MACHINERY. 



drives a rery fmall one, a cafe the judicious engineer fliould 

 always avoiti : the grand point to be attended to, is to adopt 

 fiich a contlriiSion as will infure all the teeth of a wheel 

 being precifely equal, and to make as great a number of 

 them as the ftrength will admit. This will caufe feveral teeth 

 to be in aftion at once, and make the communication of the 

 motion extremely fmooth and uniform. To obtain Itrength 

 in the cogs when they are made fine, the width or thicknefs 

 ef the wheel muft be increafed ; and this is one of the greatefl 

 praiSical improvements which has been made in machinery 

 for thefe hd twenty years. Formerly the bed engineers, 

 fuch as Smeaton, direfted the teeth of large cog-wheels to 

 be four and five inches diftant from each other, or pilch, as 

 the millwright? term it. Such wheels always aft unequally 

 tipon each other in confequence of the point of contaft 

 of the large cogs conftantly altering its pofition, becoming 

 alternately nearer or farther from the centre of one or other 

 of the wheels ; and this, tending to increafe the aning ra- 

 dius of one, whilft it diminiihes the other, caufes their 

 velocity and powers to vary at every cog that pafles by, 

 and the machine works by ftarts and jerks. The wheel- 

 work of modern machinery is conftruded with fine cogs, 

 feldom mora than one and a half or two inches pitch, and 

 as much length of cog, or breadth of the wheel, as will 

 make them uifficiently Itrong. We have feen fome wheels 

 in a large cotton mill which bore a ftrain equal to thirty 

 horfes' power, in which they were nine and twelve inches 

 broad upon the face. Cog-wheels are found to work mod 

 fmoothly when the teeth of the large wheel are m«de 

 of hard wood, and the teeth of the fmall one made of cad 

 iron, the afting furfaces being dreffed or filed fmooth and to 

 the true figiire. A mechanic, in contriving any machinery, 

 (hould always bear in mind, that where he introduces cog- 

 wheels, they (hould be as large in their diameters as is con- 

 liftent with other circumftances, becaufe this allows the 

 teeth to be made finer in proportion to the power they are 

 to bear, than if they were of fmaller radii ; and the teeth, 

 therefore, nearer the centre : it alfo occafions lefs prefTure 

 or drift upon the centre, and the wear of the whole will be 

 equable. Another circumflance is worth notice, and fhould 

 always be attended to, where it will not interfere with more 

 important confiderations ; this is, the direftion in which any 

 force is given to, and taken from, any piece of wheel-work : 

