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IN CONSTRUCTIVE WORK
WILLIAM M. BARR
by WILLIAM M. BARR
COMPOSITION, ELECTROTYPING AND PRINTING BY PUBLISHERS PRINTING COMPANY, NEW YORK CITY
DEC 13 1320 SBK B7
In the preparation of this handbook the writer attempts a systematic arrangement of a considerable volume of useful information for engineers, much of which has not been readily accessible to the public. The collection includes separate specifications relating to the chemical and physical properties of practically all of the materials entering into engineering work for the U.S. Government. The importance and economic value of the data thus presented will be recognized by manufacturers and engineers engaged in Government work not only, but this value extends into every department in industrial engineering.
The usefulness of this handbook will not rest so much upon the extent of the compilation as upon the practical nature of the data presented; a feature made possible through the free use of working drawings contributed for insertion in these pages. Selections from these drawings appear throughout the entire work in carefully prepared illustrations accompanied in most cases by tables of working dimensions; theee cover a wider range of detail than is common in books of this class. It has been the constant aim of the writer that such data shall be so complete that principal dimensions given in any table may, with suitable adaptations, be used directly in the preparation of shop drawings, and without the labor of recalculating.
Correct proportions, in series, cannot be had by selecting en acceptable detail and making one of its dimensions a unit, and then assigning proportional values to the other dimensions, except within very narrow limits. Suppose a series of strap joints as in the table, page 601; diameters ranging from a 3-inch to a 12-inch pin; the writer's method is to complete two designs similar in detail, one for the smallest and the other for the largest diameter of pin, then measuring the proportional differences graphically obtained for intermediate sizes.
There are numerous machine details which are now designed to be complete in themselves, and with very slight changes made to fit into any machine where such a detail is demanded; many examples of this kind are included in this work; in all cases the nature of the design and the properties of materials entering into it are fully considered and the proportions fixed once for all. Pulleys are a familiar example; they are designed for single or double belts, as also double extra heavy for very severe service, but once designed and patterns made, no further changes occur; the pulley becomes one of many units in a plant requiring no further attention on the part of the designer than the mere selection of size and strength.
So-called empiricism, or the reliance on direct observation and experience to the exclusion of theories or assumed principles in machine design, if it ever existed, is no longer in use; many of the so-called empirical or practical rules are in reality founded upon carefully conducted experiments, or the result of long and methodical observation in the working of machines, the ultimate proportions being fixed to safely carry the load regardless of conventional factors of safety; the latter are not believed to be "factors of ignorance" so much as they are generous allowances made to withstand the effect of forces too complex to be dealt with mathematically or physically. Rigidity depends largely upon the form and details of construction. The chemical and physical properties of any material used in engineering is now known with precision. The data relating to strength of materials in this work are wholly those obtained by direct experiment, mainly in testing machines owned and operated by the U. S. Government.
There will be noticed throughout the book a general tendency toward steam-engine details, due in large measure to the writer's long familiarity with that subject. Two satisfactory types of steam-engines are now in use the modern locomotive engine and