Fig. 25.Fig. 25.
An example of a very simple form of bearing is shown in fig. 25, which represents a brake shaft carrier of a locomotive tender. The bearing in this example is made of cast iron and in one piece. Through the oval-shaped flange two bolts pass for attaching the bearing to the wrought-iron framing of the tender. With this form of bearing there is no adjustment for wear, so that when it becomes worn it must be renewed.
Exercise25:Brake Shaft Carrier.—Draw the elevation and sectional plan of the bearing shown in fig. 25. Draw also a vertical section through the axis. The latter view to be projected from the first elevation. Scale 6 inches to a foot.
Exercise25:Brake Shaft Carrier.—Draw the elevation and sectional plan of the bearing shown in fig. 25. Draw also a vertical section through the axis. The latter view to be projected from the first elevation. Scale 6 inches to a foot.
Pillow Block,Plummer Block, orPedestal.—The ordinary form of plummer block is represented in fig. 26. A is the block proper, B the sole through which pass the holding-down bolts. C is the cap. Between the block and the cap is the brass bush, which is in halves, calledbrassesorsteps. The bed for the steps in this example is cylindrical, and is prepared by the easy process of boring. The steps are not supported throughout their whole length, but at their ends only where fitting strips are provided as shown. As the wear on a step is generally greatest at the bottom, it is made thicker there than at the sides, except where the fitting strips come in. To prevent the steps turning within the block they are generally furnished with lugs, which enter corresponding recesses in the block and cover.
Fig. 26.Fig. 26.
In the block illustrated the journal is lubricated by aneedle lubricator; this consists of an inverted glass bottle fitted with a wood stopper, through a hole in which passes a piece of wire, which has one end in the oil within the bottle, and the other resting on the journal of the shaft. The wire or needle does not fill the hole in the stopper, but if the needle is kept from vibrating the oil does not escape owing to capillary attraction. When, however, the shaft rotates, the needle begins to vibrate, and the oil runs down slowly on to the journal; oil is therefore only used when the shaft is running.
Exercise26:Pillow Block for a Four-inch Shaft.—Draw the views shown of this block in fig. 26. Make also separate drawings, full size, of one of the steps. Scale 6 inches to a foot.
Exercise26:Pillow Block for a Four-inch Shaft.—Draw the views shown of this block in fig. 26. Make also separate drawings, full size, of one of the steps. Scale 6 inches to a foot.
Proportions of Pillow Blocks.—The following rules may be used for proportioning pillow blocks for shafts up to 8 inches diameter. It should be remembered that the proportions used by different makers vary considerably, but the following rules represent average practice.
Diameter of journal=d.Length of journal=l.Height to centre= 1·05d+ ·5.Length of base= 3·6d+ 5.Width of base= ·8l.” block= ·7l.Thickness of base= ·3d+ ·3.” cap= ·3d+ ·4.Diameter of bolts= ·25d+ ·25.Distance between centres of cap bolts= 1·6d+ 1·5.” ” base bolts= 2·7d+ 4·2.Thickness of step at bottom=t= ·09d+ ·15.” ” sides= ¾t.
The length of the journal varies very much in different cases, and depends upon the speed of the shaft, the load which it carries, the workmanship of the journal and bearing, and the method of lubrication. For ordinary shafting one rule is to makel=d+ 1. Some makers use the rulel= 1·5d; others makel= 2d.
Fig. 27.Fig. 27.
Fig. 28.Fig. 28.
Exercise27:Design for Pillow Block.—Make the necessary working drawings for a pillow block for a shaft 5 inches in diameter, and having a journal 7 inches long.
Exercise27:Design for Pillow Block.—Make the necessary working drawings for a pillow block for a shaft 5 inches in diameter, and having a journal 7 inches long.
Brackets.—When a pillow block has to be fixed to a wall or column a bracket such as that shown in figs. 27 and 28 may be used. The pillow block rests between thejogglesA A, and is bolted down to the bracket and secured in addition with keys at the ends of the base of the block, in the samemanner as is shown, for the attachment of the bracket to the column.
Exercise28:Pillar Bracket.—Fig. 27 shows a side elevation and part horizontal section, and fig. 28 shows an end elevation of a pillar bracket for carrying a pillow block for a 3-inch shaft. Draw these viewsproperly projected from one another, showing the pillow block, which is to be proportioned by the rules given on page 32. Draw also a plan of the whole. Scale 4 inches to a foot.
Exercise28:Pillar Bracket.—Fig. 27 shows a side elevation and part horizontal section, and fig. 28 shows an end elevation of a pillar bracket for carrying a pillow block for a 3-inch shaft. Draw these viewsproperly projected from one another, showing the pillow block, which is to be proportioned by the rules given on page 32. Draw also a plan of the whole. Scale 4 inches to a foot.
