Modern Machine-Shop Practice Part 243

THROWING OFF A DRIVING WHEEL.--This is not a common accident, but nevertheless it sometimes occurs; they break usually just outside of the driving axle box. In this case take out the driving box and fit in its place a block of wood affording journal bearing for the axle. Let this block rest on the pedestal cap, holding the axle up in the centre of the pedestal. Then secure the piston and disconnect the valve gearing and open the cylinder c.o.c.ks as before, and the engine can be run _slowly_ to the repair shop without danger of further accident, or, if convenient, it can be towed by another engine.

BREAKING A SPRING OR SPRING HANGER.--Lift the engine with the jacks until the driving wheel axle box is about in the centre of the pedestal, and put any convenient piece of iron across the top of the driving axle box and between it and the engine frame, thus taking the weight of the engine on the frame instead of on the spring. Place also a block of iron between the end of the equalizing bar and the top of the engine frame, so as to prevent the movement of the equalizing bar, and to allow the spring at the other end of the equalizing bar to operate without moving the said bar. Every engineer should carry in his tool box pieces of metal suitable for this purpose, because this is a frequent accident. It does not, however, materially affect the working of the engine, and should not delay a train more than a few minutes.

BURSTED FLUES AND TUBES.--These are usually plugged by tapering a piece of pine wood and driving it into the bursted tube by means of an iron bar. Taper iron plugs are often carried, and then driven into the end of the tube after the wooden one has been driven in. To enable this job to be done, it is necessary to thickly cover the fire with green coal, which operates to cool the tubes and prevent the loss of the water in the boiler. Sometimes careful engineers prepare for use pine plugs turned slightly taper, and a little slack, for the inside of the tube.

In case of leak, this plug is inserted in the flue, and driven along it until it covers the fracture, the expansion due to its saturation causing it to become locked in the tube.

SLIPPING OF ECCENTRICS.--Place the reverse lever in the forward notch of the sector. Place the crank on its forward dead centre, as near as can be judged by the eye, and loosen the set screw of the forward eccentric, that is to say, the eccentric which connects to the upper end of the link. Move that eccentric round upon the axle until the slide valve leaves the steam port at the front end of the cylinder open to the amount of required valve lead. In moving the eccentric round upon the shaft, move it in the direction in which it will rotate when the engine is running forward, so as to allow for and take up any lost motion there may be in the eccentric straps, in the eccentric rod eyes and bolts, and in the other working parts of the valve gear; for if the eccentric was moved backward, all such lost motion would operate to vitiate the set of the valve. The eccentric being placed as directed fastens its set screw securely.

If the backward eccentric is the loose one, throw the reverse lever to the backward notch of the sector, lifting the link up so that the eccentric connected to the lower end of the link may be approximately adjusted by moving it around upon the axle in the direction in which it will rotate when the engine is running backward, until the back cylinder port is open to the amount of the valve lead. Another very ready plan of temporarily adjusting the eccentrics is as follows: Place the reverse lever in the end notch forward, and place the engine crank or driving crank pin as near on a dead centre as the eye will direct, and open both the cylinder waste water c.o.c.ks. Then disconnect the slide valve spindle from the rocker arm, and move the slide valve spindle until the opening of the cylinder steam port corresponding to the end of the cylinder at which the piston stands will be shown by steam blowing through the waste water c.o.c.k at that end of the cylinder; the throttle valve being opened but a trifle, to allow a small steam supply to enter the steam chest and cylinder, for if much steam is admitted, it may pa.s.s through a leak in the piston and blow through both the waste water cylinder c.o.c.ks.

The position of the valve being thus determined, the eccentric must be moved upon the driving axle until the valve spindle will connect with the rocker arm without being moved, or moving the valve at all.

HOT AXLE BOXES.--If not convenient to reduce the speed of the engine, or if that and free lubrication do not cool the box, a plentiful supply of cold water should be administered, it being well to have at hand a small hose pipe, by means of which water from the tender tank can be used. If the bra.s.ses have Babbitt metal in them and it should melt, it is better, if possible, to cool the axle box while the engine is moving, which will injure the journal less than if the journal is stopped to cool the box, because in the latter case the bra.s.s or box is apt to become soldered to the journal of the axle, and when the engine is again started, the cutting or abrasion will recommence with extreme violence.

