Cast iron guide bars wear better than either bra.s.s, iron, or steel ones, so long as they are properly lubricated. The face of each guide bar should be cut away, so that the ends of the cross head guides will travel past it. This will prevent a shoulder forming at the ends of the bar as the face wears away. Such shoulders are apt to cause a knock as the connecting rods are lined up, because in the lining the connecting rod is restored to its original length, and the path of the cross-head guides along the bars may be altered.

THE CONNECTING ROD.

There are two princ.i.p.al kinds of connecting rods, the "strap ended" and the "solid ended." The solid ended wear the best, but are more difficult to get on and off the engine.

Connecting rod straps are secured to the stub ends (as the ends of the rod are called), either by bolts or by one or two gibs, and the bra.s.ses are set up by a taper key or wedge.

The taper for connecting rod keys is about an inch per foot.

The angularity of a connecting rod is a term that applies to its path of motion, which is (during all parts of the stroke except on the dead centre) at an angle to the line of engine centres. The effect of this angularity is to cause the piston motion to be accelerated at one part of the stroke and r.e.t.a.r.ded at another, thus causing the point of cut-off to occur at different points of the two strokes.

The direction of the variation is to cause the point of cut-off to occur later on the stroke when the piston is moving from the head end of the cylinder towards the crank.

The amount of variation caused in the two points of cut off by the connecting rod depends upon the proportion that exists between the length of the crank and that of the connecting rod, and is less in proportion as the length of the connecting rod is greater than that of the crank.

An ordinary length of connecting rod is six times the length of the crank, or _six cranks_, as it is commonly termed.

Fig. 3296 represents a cylinder, piston and rod, cross head, connecting rod, and crank.

The piston _b_ is shown in the middle of the cylinder, the cross head at E, and the crank pin at B, instead of being at G', as it would but for the connecting rod, or if the connecting rod was infinitely long.

Now take a pair of compa.s.ses and set it from _b_ to E, and then try it from _a_ to D, and from _c_ to F, and it will be seen that the three cross head positions D, E, and F correspond correctly to the three piston positions _a_, _b_, _c_. Then take a pair of compa.s.ses and set them to the length of the connecting rod (from E to B) and try them from D to A, from B to E, and from C to F, and it will be seen that crank pin positions A, B, and C correspond to cross head positions D, E and F, and therefore that the crank is not at half stroke when the piston is in the middle of the cylinder. Take these same compa.s.ses, and resting one point at (G') mark the arc H, and that is where the cross head would be when the crank was at (G'). Now then we see that the connecting rod causes the piston to move slower while running from _a_ to _b_ than it does while running from _b_ to _c_.

[Ill.u.s.tration: Fig. 3296.]

THE D SLIDE VALVE.

The various events which are governed by the D slide valve of a steam engine are as follows:

The live steam period is that during which the steam is admitted from the steam chest into the cylinder and the steam admitted during this period is termed _live_ steam.

The point of cut off is that at which the valve closes the steam port, and the admission of steam into the cylinder is stopped, hence the point of cut off is at the end of the live steam period.

The period of expansion is that during which the steam is allowed to expand in the cylinder, and therefore begins at the point of cut off, and ends at the point of release.

The point of release is that at which the valve opens the port and permits the steam to escape.

The point of compression is that at which the exhaust port is closed, which occurs before the piston has reached the end of its stroke; the steam that has not pa.s.sed out of the cylinder is therefore compressed, the compression continuing until the valve opens for the lead.

The lead of the valve is the amount the port is open to the live steam when the crank is on the dead centre.

The point of admission is that at which the port opens for the live steam to enter, and it follows that the lead and compression both act as a cushion, arresting the motion of the piston when it reaches the end of the stroke.

Cushioning begins, however, at the time the exhaust port is closed enough to arrest the escape of the steam, while compression begins when the valve has closed the exhaust port.

[Ill.u.s.tration: Fig. 3297.]

The construction of a common slide valve is shown in Fig. 3297, in which the valve is shown in its mid-position. P P are the cylinder steam ports (as the openings through which the steam pa.s.ses from the steam chest to the cylinder are termed), and at X is the cylinder exhaust port, through which the steam escapes from the cylinder. Z is the valve exhaust port or exhaust cavity.

The lip of a valve is the width of its f.l.a.n.g.e face, or the distance L, which is measured from the steam edge A to the exhaust cavity Z. At the other end of the valve, H is the lip extending from the steam edge B to the exhaust cavity.

Steam lap is the distance the steam ends (or the steam edges as they are called) A, B overlap the steam ports, this distance being shown on the ends of the valve at _a_ C. If the valve had no steam lap, its steam edges would just cover the ports, as denoted by the dimension W.

Exhaust lap is the amount the exhaust cavity Z overlaps the bridges _q q'_, as at _p_, _r_.

Unequal steam lap is given to cause the point of cut off to occur at equal points in the piston stroke; thus in the figure there is more steam lap at the head end than at the crank end of the valve. But unequal lap could also be given in order to greatly vary the points of cut off for the two piston strokes, if such was desired.

Unequal exhaust lap may be given to equalize the point of release, or to equalize the points of compression.

The head end of the valve (or of the cylinder) is that which is furthest from the crank shaft, the other end, or that nearest to the crank shaft, being termed the crank end.

THE ACTION OF A COMMON SLIDE VALVE.

The action of a common slide valve may be traced as follows:

[Ill.u.s.tration: Fig. 3298.

Port _a_, open to the amount of the lead.]

[Ill.u.s.tration: Fig. 3299.

Port _a_, full open for the admission.]

[Ill.u.s.tration: Fig. 3300.

Port _a_, closed off for cut.]

[Ill.u.s.tration: Fig. 3301.

Valve opening port _a_, for the exhaust.]

[Ill.u.s.tration: Fig. 3302.

Port _a_, full open for the exhaust.]

Suppose the port _a_ to be at the head end of the cylinder and open to the amount of the lead with the crank on the corresponding dead centre, and if the valve travel be made equal to twice the lap and the lead, the various positions of the valve will be as marked in Figs. from 3298 to 3302; the event corresponding to each valve position being stated in the figures.

DOUBLE PORTED VALVES.

The term _port_ applies strictly to the area of opening of the steam pa.s.sage where it emerges upon the valve seat. The term _steam pa.s.sage_ includes the full length of the opening from the cylinder bore to the face upon which the valve is seated.

A double ported steam port is one in which there are _two_ openings or steam ports, leading into _one steam pa.s.sage_.

A double ported valve is one in which there are two ports _at each end_ of the valve. These two ports in some cases admit steam to a single cylinder port, and in others to two steam ports, terminating in one steam pa.s.sage.