Hangers.—When a shaft is suspended from a ceiling it is carried by hangers, one form of which is shown in fig. 29, and which will be readily understood. The cap of the bearing, it will be noticed, is secured by means of a bolt, and also by a square key.
Exercise29:Shaft Hanger.—Draw the two elevations shown in fig. 29, and also a sectional plan. The section to be taken at a point 5 inches above the centre of the shaft. Scale 6 inches to a foot.
Exercise29:Shaft Hanger.—Draw the two elevations shown in fig. 29, and also a sectional plan. The section to be taken at a point 5 inches above the centre of the shaft. Scale 6 inches to a foot.
Wall Boxes.—In passing from one part of a building to another a shaft may have to pass through a wall. In that case a neat appearance is given to the opening and a suitable support obtained for a pillow block by building into the wall awall box, one form of which is shown in fig. 30.
Exercise30:Wall Box.—Draw the views of the wall box shown in fig. 30, and also a sectional plan; the plane of section to pass through the box a little above the joggles for the pillow block. Scale 3 inches to a foot.
Exercise30:Wall Box.—Draw the views of the wall box shown in fig. 30, and also a sectional plan; the plane of section to pass through the box a little above the joggles for the pillow block. Scale 3 inches to a foot.
Fig. 29.Fig. 29.
Fig. 30.Fig. 30.
Velocity Ratio in Belt Gearing.—Let two pulleys A and B be connected by a belt, and let their diameters be D1and D2; and let their speeds, in revolutions per minute, be N1and N2respectively. If there is no slipping, the speeds of the rims of the pulleys will be the same as that of the belt, and will therefore be equal. Now the speed of the rim of A is evidently = D1× 3·1416 × N1; while the speed of the rim of B is = D2× 3·1416 × N2. Hence D1× 3·1416 × N1= D2× 3·1416 × N2, and therefore
N1——N2=D2——.D1
Pulleys for Flat Bands.—In cross section the rim of a pulley for carrying a flat band is generally curved as shown in figs. 31 and 32, but very often the cross section is straight. The curved cross section of the rim tends to keep the band from coming off as long as the pulley is rotating. Sometimes the rim of the pulley is provided with flanges which keep the band from falling off.
Pulleys are generally made entirely of cast iron, but a great many pulleys are now made in which the centre or nave only is of cast iron, the arms being of wrought iron cast into the nave, while the rim is of wrought sheet iron.
The arms of pulleys when made of wrought iron are invariably straight, but when made of cast iron they are very often curved. In fig. 31, which shows an arrangement of two cast-iron pulleys, the arms are straight; while in fig. 32, which shows another cast-iron pulley, the arms are curved. Through unequal cooling, and therefore unequal contraction of a cast-iron, pulley in the mould, the arms are generally in a state of tension or compression; and if the arms are straight they are very unyielding, so that the result of this initial stress is often the breaking of an arm, or of the rim where it joins an arm. With the curved arm, however, its shape permits it to yield, and thus cause a diminution of the stress due to unequal contraction.
The cross section of the arms of cast-iron pulleys is generally elliptical.
Fig. 31.Fig. 31.
Exercise 31:Fast and Loose Pulleys.—Fig. 31 shows an arrangement of fast and loose pulleys. A is the fast pulley, secured to the shaft C by a sunk key; B is the loose pulley, which turns freely upon the shaft. The loose pulley is prevented from coming off by a collar D, which is secured to the shaft by a tapered pin as shown. The nave or boss of the loose pulley is here fitted with a brass liner, which may be renewed when it becomes too much worn. Draw the elevations shown, completing the left-hand one. Scale 6 inches to a foot.By the above arrangement of pulleys a machine may be stopped or set in motion at pleasure. When the driving band is on the loose pulley the machine is at rest, and when it is on the fast pulley the machine is in motion. The driving band is shifted from the one pulley to the other by pressing on that side of the band which is advancing towards the pulleys.
Exercise 31:Fast and Loose Pulleys.—Fig. 31 shows an arrangement of fast and loose pulleys. A is the fast pulley, secured to the shaft C by a sunk key; B is the loose pulley, which turns freely upon the shaft. The loose pulley is prevented from coming off by a collar D, which is secured to the shaft by a tapered pin as shown. The nave or boss of the loose pulley is here fitted with a brass liner, which may be renewed when it becomes too much worn. Draw the elevations shown, completing the left-hand one. Scale 6 inches to a foot.
By the above arrangement of pulleys a machine may be stopped or set in motion at pleasure. When the driving band is on the loose pulley the machine is at rest, and when it is on the fast pulley the machine is in motion. The driving band is shifted from the one pulley to the other by pressing on that side of the band which is advancing towards the pulleys.
