The cranks and treadles for driving the fly wheels of foot lathes, have been made of many different forms, some of which have been shown. For the earlier foot wheels, figs. 32 - 34, the axis was of a length only sufficient to run in bearings to support the wheel; one end being formed into a short crank nearly resembling a winch handle. Sometimes, indeed, there was no crank, but a stud was inserted in the side of the wheel or in one of its spokes, or, the end of the axis was pierced transversely to receive a short straight rod, terminating in the stud for the reception of the loop of the treadle thong.
So soon as the wheel axis was extended in length, it was bent as a crank, the bend being first placed close to the wheel, but subsequently about the center of its length. The lengthened crank axis was at first forged square as in fig. 63, and it passed through a square hole in the fly wheel, the two being fixed to each other by means of wooden wedges; the crank or bend itself was also rectangular, a form still frequently met with.
The modern crank shaft, fig. 41, is round and the bend, which is nearly of the same thickness with the shaft throughout, is gradual and without angles. The fly wheel, as in the section of the single bevil wheel, fig. 42, is attached upon a round fitting, secured against a shoulder or flange upon the crank, by a screw and nut behind; it is prevented from moving upon the crank shaft by a stud fixed in the face of the flange, which enters a corresponding hole pierced in the face of the wheel.
Occasionally the cylindrical crank shaft is without a flange, and is either filed with a flat, or provided with a slot or key way; the circular fitting in the wheel has a corresponding rectangular slot, filed slightly taper, and the two are fixed together by an iron key, fig. 41. The crank shaft is sometimes left circular, the under surface of the key is then fluted to fit it. One or two light blows of a hammer upon the end of the key, effectually secures the wheel, and it is as readily loosened by driving a taper punch or chisel between the face of the wheel and the head of the key. Of these two methods of fixing, the fluted key is the less secure, but it includes an element of safety well adapted for the driving gear of machinery; the key, may be just sufficiently tightened to do the work, but immediately slips in the case of accident or undue resistance. When a lateral adjustment of the wheel upon the crank is desirable, the end of the crank shaft may be turned as a square threaded screw, the wheel fits this by a plain hole and is traversed and secured by nuts upon either face; an arrangement adopted in fig. 114.
Cranks of foot lathes to about five feet in length, are by preference made with a single bend or throw; but above that length, they are more usually made with two throws, dividing the crank shaft into three, about, equal lengths; principally necessary to afford a second support to the long treadle, which would deflect or "rack" if supported only at the center as in fig. 43. In all convenient cases, it is desirable to avoid the use of the double throw crank, which for light and smooth running requires exact vertical and horizontal parallelism in the two necks for the hooks. This is liable to disturbance, from original defect in manufacture, from elasticity, or from inequalities in the material, any of which may set up irregular wear in the crank, its hooks or centers; frequently causing the double throw crank to "grind" and to work much more laboriously than that with the single throw.
The crank has been made to run in bearings, on centers, and upon various combinations of anti-friction rollers, many of the latter very complex, but all offering but little advantage. It is now usually made of round iron steeled at the ends, in which the hollow centers are first bored with a small deep hole, and are then coned to an angle of about 60°. The crank is supported upon screwed and fixed steel points turned to a corresponding angle; the extreme points of the centers being free, and received by the small hole bored at the apex of the hollow center, which also serves to retain the oil for lubrication. The center screws of the crank and treadle are screwed up moderately tightly, until upon shaking the crank lengthways, the knocking that arises from excess of distance between the end screws just ceases; an increased and laborious friction is felt directly the centers are screwed up too tightly. Should either the wheel be loose upon the crank, or the treadle insufficiently screwed up, the circumstance is indicated by a sound resembling a knock, occurring at every depression of the foot.
The treadle of the foot lathe has been constructed in various forms and as levers of each of the different orders. For a long period it consisted of a straight piece of wood, one end jointed to the floor, and the other, attached to the cord from the pole or wheel, figs. 20, 22; this straight piece being crossed by a second for the foot. The piece for the link or cord was then fixed about parallel with the lathe axis, fig. 32, that for the foot being retained as before, or sometimes abandoned, when the treadle consisted of a single bar. The link to connect the crank and the treadle has consisted of a cord, a leathern thong, a chain, and a metal rod or hook, while elaborate contrivances of bearings in halves, universal joints and screw adjustments, to determine the exact distance of the treadle from the ground, were sometimes adopted.
The general form of the modern treadle is given by fig. 43, and some varieties of its construction may be found in later illustrations. The modern treadle is continued beyond the hook, which is attached about the middle of its length, and the width is considerably increased by the footboard attached by rails to the axis at the back. The hook is generally of iron steeled at the working parts, a thin strip of metal being some times attached to its upper edge to exclude access of shavings to the crank; the lower hook shown, is now usually exchangee for a plain hole which cannot separate from the steel pin upon which it works, unless the latter is intentionally withdrawn