Mechanisms and mechanical movements
MECHANISMS AND MECHANICAL MOVEMENTS
A treatise on different types of mechanisms and various methods of transmitting, controlling and modifying motion, to secure changes of velocity, direction, and duration of time of action.
BY FRANKLIN D. JONES
NEW YORK; THE INDUSTRIAL PRESS; 1919
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Mechanisms and mechanical movements
PREFACE
This treatise on mechanisms and mechanical movements is intended for designers of machinery and for all interested in originating new mechanical devices or in developing and perfecting those now in use. In view of the fact that there is an almost endless variety of mechanisms, it might seem impracticable to deal with such a broad subject in a single volume of this size. As the classes of mechanisms, however, which differ radically in principle, are few in comparison with those which simply vary in form, it was considered not only practicable, but very desirable, to present in one volume a variety of mechanical devices representing different types of mechanisms and selected especially to illustrate important fundamental principles.
The designers of machines or mechanisms in general are constantly engaged in the solution of problems pertaining to motion and its transmission. The motion derived from some source of power must be modified to produce certain effects, and various changes in regard to velocity, direction, and time of action may be necessary. Frequently, the same result may be obtained by forms of mechanisms which differ entirely in principle and effectiveness, and it is essential to employ an approved method. The purpose of this treatise is not only to explain how various mechanical motions may be produced and controlled, but to show the relation between the theoretical and practical sides of the subject. The examples include many ingenious mechanical combinations and are practical designs which not only illustrate the principles involved, but indicate exactly how those principles are applied. An understanding of these concrete examples will prove much more beneficial than a study of abstract theories, which only give an inadequate conception of their application in the design of mechanisms of various types.
Many technical graduates and draftsmen understand the proportioning of parts to safely withstand certain stresses more thoroughly than they do the use of different combinations of parts either for transmitting, reversing, or otherwise modifying motion to secure whatever action or effect may be required. Frequently, the stress involved or the strength of the parts is of little importance, and the principal problem is one pertaining to motion, especially in the development of new forms of mechanisms. While a general knowledge of mechanisms and their possibilities could be obtained by studying miscellaneous designs, this would involve considerable duplication of effort, because so many mechanical devices which vary as to form and purpose are identical in principle. The different forms of mechanisms described in this volume represent many distinct types, and they have been classified and arranged so that various modifications of the same general type may readily be compared.
The columns of Machinery were of valuable assistance in supplying information and illustrations regarding various types of mechanisms, especially of the classes common to the machine- building and machine-tool fields. Special mention should be made of the excellent examples of mechanisms obtained from the contributions of G. W. Armstrong and G. M. Meyncke. The study of mechanical movements is of especial importance at the present time, owing to the increasing use of automatic machines in almost every branch of manufacture, and this treatise is published in the belief that it will be of practical value to many designers, draftsmen, mechanical engineers, and inventors engaged in originating and planning new developments.
CONTENTS
Chapter I
MOTIONS AND GENERAL METHODS OF TRANSMISSION IN MACHINES
Classes of Motion - Velocity and Acceleration - Velocity Ratio - Angular Velocity - Link Mechanisms - Universal Joint - Straight-line Motions - Toggle Joint - Pantograph Mechanisms - Transmission by Frictional and Toothed Gearing - Transmission by Flexible Bands, Ropes, and Chains - Trains of Mechanism - Analyzing Action of Epicyclic Gearing
Chapter II
SPEED-CHANGING AND CONTROLLING MECHANISMS
Types of Mechanical Speed-changing Mechanisms - Arrangement of Cone-pulley Drives - Combination of Cone pulley and Gearing - Geared Speed-changing Mechanisms - Frictional Speed-changing Devices – Multiple disk Type of Speed-changing Mechanism - Friction Disk and Epicyclic Gear Combination - Governors of Centrifugal and Inertia Types
Chapter III
CONVERSION OF ROTARY AND RECTILINEAR MOTIONS
Crank and Connecting-rod - Relative Motions of Crank-pin and Cross-head - the Eccentric - the Crank and Slotted Cross-head or Scotch Yoke - Cylinders which Revolve about a Stationary Crank - Cylinders which Revolve within an Eccentric Track - Rack and Gear Combination - Methods of Doubling Stroke - Single- and Double-stroke Toggle Mechanisms - Press-bed Motions for Flat or Cylinder Presses - the Napier Motion - Reciprocating Motion from Epicyclic Gearing