Fig. 32.Fig. 32.
Exercise 32:Cast-iron Pulley with Curved Arms and Cone Keys.—Draw a complete side elevation and a complete cross section of the pulley represented in fig. 32 to a scale of 3 inches to a foot. In drawing the side elevation of the arms first draw the centre lines as shown; next draw three circles for each arm, one at each end and one in the middle; the centres of these circles being on the centre line of the arm, and their diameters equal to the widths of the arm at the ends and at the middle respectively. Arcs of circles are then drawn to touch these three circles. The centres and radii of these arcs may be found by trial. The cone keys for securing the pulley to the shaft were described on p. 23.
Exercise 32:Cast-iron Pulley with Curved Arms and Cone Keys.—Draw a complete side elevation and a complete cross section of the pulley represented in fig. 32 to a scale of 3 inches to a foot. In drawing the side elevation of the arms first draw the centre lines as shown; next draw three circles for each arm, one at each end and one in the middle; the centres of these circles being on the centre line of the arm, and their diameters equal to the widths of the arm at the ends and at the middle respectively. Arcs of circles are then drawn to touch these three circles. The centres and radii of these arcs may be found by trial. The cone keys for securing the pulley to the shaft were described on p. 23.
Pulleys for Ropes.—Ropes made of hemp are now extensively used for transmitting power. These ropes vary in diameter from 1 inch to 2 inches, and are run at a speed of about 4,500 feet per minute. The pulleys for these ropes are made of cast iron, and have their rims grooved as shown in fig. 33, which is a cross section of the rim of a pulley carrying three ropes. The angle of the V is usually 45°, and the roperests on the sides of the groove, and not on the bottom, so that it is wedged in, and has therefore a good hold of the pulley. The diameter of the pulley should not be less than 30 times the diameter of the rope. Two pulleys connected by ropes should not be less than thirty feet apart from centre to centre, but this distance may be as much as 100 feet.
Fig. 33.Fig. 33.
Exercise 33:Section of Rim of Rope Pulley.—Draw, half size, the section of the rim of a rope pulley shown in fig. 33.
Exercise 33:Section of Rim of Rope Pulley.—Draw, half size, the section of the rim of a rope pulley shown in fig. 33.
Pitch Surfaces of Spur Wheels.—Let two smooth rollers be placed in contact with their axes parallel, and let one of them rotate about its axis; then if there is no slipping the other roller will rotate in the opposite direction with the same surface velocity; and if D1, D2be the diameters of the rollers, and N1, N2their speeds in revolutions per minute, it follows as in belt gearing that—
N1——N2=D2——.D1
If there be considerable resistance to the motion of the follower slipping may take place, and it may stop. To prevent this the rollers may be provided with teeth; then they becomespur wheels; and if the teeth be so shaped that the ratio of the speeds of the toothed rollers at any instant is the same asthat of the smooth rollers, the surfaces of the latter are called thepitch surfacesof the former.
Pitch Circle.—A section of the pitch surface of a toothed wheel by a plane perpendicular to its axis is a circle, and is called apitch circle. We may also say that the pitch circle is the edge of the pitch surface. The pitch circle is generally traced on the side of a toothed wheel, and is rather nearer the points of the teeth than the roots.
Pitch of Teeth.—The distance from the centre of one tooth to the centre of the next, or from the front of one to the front of the next,measured at the pitch circle, is called thepitch of the teeth. If D be the diameter of the pitch circle of a wheel,nthe number of teeth, andpthe pitch of the teeth, then D × 3·1416 =n×p.
Fig. 34.Fig. 34.
By the diameter of a wheel is meant the diameter of its pitch circle.
Form and Proportions of Teeth.—The ordinary form of wheel teeth is shown in fig. 34. The curves of the teeth should be cycloidal curves, although they are generally drawn in as arcs of circles. It does not fall within the scope of this work to discuss the correct forms of wheel teeth. The student will find the theory of the teeth of wheels clearly and fully explained in Goodeve's 'Elements of Mechanism,' and in Unwin's 'Machine Design.'
The following proportions for the teeth of ordinary toothed wheels may be taken as representing average practice:—
Pitch of teeth=p= arca b c(fig. 34).Thickness of tooth=b c= ·48p.Width of space=a b= ·52p.Total height of tooth=h= ·7p.Height of tooth above pitch line=k= ·3p.Depth of tooth below pitch line=l= ·4p.Width of tooth= 2pto 3p.
Exercise34:Spur Wheel.—Fig. 35 shows the elevation and sectional plan of a portion of a cast-iron spur wheel. The diameter of the pitch circle is 237⁄8inches, and the pitch of the teeth is 1½ inches, so that there will be 50 teeth in the wheel. The wheel has six arms. Draw a complete elevation of the wheel and a half sectional plan, also a half-plan without any section. Draw also a cross section of one arm. Scale 4 inches to a foot.
