In outlining this course of study in machine design it has been considered wise to depart from the customary methods employed in presenting the subject, i. e., that of dealing with a number of isolated problems that have no relation one to the other, and in its place to take up the study of completed machines, analyze their mechanisms and design the various details. To design any piece of machinery in an intelligent manner it is necessary that the person be acquainted with the conditions surrounding that piece in the machine, hence the first thing to do is to analyze the mechanism of the machine and find the magnitude and direction of the acting forces; this will give him some thread and meaning to his work. After a general plan has been laid for the work then each individual piece can be taken up, in its logical order and designed to meet the conditions. All parts that admit of theoretical analysis should be so treated and when such treatment is not apparent, or for some unknown cause cannot be applied the parts may be proportioned from existing and parallel cases.

The following pages, are not presented as an exhaustive treatise on machine design but rather as a comprehensive analysis of typical machines, illustrating how such work would be handled.

In compiling the notes, frequent reference was made to the following standard works on Machine Design, to each of which the author is indebted for much valuable material. The student should make much use of these works throughout the Design Course.

-    The Constructor, Reuleaux
-    Elements of Machine Design, Parts I and II, Unwin
-    Machine Design, Parts I and 11, Jones
-    Machine Drawing and Design, Low and Bevis
-    Mechanical Drawing and Machine Design, Reid and Reid
-    Mechanical Engineers' Pocket Book. Kent
-    Cambria and other Trade Books

Machine Design - In taking up the study of the design of machine parts our attention is called to the Concrete Side of Mechanism and Mechanics; Machine Design is an analysis of the definite forces acting upon pieces of material which must in turn be designed of sufficient size to resist these forces.

Resolution of Forces - It will be assumed in this text that the student is familiar with the elementary work in forces and that a suggestion to review this topic and become thoroughly familiar with it will be all that is necessary here. Design No, i is calculated chiefly from the composition and resolution of forces hence this topic should be carefully considered.

In the above be careful to thoroughly understand and be able to apply the parallelogram, the triangle and the polygon of forces.

The following general statements concerning forces should be well understood;

(1) Any force has a definite effect on a body no matter if that body is in motion or at rest.

(2, Any number of forces acting on a body will cause it to remain at rest, or to move in the direction of the resultant of the forces.

Forces act in one of five ways relative to each other: First, in the same straight line, and in the same direction, in which case we have the sum of the forces. Illustration, a ship propelled downstream under the action of the current and the engines; Second, in the same straight line and in opposite directions in which case we have the difference of the forces. Illustration, the ship propelled upstream; Third, in separate lines that intersect, in which case we have the resultant of the forces as the ship crossing the stream under the action of her engines, and moving down stream by the current; Fourth, parallel forces acting in different lines, and in the same, or in opposite directions; Fifth, forces that are not parallel and do not intersect.

Conditions one, two and three are comprehended in the above while the fourth and fifth conditions bring in the idea of moments, a subject which will occupy the leading thought in Design No. 1, and should be considered by all as one of the fundamental principles of machine design.

Moment Of a Forces - The moment of a force is the product of the force and its lever arm. If the force be measured in pounds and the lever arm in feet the moment will be in foot pounds. If the force be in pounds and the arm in inches the moment will be inch pounds. In most machine design calculations the latter would be used. When a system of forces acting on a body is in equilibrium, the sum of the moments acting in one direction about any given axis is equal to the sum of all the moments acting in the opposite direction about the same axis.

Load - A load is a combination of external forces acting on a piece of a structure, or machine. Illustrations: A weight carried by a rope; a weight supported on a beam; power transmitted through shafting or belting, etc., etc.

Loads are classified according to action as dead load, live load, and shock. They are also classified as to distribution, as concentrated and distributed.

A dead load is one that is applied slowly and remains constant. A live load is one that is continually changing, but is not subjected to any sudden applications. Shocks are due to sudden applications and withdrawals usually alternately in opposite directions. The first and second can well be represented as the dead weight of the bridge, and the live load of the train passing over it. The third can be represented as the load on the piston rod, or the connecting rod of an engine, A concentrated load is one that is applied at one point and a distributed load is covered more or less evenly over the surface. Distributed loads are commonly uniformly distributed, as for example, the load due to the floor of a bridge, also the uniform weight of the floor supports.

Effect of a Load - When a load is applied to a body it produces a change of form, or strain, in that body. It also produces a corresponding stress on the fibres of the material. The change of form then is known as strain, and is usually expressed as a certain amount per unit of length. That internal force which is called into play to resist this deformation is called the stress, and is expressed in pounds per square inch. In machine design we are usually concerned with the stress and less often have to consider the strain.

Factor of Safety - The Factor of Safety is the ratio of the ultimate stress to the working stress. This is assumed by the designer from his knowledge of the kind of work the machine will perform To illustrate: In machine construction every piece sustains some sort of a load. If this load is a dead load the piece can be used under a greater fibre stress than if it be subjected to a shock. Consequently the factor of safety will be smaller. Conversely, if the piece is under shock the factor of safety should be taken large so the working stress would be low.

It is sometimes confusing, for the person just taking up the subject of Machine Design, to understand why the factor of safety should vary through such wide limits in the same material when used under different conditions. All materials, when subjected to continued usage, become weaker and the ultimate strength, after much service, is less than it was at first. This is more noticeable in materials subjected to live loads and shocks than in those subjected to dead loads. If the ultimate strength of new metal be taken as one, then subjecting this material continuously, for a considerable time to a live load with stress of one kind only, the ultimate strength may fall to about one-half; also if subjected to a live load of equal and opposite stresses, the ultimate strength may fall to one-third. Looking at the question from this standpoint, we see that the ultimate strength of the metal is really the variable factor.