plates 1" thick-steel plates and rivets, drilled holes (see Fig. 142a). EX. 4. Combined lap and cover-plate joint (Fig. 143)---plates "thick-steel plates and rivets, drilled holes. (Centre row of rivets ordinary pitch, outer rows as close in as possible for convenient riveting.) EX. 5.-Lozenge joint (Fig. 146)-plates 1" thick-lapped one over the other without cover plate. Width of plate 15′′. (Shearing strength of rivets to equal tensile strength of plate at weakest section-i.e., through first or second row of rivet holes.) (93) Efficiency of Riveted Joints.—It is evident that a riveted joint cannot possibly be as strong to resist tensile forces as the plate of which it is made, simply because the section through the rivet holes is always less than the section of the solid plate. The ratio between the strength of the joint and of the plate is termed the efficiency of the joint. If the tensile resistance per square inch was the same through the holes as through the solid plate, the efficiency would evidently be pdf. t.p.fr = Ρ Ρ d but as this is not the case, for reasons already seen, the efficiency is d expressed by K. where K = ratio of the resistance per Р square inch through the holes, to the resistance per square inch through the solid plate, and varies with different joints. The "theoretical efficiency » P-d p varies from 75 per cent. to 100 per cent., and the "actual efficiency" K. cent. to 72 per cent. Ρ Ρ و The student will see the importance of knowing the efficiency of the joint in designing riveted structures, as the strength of the structure cannot evidently be greater than the strength of its weakest joint. Hence, in designing a boiler of plates having a tensile resistance per square inch equal to T, the working stress must be proportioned to ET where E = efficiency of joint, since this is the actual strength of the plate. (94) Working Stress. In ordinary boiler and bridge work the working stress may be one-fourth to one-fifth of the breaking stress (Unwin). A high factor of safety is necessary to allow for loss due to corrosion BZ с (95) Connection of more than two Plates. It is necessary in boiler work to rivet joints where three or four plates join, as in the case where the longitudinal and circumferential joints meet. An example of the connection of four plates is shown in Fig. 147, the plates being marked A, B, C, D. The plates B and D have their corners thinned out as shown, so as to lap one over the other, and not exceed the thickness of one plate, each thin corner being lengthened to be held by two rivets. O B Α Fig. 147. Thus the three rivets, 1, 2, 3, pass through three plates. a с (96) Rolled Bars.-In Fig. 148, a, b, c, d, are shown sections of bars commonly used for different connections of plates. They are rolled in long lengths of various standard sizes, in either iron or steel. The width of the flanges increase by 1" for angle bars (a), and by " for other bars. Thickness of flanges increase by for angle and tee bars of small size, for larger sizes and for other bars, by ". The thickness at the root is slightly greater than at the edges. No sections should be shown on a drawing which cannot be found in the makers' lists. b d Fig. 148. (a) Angle Bars.-Made with flanges of equal or unequal width. Sizes, from 3" x 3" x 1" to 6′′ × 6′′ × 3". Weight, from 0.89 lb. to 28.7 lbs. per foot of length. 5 " 16 (b) Tee Bars. Made with the flange F and web W of equal or unequal width. Sizes, from 3′′ (the first size is width of flange). lbs. per foot of length. × 3′′ × to 7" x 10" x 3" Weight, from 7.2 lbs. to 45 (c) Zed Bars.-Made with equal or unequal flanges, and of various depths. Sizes, from 3" x 3" x 24" x 3" to 10" x 3" x 3" x 3" (the first size is the depth d1). Weight, from 10 lbs. to 44 lbs. per foot of length. (d) Channel Bars.-Made with flanges of equal size, and of different depths. Sizes, from 21" x 11" x 1" to 12" x 4" x 1" (the first size is the depth d1). Weight, from 4 lbs. to 33 lbs. per foot of length. (97) Connection of Plates and Bars.-The use of the above bars for different connections of plates in boiler and tank work is shown in Fig. 149, a, b, c, d. (a) Angle bars, used to connect plates at right angles. Notice that the rivets in one flange come between the rivets in the other flange, for convenience in riveting up. (b) Tee bars, used to connect plates in the same plane, and also of great service in stiffening the plates. (c) and (d) Zed and Channel bars, used to connect plates parallel to each other, and commonly employed to connect the inside fire box to the shell plates of a locomotive type of boiler. Notice that the use of the Zed iron gives one inside joint, which is avoided in the Channel iron. The rivets in the Channel iron should be arranged as with angle bars. (98) Flanging of Plates.