Figs. 253. and 254. are views of the piston-rod clutch or cross-head. The piston-rod passes into the round hole in the centre of the clutch, and Figs. 253, 254. Figs. 256. and 257. represent the main links; fig. 258. the pillar of the main link, which is interposed between the upper and under brasses; and figs. 259, 260, and 261. the upper brass and pillar plate of the link. Fig. 262. represents the gibs and cutter of the main links. The sectional area of the main links is usually made about th of the area of the piston, that of the piston-rod being th. To find the proper sectional area of the main links, a common rule is to divide the square of the diameter of the Figs. 259, 260, 261. Fig. 262. BRASSES OF MAIN LINKS. 1 inch 1 foot. cylinder by 144. The length of the main links is usually about the same as the length of the crank, which is half the stroke. The main beam is always somewhat longer than the distance between the cylinder and crank centres, and at the cylinder end the perpendicular centre line divides the versed sine equally. The angular motion of the beam is about 38 degrees during the whole stroke. The length of stroke is the chord of the arc the centre of the end pin describes, and the versed sine represents the amount of deviation from the perpendicular, which is called the vibration of the beam. The beam being three times the length of the stroke, the distance from the main centre to the end stud is one and a half times the length of the stroke, and with these proportions the end stud will deviate from the perpendicular one inch for every foot of stroke. To find the amount of vibration of the end stud:-from the square of the radius in inches described by the stud subtract the square of the length of the crank in inches; extract the square root of the remainder, which deduct from the radius in inches. To find the proper distance between the main centre and the centre of the cylinder:-add the above-mentioned square root to the radius of the lever in inches; half their sum will be the horizontal distance in inches. The main centre of a land engine beam is usually fixed in with keys: the other centres are sometimes fixed with keys, and at other times they are ground in, which appears to be the preferable practice. The beam is set upon its edge, on two blocks of wood: a straight edge is applied to ascertain if it is nearly straight, and if bent or twisted it is brought straight by being hammered with the face of the hammer, though this practice weakens the beam if carried far. A cross piece of wood is put into each main-centre hole, upon which the central point is marked; the beam is plumbed, the end centres are put through, staked with wedges, and levelled by means of a short level with two legs passing down from the edge of the beam. The lengths from the main centre are next ascertained to be right, and the main centre is then put in, using the end centres as points to measure from. Finally, the keys are fitted. This is the mode of procedure when the holes for the centres are not bored out. It is expedient to put a centre line on the edge of the beam to fix the position of the studs laterally, and this is generally done. The force acting at each end of an engine-beam may be taken at 14 lbs. per circular inch of the piston, or, if the beam be supposed to be supported at both ends, it may be taken at 281bs. per circular inch acting at the centre. The depth of the beam at the ends being one third of the depth at the middle, to find the dimensions at the middle, divide the weight in pounds acting at the centre by 250, and multiply the quotient by the distance in feet between the supports. To find the depth, the breadth being given; divide this product by the breadth in inches, and extract the square root of the quotient, which is the depth. It is expedient, however, to make main beams stronger than is indicated by any of these rules, as a higher pressure of steam is now used almost universally than was employed by Mr. Watt. In our table of the dimensions of beams at the centre, the figures given are understood to represent the web of the beam, or the dimensions within the beads, if the pressure of the steam be above that of the atmosphere. We now come to the parallel motion. Figs. 263. and 264. represent the radius bars, and figs. 265. and 266. the parallel bars. The screws at the end of the parallel bars enter holes in the cross bar shown in figs. 267. and 268., and to the exterior bearings of the same cross-bar the radius-bars are attached, the other ends of those bars being attached to studs fixed to the spring beams in the line of the piston-rod. Figs. 269. and 270. represent the back links, the upper brass of which encircles the air-pump stud in the beam: the middle brass receives the cross-head on the top of the air-pump rod, and the lowest brass connects with the cross-bar of back links, fig. 268., through the oval hole in which LL the air-pump rod passes. The sectional area of the back links is made the same as that of the air-pump rod, which is one tenth of the diameter of the air-pump, or one twentieth of the diameter of the cylinder. The sectional area therefore of the two back links taken together is equal to the area of a circle one twentieth of the diameter of the cylinder; but in practice they are generally made of somewhat stronger proportions. We have already given at page 18. an explanation of the manner in which the parallel motion acts, and at page 100. we have given rules for computing the lengths of the principal rods and levers, while at pages 136. and 154. we have given the strengths of the parts in engines of different powers. The best proportionment of the parallel motions of land engines, and that now followed universally, consists in making the radius and parallel rods of exactly the same length, and this length equal to half the radius of the great beam. The stud from which