worth's Tables of Standard V-Threads, Nuts and Bolt Heads -Seller's V-Thread - The Square Thread -The Round Thread - The Buttress Thread - Right- and Left-Hand Screws-The Screw Coupling for Railway Carriages-Single, Double, and Treble Threaded Screws-Backlash in Wheel and Screw Gearing-Questions PAGES 148-159 Efficiency of Combined Lever, Screw, and Pulley Gear-Example I. Bottle Screw-Jack-Example II.-Traversing Screw-Jack -Screw Press for Bales-Screw Bench Vice-Example III. -Endless Screw and Worm Wheel - Combined Pulley, General Idea of the Mechanism in a Screw-cutting Lathe-Mo- tions of the Slide Rest and Saddle-Velocity Ratio of the Change Wheels-Rules for Calculating the Required Num- ber of Teeth in Change Wheels-Examples I. II.-Movable Headstock for a Common Lathe-Description of the Screw- cutting Lathe in the Author's Electrical Engineering Workshop, with a complete set of Detail Drawings-Ques- tions 160-173 174-190 Hydraulics-Definition of a Liquid-Axioms relating to a Liquid at Rest-Transmission of Pressure by Liquids-Pascal's Law-"Head" or Pressure of a Liquid at Different Depths -Total Pressure on a Horizontal Plane immersed in a Liquid-Lord Kelvin's Wire-testing Machine-Total Pres- PAGES LECTURE XIX. -- Hydraulic Machines-The Common Suction Pump—Example I.— The Plunger, or Single-action Force Pump-Example II.- Force Pump with Air Vessel-Continuous-delivery Single- acting Force Pump without Air Vessel-Combined Plunger Bramah's Hydraulic Press-Bramah's Leather Collar Packing- Examples I. II.-Large Hydraulic Press for Flanging Boiler Plates-The Hydraulic Jack-Weems' Compound Screw and Hydraulic Jack-Example III.—The Hydraulic Bear or Motion and Velocity-Uniform, Variable, Linear, and Angular Velocity-Unit of Velocity-Acceleration-Unit of Accele- ration-Acceleration due to Gravity-Graphic Representa- tion of Velocities-Composition and Resolution of Velocities -Newton's Laws of Motion-Formula for Falling Bodies- Formulæ for Linear Velocity-With Uniform Acceleration -Centrifugal Force due to Motion in a Circle-Experiments I. II. III.-Example I.-Balancing High-speed Machinery -Centrifugal Stress in the Arms of a Fly-wheel-Example II.-Energy-Potential Energy-Kinetic Energy-Accumu- lated Work-Accumulated Work in a Rotating Body-The Fly-wheel-Radius of Gyration-Example III.—The Fly- 223-239 240-254 Some Properties of Materials employed by Mechanics-Essential Properties-Extension-Impenetrability-Contingent Pro- perties-Divisibility-Porosity-Density-Cohesion-Com- Stresses on Chains-Shearing Stress and Strain-Example I.- Torque or Twisting Movement-Strength of Solid Round Shafts-Example II.-Pressures on and Reactions from the Supports of Beams-Examples III. IV.—-Transverse Stress Bending Moment of Cantilevers, (1) Loaded at End; (2) Load Uniformly Distributed-Bending Stresses-Neutral Surface and Axis-Moment of Resistance opposed to the Bending Moment Strength of Rectangular Beams - Relative Strengths of Rectangular Beams supported and loaded in Different Ways-Illustrations, Explanations, and Formulæ for Rectangular Beams supported in Different Ways -Comparison of the Loads and Sizes of Beams by Pro- Hooke's Coupling or Universal Joint-Double Hooke's Joint- Sun and Planet Wheels-Cams-Heart Wheel or Heart- shaped Cam-Cam for Intermittent Motion-Quick Return Cam-Example-Pawl and Ratchet Wheel-Reversible Pawl Reversing Motions-Planing Machine-Reversing by Friction Cones and Bevel Wheels-Whitworth's Reversing Gear- Quick Return Reversing Motion-Whitworth's Quick Re- turn Motion-Whitworth's Slotting Machine-Common Quick Return-Horizontal Shaping Machine-Quick Re- ELEMENTARY MANUAL ON APPLIED MECHANICS. LECTURE I. CONTENTS.-Definition of Applied Mechanics-Force-Matter-Unit of Force The Elements of a Force-Graphic Representation of Forces -Forces in Equilibrium-Action and Reaction-Resultant and Com. ponents-Resultant of Forces acting in a Straight Line. Applied Mechanics is that branch of applied science which not only explains the principles upon which machines are designed, made and act, but also describes their construction and applications, as well as how to calculate and test their strength and efficiency. Before a student can successfully master any science, he must thoroughly understand the units of measurement that have been adopted in calculating results, and he should also have a clear conception of the exact meaning of the various terms employed. Consequently, we shall commence the study of Elementary Applied Mechanics with definitions and with units of force, work and power.* Force is any cause which produces, or tends to produce, motion or change of motion in the matter upon which it acts. Matter is anything which can be perceived by one or more of the senses, and which can be acted on by force. Matter exists under three conditions: (1) Solids, (2) Liquids, (3) Gases. For example, pieces of wood and of iron are solids; water and mercury are liquids; whilst air and oxygen are gases. * For the units of length, surface and cubic measure, and for the mensuration of areas and solids, the student is referred to Lectures I. II. and III. of Author's "Elementary Manual on Steam and the Steam Engine,' issued by the publishers of this book. A Bodies are therefore limited portions of matter. When the resistance to motion of a body is equal to or greater than the force applied, so that no motion takes place, the body is said to be subjected to pressure. Solids do not yield readily to pressure, for they tend to retain their original shape and size, whereas liquids and gases yield to a very slight pressure, and consequently possess no definite shape. A gas differs from a liquid since it possesses the property of indefinite expansion. A liquid has therefore a definite size, but not a definite shape, whilst a gas has neither definite shape nor definite size. Unit of Force.-The British unit of force is the pound avoirdupois, or GRAVITATION UNIT. The magnitude of a force is therefore reckoned by the number of pounds which the force would support against gravity or by the weight in pounds which would produce the same effect. For example, a force of 100 lbs. means that force which would just lift a weight of 100 lbs. if acted on by gravity alone. But the force of gravity varies at different parts of the earth's surface, being slightly greater at the Poles than at the Equator. Consequently, our unit is not an absolute or invariable one, although for applied mechanics it is the most convenient unit which could be employed.* The Elements of a Force.-When a force acts upon a body, then, in order to fully determine its effect we must know the three following elements:-(1) The point or place of application of the force. (2) The direction in which the force acts. (3) The magnitude of the force. (1) Place of Application.-In the case of the force of gravity acting on a body, the place of application may be considered to be the whole mass of the body, or we may estimate the whole weight of the body as concentrated at one point, termed the centre of *Where great accuracy of measurement is required an absolute or invariable unit of force must be selected. An absolute unit of force may be defined as that force which, acting for unit time on unit mass, will produce unit change of velocity. If the units of time, mass, and velocity be the second, pound, and foot per second respectively, then we may define the absolute unit of force (called the poundal) as that force which, acting for one second on a mass of one pound would produce a change in velocity of one foot per second. It has been determined experimentally that if a body be let fall freely in vacuo, near the earth's surface, the attractive force of the earth will produce a change of velocity every second of g (=322 nearly) feet per second. Clearly, then, the gravitation unit is g times the absolute unit. Hence the following relation between the gravitation and absolute units of force:- force of one pound g poundals, or a force of one poundal = 1/g pound. = In this book the gravitation units of force and work will be used. |