Relative proportions of the steam-engine .. by Marks William Dennis 1849-1914

Author:Marks, William Dennis, 1849-1914
Language: eng
Format: epub
Tags: Steam-engines
Publisher: Philadelphia : Lippincott

to 2j^g- greater area than that of the inserted part of the crank-pin at right angles to its axis.

In some cases the pin, crank and crank-shaft are forged in one solid piece out of wrought iron or made of cast steel in one solid mass; this method is more expensive, but more satisfactory.

The web of the crank is that part connecting its hub and eye, and is made of many differing shapes, and always with its cross-section increasing from the eye to the hub. Economy of material, as will presently be shown, dictates that this cross-section be increased by an increase of the breadth of the face of the crank— i. e., in a direction at right angles to the centre line of the crank-pin, rather than in the direction of the crank-pin.

If now we assume that the crank has a constant thickness in the direction of the centre line of the crank-pin, the longitudinal profile of the web of the crank should be a parabola (Weisbach's Mechanics of Engineering, sec. iv., art. 251-253), with its vertex at the centre of the pin, and, providing we neglect the torsional strain on the web due to the moment of the crank-pin, will be of uniform strength.

The torsion of the web due to the force acting at the crank-pin is much slighter, in fact, than might at first appear, as the pin is fastened in the connecting-rod as well as the crank; and if the pin is a little loose, as it never should be, its springing tends, by throwing the centre of pressure nearer the crank, to reduce the moment. We can, therefore, neglect the torsion of the web, and consider it as a beam required to be of uniform strength, fixed at one end and loaded at the other.

To trace the parabolic outline of the crank would be tedious and result in an ugly shape, but the property of the tangent of a parabola permits us, with an increase of less

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