Chapter IV.—SCREW THREAD.
Screw threads are employed for two principal purposes—for holding or securing, and for transmitting motion. There are in use, in ordinary machine shop practice, four forms of screw thread. There is, first, the sharp V-thread shown in [Fig. 246]; second, the United States standard thread, the Sellers thread, or the Franklin Institute thread, as it is sometimes called—all three designations signifying the same form of thread. This thread was originally proposed by William Sellers, and was afterward recommended by the Franklin Institute. It was finally adopted as a standard by the United States Navy Department. This form of thread is shown in [Fig. 247]. The third form is the Whitworth or English standard thread, shown in [Fig. 248]. It is sometimes termed the round top and bottom thread. The fourth form is the square thread shown in [Fig. 249], which is used for coarse pitches, and usually for the transmission of motion.
The sharp V-thread, [Fig. 246], has its sides at an angle of 60° one to the other, as shown; or, in other words, each side of the thread is at an angle of 60° to the axial line of the bolt. The United States Standard, [Fig. 247], is formed by dividing the depth of the sharp V-thread into 8 equal divisions and taking off one of the divisions at the top and filling in another at the bottom, so as to leave a flat place at the top and bottom. The Whitworth thread, [Fig. 248], has its sides at an angle of 55° to each other, or to the axial line of the bolt. In this the depth of the thread is divided into 6 equal parts, and the sides of the thread are joined by arcs of circles that cut off one of these parts at the top and another at the bottom of the thread. The centres from which these arcs are struck are located on the second lines of division, as denoted in the figure by the dots. Screw threads are designated by their pitch or the distance between the threads. In [Fig. 250] the pitch is 1⁄4 inch, but it is usual to take the number of threads in an inch of length; hence the pitch in [Fig. 250] would generally be termed a pitch of 4, or 4 to the inch. The number of threads per inch of length does not, however, govern the true pitch of the thread, unless it be a “single” thread.
A single thread is composed of one spiral projection, whose advance upon the bolt is equal in each revolution to the apparent pitch. In [Fig. 251] is shown a double thread, which consists of two threads. In the figure, a denotes one spiral or thread, and b the other, the latter being carried as far as c only for the sake of illustration. The true pitch is in this case twice that of the apparent pitch, being, as is always the case, the number of revolutions the thread makes around the bolt (which gives the pitch per inch), or the distance along the bolt length that the nut or thread advances during one rotation. Threads may be made double, treble, quadruple and so on, the object being to increase the motion without the use of a coarser pitch single thread, whose increased depth would weaken the body of the bolt.
The “ratchet” thread shown in [Fig. 252] is sometimes used upon bolts for ironwork, the object being to have the sides a a of the thread at a right angle to the axis of the bolt, and therefore in the direct line of the strain. Modifications of this form of thread are used in coarse pitches for screws that are to thread direct into woodwork.
A waved or drunken thread is one in which the path around the bolt is waved, as in [Fig. 253], and not a continuous straight spiral, as it should be. All threads may be either left hand or right, according to their direction of inclination upon the bolt; thus, [Fig. 254] is a cylinder having a right-hand thread at a and a left-hand one at b. When both ends of a piece have either right or left-hand threads, if the piece be rotated and the nuts be prevented from rotating, they will move in the same direction, and, if the pitches of the threads are alike, at the same rate of motion; but if one thread be a right and the other a left one, then, under the above conditions, the nuts will advance toward or recede from each other according to the direction of rotation of the male thread.
