Tips and tricks
Self taught mechanical drawing and elementary machine design
SELF TAUGHT MECHANICAL DRAWING AND ELEMENTARY MACHINE DESIGN
A Treatise Comprising the First Principles of Geometric and Mechanical Drawings Workshop Mathematics, Mechanics, Strength of Materials, and the Design of Machine Details, including Cams, Sprockets, Gearing, Shafts, Pulleys, Belting, Couplings, Screws and Bolts, Clutches, Flywheels, etc. Prepared for the Use of Practical Mechanics and Young Draftsmen.
By F. L. SYLVESTER
NEW YORK, THE NORMAN W. HENLEY PUBLISHING CO., 1910
DOWNLOAD FREE BOOK:
Self taught mechanical drawing and elementary machine design
PREFACE
The demand for an elementary treatise on mechanical drawing, including the first principles of machine design, and presented in such a way as to meet, in particular, the needs of the student whose previous theoretical knowledge is limited, has caused the author to prepare the present volume. It has been the author's aim to adapt this treatise to the requirements of the practical mechanic and young draftsman, and to present the matter in as clear and concise a manner as possible, so as to make “self-study” easy. In order to meet the demands of this class of students, practically all the important elements of machine design have been dealt with, and, besides, algebraic formulas have been explained and the elements of trigonometry have been treated in a manner suited to the needs of the practical man.
In arranging the material, the author has first devoted himself to mechanical drawing, pure and simple, because a thorough understanding of the principles of representing objects greatly facilitates further study of mechanical subjects; then, attention has been given to the mathematics necessary for the solution of the problems in machine design presented later, and to a practical introduction to theoretical mechanics and strength of materials; and, finally, the various elements entering in machine design, such as cams, gears, sprocket wheels, cone pulleys, bolts, screws, couplings, clutches, shafting, flywheels, etc., have been treated. This arrangement makes it possible to present a continuous course of study which is easily comprehended and assimilated even by students of limited previous training.
Portions of the section on mechanical drawing was published by the author in The Patternmaker several years ago. These articles have, however, been carefully revised to harmonize with the present treatise, and in some sections amplified. In the preparation of the material, the author has also consulted the works of various authors on machine design, and credit has been given in the text wherever use has been made of material from such sources.
In arranging the material, the author has first devoted himself to mechanical drawing, pure and simple, because a thorough understanding of the principles of representing objects greatly facilitates further study of mechanical subjects; then, attention has been given to the mathematics necessary for the solution of the problems in machine design presented later, and to a practical introduction to theoretical mechanics and strength of materials; and, finally, the various elements entering in machine design, such as cams, gears, sprocket wheels, cone pulleys, bolts, screws, couplings, clutches, shafting, flywheels, etc., have been treated. This arrangement makes it possible to present a continuous course of study which is easily comprehended and assimilated even by students of limited previous training.
Portions of the section on mechanical drawing was published by the author in The Patternmaker several years ago. These articles have, however, been carefully revised to harmonize with the present treatise, and in some sections amplified. In the preparation of the material, the author has also consulted the works of various authors on machine design, and credit has been given in the text wherever use has been made of material from such sources.
CONTENTS
- INSTRUMENTS AND MATERIALS
- DEFINITIONS OF TERMS USED IN GEOMETRICAL AND MECHANICAL DRAWING
- GEOMETRICAL PROBLEMS
- PROJECTION
- WORKING DRAWINGS
- ALGEBRAIC FORMULAS
- ELEMENTS OF TRIGONOMETRY
- ELEMENTS OF MECHANICS
- FIRST PRINCIPLES OF STRENGTH OF MATERIALS
- CAMS
- SPROCKET WHEELS
- GENERAL PRINCIPLES OF GEARING
- CALCULATING THE DIMENSIONS OF GEARS
- CONE PULLEYS
- BOLTS, STUDS AND SCREWS
- COUPLINGS AND CLUTCHES
- SHAFTS, BELTS AND PULLEYS
- FLY-WHEELS FOR PRESSES, PUNCHES, ETC.
- TRAINS OF MECHANISM
- QUICK RETURN MOTIONS
CHAPTER X - CAMS
General Principles. In designing machinery it is frequently desirable to give to some part of the mechanism an irregular motion. This is often done by the use of cams, which are made of such form that when they receive motion, either rotary or reciprocating, they impart to a follower the desired irregular motion.