Chapter IV
REVERSING MECHANISMS
Intermediate Spur Gears for Reversing Motion - Bevel Gear Type of Reversing Mechanism - Reversal of Motion with Friction Disks - Operation of Reversing Clutches - Controlling Point of Reversal by Special or Auxiliary Mechanism - Planer Reversing Mechanism - Reversal of Motion through Epicyclic Gearing - Automatic Ratchet Reversing Mechanism - Automatic Control of Spindle Reversal - Automatic Variation in Point of Reversal - Reversal of Motion after Predetermined Number of Revolutions
Chapter V
QUICK-RETURN MOTIONS
Quick-return Motion from Crank and Oscillating Link - Whitworth Quick-return Motion - Modification of Whitworth Motion - Quick-return Motion from Elliptical Gearing - Eccentric Pinion and Elliptical Gearing for Quick-return Motion
Chapter VI
INTERMITTENT MOVEMENTS
Ratchet Gearing - Ratchet Mechanisms for Releasing Sprockets - Automatic Disengagement of Ratchet Gearing - Escapements - Automatic Reduction of Intermittent Movement - Gearing for Uniform and Variable Intermittent Motion - High-speed Intermittent Gearing of Moving Picture Projector - the Geneva Type of Intermittent Gearing - Intermittent Gears for Shafts at Right Angles - Adjustable Intermittent Motion - Automatic Variation of Intermittent Motion - Automatic Indexing Mechanism - Action of an Adding Mechanism
Chapter VII
IRREGULAR MOTIONS
Plate Cams - Positive Motion Cams - Return Cam for Follower to Secure Positive Motion - Yoke Type of Cam Follower - Inverse Cams - Wiper and Involute Cams - Cylinder or Barrel Cam - Automatic Variation of Cam Motion - Varying Dwell of Cam Follower - Automatic Variation of Cam Rise and Drop - Sectional Interchangeable Cams for Varying Motion - Mechanism for Engaging Cams in a Group Successively - Obtaining Resultant Motion of Several Cams - Double-shifting Cam
Chapter VIII
DIFFERENTIAL MOTIONS
Differential Screw - Chinese Windlass - Differential Motions from Epicyclic Gearing - Compound Differential Gears for Varying Speeds - Differential Motion between Revolving Screw and Nut - Differential Feeding Mechanism for Revolving Spindle - Application of Floating Lever Principle - Controlling Mechanisms for Steam Steering Gears - Substitute for Floating Lever - Differential Governors for Water Turbines - Differential Gearing of Automobiles - Speed Regulation through Differential Gearing - Differential Action through a Cam-controlled Gear - Differential Mechanism of a Gear-cutting Machine - Differential Hoisting Mechanism - Differential Speed Indicator
Chapter IX
CLUTCHES AND TRIPPING MECHANISMS
Controlling Motion by Means of Clutches - Positive and Friction Clutches - Multiple-disk Friction Clutches - Rapid-acting Multiple-disk Clutch equipped with Brake - Magnetic and Induction Clutches - Clutches that Automatically Disengage - Automatic Tripping Mechanisms for Stopping a Machine or some Moving Part - Breakable Pins to Prevent Overload - Automatic Clutch Control to Prevent Overload - Pressure of Frictional Gearing varied according to Load - Automatic Relief Mechanisms for Forging Machines - Automatic Speed-limiting Devices - Electromagnetic Tripping Devices
Chapter X
AUTOMATIC FEEDING MECHANISM
Automatic Feeding Attachments having Inclined Chutes - Attachment for Automatically Feeding Pinion Staffs - Magazine Attachment for Narrow Bushings - Revolving Magazine Attachment - Hopper Feeding Mechanism for Screw Blanks - Simple Arrangement for Feeding Shells with Closed Ends Foremost - Feeding Bullets with Pointed Ends Foremost - Feeding Shells Successively and in any Position - Feeding Shells Successively and Gaging the Diameters
CHAPTER I - MOTIONS AND GENERAL METHODS OF TRANSMISSION IN MACHINES
Machines of various classes are designed to modify energy and adapt it to useful work. The energy is derived from some natural source and is transmitted through the members composing the mechanical device to the place where work is to be performed. The construction of any machine or mechanism involves, first, a combination of parts which will produce the necessary motion, and, second, the formation and proportioning of these parts so that the required amount of energy may be transmitted. In the design of any machine, then, there are two distinct branches of work. One branch pertains to motion and the other to the magnitude of the forces involved, and the mechanical means for transmitting them without breakage or excessive distortion of the different machine members. Evidently, the means for obtaining the right kind of motion and of modifying it to suit specific purposes may be studied without considering the forces which are to act upon the machine parts or the proportioning of these parts with reference to stresses, etc. This volume deals principally with various well-known mechanical movements and contains illustrated descriptions of mechanisms which have been applied to many different types of machinery.
