Machinery's Reference Series ... by Unknown

Author:Unknown
Language: eng
Format: epub, pdf
Publisher: The Industrial Press
Published: 1910-03-25T05:00:00+00:00

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enough be allowed. A toy engine, for example, might do 1,000,000 foot>pounds of work in a few hours, while an engine of moderate proportions would accomplish as much during a few strokes of the piston. Foot-pounds of work, merely, with time left out of account, would form no basis by which the capacities of the two engines could be compared. Hence, to compare the work done, either by or upon some agent, the time required must be considered.

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The term pother is employed to indicate the quantity of work done in a given time. "One million foot-pounds" is an expression indicating work; 1,000,000 foo^pounds of work performed in a day, or an hour or minute indicates power. Work has the two elements, force and the distance through which the force acts; power has three elements: force, distance, and time.

'The unit of power adopted for engineering work is the hone-power (abbreviated H.P.). One horse-power is equal to 83.000 foot-pounds

No. s-^PRlNCIPLES OF MECHANICS

adhesion can generally be neglected, and the whole resistance considered as the friction.

Kinds of Friction

(a) A distinction is usually made between Iriction of rent and frio-Hon of motion^ the former being the frictional resistance to be overcome in starting a body into motion, and the latter the resistance that continually accompanies the motion. Friction of rest is generally greater than friction of motion, other conditions being equal.

(2)) When friction is mentioned, sliding friction is understood, i.e., such as that between an engine crosshead and its guides, or between a journal and its bearing. It is due to the roughness of the surfaces in contact Whenever wheels are employed, or rollers or balls placed between the surfaces, the resistance is called rolling friction, the nature of which is somewhat different; it is then due to the fact that the rolling body makes a greater or less depression in the surface of the other, so that it has continually to rise out of a hollow, as it were.

(c) Frictional resistance also occurs between the molecules of

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