In the larger back geared lathes driven by power, the round driving band is replaced by a flat leather or other belt, and the manner in which these driving straps and their pulleys are employed, may be briefly referred to. The power of the strap, which depends upon its wrapping contact, increases directly with its width; the latter therefore is always proportioned to the magnitude of the work to be performed. The leather driving straps, which in most particulars hold their ground against various competing substances, and are most general, when of moderate dimensions are made of one thickness; the larger sizes are of two or more thicknesses of leather, sewn together by seams along their entire length. The ends of the strap which overlap, are punched with a series of holes and are joined by lacing with narrow leather thongs, which latter are indurated and toughened by various processes of manufacture. The joining by lacings, has the advantage, that it enables the belt to be tightened by the extent of the difference between one or more holes; but in addition to the lacing, portions of the driving straps are frequently joined by various forms of metal rivets, useful when a strap requires lengthening or piecing for repairs; sometimes these belts are entirely joined by rivets or other forms of metal couplers. The driving straps for lathes, range from about one and a half to about eight inches in width, two to three inches being perhaps the average size.
The mandrel pulley has the grooves replaced by a series of quasi-flat fillets, fig. 84, each being turned to a flat arc, so as to be slightly higher in the center of its width, the corresponding speed pulley on the countershaft, and all the driving pulleys being turned in a similar manner. This formation of the edge of the pulley is necessary to neutralize a characteristic of the wrapping contact of the flat band, viz., to run up hill. A band wrapped around a cone leads to the larger diameter, and if the rim of the pulley be any larger at one edge than the other, the band in its revolution continues to find its way to that side, and if free to escape. runs itself off the pulley. A trifling difference from the cylinder suffices to prevent this accident, and the formation equivalent to a double cone base to base, forces the strap to take the middle of the width of the rim for its seat. The action of the double cone is very visible when the running strap is shifted from one to the other of the fast and loose pulleys, running side by side upon the shaft. The strap, from the draw of the cone or first half of the width, invariably finds its way beyond the center of the pulley on to which it is carried, but this is immediately corrected so soon as it meets with the contrary influence of the reverse cone, forming the other half; when the strap returns and settles itself under the combined influence, exactly centrally upon the curved edge of the pulley.
The driving shafts and pulleys occupy definite positions; they are generally fixed to or near the ceiling, one driving or "lying" shaft usually running the entire length of the shop. This moves constantly in one direction and at one velocity, and generally communicates the power to several machines, each of which therefore requires means of changing both the velocity and direction of the motion received. The method formerly employed by the millwrights for driving the lathe by power, is explained by the diagram fig. 93; the swing arm contained in the arrangement is very convenient and is still often employed for this and many other purposes. The lying shaft L, carries a large driving wheel; M, is the mandrel pulley, and S, the swing or tension pulley, mounted in a jointed frame attached to the wall; the motion is led from L to M, by two belts, the second being placed on any of the diameters of S, and M. The belts are stretched by two independent weights attached to the tension frame, which are led away by a suitable arrangement of chains and pulleys, to any part of the shop where their presence is not inconvenient; these chains and weights are now generally replaced by tension screws.
With both bands "open," the direction of motion is alike in all three axes, with one "crossed," the direction of the mandrel is reversed. The pulley M, runs either loose or fixed on the mandrel, by one or two studs, placed and retained in or out of action by a gearing fork or lever. This latter contrivance, known as a clutch or catch box, is inconvenient when used for this purpose in the lathe, or in any other machine moving rapidly; as it communicates the whole velocity and power at once to a mass previously at rest, causing a blow or jar highly objectionable.
The ordinary modern arrangement is that indicated in plan by fig. 94, but the mandrel pulley M, must be assumed to be vertically beneath C. The driving shaft L, and the mandrel M, remain as before, but the swing arm is replaced by the countershaft C, which runs in fixed bearings attached to the wall or ceiling. The center of the countershaft carries a fixed cone or step pulley, the diameters of which agree with those of the mandrel pulley; at either end, it carries three pulleys, two loose and one fixed, for the driving straps from L.
The fixed pulley 1, and the two loose pulleys 2 and 3, are drawn of the same diameter as the one wide driving pulley 7, on the shaft L; when therefore the hand connects 7 and 1, the two shafts travel with equal velocities and in the same or opposite directions as the hand may be open or crossed, and when the band is shifted by the forked strap lever, from the fixed pulley 1, to the loose pulley 2, the latter alone revolves without moving the countershaft. The fixed pulley 4, and the two loose pulleys 5 and 6, are under precisely the same condition with respect to their driving pulley 8, but, they are only of half the diameter of that and of all the other pulleys; 4 therefore drives C, at twice the velocity of the driving shaft.