.
The dimensions of the rod are based on empirical relationship and are checked finally for strength. The proportions of the ends are largely governed by the size of the bearings.
An eccentric converts the rotating motion of the crankshaft into a short reciprocating motion of the slide value of a steam engine. The reverse operation (reciprocating motion to rotary motion) is not advisable due to excessive friction between the moving parts (sheave and strap). Fig. 16.15 illustrates the various parts of an eccentric. It consists of a sheave and two straps, all made of cast iron. The sheave is in the form of circular disc with a stepped rim at the outer edge. It is secured onto the shaft by means of a key. The sheave rotates eccentrically as the shaft hole is placed eccentrically within the sheave.
FIG. 16.15 Eccentric
The two straps are semicircular elements with an annular recess cut on the inside radial surfaces to accommodate the projected rim of the sheave. The eccentric rod (not shown) is connected to the straps through the two studs and nuts. Thus the rotary motion of the sheave is converted to reciprocating motion of the eccentric rod which actuates the value motion. Packing pieces or shims are employed between the two straps, for adjustment for wear at a later period.
The distance between the axes of the shaft and the sheave (25 mm in Fig. 16.15) is referred to as the amount of eccentricity and is equal to half the travel (throw) of the valve. Eccentrics are also used for operating small pump plungers, shakers, and so on.
INTERNAL COMBUSTION ENGINE
In an internal combustion engine, the fuel burns within the cylinder and generates heat energy which is converted into mechanical energy through the movement of the piston, connecting rod, and crankshaft. A detailed description of an I.C. engine is beyond the scope of this book. However, to understand the operation of the engine, a diagram is presented in Fig. 16.16 indicating its essential components.
Unlike in a steam engine, fuel is burnt on one side of the cylinder and hence the piston is directly connected to the crank by means of a connecting rod. Due to design optimisation, the piston is made hollow with one end open and attached to the connecting rod with the help of a gudgeon pin, also known as a piston pin. Fuel is injected in the cylinder through the inlet port and exits after burning through the exhaust port. The inlet and outlet flow are controlled through the respective valves. In a petrol engine, a spark plug ignites the fuel at the appropriate time. This entire arrangement of fuel injection and exhaust is made in the cylinder head as shown in Fig. 16.16.
FIG. 16.16 Nomenclature of I.C. engine
In the following section, we shall discuss two of the major parts, namely, piston and connecting rod, of an I.C. engine, to get a basic idea of these two complicated, yet important components.
A piston is a cylindrical piece that reciprocates within a cylinder, being actuated by the force exerted on it due to the burning of fuel. The piston for I.C. engines may take different forms depending on the type of engines—petrol, diesel, gasoline, and so on.
The high temperature range to which the piston is subjected makes its design a critical issue for engineers. The piston is made slightly smaller than the bore of the cylinder. Leakage of fluid past the piston is prevented by fitting split metal rings (not shown), known as piston rings, into the grooves (Fig. 16.17) provided on the outside surface of the piston.
The manufacturing and subsequent assembly of piston rings into the piston grooves require special technique. Each of the rings is machined to a diameter larger than that of the cylinder bore, and a gap is cut across it. The ends are then drawn together and outside diameter of the piston is machined again making it equal to the diameter of the cylinder bore. When the rings are placed into the grooves, they spring out and exert a constant pressure on the cylinder wall, thus preventing the leakage of fluid past the cylinder. Both the piston ring and cylinder bore demand very high level of surface finish and tolerance; hence, sophisticated machines are employed for this purpose. The nominal dimensions of piston ring grooves are shown in the blown up view.
Fig. 16.17 shows the construction of a typical aluminium alloy piston. Petrol engine pistons are generally die-cast. The top portion of the piston is reffered to as a piston head and the portion below the ring grooves is known as the skirt. The piston is attached to the connecting rod by means of a gudgeon pin made of nickel steel, hardened and ground.
The hole for the gudgeon pin is shown in Fig. 16.17. An abrupt variation in the surface contour of a casting may lead to a crack due to thermal differentials. Hence, it is essential to avoid all sharp edges in the design as cast. Another piston with a slightly modified design is shown in Fig. 16.18.
FIG. 16.17 Aluminium alloy piston of an I.C. engine
FIG. 16.18 Piston for an internal combustion engine
The connecting rod joins the gudgeon pin inserted in the piston to the crank pin of the crankshaft. Fig. 16.19 presents the general design and principal dimensions of a steel connecting rod for an oil engine piston. The small-end bush (not shown) is made of G.M. sunk into the eye. The large-end bearing is of C.S.lined with W.M. and is shown only in outline.
The corresponding piston is already shown in Fig. 16.17. The gudgeon pin of 79 mm dia has to be prevented from turning and shaped cover plates have to be bolted to the piston to provide oil-tight joints at the pin ends.