BREAKING A LIFTING LINK OR THE SADDLE PIN THAT CONNECTS THE SLOT LINK TO THE REVERSE SHAFT.--Cut a piece of wood and tie it into the slot of the link, over the link block or die, making it of a length to keep the link in the position for hauling the train. Then fasten another piece of wood in the link slot beneath the sliding block or die, thus securing that die in the proper position for the engine to go ahead. In this case, the engineer must be careful in stopping, as he cannot reverse the engine on the crippled side.

Secondary accidents are almost sure to occur if a disconnected piston is not securely blocked in the cylinder, or from blocking the piston aright and attempting to let the slide valve run, or from attempting to run with the parallel rods on one side only disconnected. There are numerous accidents, which only common sense and a familiarity with the locomotive can provide a temporary remedy for, but those here enumerated are by far the most common.

ADJUSTING THE PARTS OF A LOCOMOTIVE.

When the wedges of the axle boxes are to be adjusted for fit to the pedestal shoes, the engine should be moved until the coupling rods on one side of the engine are in line with the piston rod, because in this position the rod will, to a certain extent, act as a guide in keeping the axles parallel to each other, and at a right angle to the line of engine centres.

Bear in mind that the distance from the centre to centre of axle boxes must be the same as the distance from centre to centre of the crank pins, and that when the coupling or side rods are in line with the piston rod, they act to resist the axle boxes from being set up too close together.

[Ill.u.s.tration: Fig. 3350.]

The importance of a proper adjustment of the axle boxes, coupling boxes, and connecting rods cannot be overestimated, and it is necessary therefore to explain it thoroughly. In Fig. 3350, then, _s_, _s_ represent two wheel axles, whose boxes are between their wedges. At S, S' are the screws for setting up the shoes or wedges W and W'

respectively. The axles are shown on the line of centres C, C of the engine, the piston being at the head end of the cylinder, and the crank pins on the line of centres as denoted by the small black circles. The wedges W and W' are shorter than the leg of the pedestal, so that they may be set up by the set screws S and S', and take up the wear.

In some engines the wedges V and V' are also shorter for the same purpose. Now it is clear that setting up the screws S and S' will move the axles _s_, _s'_ to the left, and this will alter the clearance between the piston when it is at the end of the stroke and the cylinder cover.

It is clear that the distance between the centres of the two axles must be the same as the distance between the centres of the two crank pins, or else the frame will be subjected to a great strain, tending to break the crank pins and the side rods.

In order to keep the clearance equal and to know when it is equal, it is necessary, at some time when the cylinder cover at the head end is off, to disconnect the connecting rod and push the piston clear up against the left hand cylinder cover, and from the cross head as a guide, make on the side of the guide a line L'. Then put on the cylinder cover at the head end and push the piston up against it and mark a line L. Then when the connecting rod is put on again, the wheels may be moved around if the engine is jacked up, or, if not, the engine may be moved along the rails with a pinch bar, and the clearance will be equal when the cross head (at the ends of the stroke) comes within an equal distance of the respective lines L', L when the crank is on the dead centres, and it is well to adjust the wedges W W' so that the cross head does travel within an equal distance, and mark on the guide bar two more lines, one at each end of the bar.

These lines are a permanent guide in setting up the shoes or wedges, and lining up the connecting rods, and coupling or side rods, because it is clear that from the method employed in marking them the distance between the end of the cross head, when at the end of its stroke, and the line L, and that between the face of the piston and the cylinder cover, will be equal.

A proper adjustment, therefore, should be made as follows: The piston should be at the end of its stroke, the crank pins being on the line of centres.

Screw S should be operated to set up the wedge W, taking up the wear of the sides of the box, and bringing the edge of the cross head the proper distance from the line L. The connecting rod bra.s.ses should then be set up to fit the pins, and the screw S' operated to set up wedge W' to have easy contact with the side of its axle box. If, however, there has been so much wear on the axle boxes that they are still too loose between the wedges, both wedges may be set up to take up this wear, since it is more important to have the axle boxes a proper fit between the wedges than it is to maintain an exactly equal amount of clearance at each end of the cylinder.

The engine will then be in proper tram on this side, or, in other words, the distance from the centre to centre of the crank pins will be the same as that from centre to centre of the axles.

On the other side of the engine the process is the same, the engine being moved until the crank pins are on the line of centres C C and the wedges set up according to the lines.

