An Introduction to Machine Drawing and Design Part 7

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 a _coupling 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 bra.s.s bushes, _without any adjustment for wear_. This form of coupling rod end is found to answer very well in locomotive practice where the workmans.h.i.+p and arrangements for lubrication are excellent. When the bra.s.s 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 bra.s.s 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 bra.s.s plug. The bra.s.s plug 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 s.c.r.e.w.i.n.g into the hole in the bra.s.s 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.

[Ill.u.s.tration: 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.

XIII. CROSS-HEADS.

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.

[Ill.u.s.tration: 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 bra.s.s 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.

XIV. PISTONS.

A _piston_ is 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 a _piston rod_, which pa.s.ses 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, or _vice versa_.

[Ill.u.s.tration: FIG. 46.]

A _plunger_ is 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 which allow the fluid to pa.s.s through it from one side to the other is called a _bucket_.

_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 bra.s.s 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 bra.s.s, 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.

_Pump Bucket._--The next form of piston which we ill.u.s.trate is shown in fig. 47. This represents the air-pump bucket of a marine engine. The bucket is made of bra.s.s, 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 bra.s.s. 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-fas.h.i.+oned form of packing, and is now only used for pump buckets.

[Ill.u.s.tration: FIG. 47.]

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.

_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 ill.u.s.trated is made of bra.s.s, and is secured to a wrought-iron piston rod by a bra.s.s nut. Two circ.u.mferential grooves of rectangular section are turned out of the piston, and into these fit two corresponding rings, which may be of bra.s.s, 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 example 1/2 inch), 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.

[Ill.u.s.tration: FIG. 48.]

EXERCISE 48: _Locomotive Piston._--A part elevation and part section of a locomotive piston, for a cylinder having a bore 18 inches in diameter, is shown in fig. 48. Draw this, and also a view looking on the nut in the direction of the axis of the piston rod.

Scale 6 inches to a foot.

_Note._--The reason why the part of the piston rod within the piston has such a quick taper is that the piston has to be taken off the rod while it is in the cylinder. The cross-head being forged on the end of the piston rod prevents the piston and piston rod being withdrawn together.

_Large Pistons._--Pistons of large diameter are generally provided with two cast-iron packing rings placed within the same groove. These rings are pressed outwards against the cylinder, and also against the sides of the groove by one or more springs. One form of this packing (Lancaster's) is shown in fig. 49. Here one spring only is used, and it is first made a straight spiral spring, and then bent round and its ends united. The action of the spring will be clearly understood from the ill.u.s.tration. For the purpose of admitting the packing rings the piston is divided into two parts, one the piston proper, and the other the _junk ring_. In fig. 49, A is the junk ring, which is secured to the piston by means of bolts as shown.

[Ill.u.s.tration: FIG. 49.]

EXERCISE 49: _Marine Engine Piston._--The piston ill.u.s.trated by fig. 49 is for the high-pressure cylinder of a marine engine. The piston, junk ring, and packing rings are of cast iron. The piston rod and nut are of wrought iron, so also are the junk ring bolts.

The nuts for the latter are of bra.s.s. The spiral spring is made from steel wire 3/8 inch diameter. An enlarged section of one of the packing rings is shown at (_a_). A front elevation of the locking arrangement for the piston rod nut is shown at (_b_). A sectional plan of one of the nuts for the junk ring bolts is shown at (_c_).

First draw the vertical section of this piston, next draw a plan, one-third of which is to show the piston complete, one-third to show the junk ring removed, and the remaining third to be a horizontal section through between the packing rings. The details (_a_) and (_c_) need not be drawn separately. Scale 3 inches to a foot.

_Proportions of Marine Engine Pistons._--Mr. Seaton, in his 'Manual of Marine Engineering,' gives the following rules for designing marine engine pistons:--

D = diameter of piston in inches.

_p_ = effective pressure in lbs. per square inch.

_x_ = D/50 [sqrt (_p_)] + 1.

Thickness of front of piston near boss 0.2 _x_.

" " " rim 0.17 _x_.

" back of piston 0.18 _x_.

" boss around rod 0.3 _x_.

" f.l.a.n.g.e inside packing ring 0.23 _x_.

" " at edge 0.25 _x_.

" junk ring at edge 0.23 _x_.

" " inside packing ring. 0.21 _x_.

" " at bolt-holes 0.35 _x_.

" metal around piston edge 0.25 _x_.

Breadth of packing ring 0.63 _x_.

Depth of piston at centre 1.4 _x_.

Lap of junk ring on piston 0.45 _x_.

s.p.a.ce between piston body and packing ring 0.3 _x_.

Diameter of junk-ring bolts 0.1 _x_ + .25 inch.

Pitch of junk-ring bolts 10 diameters.

Number of webs in piston (D + 20)/12.

Thickness " 0.18 _x_.

EXERCISE 50: _Design for Marine Engine Piston._--Calculate by Seaton's rules the dimensions for a marine engine piston 40 inches in diameter, and subjected to an effective pressure of 36 lbs. per square inch. Then make the necessary working drawings for this piston to a scale of, say, 3 inches to a foot.

_Note._--Take the dimensions got by calculation to the nearest 1-16th of an inch.