Exercise34:Spur Wheel.—Fig. 35 shows the elevation and sectional plan of a portion of a cast-iron spur wheel. The diameter of the pitch circle is 237⁄8inches, and the pitch of the teeth is 1½ inches, so that there will be 50 teeth in the wheel. The wheel has six arms. Draw a complete elevation of the wheel and a half sectional plan, also a half-plan without any section. Draw also a cross section of one arm. Scale 4 inches to a foot.
Fig. 35.Fig. 35.
Mortise Wheels.—When two wheels gearing together run at a high speed the teeth of one are made of wood. These teeth, or cogs, as they are generally called, have tenons formed on them, which fit into mortises in the rim of the wheel. This wheel with the wooden teeth is called amortise wheel. An example of a mortise wheel is shown in fig. 36.
Fig. 36.Fig. 36.
Bevil Wheels.—In bevil wheels the pitch surfaces are parts of cones. Bevil wheels are used to connect shafts which are inclined to one another, whereas spur wheels are used to connect parallel shafts. In fig. 36 is shown a pair of bevil wheels in gear, one of them being a mortise wheel. At (a) is a separate drawing, to a smaller scale, of the pitch cones. The pitch cones are shown on the drawing of the complete wheels by dotted lines.
The diameters of bevil wheels are the diameters of the bases of their pitch cones.
Exercise35:Pair of Bevil Wheels.—Draw the sectional elevation of the bevil wheels shown in gear in fig. 36. Commence by drawing the centre lines of the shafts, which in this example are at right angles to one another; then draw the pitch cones shown by dotted lines. Next put in the teeth which come into the plane of the section, then complete the sections of the wheels. The pinion or smaller wheel has 25 teeth, and the wheel has 50 teeth, which makes the pitch a little over 3 inches. Each tooth of the mortise wheel is secured as shown by an iron pin5⁄16inch diameter. Scale 3 inches to a foot.
Exercise35:Pair of Bevil Wheels.—Draw the sectional elevation of the bevil wheels shown in gear in fig. 36. Commence by drawing the centre lines of the shafts, which in this example are at right angles to one another; then draw the pitch cones shown by dotted lines. Next put in the teeth which come into the plane of the section, then complete the sections of the wheels. The pinion or smaller wheel has 25 teeth, and the wheel has 50 teeth, which makes the pitch a little over 3 inches. Each tooth of the mortise wheel is secured as shown by an iron pin5⁄16inch diameter. Scale 3 inches to a foot.
The most important application of the crank is in the steam-engine, where the reciprocating rectilineal motion of the piston is converted into the rotary motion of the crank-shaft by means of the crank and connecting rod.
At one time steam-engine cranks were largely made of cast iron, now they are always made of wrought iron or steel. The crank is either forged in one piece with the shaft, or it is made separately and then keyed to it.
Overhung Crank.—Fig. 37 shows a wrought-iron overhung crank. A is the crank-shaft, B the crank arm, provided at one end with a boss C, which is bored out to fit the shaft; at the other end of the crank arm is a boss D, which is bored out to receive the crank-pin E, which works in one end of the connecting rod. The crank is secured to the shaft by thesunk key F. It is also good practice toshrinkthe crank on to the shaft. The process of shrinking consists of boring out the crank a little smaller than the shaft, and then heating it, which causes it to expand sufficiently to go on to the shaft. As the crank cools, it shrinks and grips the shaft firmly. The crank may also be shrunk on to the crank-pin, the latter being then riveted over as shown in fig. 37.
Fig. 37.Fig. 37.
A good plan to adopt in preference to the shrinking process is to force the parts together by hydraulic pressure. This method is adopted for placing locomotive wheels on their axles, and for putting in crank-pins. As to the amount of pressure to be used, the practice is to allow a force of 10 tons for every inch of diameter of the pin, axle, or shaft.
Instead of being riveted in, the crank pin may be prolonged and screwed, and fitted with a nut. Another plan is to put a cotter through the crank and the crank-pin.
The distance from the centre of the crank-shaft to the centre of the crank-pin is called the radius of the crank. Thethrowof the crank is twice the radius. In a direct-actingengine the throw of the crank is equal to the stroke of the piston.
Exercise36:Wrought-iron Overhung Crank.—Draw the two elevations shown in fig. 37, also a plan. Scale 1½ inches to a foot.Proportions of Overhung Cranks.D = diameter of shaft.d= ” crank-pin.Length of large boss= ·9 D.Diameter ”= 1·8 D.Length of small boss= 1·1d.Diameter ”= 1·8d.Width of crank arm at centre of shaft= 1·3 D.” ” crank-pin= 1·5d.The thickness of the crank arm may be roughly taken as = ·7 D.Exercise37.—Design a wrought-iron crank for an engine having a stroke of 4 feet. The crank-shaft is 9 inches in diameter, and the crank-pin is 4¾ inches in diameter and 6½ inches long.