the back links are hung is in this case si Fig. 267. -1.37 18 CROSS BAR OF BACK LINKS. SIDE VIEW. 14 inch 1 foot. tuated midway between the main centre and the end stud of the beam, and the studs in the spring beams round which the radius bars move, are in the same vertical line as the centre of the cylinder. To find, therefore, the right position for those studs, measure down perpendicularly from the centre of the end stud, when the beam is level, to a distance equal to the length of the main or back links, and at this distance draw a horizontal line on the inner sides of the spring beams. Then set off from the main centre on this line the distance between the main centre and centre of the cylinder: the point of intersection is the right position of the studs in the spring beams to which the radius or bridle rods are attached. In Mr. Watt's early engines the radius rods were made longer than the parallel rods, and were attached to a shaft which passed across between the spring beams clear of the end of the working beam; but the universal practice now is to introduce studs into the spring beams, whereby BACK LINKS Figs. 277, 278. the parallel and radius bars may be made of the same length, and this is in every way a preferable arrangement. The back links and the main links are always of the same length, and their length was, according to Mr. Watt's practice, three-sevenths of the stroke, but they are now generally made half the length of the stroke, or the length of the crank, as we have already stated. The air-pump cross head is inserted in the back links at the middle of their length. This point in the back links moves, it is obvious, in the vertical line, for as the top of the links follows the motion of the main beam, and the bottom that of the radius bars, which have the same radius and the same length of motion as the stud in the beam from which the links are suspended, the central point of the links will have motion in a curve equally removed from that of each end, which will be a straight line very nearly. The line traced by the parallel motion is not precisely a straight line, but a species of S curve; it approaches to a straight line, however, with sufficient nearness for every practical purpose. Notwithstanding the elegance of the parallel motion, as an expedient for maintaining the perpendicular position of the piston-rod, it is questionable whether guides are not to be preferred. In America they are very generally used, even with very long strokes and very short beams; and in some of the steam vessels in this country they have been substituted with advantage. The adjustment of parallel motions is a difficult task in the hands of ignorant persons; and unless the parallel motion be very true it will be difficult to keep good packing in the stuffingbox, and the cylinder will speedily be worn oval. If guides be used, it appears to us expedient that they should consist of strong round rods, and that the eyes at the ends of the cylinder cross-head should be formed into stuffing-boxes, which may be tightened up when the holes wear. The ends of the rods must rest in sockets cast on the cylinder, and the cylinder cover should not be made tight with gasket, which may be compressed more in one part than in another, but should be formed with a metallic joint. If a parallel motion, however, be preferred to the guides, we should suggest its being made in fewer pieces. We see no use whatever in making the main or back links to consist of pillars and straps. Engineers have become wedded to this species of architecture, and the combination is satisfactory to their associations, but unbiassed judges, we humbly conceive, would prefer suitable pieces of solid iron, with holes and brasses in the right places. If guide-rods be used, however, such as we have suggested, it will be necessary to make the lower portions of the main links with straps and cutters, as in the connecting-rod of a marine engine; as the stuffing-boxes at the ends of the cross-head, for the reception of the guiderods, could not be passed through holes in the links. Figs. 271. and 272. represent the air-pump cross-head, which fits into the central brasses of the back links. Fig. 273. is the pillar which fits Figs. 271, 272. -34-- -3% -1,37 Fig. 279. leable iron connecting rods are now coming into use for land engines, and they are in every way preferable. When the connecting-rod is of castiron, of the form represented in the figure, the breadth across the arms of the cross is made about th of the length of the rod; the sectional area at centre of rodth of the area of the cylinder, and the sectional area at ends of rod th of the area of the cylinder. Fig. 279. shows the form of sectional area at centre. The length of the connecting-rod is generally made about three times the length of the stroke. The diameter of the crank pin is about one sixth of the diameter of the cylinder, and is generally made of cast-iron in land engines. The gudgeons of water wheels are generally loaded with about 500 lbs. for every circular inch of their transverse section, which is nearly the proportion which obtains in the end studs of engine-beams, but the main centre is usually loaded beyond this proportion. To find the proper size of a cast-iron gudgeon adapted to sustain a SECTION OF CONNECTING. ROD AT CENTRE. inch=1 foot. LL 2 given weight, multiply the weight in pounds by the intended length of bearing expressed in terms of the diameter, divide the product by 500, and extract the square root of the quotient, which is the diameter in inches. For malleable iron the operation is the same, but the divisor may be made 1000 instead of 500. These strengths are not intended to resist torsion, but are those proper for gudgeons. Experiments upon the force requisite to twist off cast-iron necks show, that if the cube of the diameter of the neck in inches be multiplied by 880, the