FIG. 16.19 Details of a connecting rod
TOOLHEAD OF A SHAPING MACHINE
The toolhead of a shaping machine is designed to hold the cutting tool. It is fixed to the front face of the ram of a shaper as shown in Fig. 16.20a. The ram reciprocates and the tool removes material only in the forward stroke and drags on the work piece during the return stroke. Therefore, it is essential to provide some relief arrangement to the cutting tool during its return stroke so that it does not rub the already finished surface of the material. This is achieved by providing a drag release plate hinged at its top, as shown in Fig. 16.20b. The figure illustrates detailed drawings of the individual components of the toolhead. To understand the basic working principle, an isometric illustration of the assembly is also presented. Its major parts—swivel plate and drag release plate—are made of C.I., while the tool holder, swivel pin and clamping screw are made of M.S. The actual tool is placed within the elliptical slot between the hardened steel washer and the tightening screw. The tool holder is fitted in the swivel plate which in turn is fitted to the swivel plate by means of a swivel pin about the axis of which the swivel plate can be swivelled to any desired angle to facilitate inclined shaping operations. The swivel plate is clamped to the vertical slide with the help of a clamping screw. The extent of the circular slot on either side of the vertical line limits the swivel angle.
FIG. 16.20 Details of toolhead of a shaping machine
TAILSTOCK OF A LATHE
The lathe is the father of all machine tools—it is the oldest tool recorded in the history of mankind. Previously, it was extensively used for turning of wood. For its present form, we owe much to Henry Mandsley, who first introduced the sliding carriage, and built a screw cutting lathe, way back in 1800. The pictorial view of a lathe is displayed in Fig. 16.21a where the tailstock is situated to the right of the picture.
FIG. 16.21a Lathe
During its operation, the lathe holds a piece of material either between two rigid supports, called centers, or by other devices such as a chuck or face plate. The spindle carrying the work is rotated whilst a cutting tool supported in a tool-post is allowed to travel in a certain direction to remove material from the work. The head stock usually carries a dog-chuck, faceplate, or center and is fixed to the left hand end of the lathe bed.
FIG. 16.21b Tailstock detail
Tailstock is the counterpart of the head stock and carries the right hand center for supporting work when turning on centers. It is also used for supporting and feeding drills, reamers, and so on. The tailstock can slide on the lathe bed and can be clamped in any desired position to accommodate different jobs of varying length. Fig. 16.21b illustrates the detailed drawing of a tailstock used on a light duty lathe. Materials used for different parts are also mentioned in the figure. The body or carriage of the tailstock is placed with four feet to fit into the parallel machined ways of the lathe bed. The body developed in solid model is also shown in Fig. 16.22. The upper part of the body is made hollow to receive the barrel that carries the center at one end in line with the spindle. The other portion of the barrel is provided with threads in which the screw or spindle moves (Fig. 16.21b). The barrel has a keyway cut in it. Rotation of the barrel is prevented by inserting the key at the appropriate place. When the spindle is turned by the handwheel, it rotates about its axis. However, it is not allowed to move along the axis. As a result, the barrel will slide within the body along the axis.
FIG. 16.22 Solid model of the tailstock body
Fig. 16.23 shows the assembly view of another tailstock in which the left hand view shows the profile of the body and the invisible part from left is shown by dotted lines in the sectional view (Section A-A). It may be pointed out that the design of the detailed drawing (Fig. 16.21b) is slightly different from the assembled drawing (Fig. 16.23) as the handle is not present in the first case.
FIG. 16.23 Tailstock assembly
- Describe, with the aid of sketches, the different types of bearings used in industry.
- What do you understand by antifriction bearing?
- When do you use thrust bearing?
- Draw the assembled views of the pedestal bearing from the detailed drawings of the components shown in Fig. 16.4.
- Draw the detailed drawing of the plummer block shown in Fig. 16.3. One of the views of each component should be in half section.
- Develop the solid model of the plummer block cap shown in Fig. 16.4.
- Draw the sectional front view, side view, and top view of the radial ball bearing shown in Fig. 16.8.
- Explain, with a sketch, the function of a taper roller bearing.
- Draw the half-sectional front view, top view, and right hand side view of a stuffing box shown in Fig. 16.11.
- Develop the solid model of the stuffing box shown in Fig. 16.12.
- Draw the two views of a connecting rod for a steam engine shown in Fig. 16.13.
- Draw three views of the small end of a connecting rod shown in Fig. 16.14.
- Draw the detailed drawings of the eccentric shown in Fig. 16.15.
- Develop the solid model of the eccentric sheave and strap shown in Fig. 16.15.
- Draw the half-sectional front view, top view, and side view of the piston shown in Fig. 16.17.
- Draw the three views of the connecting rod (Fig. 16.19) and piston (Fig. 16.17) assembly of an I.C. engine.
- Develop the solid model of the aluminium connecting rod shown in Fig. 16.19.
- Describe the essential components of a steam engine by means of a line diagram.
- Draw the sectional front view, side view, and top view of a tool head in assembled condition.
- Develop the solid model of the tool holder of the tool head of a shaping machine. Take the necessary dimensions from Fig. 16.20.
- Draw the assembly views (three views) of a tail stock from the detailed drawings shown in Fig. 16.21.
- Draw the detailed drawing of the tail stock from its assembly view shown in Fig. 16.22. For each component, one of the views must be in full section.