The follower is sometimes flat, and sometimes round. When the follower is round it is usually made in the form of a wheel or roller, so as to lessen the wear and the friction. The follower may work upon the edge of the cam, or if round, it may work in a groove formed either on the face or on the side of the cam.
The working surfaces of cams with round followers are laid out from a pitch line, so called, which passes through the center of the follower. The shape of this pitch line determines the work which the cam will do. The working surface of the cam is at a distance from the follower equal to one-half the diameter of the follower. This principle of a pitch line holds good whether the cam works only upon its edge like the one shown in Fig. 139, or whether it has an outer portion to insure the positive return of the follower. This outer portion is frequently made in the form of a rim of uniform thickness around the groove.
Design a Cam Having a Straight Follower Which Moves Toward or From the Axis of the Cam, as Shown in Fig. 136. Let it be required that the follower shall advance at a uniform rate from a to b as the cam makes a half revolution, this advance being preceded and followed by a period of rest of
a twelfth of a revolution of the cam.
Divide that half of the cam during the revolution of which the follower is to be raised from a to b, in this case the half at the right of the vertical center line, into a number of equal angles, and divide the distance from a to b into the same number of equal spaces. Mark off the points so obtained onto the successive radial lines as indicated by the dotted lines, and at the points where these dotted lines intersect the radial lines draw lines at right angles to the radial lines to represent the position of the follower when these radial lines become vertical as the cam revolves.
A period of rest in a cam is represented by a circular portion, having the axis of the cam as its center. In order, therefore, to obtain the required periods of rest, the distances of a and b from the center are marked off upon the radial lines c and d, these lines being made a twelfth of a revolution from the vertical center line, and lines representing the follower are drawn at these points as before. To get the return of the follower the space from c to d is divided into a number of equal angles, and the distance from e to f is divided off to represent the desired rate of return of the follower. In this case the rate of return is made uniform, so the distance ef is spaced off equally. The distance of these points from the axis is marked off upon the radial lines between c and d, and lines representing the follower are drawn.
A curved line, which may be made with the aid of the irregular curves, which is tangent to all of the lines representing the follower, gives the shape of the cam.
Fig. 137 shows a cam having the conditions as to the rise, rest and return of the follower the same as the one shown in Fig. 136, the follower, however, being pivoted at one end.
Draw the arc ab representing the path of a point in the follower at the vertical center line, and divide that part of the arc through which the follower rises into the same number of equal spaces as the half circle at the right of the vertical center line is divided into angles. Through these points draw lines, as shown, representing consecutive positions of the working face of the follower. The various distances of the follower from the axis of the cam are now marked off upon the corresponding radial lines as before. Lines to represent the follower are now drawn across each of these radial lines, at the same angle to them that the follower makes with the vertical center line when at that part of its stroke corresponding to the particular radial line across which the line representing the follower is being drawn. A curved line passing along tangent to all of these lines gives the shape of the cam as before.
Design a Cam with a Round Follower Rising Vertically. In Fig. 138 the follower has the same uniform rise, and the same periods of rest as before.
A cam with a round follower is less limited in its capabilities than one with a straight follower; in the one here shown the follower on its return drops below the position in which it is shown. That part of the cam during which the conditions are the same as in the others is divided off and the position of the center of the follower upon the radial lines is obtained in the same manner as before. That part of the cam representing the return of the follower is divided into such angles as desired, and the distance through which the follower is to drop as the cam revolves through each of these angles is marked off upon the proper radial line. A curved line which is now made to pass through all of the points so obtained gives the pitch line of the cam.
In drawing such a cam it is not always necessary to fully draw the working faces. The pitch line and the method of obtaining it being shown, a number of circles representing consecutive positions of the follower may be drawn. This will usually be sufficient. The side view of the cam, which in a case like this would naturally be made in section, will give opportunity to show any further detail that may be desired.
Design a Cam with a Round Follower Mounted on a Swinging Arm. Fig. 139 shows such a cam, all of the conditions as to rise, rest and return of the follower being the same as in the cam shown in Fig. 138. The cam is divided into the same angles as before, and the position of the follower is laid out on these radial lines as though it moved vertically.