Exercise36:Wrought-iron Overhung Crank.—Draw the two elevations shown in fig. 37, also a plan. Scale 1½ inches to a foot.
Proportions of Overhung Cranks.
D = diameter of shaft.d= ” crank-pin.Length of large boss= ·9 D.Diameter ”= 1·8 D.Length of small boss= 1·1d.Diameter ”= 1·8d.Width of crank arm at centre of shaft= 1·3 D.” ” crank-pin= 1·5d.
The thickness of the crank arm may be roughly taken as = ·7 D.
Exercise37.—Design a wrought-iron crank for an engine having a stroke of 4 feet. The crank-shaft is 9 inches in diameter, and the crank-pin is 4¾ inches in diameter and 6½ inches long.
Fig. 38.Fig. 38.
Locomotive Cranked Axle.—As an example of a cranked shaft we take the cranked axle for a locomotive with inside cylinders shown in fig. 38; here the crank and shaft or axle are forged in one piece. A is the wheel seat, B the journal, C the crank-pin, and D and E the crank arms. Only one half of the axle is shown in fig. 38, but the other half is exactly the same. The cranks on the two halves are, however, at right angles to one another. The ends of the crank arms are turned in the lathe, the crank-pin ends being turned at the same timeas the axle, and the other ends at the same time as the crank-pin. This consideration determines the centres for the arcs shown in the end view.
Exercise38.—Draw to a scale of 2 inches to a foot the side and end elevations of the locomotive cranked axle partly shown in fig. 38. The distance between the centre lines of the cylinders is 2 feet.
Exercise38.—Draw to a scale of 2 inches to a foot the side and end elevations of the locomotive cranked axle partly shown in fig. 38. The distance between the centre lines of the cylinders is 2 feet.
Fig. 39.Fig. 39.
Built-up Cranks.—The form of cranked shaft shown in fig. 38 is largely used for marine engines, but for the very powerful engines now fitted in large ships this design of shaft is very unreliable, the built-up crank shown in fig. 39 being preferred, although it is much heavier than the other. It will be seen from the figure that the shaft, crank arms, and crank-pin are made separately. The arms are shrunk on to the pin and the shaft, and secured to the latter by sunk keys. These heavy shafts and cranks are generally made of steel.
Exercise39.—Keeping to the dimensions marked in fig. 39, draw the views there shown of a built-up crank-shaft for a marine engine. Scale3⁄4inch to a foot.
Exercise39.—Keeping to the dimensions marked in fig. 39, draw the views there shown of a built-up crank-shaft for a marine engine. Scale3⁄4inch to a foot.
Theeccentricis a particular form of crank, being a crank in which the crank-pin is large enough to embrace the crank-shaft. In the eccentric what corresponds to the crank-pin is called the sheave or pulley. The advantage which an eccentric possesses over a crank is that the shaft does not require to be divided at the point where the eccentric is put on. The crank, however, has this advantage over the eccentric, namely, that it can be used for converting circular into reciprocating motion, orvice versâ, while the eccentric can only be used for converting circular into reciprocating motion. This is owing to the great leverage at which the friction of the eccentric acts.
The chief application of the eccentric is in the steam-engine, where it is used for working the valve gear.
To permit of the sheave being placed on the shaft without going over the end (which could not be done at all in the case of a cranked axle, and would be a troublesome operation in most cases) it is generally made in two pieces, as shown in fig. 40, which represents one of the eccentrics of a locomotive. The two parts of the sheave are connected by two cotter bolts. The part which embraces the sheave is called the eccentric strap, and corresponds to, and is, in fact, a connecting rod end: the rod proceeding from this is called the eccentric rod.
The distance from the centre of the sheave to the centre of the shaft is called theradiusoreccentricityof the eccentric. Thethrowis twice the eccentricity.
The sheave is generally made of cast iron. The strap may be of brass, cast iron, or wrought iron; when the strap is made of wrought iron it is commonly lined with brass.
Exercise40:Locomotive Eccentric.—In fig. 40 D E is the sheave, F H the strap, and K the eccentric rod. The sheave and strap are made of cast iron, and the eccentric rod is made of wrought iron. (a) is a vertical cross section through the oil-box of the strap; (b) is a plan of the end of the eccentric rod and part of thestrap. All the nuts are locked by means of cotters. Draw first the elevation, partly in section as shown. Next draw two end elevations, one looking each way. Afterwards draw a horizontal section through the centre, and also a plan. Scale 4 inches to a foot.
