TIIEOKY AND PRACTICE OF ENGINEERING.
Book 1 I.
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by that without reference to the other ; a diminution of the impelling power accompaniesan increase of velocity, and in every machine there is an inertia to be overcome, before wearrive at a motion which will produce the useful effect.
Plumbers Blocks and Steps are the bearingsupon which the gudgeons of shafts revolve.
The pillows or brasses are made of gun metal,or a composition of copper and tin, which whenworking with cast-iron acts with little friction.
The internal brass or pillow is fitted within aniron frame or casting, one half attached to thelower plate, the other to the upper, and bymeans of screws and nuts arc securely drawntogether. In order that the two halves of thebrasses may be true, they are turned previously ina lathe, or bored out of the solid, and afterwardsseparated and put into their respective situations.
Friction Rollers, or wheels, are used to diminishfriction, and their effect is produced by convertinga sliding into it rolling motion. Wheels aresometimes employed for this purpose, but thestress is then thrown on their axles, and is notborne on the surface as in the rollers. Frictionrollers consist of an iron plate bolted down tothe framing, upon which are cast circular rings,whose interior surface is accurately turned;the gudgeon rests in the middle of six rollers,placed at equal distances, and of such a diameteras exactly to fill up the space between the gudgeonand the outer ring. When the revolution of thegudgeon takes place, as the rollers are free tomove, they roll round in the same direction, butwith a velocity in proportion to the difference of thediameter of the gudgeon and the ring. A grooveis formed in these rollers, and an iron ring made to fit upon them,so that they are kept asunder at their proper distances, and trulyparallel after the gudgeon is introduced in the middle. Thenotches in the ring are not made to fit the rollers tightly, but toallow sufficient play, and little friction. The sidds of the ring areboxed up, that no dust or dirt can enter to derange the rollers.
Conical Friction Rollers, if not executed very accurately, willwear out of adjustment, and are not of frequent introduction ;they are sometimes used to vertical shafts; two conical frictionrollers introduced under the end of a shaft, also made conicalaround the pivot, to play on them, prevents the weight fromresting upon the pivot.
Having described some of the first movers employed in theconstruction of machines, the effect of which is usually rectili-near, it is now necessary to consider them as acting tangentiallyon the circumference of a circle, producing rotary motion ;also to describe the great variety of contrivances to vary thesemotions, and adapt them to the constructionof machines.
Alternate cones are frequently made use offor changing the velocity of motion, whenthis is required to be gradually and uniformlyaccelerated: one cone gives motion to theother by a belt, carried along by a guide,by which means the motion is acceleratedor retarded. One of the earliest methodsadopted for the transmission of rotary motion,with or without a change of speed, is bybands or straps. A series of pulleys orgrooves in one common pulley, correspondingwith others in another such wheel, so as topreserve always the same length of band,allows a great variety of speed to be obtainedb. merely shifting the band from one pair of
Fig 1317.