Elements of machine design - Leutwiler

The purpose of the author, in preparing this book, has been to present in fairly complete form a discussion of the fundamental principles involved in the design and operation of machinery. An attempt is also made to suggest or outline methods of reasoning that may prove helpful in the design of various machine parts. The book is primarily intended to be helpful in the courses of machine design as taught in the American technical schools and colleges, and it is also hoped that it may prove of service to the designers in engineering offices.

Since a text on machine design presupposes a knowledge of Strength of Materials and Mechanics of Machinery, a chapter reviewing briefly the more important straining actions to which machine parts are subjected is included as well as a chapter discussing briefly the properties of the common materials used in the construction of machinery. Furthermore, throughout the book, the question of the application of mechanical principles to machines and devices has not been overlooked, and many recent devices of merit are illustrated, described and analyzed. A considerable amount of the material in this book was published several years ago in the form of notes which served as a text in the courses of machine design at the University of Illinois.

In the preparation of the manuscript the author consulted rather freely the standard works on the subject of machine design, the transactions of the various national engineering societies and the technical press of America and England. Whenever any material from such sources of information was used, the author endeavored to give suitable acknowledgment. The numerous illustrations used throughout the book have been selected with considerable care and in the majority of cases they represent correctly to scale the latest practice in the design of the parts of modern machines. At the close of nearly every chapter a brief list of references to sources of additional information is given.

Through the generosity of various manufacturers, drawings illustrating the prevailing practice in America were placed at the author's disposal thus making it possible to use scale drawings for illustrating the various machine parts. To all such manufacturers he is especially indebted. The author is also indebted to Mr. H. W. Waterfall of the College of Engineering of the University of Illinois for the many helpful suggestions and criticisms received during the preparation of the manuscript. To his friend and colleague Professor G. A. Goodenough, the author is deeply indebted for much valuable advice and the many suggestions received in preparing the manuscript, also for the critical reading of the proof.

The transmission of power by means of belting may be accomplished satisfactorily and efficiently when the distances between the pulleys are not too great. When the power to be transmitted is not large, round or V-shaped belts are used, the latter form also being used for drives with short centers. The materials used in the construction of belting are leather, rubber, cotton, and steel.

Leather Belting. The highest grade of leather belting is obtained from the central portion of the hide. This central area is cut into strips which are cemented, sewed, or riveted together to form the desired thickness and width of belt. The thicknesses vary from a single hide thickness to that of four, the former being known as a single leather belt and the latter as a quadruple belt. The terms double and triple belt are used when two or three thicknesses are employed in the construction. The hides from which leather belts are made may be tanned by different processes. For ordinary indoor installations, the regular oak-tanned leather belting is well-adapted. For service in which the belt is exposed to steam, oil or water, a special chrome-tanned leather is recommended. This special tanning process is more or less secret and is guarded by patents. The users of this process claim that a more durable leather is produced, due to the fact the fibrous structure of the hide is preserved and not weakened as may result in the oak-tanning process. Leather belting weighs on an average about 0.035 pounds per cubic inch.

Rubber Belting. Rubber belting is made by fastening together several layers of woven duck into which is forced a rubber composition which subsequently is vulcanized. Belting of this description is used to some extent in damp places, as for example in paper mills and saw mills.

A material resembling rubber, known as balata, is now used extensively in the manufacture of an acid- and water-proof belt. Balata is made from the sap of the boela tree found in Venezuela and Guiana. It does not oxidize or deteriorate as does rubber. The body of the belt, consisting of a heavy woven duck, is impregnated and covered with the balata gum, producing a belting material which is acid and water-proof, and according to tests is about twice as strong as good leather. It is claimed that the heating of the belt due to excessive slippage softens the balata and thereby increases its adhesive properties. Due to this fact, it appears that balata belting is unsuitable for installations where temperatures of over 100F. prevail.

Textile Belting. Textile belts are made by weaving them in a loom or building them up of layers of canvas stitched together. The woven body or strips of canvas are treated with a filling to make them water-proof, and in some cases oil-proof. Generally, belts treated with a cheap filling are very stiff and hence do not conform to the pulley, making it more difficult to transmit the desired power. Textile belts are used more for conveyor service than for the transmission of power.

119. Steel Belting. The transmission of power by means of steel belts was first introduced in 1906 by the Eloesser Steel Belt Co. of Berlin, Germany, and at the present time this method of transmitting power is recognized by many German engineers as being superior to that in which leather belting or ropes are used.

The steel belt is used in the same manner as the leather belt, except that it is narrow, thin and of very light weight. It is put on the pulley with a fairly high initial tension and hence runs without sag. The material used in making steel belts is a char- coal steel, prepared and hardened by a secret process.

The pulleys upon which these belts run are preferably flat, and are covered with layers of canvas and cork so as to increase the coefficient of friction. A crowned pulley may be used, provided the crown does not exceed approximately 33 ten-thousandths of the width of the belt. Steel belts are not adapted to tight and loose pulleys, but crossed belts will work satisfactorily, provided the distance between the shafts is about seventy times the width of the belt.

In case the power transmitted is large, so that a single belt of sufficient width to give the required cross-sectional area cannot be obtained, two or more belts are run side by side. In putting steel belts on pulleys, a special clamp is used in order to measure correctly the initial tension and at the same time to facilitate fitting the special plates necessary to make the joint. The design of a proper fastening for steel belts presented a difficult problem, but after considerable experimental work D. Eloesser, now head of the firm that bears his name, perfected a joint that has proven very satisfactory. His first design was made of one piece and the ends of the belt had to be soldered in place at the installation. The latest design, shown in Fig. 57, consists of several parts fastened together by screws e that are removable. The ends of the steel band are soldered to the main parts of the joint and the small screws/ and g passing through the triangular-shaped steel pieces c and d give added strength to the fastening. The plates a and b that form the main parts of the joint are curved, the curvature depending upon the size of the pulley upon which the belt is to run.

Belt Fastenings. Fastenings of various forms are used for joining the ends of a belt, but none of them is as strong and durable as the scarfed and glued splice, which when made carefully is but little weaker than the belt proper. Of necessity, the scarfed and glued joint or cemented splice is adapted to installations in which the slack of the belt is taken up by mechanical means, and where careful attention is given to belting by competent workmen. Probably the oldest form of fastening, as well as that used most commonly, is to join the ends of a belt by means of rawhide lacing. Not infrequently belts are laced together with wire, and such joints run very smoothly, especially if made with a machine, and are considerably stronger than the rawhide laced joint, as is indicated in Table 36. Patented metal fasteners in the form of hooks, studs, and plates are also in use and have the advantage that they are cheap and applied very easily and quickly. Some of the metal fasteners are too dangerous to be used on belts that must be touched by hand, and for that reason some states have legislated against their use.