INTRODUCTION FOR the operating engineer or the prospective purchaser it may be of interest to know which design and construction of different parts of an engine may best be selected and which should be avoided. In other words, it would be a good thing if engine troubles in general and particular could be listed, so as to give interested parties a chance to forestall costly and dangerous accidents. In the following pages the troubles which the principal parts of steam engines are subject to are described, good design is contrasted with bad, the most suitable material for certain parts and the most approved construction of the same is pointed out. The advantages and disadvantages of many designs are discussed, the troubles they are heir to and ways to overcome or minimize them. The directions given for correcting existing evils are easily understood, by following them breakdowns and costly accidents can be eliminated. The principal considerations in the building of foundations, setting of templets and lining up an engine are given with a complete account of how to erect the engine in the language of the man on the job. There are directions for lubrication, valve setting and testing, also how to locate trouble with an indicator, and what adjustments are to be made on new engines and maintained on old ones. Speed regulations, so essential to good service, are treated. Simple rules for calculating safe pressures of mainbearings, stresses in flywheels, strength of cylinder walls, etc., are given which enable an engineer to find out whether the engine he runs is constructed on sound principles or not. January, 1919. H. HAMKENS. STEAM ENGINE TROUBLES CHAPTER I CYLINDERS Engine cylinders must be of correct size. Stresses in cylinder walls. Radiation losses. Bolts and nuts. Slide-valve cylinder. Square-corner Corliss cylinder. Lagging. Built-up cylinders. Steam jackets. Sleeves for cylinders. Material for cylinder castings. Insufficient drainage. Location of valves. Inaccessibility of cylinders with valves in heads. Ground joints. Regular joints. THE power of a steam engine is, to a large extent, determined by the size of its cylinder, and its usefulness depends on the construction and material of the same. Many engine troubles are due to the size of the cylinder which is used, and to the power developed; the cylinder may be too large or too small, and the engine may not carry a sufficient load or be overloaded, either one is liable to be a source of trouble. Overloading will put too heavy a strain on the moving parts, while too light a load will be a heavy drain on the coalpile, altogether out of proportion to the work done. Since most of the engines in use are manufactured, that is, made to standard designs and from existing patterns, their power and economical range is dependent on certain predetermined factors, principally steam pressure and speed. Twenty years ago the majority of stationary engines were designed for 80 to 100 lbs. boiler pressure and a piston speed not to exceed 600 ft. per minute. The cylinder walls, size of bolts, thickness of flanges, piston rods and other working parts were designed for the prevailing boiler pressure with a factor of safety more or less determined by practice; tne dimensions of many engines were fixed by a rule of thumb, which answered very well under the existing conditions. As long as boilers and engines are well matched and of the proper size this method may give fairly good results, but as soon as a change is made either by increasing the steam pressure or speeding up the engine in order to get more power the whole arrangement is thrown out of balance and trouble may be expected. Engine cylinders of small bore, 12 ins. or less, have, as a rule, a much larger factor of safety for the cylinder walls and other parts than the larger sizes, for the simple reason that on small castings time is saved in molding and casting by making the castings a little heavier than is absolutely necessary for safety's sake. The time saved more than compensates for the cost of the extra metal used. Medium and larger size cylinders, however, are made mostly without any excess of metal and they are, therefore, more limited in regard to pressure. If the steam pressure is raised to any considerable extent it is well to do some investigating and a little figuring; not only should the strength of the cylinder walls be ascertained, but also that of the steam chest, bolts and other parts, which are connected with the cylinder. It is a good rule to let the combined stresses in the cylinder walls in no part exceed 1500 lbs. per square inch, whether due to steam pressure or other forces. The principal stresses due to steam pressure in the cylinder barrel are tensional, tending to disrupt the metal in two direc 1 tions; one of which is longitudinal and the other circumferential. The latter is practically twice as great as the former, as a simple calculation will show. To get the correct idea of this we will take the two views of a cylinder, shown in Fig. 1, and assume that the inside diameter of the same is I in., that the cylinder walls are 1 in. thick and the steam pressure 100 lbs. per square inch; then the pressure on each half of the circumference for 1 in. in length will be 100 lbs. and the tension on each side will be HO FIG. 1. Stresses in cylinder walls. 50 lbs. per square inch. If both ends are closed the end pressure in both directions will be 0.785 X 100 3.14 = 25 lbs. per square inch. The combined stresses will be 50+25 =75 lbs. per square inch. The cylinder would, there 2000 IO 75 =2000 lbs. per square inch. 10 ins. the safe pressure would be =200 lbs., etc. Without any complications and great refinements a simple calculation of this kind, if applied to a cylinder, will indicate the danger point |
