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LECTURE XIV. QUESTIONS.

1. Make a diagram of the valve and ports of a steam engine with crank and eccentric circles, showing the positions of the crank, of the centre of the eccentric, and of the slide valve, when the crank is on its dead centre. (S. and A. Exam., 1888.)

2. Make a free-hand sketch of the first figure in this Lecture, but place the cylinder on the right hand and the crank on the left hand of your paper. Trace the course of the steam in entering and leaving the cylinder, and write out a complete "Index to Parts."

3. Show, by sketches, &c., how the motion of the piston is converted into that of the crank, and that of the eccentric into that of the slide valve. Why is the eccentric fixed so as to move ahead of the crank?

4. A crank is I foot long, while the connecting rod is 6 feet: find graphically how far the piston is from the beginning of its stroke when the crank has moved through 45 degrees from the inner dead centre. Ans. 4 inches nearly.

5. In a direct-acting horizontal engine the lengths of the crank and connecting rod are 1 and 5 feet respectively. How far is the piston from the middle of its stroke when the crank is vertical? Ans. 123 inch.

6. In a direct-acting engine, plot out by a diagram the relative positions of the piston and crank during a stroke, on the supposition that the connecting rod is of infinite length or remains parallel to itself. How is this diagram altered when a definite length is assigned to the connecting rod?

7. What is meant by the "dead centres" in the case of a crank and connecting rod? Sketch four figures in mere skeleton lines, showing a crank, connecting rod, piston rod and piston, and cylinder, when the crank is at each dead centre, and also when the force transmitted through the connecting rod has the greatest turning effect.

8. Sketch an ordinary eccentric in side elevation and in sectional plan. Give an index to the parts, and show how the motion imparted by it is the same as by a crank of the same throw or radius. Why cannot an eccentric be used for converting reciprocating into circular motion? In what cases are eccentrics applied in preference to cranks, and why?

9. Compare the crank with the eccentric. Show that they both produce the same motion. State reasons for employing one or the other in particular cases. (S. and A. Exam. 1889.)

10. Describe, with sketches, the construction of a horizontal direct acting engine, working with high pressure steam and without condensation, showing how the steam is admitted into the cylinder and let out again as required. (S. and A. Exam. 1889.)

II. Give a longitudinal section through the cylinder and steam-chest of an engine, showing the steam and exhaust ports, the steam slide-valve, the valve-rod, and stuffing-box for the latter. Put the valve in its mid-position. (S. and A. Exam. 1891.)

12. The crank of a direct-acting engine is 2 feet in length and the connecting rod 5 feet. Find the positions of the piston when the crank has described an angle of 60° from the dead centre in both the forward and backward strokes. (S. & A. Exam. 1895.)

LECTURE XV.

CONTENTS.-Lap, Lead, and Travel of a Slide Valve-Angle of Advance and Throw of an Eccentric-Relative Positions of the Crank and the Slide Valve-Cause of the Unequal Distribution of Steam during the Forward and the Back Stroke.

Lap and Lead of a Valve, &c.-The slide valve shown in the following figure is purposely placed at the centre of its stroke, in order to facilitate an explanation of what is meant by lap. The valve, like that in the figure, p. 125, is that known as the Locomotive D Slide Valve. It consists of a hollow box with projecting ends, the lower face being accurately planed and fitted, so as to be steam tight on the valve port face. The hollow arch of the valve just covers the distance between the inner edges of the steam ports, so that the moment the valve cuts off the exhaust from one end of the cylinder, it opens the other end of the cylinder to exhaust.

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Now, looking at the left-hand figure, we see the three dotted vertical lines drawn above the valve face at each end of the valve. The distance, O to L, is the amount by which the valve overlaps the steam port at each end. This is termed the outside lap of the valve, while the distance between L and M is the amount the valve (when at the end of its stroke) opens the steam port for admission of steam into the cylinder. This distance, LM, is frequently less than the breadth of the steam port, because the same passage serves both for inlet of the steam to, and its exit from, the cylinder; and, seeing that the steam has expanded while

doing work in the cylinder, the larger the opening to exhaust, the less will be the back or obstructive pressure.

The vertical dotted lines drawn from, N, below the valve face near the outside edge of each steam port, indicate the lead.