CHAPTER x.x.xIX.--THE MECHANICAL POWERS. LEVERS, PULLEYS, GEAR WHEELS, ETC.

Power is distinguishable from force or pressure in that the term power means force or pressure in motion, and since this motion cannot occur without the expenditure of the force or pressure, power may, with propriety, be termed the expenditure of force or pressure.

If we suppose a piston to stand in a vertical cylinder sustaining a weight upon its surface and compressing the air within the cylinder, so long as there is no motion no work is done, as the term "work" is understood in a mechanical sense, and the weight merely produces a pressure. If, however, the weight be removed, the compressed air will force the piston upward, performing a certain quant.i.ty of work which may best be measured by the amount of power exercised or expended.

The mechanical value of a given amount of power cannot be either increased, diminished or destroyed by means of any mechanical device or appliance whatsoever through which it may be transmitted.

It may be concentrated, as it were, by decreasing the amount of its motion. It may be distended, as it were, by increasing the distance through which it moves, or it may be expended in giving or producing motion, but in either case the amount of duty or work done is the exact equivalent of the amount of power applied.

A gain or increase in speed is not, therefore, a loss of power, but merely a variation in the mode of using or utilizing such power.

For instance, 1 lb. moving through a distance of 12 inches in a given time represents an amount of power which may be employed either as 1 lb.

moving a foot, 2 lbs. moving six inches, or 1/2 lb. moving through 24 inches, in the same s.p.a.ce of time, the amount of the power or duty remaining the same in each case, the method of utilization merely having differed.

It is an inexorable law of nature that power is concentrated in proportion as the amount of its motion is diminished, or distended in precise proportion as such motion is increased.

[Ill.u.s.tration: Fig. 3351.]

Suppose, for example, that in Fig. 3351 L is a lever having its fulcrum at F, which is 4 inches from end A, and 8 inches from end B, and (leaving the weight of the lever out of the question) if we place an 8 lb. weight on a it will just balance 4 lbs. at B.

If the lever is moved, the amount of motion will be twice as much at end B as it is at end A.

If we apply the power at A, the lever has become a means of converting 8 lbs. moving a certain distance into 4 lbs. moving twice that distance, and nothing has been either gained or lost.

If we apply the power at B, the lever has merely been used as a means of converting 4 lbs. moving a certain distance into 8 lbs. moving one half that distance, and nothing has been gained or lost.

Suppose that end A was moved an inch, and the power at that end will be 8 inch pounds or 8 lbs. moving an inch, whereas at the end B the power is 4 lbs. moving 2 inches; we have, therefore, reduced the weight in the same proportion that we have increased the distance moved through.

Suppose now that the lever is moved to the position denoted by the dotted line M M, and the leverages will be altered; that at end A becoming that denoted by the distance from F to the vertical C, and that for end B being denoted by the distance from F to the vertical D.

This occurs because we are dealing with gravity, which always acts in a vertical line.

[Ill.u.s.tration: Fig. 3352.]

A crow bar is an excellent example of the application of the lever. In Fig. 3352, for example, we have a 1 lb. weight on the long end of the lever, and as we are dealing with a weight, the effective length of the long end of the lever is from the fulcrum _f_ to _w_, which is divided into 10 equal divisions. The short end of the lever is from _f_ to _p_, which is equal to one division, hence the 1 lb. is balanced by the 10 lbs.

[Ill.u.s.tration: Fig. 3353.]

A simple method of distending power is by means of pulleys or gear wheels. Suppose, for example, that in Fig. 3353, we have a weight of 12 lbs. suspended from a shaft or drum, whose radius _a_ is 10 inches, and that on the same shaft there is a pulley, whose radius _b_ is 20 inches, and the two weights will balance each other.

In this case the falling of either weight would not effect the leverage, because the distance of both weights would remain the same from the centre of the shaft. The leverage of the 12 lbs. is denoted by the line _a_, and that of the 6 lbs. by _b_.

So far as the transmission of power is concerned, therefore, pulleys are in effect revolving levers, which may be employed to concentrate or to distend power, but do not vary its amount.

[Ill.u.s.tration: Fig. 3354.]

Suppose we have two shafts, on the first of which are two pulleys, B and C, Fig. 3354, while upon the second there are two pulleys D and E. A belt H, connecting C to D. Let the pulleys have the following dimensions: