of the eccentric or it is the radius of a circle described during one revolution of the eccentric, equal line, A D, Fig. 23. When the eccentric works the valve direct, the throw of the eccentric is equal to one-half the travel of the valve; or equal the greatest width the port is opened for the admission of steam lap of the valve.

Advance of an Eccentric.-The angular-advance of an eccentric is

E

B

the angular measurement of the arc E in Fig. 23. The linear-advance of an eccentric is the distance travelled by the slide-valve during the time the eccentric moves through its angular-advance. It is equal to the lap plus the lead of the valve, and equal A F, Fig. 23.

Position of the Keybed of the Eccentric. The position of the eccentric-sheaves and keybeds on a crank

Fig. 23.-Diagram showing the position, shaft, neglecting the obliquity of the

throw, and advance of an Eccentric.

C

H

eccentric-rod, may be found as follows: -Place the crank in the position shown in Fig. 24, with the horizontal centre-line A B level: draw the vertical centreline C D. On the end of the crank-shaft, from its centre O, with a radius equal to one-half the travel of the slide-valve, describe the travel-circle E. Draw F G, parallel to CD, at a distance equal to the sum of the lap and lead of the valve. Draw the radial lines

F

Fig. 24.-Diagram showing the position of the Keybed

of an Eccentric.

--B

OH and OI through the points of intersection of the travel-circle with the line F G. Then, if the keyway is cut on the centre-line, or throw-line, of the eccentric, OH is the centre-line of the forward-eccentric, and O I that of the backward-eccentric, on which lines the keybeds must be cut in the crank-shaft.

Steam-Ports and Passages.-There is generally more or less loss of initial pressure from the resistance due to friction in the passing of the steam through valves, pipes, steam-chest, ports, and passages to the cylinder. The ports and passages of a cylinder should be sufficiently large to permit the steam to follow the piston to the point of cut-off without loss of pressure. As the steam is exhausted, in most cases, from a cylinder through the same passages and ports by which it entered, the size of the steam-ports and passages should be proportioned to the velocity of the

exhaust-steam. If the ports and passages are too large, it causes loss of steam from excessive steam-space. If they are too small, the steam cannot follow the piston without expanding before being cut off, and there is loss of efficiency from wire-drawing the steam in passing in and out of the cylinder. The velocity of the exhaust-steam through the ordinary ports and bent passages of a cylinder with a slide-valve may be 75 feet per second, and should not exceed = 95 feet per second × 60 = 5700 feet per minute. For very short and straight passages, it may be = 125 feet × 60 = 7500 feet per minute. The area of each steam-port and the sectional area of each steam-passage, P, in square inches, may be found by this Rule:Area of cylinder in square inches x piston-speed in feet per minute P = Velocity of exhaust-steam in feet per minute.

Steam-Port-Opening.-The area of the steam-passages and ports, is generally at least double that of the greatest opening of the port by the slide-valve for the admission of steam, in order to provide free exhaust of the steam. The velocity of the steam through the port-opening, on its admission to the cylinder should not be higher than 11,400 feet per minute, through ordinary bent passages, and 15,000 feet per minute through very short straight passages.

The area of the steam-port-opening, O, in square inches, may be found by this Rule:

Area of cylinder in square inches x piston-speed in feet per minute Velocity of steam through the steam-port-opening in feet per minute. The steam is generally more or less wire-drawn, resulting from the valve not opening and closing the port quickly enough. The loss of pressure due to wire-drawing the steam, from the gradual opening and closing of the steam-port, may be partly provided for by making the cut-off a little later than theoretically required.

Steam-Pipe. In order to prevent loss of pressure between the boiler and the engine, the velocity of the steam through a steam-pipe should not in ordinary cases, exceed 85 feet per second x 60 = 5100 feet per minute. For very short, straight steam-pipes, the velocity may be 6600 feet per minute. The area of the steam-pipe, S, may be found by this rule :—

S=

Area of cylinder in square inches x piston-speed in feet per minuteVelocity of steam through the steam-pipe in feet per minute. Steam-pipes should be as short and straight as possible.

STEAM ENGINE GOVERNORS.

The Function of a Governor is to regulate the supply of steam to the cylinder according to the variations or sudden changes in the load on the engine. The efficiency of a governor depends principally upon its sensitive

ness.

The Action of the Governor of a steam-engine is controlled by two forces, viz., centrifugal force, or the tendency of the revolving balls to fly away from the spindle or vertical axis, and centripetal force, or the tendency of the balls to hang in a vertical line from the centre of the pin suspending the arm, due to the force of gravity.

To find the centrifugal force of a governor in terms of the weight of the balls. Multiply the square of the number of revolutions per minute by the radius of the circle described by the centres of the balls in inches, and divide the product by the constant number 35,226.

To find the centripetal force of a governor in terms of the weight of the balls. Divide the horizontal distance of the balls from the centre of the suspending pin, by the vertical height of the same centres.

