A TREATISE ON MILLS AND MILL-WORK. SECTION IV. ON MACHINERY OF TRANSMISSION. CHAPTER I. ON WHEELS AND PULLEYS. THE elementary principles of motion by rolling contact and by wrapping connectors have been explained in Section II., so that in the present Chapter we have only to examine in detail the methods of applying these principles and their respective advantages, and especially the mode of constructing wheels in gear, so that the resulting motion shall most nearly approach the condition of perfect rolling contact. We saw in the preliminary Chapter that there were two methods of transmitting power through trains of wheel-work, the first being through the agency of wrapping connectors, and the second by rolling contact. Wrapping Connectors.—Considerable difference of opinion exists as to the best and most effective principle of conveying motion from the source of power to the machinery of a mill. The Americans prefer leather straps, and large pulleys or * I have selected the word strap, instead of belts or bands, as a term more generally applied to wrapping connectors in the northern districts. riggers. In this country, and especially in the manufacturing districts, toothed wheels are almost universally employed. In some parts of the South, and in London, straps are extensively used; but in Lancashire and Yorkshire, where mill-work is carried out on a far larger scale, gearing and light shafts at high velocities have the preference. Naturally, I am of opinion that the North is right in this matter, and that consistently, as I was to a great extent the first to introduce that new system of gearing which is now general throughout the country, and to which I have never heard any serious objection. I have been convinced by a long experience that there is less loss of power through the friction of the journals, in the case of geared wheel-work, than when straps are employed for the transmission of motive power. Carefully-conducted experiments confirm this view, and it is therefore evident which mode of transmission is, as a general rule, to be preferred. There are certain cases in which it is more convenient to use straps than to employ gearing. With small engines driving saw-mills, and other machinery where the action is irregular, the strap is superior to wheel-work, as it lessens the shocks incidental to this description of work. So, also, when the motive power has been conveyed by wheel-work and shafting to the various floors of a mill, it is best distributed to the machines by means of straps. In some of the American cotton factories, however, there is an immense drum on the first motion, with belts or straps from two to three feet wide, transmitting the power to various lines of shafting, and these in turn through other pulleys and straps giving motion to the machinery. From this description it will be seen that the whole of the mill is driven by straps alone, without the intervention of gearing. The advantages of straps are, the smooth and noiseless motion. The disadvantages are, increased friction on the journals of the shafts, cumbrous appearance, the expense of their renewal, and the necessity for frequent repairs. They are inapplicable in cases where the motion must be transmitted in a constant ratio, because as the straps wear slack, they tend to slip over the pulleys and thus lose time. In other cases, as has been observed, this slipping becomes an advantage, as it reduces the shock of sudden strains and lessens the danger of breaking the machinery. Very various materials are employed for straps, the most serviceable of all being leather spliced with thongs of hide or cement. Gutta percha has been employed with the advantage of dispensing with joints, but it is affected by changes of temperature, and it stretches under great strains. Flat straps are almost universally employed, in consequence of the property they possess of maintaining their position on pulleys, the edge of which is slightly convex (fig. 177). Round belts of catgut or hemp are sometimes used, running in grooves, which are better made of a triangular than a circular section-so that the belt touches the pulley in two lines only, tangential to the sides of the groove; in this case the friction of the belt is increased in proportion to the decrease of the angle of the groove. Fig. 177. The strength of straps must be determined by the work they have to perform. Let a strap transmit a force of n horses' power at a velocity of v feet per minute, then the tension on 33000 n the driving side of the belt is lbs. independent of the initial tension producing adhesion between the belt and pulley. For example, let v be 314.16 feet per minute, or the velocity of a 24-inch pulley at 50 revolutions per minute, and let 3 horses' power be transmitted; then 33000 × 3 = 312 lbs., the strain on the pulley due to the force transmitted. The following table has been given for determining the least width of straps for transmitting various amounts of work over different pulleys. The velocity of the belt is assumed to be between 25 and 30 feet per second, and the widths of the belts are given in inches. With greater velocities the breadth may be proportionably decreased. TABLE I.-APPROXIMATE WIDTHS OF LEATHER STRAPS, IN INCHES, NECESSARY TO TRANSMIT ANY NUMBER OF HORSES' POWER. * Toothed Wheels.-The second method of communicating motion is by rolling contact, as explained in the preliminary Chapter. But, in practice, the adhesion between the surfaces is seldom sufficient to communicate the necessary power, and hence various contrivances-such as the wheel and trundle, and toothed wheels-have been substituted. The general equations for velocity, ratio, &c. are the same as if the wheels rolled on each other at the pitch circles, but in fact each tooth slides upon its fellow. The determination of the best forms of these teeth so that the friction shall be a minimum and the motion uniform, is one of the most important contributions of applied mathematics to practical engineering. Of the introduction of toothed wheels and toothed gearing we know very little. Hero of Alexandria, who wrote two centuries before our era, speaks of toothed wheels and toothed bars in a * Part i. page 40. way which seems to indicate that he was not altogether ignorant of this method of transmitting motion. Later forms are figured in great variety in the different collections of mechanical appliances of the sixteenth and seventeenth centuries. Fig. 178. Spur gearing is employed where the axes on which the wheels are placed are parallel to one another. The smaller wheel in a combination of this sort is termed the pinion. Annexed (fig. 178) is a pinion from Ramelli (A.D. 1588), which, from its form, may be surmised to be of metal. The principle on which spur gearing is constructed is primarily the communication or motion through the rolling of two cylinders on one another. The teeth are introduced to prevent slipping, and thus to insure the regular communication of the motive power. In the older wooden wheels, the teeth were usually formed of hard wood, and driven into mortises on the periphery of a wooden wheel. The pinions were generally replaced by trundles, in which cylindrical staves, fixed at equal distances round the periphery of two discs, were driven by the teeth of the wheel. The mortise wheels are still retained in countries where iron is expensive, and even in this country they are employed in a modified form. Iron pinions, with wooden cogs fixed in the periphery, are used to receive the motion from the flywheels of engines, with a view to reduce the noise and to increase the smoothness of the motion; and many millwrights prefer, in all cases where large wheels are required to run at high velocities, to make one of them a mortise-wheel, with wooden cogs. There does not appear to have been much improvement in the construction of wood and iron gear since it was first introduced by Mr. Rennie; the only exception being the introduction of a machine for cutting out the form of the teeth,* which in * Mr. Smiles states, in his 'Lives of the Engineers,' that Brindley, more than a century ago, invented machinery for the manufacture of tooth and pinion wheels, 'a thing,' as stated by the author, 'that had not before been attempted, all such wheels having, until then, been cut by hand, at great labour and cost. |