DOWNLOAD FREE BOOK:
Self taught mechanical drawing and elementary machine design
The follower is sometimes flat, and sometimes round. When the follower is round it is usually made in the form of a wheel or roller, so as to lessen the wear and the friction. The follower may work upon the edge of the cam, or if round, it may work in a groove formed either on the face or on the side of the cam.
The working surfaces of cams with round followers are laid out from a pitch line, so called, which passes through the center of the follower. The shape of this pitch line determines the work which the cam will do. The working surface of the cam is at a distance from the follower equal to one-half the diameter of the follower. This principle of a pitch line holds good whether the cam works only upon its edge like the one shown in Fig. 139, or whether it has an outer portion to insure the positive return of the follower. This outer portion is frequently made in the form of a rim of uniform thickness around the groove.
Design a Cam Having a Straight Follower Which Moves Toward or From the Axis of the Cam, as Shown in Fig. 136. Let it be required that the follower shall advance at a uniform rate from a to b as the cam makes a half revolution, this advance being preceded and followed by a period of rest of
a twelfth of a revolution of the cam.
Divide that half of the cam during the revolution of which the follower is to be raised from a to b, in this case the half at the right of the vertical center line, into a number of equal angles, and divide the distance from a to b into the same number of equal spaces. Mark off the points so obtained onto the successive radial lines as indicated by the dotted lines, and at the points where these dotted lines intersect the radial lines draw lines at right angles to the radial lines to represent the position of the follower when these radial lines become vertical as the cam revolves.
A period of rest in a cam is represented by a circular portion, having the axis of the cam as its center. In order, therefore, to obtain the required periods of rest, the distances of a and b from the center are marked off upon the radial lines c and d, these lines being made a twelfth of a revolution from the vertical center line, and lines representing the follower are drawn at these points as before. To get the return of the follower the space from c to d is divided into a number of equal angles, and the distance from e to f is divided off to represent the desired rate of return of the follower. In this case the rate of return is made uniform, so the distance ef is spaced off equally. The distance of these points from the axis is marked off upon the radial lines between c and d, and lines representing the follower are drawn.
A curved line, which may be made with the aid of the irregular curves, which is tangent to all of the lines representing the follower, gives the shape of the cam.
Fig. 137 shows a cam having the conditions as to the rise, rest and return of the follower the same as the one shown in Fig. 136, the follower, however, being pivoted at one end.
Draw the arc ab representing the path of a point in the follower at the vertical center line, and divide that part of the arc through which the follower rises into the same number of equal spaces as the half circle at the right of the vertical center line is divided into angles. Through these points draw lines, as shown, representing consecutive positions of the working face of the follower. The various distances of the follower from the axis of the cam are now marked off upon the corresponding radial lines as before. Lines to represent the follower are now drawn across each of these radial lines, at the same angle to them that the follower makes with the vertical center line when at that part of its stroke corresponding to the particular radial line across which the line representing the follower is being drawn. A curved line passing along tangent to all of these lines gives the shape of the cam as before.
Design a Cam with a Round Follower Rising Vertically. In Fig. 138 the follower has the same uniform rise, and the same periods of rest as before.
A cam with a round follower is less limited in its capabilities than one with a straight follower; in the one here shown the follower on its return drops below the position in which it is shown. That part of the cam during which the conditions are the same as in the others is divided off and the position of the center of the follower upon the radial lines is obtained in the same manner as before. That part of the cam representing the return of the follower is divided into such angles as desired, and the distance through which the follower is to drop as the cam revolves through each of these angles is marked off upon the proper radial line. A curved line which is now made to pass through all of the points so obtained gives the pitch line of the cam.
In drawing such a cam it is not always necessary to fully draw the working faces. The pitch line and the method of obtaining it being shown, a number of circles representing consecutive positions of the follower may be drawn. This will usually be sufficient. The side view of the cam, which in a case like this would naturally be made in section, will give opportunity to show any further detail that may be desired.
Design a Cam with a Round Follower Mounted on a Swinging Arm. Fig. 139 shows such a cam, all of the conditions as to rise, rest and return of the follower being the same as in the cam shown in Fig. 138. The cam is divided into the same angles as before, and the position of the follower is laid out on these radial lines as though it moved vertically.
DOWNLOAD FREE BOOK:
Self taught mechanical drawing and elementary machine design
Free books category: