THE WHEEL, AND AXLE.
The advantage gained is in proportion as the circumference of the wheel exceeds that of the axle; therefore, the larger the wheel, and the smaller the axle, the stronger is the power of this machine, but then the weight will rise proportionally slower. A winch may be used instead of a wheel, for in turning the winch the hand will describe a circle, and there is no difference in the result, whether an entire wheel be turned, or a single spoke which the winch as a lever represents.
Rule.—As the radius of the wheel is to the radius of the axle, so is the effect, to the power.
Example.—A weight of 50 lb. is exerted on the periphery of a wheel, whose radius is 10 feet; required the weight raised at the extremity of a cord wound round the axle, the radius being 20 inches.
50 lb. × 10 feet × 12 inches 20 inches = 300 lb. Ans.
THE PULLEY.
The pulley consists of a grooved wheel, called a sheave, moveable on an axis, or gudgeon, and enclosed in a frame, or case, called a block. By passing a cord over the pulley, a man will be enabled to draw up a weight equal to that which his own body supplies in pulling downwards.
By combining a number of pulleys, as many assistants are obtained as there are wheels: thus, two pulleys will have double the power of one, because half the weight is sustained by the frame to which one end of the cord is attached; but then it requires double the time to do the work. As the friction of the pulley is very great, particular attention must be paid that all the turns or kinks of a rope be taken out, before it is made use of, and it should enter easily into the grooves of the sheaves.
Rule.—Divide the weight to be raised by twice the number of pulleys in the lower block; the quotient will give the power necessary to raise the weight.
Example.—What power is required to raise 600 lb., when the lower block contains six pulleys?