PART XVI.
SCREW-THREADS, BOLTS AND NUTS.
QUESTION 335. _How must the screws of bolts and nuts be made, in order to fit each other?_
_Answer_. Each size of screw must be made of exactly the same diameter, and their threads of the same form and proportions and _pitch_.
QUESTION 336. _What is meant by the “pitch” of a thread?_
_Answer_. It is the distance the thread progresses lengthwise of the screw in one revolution. Thus if a single-threaded screw has one-eighth of an inch pitch, it means that the threads are ¹⁄₈ of an inch apart measured from the centre of one thread to the centre of that next to it, and therefore there are eight threads to each inch in length of the screw.
QUESTION 337. _What is meant by a “single-threaded” screw?_
_Answer_. It means a screw with but one thread instead of two or more. Thus if we take a string and wind it around a pencil, it will represent a single-threaded screw, and if we take two or three strings and wind them parallel to each other, they will represent a double or treble-threaded screw. The latter kinds are seldom or never used on locomotives, so that in the following discussion only single-threaded screws will be referred to.
[Illustration: Fig. 200.
Full Size.]
QUESTION 338. _What is the usual form of the threads of screws?_
_Answer_. The most common is what is called the V-thread, represented in fig. 200, which is made sharp at both the top and bottom. If such a thread for one screw is made very pointed and that for another is blunt, it is plain that the nut for the one will not fit the other accurately, and also that if a nut has eight threads to the inch, it will not fit on a bolt with nine. Owing to the fact that, until within a few years, no common standard has been agreed upon for the form, proportions or pitch of screws there has been very great diversity in these respects in the screws which have been used in the construction of locomotives and other machinery. In 1864 the inconvenience and confusion from this cause became so great that it attracted the attention of the Franklin Institute of Philadelphia, and a committee was appointed by that association to investigate and report on the subject. That committee recommended the adoption of the Sellers system of screw-threads and bolts, which was devised by Mr. William Sellers, of Philadelphia. This same system was subsequently adopted as the standard by both the Army and Navy Departments of the United States, and then by the Master Mechanics’ and Master Car Builders’ associations, so that it may now be regarded, and in fact is called, the United States standard, but the design is due to Mr. Sellers.
QUESTION 339. _In establishing a standard system of screws and threads, what is the first thing which must be determined?_
_Answer._ The number of threads to the inch, or the pitch of the threads for screws of different diameters.
[Illustration: Fig. 201. Full size.]
QUESTION 340. _What is the standard for the number of threads to the inch for the different sized screws of the Sellers system?_
_Answer._ The number of threads with their other proportion is given in the table at the end of this chapter on page 348.
QUESTION 341. _What is the form of the thread of this standard?_
_Answer._ The form is shown in fig. 201, and is similar to the V-thread, excepting that it is flattened at the top and bottom.
QUESTION 342. _What are the reasons for the adoption of this form of thread?_
_Answer._ It has already been pointed out that if a screw is made with a “blunt” thread it will not fit a nut with very acute or “sharp” threads; or, if the thread of the bolt was acute and that in the nut obtuse, they would fit imperfectly. It is therefore necessary in a standard system to fix upon the angle which the sides of the thread shall bear to each other. This in the United States standard system was determined by Mr. Sellers at 60 degrees, because that angle is easily laid off without special instruments[81] and is perhaps as good as or better than any other form for the threads.
[81] This can be done by drawing a circle of any diameter, and subdividing the circumference into six equal parts with the radius. Lines, drawn from the points of division to the centre will have an inclination of 60 degrees to each other.
It is obvious that if a tool is ground with its sides at an angle of 60 degrees to each other, if the point is made sharp after a very little use it will be worn more or less so that the bottom of the thread will not be cut perfectly sharp, and therefore it will be difficult to make bolts and nuts with sharp threads fit each other accurately. It is also plain that the sharp edge of a thread gives very little strength to the screw, and yet diminishes that of the bolt very materially. It will also be impossible to measure the diameter of the screw at the bottom of the thread if it is made sharp, as its depth will vary as the point of the tool wears, and it is almost impossible to measure the diameter of such a screw accurately with ordinary calipers. A sharp-edged thread in a bolt is also very liable to be injured and bruised by coming in contact with other objects. To obviate these evils the standard threads are therefore made flat on the top, and it is evident that a similar shape at the bottom will give increased strength to the bolt as well as conform to and fit the thread in the nut. To give this form requires only that the point of the cutting tool shall be taken off, and then it is evident this form of thread can be cut in a lathe with the same tool and in the same manner as the sharp thread. The width of the flat top and bottom should of course bear a definite proportion to the size or pitch of the thread.
