# A Catechism of the Steam Engine eBook

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22. Q.—–­Can you give a practical rule for determining the proper quantity of cast iron for the rim of a fly-wheel in ordinary land engines?

A.—­One rule frequently adopted is as follows:—­Multiply the mean diameter of the rim by the number of its revolutions per minute, and square the product for a divisor; divide the number of actual horse power of the engine by the number of strokes the piston makes per minute, multiply the quotient by the constant number 2,760,000, and divide the product by the divisor found as above; the quotient is the requisite quantity of cast iron in cubic feet to form the fly-wheel rim.

23. Q.—­What is Boulton and Watt’s rule for finding the dimensions of the fly-wheel?

A.—­Boulton and Watt’s rule for finding the dimensions of the fly-wheel is as follows:—­Multiply 44,000 times the length of the stroke in feet by the square of the diameter of the cylinder in inches, and divide the product by the square of the number of revolutions per minute multiplied by the cube of the diameter of the fly-wheel in feet.  The resulting number will be the sectional area of the rim of the fly-wheel in square inches.

## CENTRAL FORCES.

24. Q.—­What do you understand by centrifugal and centripetal forces?

A.—­By centrifugal force, I understand the force with which a revolving body tends to fly from the centre; and by centripetal force, I understand any force which draws it to the centre, or counteracts the centrifugal tendency.  In the conical pendulum, or steam engine governor, which consists of two metal balls suspended on rods hung from the end of a vertical revolving shaft, the centrifugal force is manifested by the divergence of the balls, when the shaft is put into revolution; and the centripetal force, which in this instance is gravity, predominates so soon as the velocity is arrested; for the arms then collapse and hang by the side of the shaft.

25. Q.—­What measures are there of the centrifugal force of bodies revolving in a circle?

A.—­The centrifugal force of bodies revolving in a circle increases as the diameter of the circle, if the number of revolutions remain the same.  If there be two fly-wheels of the same weight, and making the same number of revolutions per minute, but the diameter of one be double that of the other, the larger will have double the amount of centrifugal force.  The centrifugal force of the same wheel, however, increases as the square of the velocity; so that if the velocity of a fly-wheel be doubled, it will have four times the amount of centrifugal force.

26. Q.—­Can you give a rule for determining the centrifugal force of a body of a given weight moving with a given velocity in a circle of a given diameter?

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