An Introduction to Machine Drawing and Design Part 6

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 the sunk key F. It is also good practice to _shrink_ the 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.

[Ill.u.s.tration: 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. The _throw_ of the crank is twice the radius. In a direct-acting engine the throw of the crank is equal to the stroke of the piston.

EXERCISE 36: _Wrought-iron Overhung Crank._--Draw the two elevations shown in fig. 37, also a plan. Scale 1-1/2 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 = 1.1 _d_.

Diameter " = 1.8 _d_.

Width of crank arm at centre of shaft = 1.3 D.

" " crank-pin = 1.5 _d_.

The thickness of the crank arm may be roughly taken as = .7 D.

EXERCISE 37.--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 4-3/4 inches in diameter and 6-1/2 inches long.

[Ill.u.s.tration: 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 time as 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.

EXERCISE 38.--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.

[Ill.u.s.tration: 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 s.h.i.+ps 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.

EXERCISE 39.--Keeping to the dimensions marked in fig. 39, draw the views there shown of a built-up crank-shaft for a marine engine. Scale 3/4 inch to a foot.

XI. ECCENTRICS.

The _eccentric_ is 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, or _vice versa_, 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 the _radius_ or _eccentricity_ of the eccentric. The _throw_ is twice the eccentricity.

The sheave is generally made of cast iron. The strap may be of bra.s.s, cast iron, or wrought iron; when the strap is made of wrought iron it is commonly lined with bra.s.s.

[Ill.u.s.tration: FIG. 40.]

EXERCISE 40: _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 the strap. 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.

XII. CONNECTING RODS.

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 b.u.t.ts against the bra.s.s bush C D, which is in two pieces. A _strap_ E pa.s.ses 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 or _cotter_ F 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, a _gib_ H 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 the _clearance_ or _draught._

[Ill.u.s.tration: FIG. 41.]

EXERCISE 41: _Connecting Rod End._--Make the following views of the connecting rod end ill.u.s.trated 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 bra.s.s 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.

_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 bra.s.s bush is provided with a f.l.a.n.g.e all round on one side, but on the opposite side the f.l.a.n.g.e 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 bra.s.s 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 p.r.o.ngs of the fork pa.s.ses 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.

_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 bra.s.s steps, which are lined with white metal. The cross-head end is forked, and through the p.r.o.ngs of the fork pa.s.ses a pin D, which also pa.s.ses through the cross-head, which is forged on to the piston rod or attached to it in some other way.

[Ill.u.s.tration: FIG. 42.]

[Ill.u.s.tration: FIG. 43.]