 fuppofe, for inftance, a water-wheel turning its axis, upon 

 vhich ii fixed a cog-wheel to give motion to a fecond wheel, 

 for the purpofe of driving any machinery ; now if this fe- 

 cond cog-wheel is applied on that fide of the firft cog- 

 wheel which is afcending, it will be oppofite to that fide of 

 the wheel which is loaded with water, and is confequently 

 defcending. In this ftate the gudgeons of the water-wheel 

 will have to bear (in fome cafes) double the ftrain of the 

 power of the machine ; becaufe the power, wliich is the 

 weight of the v>'ater, is applied on one fide the centre of 

 the wheel, and is taken off by turning the fecond cog- 

 v.'heel on the other fide : the centre, or fulcrum, therefore, 

 bears the whole power, and alfo the re-a£lion to that power, 

 in addition to the weight of its own parts ; in the fame man- 

 ner as the fulcrum of a fleelyard or balance beam bears the 

 whole of the weight fufpended from cither end, and its own 

 weight alfo. On the other hand, fuppofe the fecond wheel 

 applied on the defcending fide of the water whtel, this 

 being on the fame fide of the centre, the prefTure thereon 

 will be far lefs than the power of the machine. In fome 

 cafes (but not in a water-wheel), by the proper arrangement 

 of the wheel work, the power may be m.ade to operate to 

 lift the centres, and thus in part relieve them from the 

 weight of the wheil, fo as aftually to dimi:iifli the prefTure of 



friftion of the pivots, when, by a contrary application, k 

 would have increafed it in the fame degree. Similar ad- 

 vantages will attend the precaution of adapting the pofitiong 

 <»f different wheels upon their ftiafts to the different weights 

 or drains they have to bear, fo that the gudgeons at the two 

 ends of any fhaft may have an equal drift or prefTure upon 

 them. This will caufe them to wear equally, and to have lefs 

 friftion, becaufe they may be made fmaller than where no 

 fuch care is taken, dill having fuflicient drength. It is ac- 

 complifhed by confidering the drift or prefl'ure upon the 

 centre of every wheel upon any axis, and placing the two- 

 gudgeons or pivots of the axis at a didance from each of 

 tile wheels, proportionate to the drift upon its centre. Thus, 

 fuppofe a diaft has a cog-wheel fixed upon it, and a fmall 

 wheel or pinion alio fixed upon it at fome didance from the 

 wheel, the power is given to the axis by wheel-work ope- 

 rating upon the teeth of the pinion, and the re-action to this 

 power is given by fome machinery which the teeth of the 

 large wheel actuates. In this cafe the drift on the centre of 

 the pinion will be very confiderable, becaufe the power is 

 applied near the centre of the axis ; but the wheel tranf- 

 mitting the power at a greater radius, will, perhaps, have 

 much lefs drift on its centre (the proportion depending in 

 fome degree upon the direction in which the power and re- 

 ailion are applied, as dated in our lait obfervation) : if this 

 is the cafe, the gudgeon at that end of the fhaft, where the 

 pinion is placed, fhould be lengthened out, fo as to give the 

 bearing point at a greater didance from it than the wheel, 

 which fhould have its gudgeon placed much nearer to it, 

 becaufe lefs drain is to be borne. By this means the drift 

 upon the two ends of the fhaft will be equally divided be- 

 tween them : and though this proportion of the centre can- 

 not be always accomplifhed without inconvenience, the en- 

 gineer diould always have it in view ; and then, where it is not 

 praiElicable, he fhould attain the fame end, by apportioning 

 the ftrength or diameter of the gudgeons to the relative 

 drains they have to bear. 



An endlefs belt or drap is a very general method of 

 tranfmitting rotatory motion : it is ufually employed in cafes 

 where a very quick motion is to be created, and the re-aftioa 

 to be overcome is nearly equable. In fuch cafes it has the 

 advantage of wheel-work from its fimplicity and the eafe of 

 its motion. Some curious properties belong to the ecdiefs 

 ftrap, I'iz. that the pulley or rigger it works t-pon muft be 

 larged in the middle, that is, the diameter mull be greater 

 in the middle of the pulley than at the edges, becaufe the 

 ftrap always rides on to the largeit diameter of the pulley, 

 and if this is not in the centre it will flip off at one fide. 

 It is not eafy to give any fatisfadlory explanation of this 

 faft, nor of another, that if, by accident, one of the puUies 

 is ftopped while the ftrap is urged round by the motion of 

 the other, it inftantly flies off its pulley, unlefs the edge of 

 the pulley fhould be much wider than tbe drap. This pro- 

 perty is a great recommendation of it for fome purpofes, 

 fuch as threfhing mills, fiour-dreffing machines, lathes, cot- 

 ton machines, &.c. where any thing accidentally dopping the 

 machines would dedroy them if driven by wheel-work, but 

 the drap flips round, and very foon comes off, fo as to 

 avoid all further danger. Belts of girt-'unb, fuch as are ufed 

 for faddle girts, are fometimes uied inftead of leather ftraps, 

 though thele are undoubtedly preferable. The ftrsp Jliould 

 be dreffed to an eqii.il thicknefs and breadth throughout, and 

 the ends very neatly joined ; that i?, of the fame thicknefs 

 there as at every other part. It is fometimes done by few- 

 ing, but the bell method is by gluing them together, with a 

 glue compounded of Irifh glue, ifinglafs, ale grounds, and 

 boiled linfeed oil. The two ends being tapered away and. 



overlapped 