Exercise40:Locomotive Eccentric.—In fig. 40 D E is the sheave, F H the strap, and K the eccentric rod. The sheave and strap are made of cast iron, and the eccentric rod is made of wrought iron. (a) is a vertical cross section through the oil-box of the strap; (b) is a plan of the end of the eccentric rod and part of thestrap. All the nuts are locked by means of cotters. Draw first the elevation, partly in section as shown. Next draw two end elevations, one looking each way. Afterwards draw a horizontal section through the centre, and also a plan. Scale 4 inches to a foot.
Fig. 40.Fig. 40.
The most familiar example of the use of a connecting rod is in the steam-engine, where it is used to connect the rotating crank with the reciprocating piston. The rod itself is made of wrought iron or steel, and is generally circular or rectangular in section. The ends of the rod are fitted with steps, which are held together in a variety of ways.
Strap End.—A form of connecting rod end, which is not so common as it used to be, is shown in fig. 41. At (a) is shown a longitudinal section with all the parts put together, while at (b), (c),(d)and (e) the details are shown separately. A B is the end of the rod which butts against the brass bush C D, which is in two pieces. AstrapE passes round the bush and on to the end of the rod as shown. The arms of the strap have rectangular holes in them, which are not quite opposite a similar hole in the rod when the parts are put together. If a wedge orcotterF be driven into these three holes they will tend to come into line, and the parts of the bush will be pressed together. To prevent the cotter opening out the strap, and to increase the sliding surface, agibH is introduced. The gib is provided with horns at its ends to keep it in its place. Sometimes two gibs are used, one on each side of the cotter; this makes the sliding surface on both sides of the cotter the same. The cotter is secured by a set screw K. The unsectioned portion of fig. (a) to the right of the gib, or to the left of the cotter, is called theclearanceordraught.
Exercise41:Connecting Rod End.—Make the following views of the connecting rod end illustrated by fig. 41. First, a vertical section, the same as shown at (a). Second, a horizontal section. Third, side elevation. Fourth, a plan. Or the first and third views may be combined in a half vertical section and half elevation; and the second and fourth views may be combined in a half horizontal section and half plan.All the dimensions are to be taken from the detail drawings (b), (c), (d), and (e),but the details need not be drawn separately. The brass bush is shown at (d) by half elevation, half vertical section,half plan, and half horizontal section. The draught or clearance is 7-16ths of an inch.
Exercise41:Connecting Rod End.—Make the following views of the connecting rod end illustrated by fig. 41. First, a vertical section, the same as shown at (a). Second, a horizontal section. Third, side elevation. Fourth, a plan. Or the first and third views may be combined in a half vertical section and half elevation; and the second and fourth views may be combined in a half horizontal section and half plan.
All the dimensions are to be taken from the detail drawings (b), (c), (d), and (e),but the details need not be drawn separately. The brass bush is shown at (d) by half elevation, half vertical section,half plan, and half horizontal section. The draught or clearance is 7-16ths of an inch.
Fig. 41.Fig. 41.
Box End.—At (a), fig. 42, is shown what is known as a box end for a connecting rod. The part which corresponds to the loose strap in the last example is here forged in one piece with the connecting rod. In this form the brass bush is provided with a flange all round on one side, but on the opposite side the flange is omitted except at one end; this is to allow of the bush being placed within the end of the rod. The construction of the bush will be understood by reference to the sketch shown at (b). The bush is in two parts, which are pressed tightly together by means of a cotter. This cotter is prevented from slackening back by two set screws. Each set screw is cut off square at the point, and presses on the flat bottom of a very shallow groove cut on the side of the cotter.
The top, bottom, and ends of this box end are turned in the lathe at the same time as the rod itself; this accounts for the curved sections of these parts.
It is clear from the construction of a box end that it is only suitable for an overhung crank.
Exercise 42:Locomotive Connecting Rod.—In fig. 42 is shown a connecting rod for an outside cylinder locomotive. (a) is the crank-pin end, and (c) the cross-head end. The end (a) has just been described under the head 'box end.' We may just add that in this particular example the brass bush is lined with white metal as shown, and that the construction of the oil-box is the same as that on the coupling rod end shown in fig. 44. The end (c) is forked, and through the prongs of the fork passes the cross-head pin, of which a separate dimensioned drawing is shown at (d). Observe that the tapered parts A and B of this pin are parts of the same cone. The rotation of the pin is prevented by a small key as shown. The cross-head pin need not be drawn separately, and the isometric projection of the bush at (b) may be omitted, but all the other views shown are to be drawn to a scale of 6 inches to a foot.