product will be the force of torsion in pounds which will twist them off when acting at 6 inches radius. The strength for cast-iron crank shafts may be determined by multiplying the square of the diameter of the cylinder in inches by the length of the stroke in feet, multiplying by the decimal 15, and extracting the cube root of the product, which is the proper diameter of the shaft-neck in inches. This rule has reference not merely to torsion, but also to the strength as a gudgeon necessary to sustain the fly-wheel. We have already, in pages 157. and 165., given rules for determining the proportions of fly-wheels. Messrs. Fenton and Murray use the following rule for determining the weight of the fly-wheel, which is simpler perhaps than those we have given:- Multiply the number of horses power of the engine by 2000, and divide the product by the square of the velocity of the circumference of the fly-wheel in feet per second: the quotient is the proper weight of the fly-wheel in hundred weights. To find the weight of the rim of a fly-wheel in pounds, multiply the mean diameter of the rim in feet by the area of its transverse section in square inches, and multiply the product by 9-817 lbs. This gives the weight of the rim in pounds when the sectional area is determined, as may be done by the rule given at page 157. Mr. Farey gives the following rule for determining the proper quantity of cast-iron in a fly-wheel in cubic feet: -Multiply the mean diameter of the rim by the number of its revolutions per minute, and square the product for a divisor; divide the number of horse power exerted by the engine by the number of strokes the piston makes per minute; multiply the quotient by the constant number 2,760,000, and divide the product by the divisor found as above. The quotient is the requisite quantity of castiron in cubic feet to form the fly-wheel rim. In large engines each arm is cast separate, and after having been fitted to the central boss, the rim of the wheel is fitted to the arms in segments. In small engines, such as that of which the fly-wheel is shown in fig. 280., an arm and a segment are generally cast together. In mill engines it appears expedient to work with a short stroke and rapid piston, whereby the fly-wheel is made more effectual, or a smaller one will suffice. Combined oscillating engines, working with a high speed, will probably come into extensive use for turning mills; and if the arrangements be judiciously made, the fly-wheel may in time be dispensed with altogether. We have given rules for proportioning cast-iron cranks in page 156., and have little now to add upon the subject. We do not approve of the plan of putting cast-iron cranks on hot, as the eye is liable to be cracked in the process: it is preferable, we conceive, to grind As nearly all rotative land engines give mo- diameters of toothed wheels should always be such as to enable a number of teeth to be in action at the same time; and pinions should not have less than thirty or forty teeth to enable them to work satisfactorily. Bevelled wheels act better than spur wheels, and wheels with internal teeth better than either; for the more nearly the lines of motion approximate, with the less velocity and shock will the teeth come together. Wheels are usually made with one tooth more or less than a number that will divide the teeth of each equally: this tooth is called a "hunting cog," and its effect is to bring every tooth of the one wheel successively in contact with every tooth of the other. In speeds above 220 feet in the minute, wooden teeth should be introduced in the larger wheel, and these teeth should be a little thicker than the iron teeth, to make them of equal strength. To find the proper dimensions of the teeth of a cast-iron wheel which is required to transmit a given power, Mr. Farey proposes to multiply the diameter of the pitch circle in feet by the number of revolutions to be made per minute, and reserve the product for a divisor. Multiply the number of horse power to be transmitted by 240, and divide the product by the above divisor: the quotient is the strength. If the pitch be given, to find the breadth divide the above strength by the square of the pitch in inches; or if the breadth be given, then to find the pitch divide the strength by the breadth in inches, and extract the square root of the quotient, which is the proper pitch for the teeth in inches. Mr. Hick gives the following rule for computing the power that the teeth of wheels are capable of transmitting:- Multiply one fourth of the square of the pitch in inches by the breadth of the teeth in inches: the product is the number of horse power that the teeth will transmit when the pitch line passes through 4 feet per second. The length of the teeth, or their projection from the rim of the wheel, is usually about five eighths of the pitch. The breadth of teeth varies from 1 to 4 times the pitch, the greatest breadth being made where there is the greatest liability to wear. If only one pair of teeth be supposed to be in contact, the force transmitted by teeth in average cases may be taken at 550 lbs. for each square inch of surface in contact. In several recent instances wheels have been made in steps, or a wide wheel has been compounded of several narrow wheels set in contact on the same shaft, but the teeth of each slightly in advance of the teeth of the next succeeding, so that the pitch is divided, and the necessary strength is reconciled with what is virtually a fine pitch. Figs. 283. and 284. are representations of the eccentric rod, and figs. 285. to 289. are views of the several parts of the governor. We have given in page 168. rules for determining the proportions of governors, and the arrangement of the parts here figured will be apprehended by a reference to fig. 243. Fig. 285. is the upright revolving spindle on which the collar, ECCENTRIC HOOP AND ROD. |