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Modern Mechanical Engineering

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Modern Mechanical Engineering MODERN MECHANICAL ENGINEERING MODERN MECHANICAL ENGINEERING A PRACTICAL TREATISE WRITTEN BY SPECIALISTS EDITED BY A. H. GIBSON, D.Sc. M.Inst.C.E., M.I.Mech.E. AND ALAN E. L. CHORLTON C.B.E., M.Inst.C.E., M.I.Mech.E,, M.I.E.E. VOLUME IV THE GRESHAM PUBLISHING COMPANY LTD * 66 Chandos Street, Covent Garden, London 1923 <.r>' ..! Printed and found in (trftft /Iritoftr OONTRNTS IV MKl'II AX1SM iAMi-:s j. t;rt.sr, ,M.A,, MACUINK DRAWING AND DKSltJN !.; !!* UAUOUJ UAVIH II. HrAWSii AmtHMwm ...... fu ill, Slum AW* KuTATlNc; TAtttJi - fK IV, (V>KsttiwMKn AWI> lUn.ia .,... 74 V, |*l*lMtt AW I*lttrtN*ltrim ^ ...... 79 VI. (*|t^ttUt^, Ut>W>lP*( AWtt Ktt^KTItlt^ &4 vn, cvi.i?w n\w ........ 87 VIII, VM,VI .......... 91 IX. CfKMtiNO If*i,r, Rom, A*w 'IVx/ru ^8 RIViTRI AND R1VKTO1) t)y ANDRKW . UNIWAY APPLIED 1IKAT m w. H. IIAVIIIV'MN'. ns, . rtin,, IMC. ,-<i Ih K V. V, tttSAC'ttt,. U.U.K., M.A,, H,iteN A,M,tiwi . II, T 1(1. i* vi CONTENTS CHAP. Page IV. COMBUSTION 153 V. THE LAWS OF CHANCE OF PHYSICAL STATE .... 170 VI. TABLES OF THE PROPERTIES OF VAPOURS 180 VII. THERMODYNAMICS 185 VIII. THE EFFICIENCY OF THE THERMAL CYCLES OF INTERNAL-COMBUS- TION ENGINES 209 IX. THE EFFICIENCY OF THE THERMAL CYCLES OF STEAM-ENGINES AND REFRIGERATING MACHINES - 215 - APPENDIX I STEAM TABLES 225 APPENDIX II CHARTS AND TABLES SHOWING PROPERTIES OF SUB- STANCES USED IN REFRIGERATION - - - - - 231 STEAM BOILERS By J. M. DICKSON, B.Sc., A.M.I.Mech.E. INTRODUCTORY - 237 I. VERTICAL BOILERS 237 II. HORIZONTAL CYLINDRICAL BOILERS (SMOKE-TUBE TYPE) - - 243 III. WATER-TUBE BOILERS 263 IV. SUPERHEATERS ---, 286 V. MECHANICAL STOKERS -------- 296 VI. FEED-WATER HEATERS 305 VII. OIL FIRING 312 MECHANISM BY JAMES J. GUEST, M.A., M.I.Mech.E. Professor of Mechanical and Electrical Engineering, Artillery College, Woolwich vox, rv. i 48 Mechanism In General. A machine consists of an assembly of parts grouped together to serve a special purpose and usually inter-related in such a manner that there is only one degree of relative freedom between the parts. These parts are either rigid, which have contacts of the higher or lower type (Vol. II, p. 90); elastic, as springs; flexible, as belts; and sometimes fluid. One part in a machine is usually regarded as fixed and is termed the frame. If the relation- ship of the parts is such that the position of a part is definitely controlled by the position of the driving member of the machine, the part is said to be positively driven, distinguishing it from cases, such as those including a belt or hydraulic transmission, in which the relative positions of the parts may change owing to slip or leakage. The primary conception of a machine for any purpose is geometrical, i.e. the parts must go through the sequence of constrained movements required. In pure mechanism this is the view taken, the action between the individual parts and simple, or otherwise important, combinations being geometrically analysed. The next view is that which regards it as trans- mitting energy (by means of forces and stresses) involving the use of suit- able resistant material, the mass of which may demand dynamical con- sideration. The machine is then to be regarded from the point of view of use, and provision made for suitable lubrication and for the adjustment (in necessary cases) for the wear which is expected. Beyond these there are further aspects, such as the convenience and general suitability of the machine, both for its special purpose and for its manufacture. The three lower pairs, involving continuous surface contact, are suited for the transmission of large forces: they are comparatively easy to produce accurately, and wear is readily detected and usually easily compensated for. Hence they are of high importance in mechanism. Lubrication. While bearings have been lubricated from time imme- morial, the first critical study of the subject was the experimental work carried out by Mr. Beauchamp Tower, under the auspices of the Institution of Mechanical Engineers (Proceedings, 1883, 1885), anc^ *^e analysis of this work by Professor Osborne Reynolds. The main actions in lubrication were then elucidated, but experimental work continues on some aspects of the problem. Surfaces working together must not seize and should run with the least MECHANISM possible friction, which turns energy into heat, mechanically wasting it. The lubricating oil becomes more fluid with rise of temperature, lowering the friction but becoming more liable to be squeezed out by the pressure. Thus oil must be selected to have suitable properties at