Exercise 42:Locomotive Connecting Rod.—In fig. 42 is shown a connecting rod for an outside cylinder locomotive. (a) is the crank-pin end, and (c) the cross-head end. The end (a) has just been described under the head 'box end.' We may just add that in this particular example the brass bush is lined with white metal as shown, and that the construction of the oil-box is the same as that on the coupling rod end shown in fig. 44. The end (c) is forked, and through the prongs of the fork passes the cross-head pin, of which a separate dimensioned drawing is shown at (d). Observe that the tapered parts A and B of this pin are parts of the same cone. The rotation of the pin is prevented by a small key as shown. The cross-head pin need not be drawn separately, and the isometric projection of the bush at (b) may be omitted, but all the other views shown are to be drawn to a scale of 6 inches to a foot.
Fig. 42.Fig. 42.
Fig. 43.Fig. 43.
Marine Connecting Rod.—The form of connecting rod shown in fig. 43 is that used in marine engines, but it is also used extensively in land engines. A B is the crank-pin end, and C the cross-head end. The end A B is forged in one piece, and after it is turned, planed, and bored it is slotted across, so as to cut off the cap A. The parts A and B are held together by two bolts as shown. This end of the rod is fitted with brass steps, which are lined with white metal. The cross-head end is forked, and through the prongs of the fork passes a pin D, which also passes through the cross-head, which is forged on to the piston rod or attached to it in some other way.
Exercise 43:Marine Connecting Rod.—Draw all the views shown in fig. 43 of one form of marine connecting rod. For detail drawings of the locking arrangement for the nuts see fig. 19, page 21. Scale 4 inches to a foot.
Exercise 43:Marine Connecting Rod.—Draw all the views shown in fig. 43 of one form of marine connecting rod. For detail drawings of the locking arrangement for the nuts see fig. 19, page 21. Scale 4 inches to a foot.
Coupling Rods.—A rod used to transmit the motion of one crank to another is called acoupling rod. A familiar example of the use of coupling rods will be found in the locomotive. Coupling rods are made of wrought iron or steel, and are generally of rectangular section. The ends are now generally made solid and lined with solid brass bushes,without any adjustment for wear. This form of coupling rod end is found to answer very well in locomotive practice where the workmanship and arrangements for lubrication are excellent. When the brass bush becomes worn it is replaced by a new one.
Fig. 44 shows an example of a locomotive coupling rod end for an outside cylinder engine. In this case it is desirable to have the crank-pin bearings for the coupling rods as short as possible, for a connecting rod and coupling rod in this kind of engine work side by side on the same crank-pin, which, being overhung, should be as short as convenient for the sake of strength. The requisite bearing surface is obtained by having a pin of large diameter. The brass bush is prevented from rotating by means of the square key shown. The oil-box is cut out of the solid, and has a wrought-iron cover slightly dovetailed at the edges. This cover fits into a check round the top inner edge of the box, which is originally parallel, but is made to close on the dovetailed edges of the cover by riveting. A hole in the centre of this cover, which gives access to the oil-box, is fitted with a screwed brass plug. The brassplug has a screwed hole in the centre, through which oil may be introduced to the box. Dust is kept out of the oil-box by screwing into the hole in the brass plug a common cork. The oil is carried slowly but regularly from the oil-box over to the bearing by a piece of cotton wick.
Fig. 44.Fig. 44.
Exercise 44:Coupling Rod End.—Draw first the side elevation and plan, each partly in section as shown in fig. 44. Then instead of the view to the left, which is an end elevation partly in section, draw a complete end elevation looking to the right, and also a complete vertical cross section through the centre of the bearing. Scale 6 inches to a foot.
Exercise 44:Coupling Rod End.—Draw first the side elevation and plan, each partly in section as shown in fig. 44. Then instead of the view to the left, which is an end elevation partly in section, draw a complete end elevation looking to the right, and also a complete vertical cross section through the centre of the bearing. Scale 6 inches to a foot.
An example of a steam-engine cross-head is shown in fig. 45. A is the end of the piston rod which has forged upon it the cross-head B. The cross-head pin shown at (d), fig. 42, and to which the connecting rod is attached, works in the bearing C. Projecting pieces D, forged on the top and bottom of the cross-head, carry the slide blocks E which work on the slide bars, and thus guide the motion of the piston rod.
Fig. 45.Fig. 45.
Exercise 45:Locomotive Cross-head.—In fig. 45 are shown side and end elevations, partly in section, of the cross-head and slide blocks for an outside cylinder locomotive. Draw these views half size, showing also on the end elevation the cross-head pin and a vertical section of the connecting rod end from fig. 42. The bush in the cross-head which forms the bearing for the cross-head pin is of wrought iron, case-hardened, and is prevented from rotating by the key shown. The cross-head is of wrought iron, and the slide blocks are of cast iron, and are fitted with white metal strips as shown. A short brass tube leads oil from the upper slide block into a hole in the cross-head as shown, which carries it to a slot in the bush which distributes it over the cross-head pin.