B

Simple Governors, shown in Fig. 25.-The centre of the suspension of

P&A

Fig. 25.-Simple Governor.

Fig. 26.
Cross-armed Governor.

Fig. 27.-Governor-Arms.

the arms should invariably be placed in the centre of the spindle, unless it be placed beyond it, as in Fig. 26; because it is essential for a governor to work with the least possible variation in speed, and the placing of the point of suspension away from the centre of the spindle causes considerable variation in velocity. The variation in velocity increases as the distance is increased of the centre of the suspension-pin from the centre of the spindle. Although wrong in principle, the arms are frequently hung away from the centre of the spindle, as in Fig. 27; and in calculating such governors, the vertical height is to be taken from the plane-line, P, to the top of the cone, T, instead of the actual centre of suspension.

To find the power of a governor, multiply the weight of the balls in lbs. by the vertical height they are lifted.

To find the vertical height, H, between the point of suspension and the plane of revolution, P, divide the constant number 1875 by the number of revolutions of the governor, and square the quotient, which will give the height in inches.

Diameter of Cast-iron Balls for Ordinary Governors, B.-The weight of the balls must be sufficient to overcome the resistance of the valve

and its connections. In ordinary cases the diameter of each ball may be equal to one half the height of plane-line, H, in inches.

Length of Governor-Arms.-First determine the vertical height from the plane of revolutions to point of suspension of

arm, H, Fig. 28; then set out the centre-lines of the arms at an angle of 60°, as their position at the proper speed of the governor, and where the said centre-lines of arms cut the plane-line will be the centres of the balls, and the length of arm will be the distance between the centre of suspension and the centre of the ball thus found. The speed required to maintain the balls at that height is obtained by the following rule:

Fig. 28.-Governor-Arms.

To find the Speed of Simple Governors, divide the constant number, 1875, by the square root of the vertical height in inches between the plane of revolution and centre of suspension, and the quotient will be the number of revolutions per minute required to maintain the balls at that height.

Governors are driven from the engine crank-shaft by means of pulleys or gearing, and the diameter of pulley, or number of teeth in the wheel, to produce the proper velocity may be found by the following rules:

To find the Diameter of Pulley (or number of teeth in the wheel) on the driving shaft of the governor. Multiply the number of revolutions of the engine per minute by the diameter of pulley (or number of teeth in the wheel) on the engine crank-shaft, and

divide by the required number of revolutions per minute of the governor.

To find the diameter of pulley (or number of teeth in the wheel) on the engine crank-shaft. Multiply the diameter of pulley (or number of teeth in the wheel) on the governor driving-shaft by the number of revolutions per minute of the governor, and divide by the number of revolutions per minute of the engine.

Spring-Governor.-In small vertical engines, the governor is often placed horizontally on the top of the cylinder, as shown in Fig. 29, the centrifugal force being balanced by a spring placed inside the governor on the spindle. The tension of the spring is regulated by nuts to suit the required speed.

Cross-armed Governor with Centre-weight, Fig. 26.-In this class of governor, the centre of suspension must be calculated from the point where the arms cross each other in the centre-line of the spindle, and the vertical height is the distance from that point to the plane of revolution. By

crossing the arms in this way the governor becomes very sensitive; when the speed is increased, the point of intersection of the crossed arms rises at the same rate as the plane of revolution, and the governor balls will remain in equilibrium in every angular position at the proper speed of the governor. This kind of governor is run at a high speed; the proportions may be calculated by the following rules for centre-weighted governors.

STEAM ENGINE GOVERNORS WITH CENTRE-WEIGHT. Governor with Centre-weight, shown in Fig. 30.-This form of governor requires to be driven at a high speed, so that the centrifugal force of the balls may overcome the gravity of the centre-weight. Its advantages over the simple governor are: its extreme sensitiveness, whereby uniformity of speed is maintained under varying and sudden changes of the load on the engine; and its great power. enabling a much smaller governor to be used.

Fig 30.-Centre-weighted Governor.

Fig. 31.-Governor, with Centre-weight.

To find the vertical height from the plane of revolution to the point of suspension of a governor with centre-weight. First, fix upon the number of revolutions, divide the constant number 187.5 by the number of revolutions the balls will make when the engine is at its proper speed, and square the quotient, which will give the height in inches for an ordinary governor, H; then add together the weight of the revolving balls and twice the weight of the centre-weight, which sum multiply by the height, H (found as for an ordinary governor), and divide the product by the sum of the weights of the revolving balls, the quotient will be the height of a centre-weighted governor. If the centre-weight is hung by links at a point in the arm above the centre of the balls, like Fig. 31, then use the above rule, but instead of twice the weight of the centre-weight named above, use the product of twice the weight of the centre-weight, multiplied by the result of the length between the centre of suspension of the arm and the point where the link