16. That the introduction of guide and carrying pulleys adds to the internal resistances an amount proportional to the friction of their journals.
17. That there is still need of more light on the subject.
Chapter XXXIII.—FORGING.
Forging.—The operation of forging consists in beating or compressing metal into shape, and may be divided into five classes, viz., hand-forging, drop-forging, machine-forging, forging under trip or steam hammers, and hydraulic forging. In purely hand forging much work is shaped entirely by hand tools, but in large shops much work is roughed out under trip or steam hammers, and finished by hand, while some work is finished under these hammers. In drop forging the work is pressed into shape by dead blows, which compress it into shape in dies or moulds. In machine forging the work is either formed by successive quick blows rather than by a few heavy ones, or in some machines it is compressed by rolling. In hydraulic forging the metal is treated as a plastic material, and is forced into shape by means of great and continuous pressure.
In all forging the nature or quality of the iron is of primary importance; hence the following (which is taken from The English Mechanic), upon testing iron, may not be out of place.
“The English Admiralty and Lloyds’ surveyor’s tests for iron and steel are as follows:—
“Two strips are to be taken from each thickness of plate used for the internal parts of a boiler. One-half of these strips are to be bent cold over a bar, the diameter of which is equal to twice the thickness of the plate. The other half of the strips are to be heated to a cherry-red and cooled in water, and, when cold, bent over a bar with a diameter equal to three times the thickness of the plate—the angle to which they bend without fracture to be noted by the surveyor. Lloyds’ Circular on steel tests states that strips cut from the plate or beam are to be heated to a low cherry-red, and cooled in water at 82° Fahr. The pieces thus treated must stand bending double to a curve equal to not more than three times the thickness of the plate tested. This is severe treatment, and a plate containing a high enough percentage of carbon to cause any tempering is very unlikely to successfully stand the ordeal. Lloyds’ test is a copy of the Admiralty test, and in the Admiralty Circular it is stated that the strips are to be one and a half inches wide, cut in a planing machine with the sharp edges taken off. One and a half inches will generally be found a convenient width for the samples, and the length may be from six to ten inches, according to the thickness of the plate. If possible, the strips, and indeed all specimens for any kind of experimenting, should be planed from the plates, instead of being sheared or punched off. When, however, it is necessary to shear or punch, the piece should be cut large and dressed down to the desired size, so as to remove the injured edges. Strips with rounded edges will bend further without breaking than similar strips with sharp edges, the round edges preventing the appearance of the small initial cracks which generally exhibit themselves when bars with sharp edges are bent cold through any considerable angle. In a homogeneous material like steel these initial cracks are apt to extend and cause sudden fracture, hence the advantage of slightly rounding the corners of bending specimens.