Exercise 45:Locomotive Cross-head.—In fig. 45 are shown side and end elevations, partly in section, of the cross-head and slide blocks for an outside cylinder locomotive. Draw these views half size, showing also on the end elevation the cross-head pin and a vertical section of the connecting rod end from fig. 42. The bush in the cross-head which forms the bearing for the cross-head pin is of wrought iron, case-hardened, and is prevented from rotating by the key shown. The cross-head is of wrought iron, and the slide blocks are of cast iron, and are fitted with white metal strips as shown. A short brass tube leads oil from the upper slide block into a hole in the cross-head as shown, which carries it to a slot in the bush which distributes it over the cross-head pin.
Apistonis generally a cylindrical piece which slides backwards and forwards inside a hollow cylinder. The piston may be moved by the action of fluid pressure upon it as in a steam-engine, or it may be used to give motion to a fluid as in a pump.
A piston is usually attached to a rod, called apiston rod, which passes through the end of the cylinder inside which the piston works, and which serves to transmit the motion of the piston to some piece outside the cylinder, orvice versâ.
Fig. 46.Fig. 46.
Aplungeris a piston made in one piece with its piston rod, the piston and the rod being of the same diameter.
A piston which is provided with one or more valves whichallow the fluid to pass through it from one side to the other is called abucket.
Simple Piston.—The simplest form of piston is a plain cylinder fitting accurately another, inside which it moves. Such a piston works with very little friction, but as there is no adjustment for wear, such a piston is not suitable for a high fluid pressure if it has to work constantly. This simple form of piston is used in the steam-engine indicator, and also in pumps.
Fig. 46 shows the piston of the circulation pump of a marine engine. A is the cast-iron casing or barrel of the pump; B is a brass liner fitting tightly into the former at its ends, and secured by eight screwed Muntz metal pins C, four at each end; D is the piston, which is made of brass, and is attached to a Muntz metal piston rod E. The liner is bored out smooth and true from end to end, and the piston is turned so as to be a sliding fit to the liner. The wear in this form of piston is diminished by making the rubbing surface large.
Exercise 46:Piston for Circulating Pump.—Draw the vertical sectional elevation of the piston, &c., shown in fig. 46, also a half plan and half horizontal section through the centre. Scale 4 inches to a foot.
Exercise 46:Piston for Circulating Pump.—Draw the vertical sectional elevation of the piston, &c., shown in fig. 46, also a half plan and half horizontal section through the centre. Scale 4 inches to a foot.
Pump Bucket.—The next form of piston which we illustrate is shown in fig. 47. This represents the air-pump bucket of a marine engine. The bucket is made of brass, and is provided with six india-rubber disc valves. The rod is in this case made of Muntz metal. Air-pump rods for marine engines are very often made of wrought iron cased with brass. It will be observed that there is a wide groove around the bucket, which is filled with hempen rope or gasket. This gasket forms an elastic packing which prevents leakage. This is an old-fashioned form of packing, and is now only used for pump buckets.
Exercise 47:Air-pump Bucket.—Draw the sectional elevation of the air-pump bucket shown in fig. 47. Also draw a half plan looking downwards and a half plan looking upwards. Scale 4 inches to a foot.
Exercise 47:Air-pump Bucket.—Draw the sectional elevation of the air-pump bucket shown in fig. 47. Also draw a half plan looking downwards and a half plan looking upwards. Scale 4 inches to a foot.
Fig. 47.Fig. 47.
Fig. 48.Fig. 48.
Ramsbottom's Packing.—The form of packing used in the air-pump bucket, fig. 47, is not suitable for steam pistons. For the latter the packing is now always metallic. The simplest form of metallic packing is that known as Ramsbottom's. This form is very largely used for locomotive pistons, and for small pistons in many kinds of engines besides. A locomotive piston for an 18-inch cylinder with Ramsbottom's packing is shown in fig. 48. The particular piston there illustrated is made of brass, and is secured to a wrought-iron piston rod by a brass nut. Two circumferential grooves of rectangular section are turned out of the piston, and into these fit two corresponding rings, which may be of brass, cast iron, or steel. In this example the rings are of cast iron. These rings are first turned a little larger in diameter than the bore of the cylinder (in this example1⁄2inch), and then sprung over the piston into the groves prepared for them. Their own elasticity causes the rings to press outwards on the cylinder. At the point where a ring is split a leakage of steam will take place, but with quick-running pistons this leakage is unimportant. The points where the rings are cut should be placed diametrically opposite, so as to diminish the leakage of steam.