The circle in the right-hand figure is intended to represent the path of the centre of the eccentric pulley, which works the slide valve. The radius, OM, is, therefore, equal to the throw of the eccentric, or half the travel of the valve. Now, supposing the crank to be in the position, OC, or level at the inner dead centre in a horizontal engine (i.e., the piston is just at the commencement of the outgoing stroke), mark off the distance, ON, equal to the outside lap, OL, plus the lead, LN, draw NE perpendicular to OM, and join OE; then (neglecting the obliquity of eccentric rod) we have

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We thus see that the centre line of the eccentric must be in advance of the centre line of the crank, by (90° + 0°), where ✪° is called the angle of advance.

If there was neither lap nor lead, then the centre line of the eccentric would be at right angles to the centre line of the crank, or the eccentric only 90° ahead of the crank.

Sometimes slide valves have what is termed inside lap, that is, an inner projection at each end of the arch of the valve, marked in dotted lines by, x, in the figure. This causes the exhaust to take place later on the one side and to be cut off sooner on the other side of the piston. The effect of this is twofold-(1) a later release causing a higher back pressure, (2) compression before the end of the stroke. The latter effect is useful, as we shall see later on, owing to its assisting in arresting the momentum of the moving piston, piston rod, crosshead, and connecting rod, and thus lessening what would otherwise cause a sudden stress or jerk on the crosshead and crank-pin bearings, and therefore undue wear and tear. More frequently, however, a part of the necessary cushioning is effected by giving "lead" to the slide valve, that is, allowing it to open the steam port before the piston has come to the end of its stroke.

In order to impress these various parts and positions of the slide valve, we here enumerate them as definitions.

Lap or cover of a slide valve is the amount by which the edge of the valve overlaps the adjoining edge of the steam port, when the valve is in the middle of its stroke, and is termed outside or

steam lap, and inside or exhaust lap, according as we refer to the outside or inside of the slide valve.

Lead is the amount of the opening of the steam port at the beginning of the piston's stroke.

Angle of advance of eccentric is the angle by which the centre line of the eccentric stands in advance of that position, which would bring the valve to its mid-stroke when the crank is on the dead point; or, in other words, the angle between the crank and the centre line of the eccentric minus 90°.

The throw of an eccentric* is the distance between the centre of crank shaft and the centre of eccentric pulley.

The travel of a slide valve is equal to the distance the valve moves to and fro in one stroke of the piston, or twice (the lap + opening to steam). It is equal to twice the throw of eccentric.

Relative Positions of the Crank and Slide Valve.-The following diagram illustrates the four principal points in the motion of the simple D slide valve of the working model explained at the beginning of last Lecture, p.125,as well as the corresponding positions of the crank, and also the probable distribution of steam in the cylinder or "diagram of work.”

1. The point of admission of steam to the cylinder.
2. The point of cut off of steam from the cylinder.
3. The point of release, or when exhaust begins.
4. The point of compression, or when exhaust stops.

The diagram is self-explanatory, in as far as it shows how each of these points marked on the crank-pin circle is projected on to the "diagram of work" (or piston's stroke) below, with the corresponding positions of the slide valve sketched on the lines of projections. The direction of motion of the crank and of the slide valve at each point is also made clear by arrows. It will be observed that the slide valve is at the same position with respect to the steam ports when beginning to admit steam to the cylinder and to cut off the supply of steam from the same, and that its direction of motion is in each case opposite to the direction of the piston's motion. It is also evident that the slide valve is at the middle of its stroke when release and when compression begins, and that its motion is opposite in each case to that of the piston's motion, as indicated by the straight arrows placed directly above the figures of the slide valve.

* Several authors-e.g., Prof. Goodeve-state that the throw of an eccentric is equal to the diameter of the circle described by the centre of the eccentric pulley. The word "throw" is ambiguous, and might be discarded, for it is liable to lead to confusion. See The Practical Engineer, Nov. 18, 1887, p. 521.

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RELATIVE POSITIONS OF CRANK AND SLIDE VALVE, WITH CURVE SHOWING THE DISTRIBUTION OF STEAM IN THE CYLINDER, WHEN OBLIQUITY OF THE CONNECTING AND ECCENTRIC RODS IS NEGLECTED.

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