Classes of Motion. - When motion of a machine part does not vary in direction, it is said to be continuous; when the direction of motion reverses, it is reciprocating, and when there are periods of rest, it is intermittent, A body in motion may be free or constrained. The motion is said to be free when the body may move in any direction in accordance with the forces acting upon it, whereas the term constrained motion means that the direction of movement is confined to a restricted path. The planets in their flight through space are examples of free motion, the path or orbit of a planet being determined by the resultant of all the forces acting upon it. The moving parts of every machine represent examples of constrained motion. For instance, the cross-head of an engine is constrained and caused to move in a straight path by a guide or guides. Owing to the angular positions of the connecting-rod, the cross-head is subjected to thrusts which would cause it to move laterally were it not for the straight guiding surfaces that are strong enough to resist the opposing force. A shaft which revolves in fixed bearings is another simple example of constrained motion. The characteristic feature of constrained motion is that all points in a body having such motion follow definite paths when the action of any force produces motion. In ordinary machine construction, the forces tending to move a constrained part from the desired path are not absolutely counteracted, because the fixed members are deformed somewhat under stress; the degree of such deflection may readily be reduced to practicable limits, as it depends upon the dimensions, form, and physical characteristics of the parts which oppose the stresses.
Plane Motion. - Practically all of the movable parts of machines have either a plane motion, a helical motion, or a spherical motion. A body has a plane motion when all points in that body move in parallel planes. Nearly all movable machine parts have plane motion.
Helical Motion. - When all points in a body have a motion of rotation about a fixed axis, combined with a translation parallel to the axis, this is known as “helical motion.” The movement of a nut along a screw is a common example of helical motion.
Spherical Motion. - When all points in a body move in the surfaces of imaginary spheres and at constant distances from a fixed point or common center, the motion is "spherical." There are comparatively few examples of spherical motion in machine construction.
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Classes of Motion. - When motion of a machine part does not vary in direction, it is said to be continuous; when the direction of motion reverses, it is reciprocating, and when there are periods of rest, it is intermittent, A body in motion may be free or constrained. The motion is said to be free when the body may move in any direction in accordance with the forces acting upon it, whereas the term constrained motion means that the direction of movement is confined to a restricted path. The planets in their flight through space are examples of free motion, the path or orbit of a planet being determined by the resultant of all the forces acting upon it. The moving parts of every machine represent examples of constrained motion. For instance, the cross-head of an engine is constrained and caused to move in a straight path by a guide or guides. Owing to the angular positions of the connecting-rod, the cross-head is subjected to thrusts which would cause it to move laterally were it not for the straight guiding surfaces that are strong enough to resist the opposing force. A shaft which revolves in fixed bearings is another simple example of constrained motion. The characteristic feature of constrained motion is that all points in a body having such motion follow definite paths when the action of any force produces motion. In ordinary machine construction, the forces tending to move a constrained part from the desired path are not absolutely counteracted, because the fixed members are deformed somewhat under stress; the degree of such deflection may readily be reduced to practicable limits, as it depends upon the dimensions, form, and physical characteristics of the parts which oppose the stresses.
Plane Motion. - Practically all of the movable parts of machines have either a plane motion, a helical motion, or a spherical motion. A body has a plane motion when all points in that body move in parallel planes. Nearly all movable machine parts have plane motion.
Helical Motion. - When all points in a body have a motion of rotation about a fixed axis, combined with a translation parallel to the axis, this is known as “helical motion.” The movement of a nut along a screw is a common example of helical motion.
Spherical Motion. - When all points in a body move in the surfaces of imaginary spheres and at constant distances from a fixed point or common center, the motion is "spherical." There are comparatively few examples of spherical motion in machine construction.
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