-The simplest connection of plates at right angles is shown in Fig. 150, where one of the plates is bent to form a flange, which is then riveted to the other plate. The inside radius of the flanged plate should not be less than four times the plate thickness. This is the common construction in boiler work, where the flanged plate is the flat end plate, riveted to the circumferential shell plate. It possesses the great advantage over the angle bar, Fig. 149, a, as giving only one riveted joint. EXAMPLES. Fig. 150. Make working dimensioned drawings, half full size, showing two views of the following connections of plates and bars : EX. 6.-Connection of four iron plates" thick, as in Fig. 147, single riveted iron plates, drilled holes. EX. 7.-Connections of plates and bars as follows, all iron, with iron rivets and drilled holes : THE design of lubricated bearings is a subject of great importance, affecting as it does the efficient working of nearly all classes of machinery. It is far too large to permit of any but general principles being explained here, the student being referred for fuller information to the chapter on "Journals" in Professor Unwin's Machine Design, and to the proceedings of the Institution of Mechanical Engineers' Research Committee on "Friction." All rotating shafts and axles of machines require supporting in such a way that as little as possible of the power they transmit is lost. Such supports have received the general name of "bearings," common examples of which will be familiar to engineering students in their application to shafting in workshops, spindles of lathes, and to the crank shafts of engines. (99) Journals.-That part of the shaft or axle supported by the bearing is known as the "journal." Evidently its diameter and length must be proportioned according to the forces acting upon it and to the power it transmits. Such considerations of strength are, however, too advanced for the limits of this book. But apart from the question of strength a journal must be proportioned according to the pressure it exerts upon the bearing which supports it, and to its other working conditions. (100) Friction of Journals.-One effect of the pressure exerted by a journal upon its bearing is to produce a frictional resistance to movement. Part of the work which the journal is transmitting must be expended in overcoming this resistance, which work is converted into heat, and raises the temperature of the rubbing surfaces. If this increase of temperature continues, the journal will seize in the bearing, producing more or less serious results. The object of lubrication is to reduce this frictional resistance to a minimum, thereby ensuring a good working bearing and making the useful work performed by the shaft as large as possible. = Generally speaking, the friction of rubbing surfaces may be regarded as follows:-Let W = the pressure between the surfaces producing the friction. F the force required to overcome the friction, and, therefore, a measure of the "frictional resistance." Then, in general terms, F = W, where is a certain factor known in mechanics as the "coefficient of friction," but which for journals is not necessarily a simple factor. Evidently then, with a given value of the load W, the smaller the coefficient of friction, the less is the frictional resistance F. The aim in lubricated bearings is to keep as small as possible, and, therefore, to reduce the frictional resistance F to a minimum. The frictional resistance of lubricated bearings may be stated in general terms to depend upon the following conditions: I. The pressure per square inch between the sliding surfaces. II. The velocity of the sliding surfaces (ft. per min.) III. The materials of which the sliding surfaces are composed. VI. The kind of lubricant used, and the method of lubrication. (101) I. Pressure between the Sliding Surfaces.-The effect of increasing the pressure between a journal and bearing is that the lubricant is squeezed out from between the sliding surfaces, and the journal heats and seizes. Evidently, then, the safe working pressure depends entirely upon the manner of lubrication, and hence a bearing which may seize with one method of lubrication will run quite smoothly with another method. For bath lubrication, where part of the journal runs |