[Illustration: Fig. 202.]
QUESTION 343. _What are the proportions of the standard threads?_
_Answer_. The rule given by Mr. Sellers for proportioning the thread is as follows: “DIVIDE THE PITCH, OR, WHAT IS THE SAME THING, THE SIDE OF THE THREAD, INTO EIGHT EQUAL PARTS; TAKE OFF ONE PART FROM THE TOP AND FILL IN ONE PART IN THE BOTTOM OF THE THREAD: THEN THE FLAT TOP AND BOTTOM WILL EQUAL ONE-EIGHTH OF THE PITCH, THE WEARING SURFACE WILL BE THREE-QUARTERS OF THE PITCH AND THE DIAMETER OF SCREW AT BOTTOM OF THE THREAD WILL BE EXPRESSED BY THE FORMULA:
_diameter_ - 1.299 ------------------------ _no. threads per inch._”
In order to make the form and proportions of this standard thread as plain as possible, we have had an enlarged diagram, fig. 202, engraved, so as to represent the different parts clearly. _x_ represents the pitch, _d_ the diameter of the screw in inches, _v_ the number of threads to the inch, _n_ the diameter at bottom of thread, _m_ the width of back part of thread at the top and bottom, and _s_ the length of the side of thread.
From Mr. Sellers’ rule the following formulæ can be deduced:
1.299 _n_ = _d_ - ----- _v_
_x_ _m_ = --- 8
3_x_ _s_ = ---- 4
From this rule any thread can be constructed, it being only necessary to know the pitch or number of threads to the inch. This, with all the dimensions of the standard threads for bolts from ¹⁄₄ to 2 inch diameter, is given in the table on page 348.
For practical use in the shop a gauge like that shown in fig. 203 will be found most convenient for grinding the tools to the proper form for making the standard screws. With this gauge the screw-cutting tool can first be ground to the proper angle by fitting it to the deepest notch, and the requisite quantity should then be taken off the point by fitting it to the notch representing the form of thread for the sized bolt or number of threads to the inch which it is intended to cut.
Wherever this standard for threads is used, if any pretense at all is made to accuracy of workmanship, careful attention should be given to the form and proportion of the threads as well as to the number to the inch. In buying taps and dies the purchaser should see that they conform in every respect to the standard, and in making specifications for new work similar care should be exercised to secure the true standard, form and proportion of screws. In many shops the workmen who have the care of those tools are entirely ignorant of the peculiarities of the Sellers system, and have only the vague idea that so long as they get the proper number of threads to the inch they are doing all that is necessary to secure uniformity. Unless, therefore, some care is exercised to insure accuracy of workmanship in this department, the adoption of a “standard” for screws will not insure the advantages which would result from uniformity of screws and threads.
[Illustration: Fig. 203. Full size.]
QUESTION 344. _How thick must a nut be, measuring lengthwise of the bolt, so that the thread in the nut will be of equal strength to that of the bolt?_
_Answer._ Its thickness must be equal to the outside diameter of the screw.
TABLE GIVING PROPORTIONS OF THE UNITED STATES OR SELLERS STANDARD SYSTEM OF SCREW-THREADS.
=========+=========+============+============= | |Diameter of | | |screw at the|Width of top Outside | |root of the |and bottom of diameter|Number of| thread in | thread in of screw| threads |decimals of |decimals of in inches| per inch| an inch | an inch ---------+---------+------------+------------- ¹⁄₄ | 20 | .185 | .0062 ⁵⁄₁₆ | 18 | .240 | .0074 ³⁄₈ | 16 | .294 | .0078 ⁷⁄₁₆ | 14 | .344 | .0089 ¹⁄₂ | 13 | .400 | .0096 ⁹⁄₁₆ | 12 | .454 | .0104 ⁵⁄₈ | 11 | .507 | .0113 ³⁄₄ | 10 | .620 | .0125 ⁷⁄₈ | 9 | .731 | .0138 1 | 8 | .837 | .0156 1¹⁄₈ | 7 | .940 | .0178 1¹⁄₄ | 7 | 1.065 | .0178 1³⁄₈ | 6 | 1.160 | .0208 1¹⁄₂ | 6 | 1.284 | .0208 1⁵⁄₈ | 5¹⁄₂ | 1.389 | .0227 1³⁄₄ | 5 | 1.491 | .0250 1⁷⁄₈ | 5 | 1.616 | .0250 2 | 4¹⁄₂ | 1.712 | .0277 =========+=========+============+=============