the running tem- perature, and the rise in temperature is often considerable when the speed is high. " " The ordinary laws of friction were deduced by Morin from his experiments on sliding surfaces, under conditions of small velocities and low pres- sures, and the results obtained were the frictional force F was proportional to the normal force R, and independent of the velocity of sliding and of the area of contact, so that F = M&, the coefficient of friction /u. being about 0-15 for metal on metal, dry " " or smeared with a contamination film only, and " 0-075 when the surfaces were well lubricated ". In Mr. Tower's ex- periments, made on a journal 4 in. diameter X 6 in. long, with a loaded gun-metal bearing on the upper part of the journal, loaded and speeded as in railway practice, and lubricated in some experiments with an oil bath in others and by an oiled pad or siphon, entirely different laws with much lower coefficients of friction were found. The results were most consistent with the perfect lubrication of the oil bath, and it was found that a con- Fig. 2 tinuous film of lubricant was then carried by the journal into the space between it and the bearing, a high pressure being generated in the film. The oil travelled through the small space between the journal and bearing and was delivered on the other side. For lubrication purposes a certain small difference of diameter of journal and bearing is necessary, and in action the axes of journal and bearing are not quite coincident but somewhat as sketched in fig. i, the oil film varying in thickness. 1 Oil -film Pressure. The pressure in the film at various points was to 2, measured and was found be as shown in fig. (Proc. Inst. Mech. E., 1883). MECHANISM When the oil film is perfect and continuous, the surfaces of journal and a viscous bearing are completely separated by it, the action consisting of shear in the film so that the materials of which the parts are composed are then of no importance. If the load be continuously increased the pressure ultimately ruptures the film and later squeezes out the oil until the sur- faces of journal and bearing come into contact, and if the pressure be high enough the bearing seizes, the materials becoming united by a kind of cold welding. To lessen the risk of seizing, with its consequent damage, the materials of journal and bearing should be different with the exception of cast iron, which works well on cast iron under low pressures. The pairs of metals for journal and bearing are usually considered to be in the follow- ing order of merit : Hardened steel and hardened steel. Hardened steel and bronze (preferably phosphor bronze). Mild steel and white metal. Mild steel and brass. Cast iron and cast iron. Mild steel and bronze. Mild steel and mild steel. As seizing depends upon squeezing out the lubricant, the more viscous it is the less the risk, and this can be further lessened by the use of solid lubricants (graphite, &c.), but the friction of these is so high as to prevent " " their general use. Besides viscosity a certain oiliness is necessary in lubricating oils: it appears to be dependant on the presence of a small amount of fatty acid. For that reason mineral oils are blended with those of an animal or vegetable origin to produce satisfactory lubricants. When the oil film is perfect the friction is very low compared with any of Morin's values, and it is nearly independent of the total load and hence varies inversely as the pressure. Some of Mr. Tower's results showing the variation of friction with load and with speed are given in the following table: TABLE VI OF MR. TOWER'S PAPER IN PROC. INST. MECH. ENG., 1883, p. 146 (Bath of mineral oil. Temperature 90 F. Medium oil fluid at 50 F. Journal 4 in. diameter X 6 in. long. Chord of arc of contact of mass = 3-92 in. Actual load = 4 X 6 X Normal load). MECHANISM The oil film is of the order of a thousandth of an inch in thickness, and in the of limits for fits the thickness is an important factor fixing running ; can decrease until films of single molecule thickness corresponding nearly to Morin's dry metals are reached. These ultimate films are difficult to remove unless abrasion, as in brake blocks, occurs. There appears to be no actual discontinuity between starting and running than friction, the coefficient, after becoming slightly higher at a low velocity initially, falls as the speed increases and the liquid film establishes itself, and after a certain velocity the friction varies as the square root of the velocity. When the oil film is broken the action becomes irregular, the friction lying between oil-film values and those Fig. 3 found by Morin.
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