Fig. 255.
In [Fig. 255] is represented a form of thread designed to enable the nut to fit the bolt, and the thread sides to have a bearing one upon the other, notwithstanding that the diameter of the nut and bolt may differ. The thread in the nut is what may be termed a reversed ratchet thread, and that in the bolt an undercut ratchet thread, the amount of undercut being about 2°. Where this form of thread is used, the diameter of the bolt may vary as much as 1⁄32d of an inch in a bolt 3⁄4 inch in diameter, and yet the nut will screw home and be a tight fit. The difference in the thread fit that ordinarily arises from differences in the standards of measurement from wear of the threading tools, does not in this form affect the fit of the nut to the bolt. In screwing the nut on, the threads conform one to the other, giving a bearing area extending over the full sides of the thread. The undercutting on the leading face of the bolt thread gives room for the metal to conform itself to the nut thread, which it does very completely. The result is that the nut may be passed up and down the bolt several times and still remain too tight a fit to be worked by hand. Experiment has demonstrated that it may be run up and down the bolt dozens of times without becoming as loose as an ordinary bolt and nut. On account of this capacity of the peculiar form of thread employed, to adapt itself, the threads may be made a tight fit when the threading tools are new. The extra tightness that arises from the wear of these tools is accommodated in the undercutting, which gives room for the thread to adjust itself to the opposite part or nut.
Fig. 256.
In a second form of self-locking thread, the thread on the bolt is made of the usual V-shape United States standard. The thread in the nut, however, is formed as illustrated in [Fig. 256], which is a section of a 3⁄4-inch bolt, greatly enlarged for the sake of clearness of illustration. The leading threads are of the same angle as the thread on the bolt, but their diameters are 3⁄4 and 1⁄16th inch, which allows the nut to pass easily upon the bolt. The angle of the next thread following is 56°, the succeeding one 52°, and so on, each thread having 4° less angle than the one preceding, while the pitch remains the same throughout. As a result, the rear threads are deeper than the leading ones. As the nut is screwed home, the bolt thread is forced out or up, and fills the rear threads to a degree depending upon the diameter of the bolt thread. For example, if the bolt is 3⁄4 inch, its leading or end thread will simply change its angle from that of 60° to that of 44°, or if the bolt thread is 3⁄4 and 1⁄64th inch in diameter, its leading thread will change from an angle of 60° to one of 44°. It will almost completely fill the loose thread in the nut. The areas of spaces between the nut threads are very nearly equal, although slightly greater at the back end of the nut, so that if the front end will enter at all, the nut will screw home, while the thread fit will be tight, even under a considerable variation in the bolt itself. From this description, it is evident that the employment of nuts threaded in this manner is only necessary in order to give to ordinary bolts all the advantages of tightness due to this form of thread.
The term “diameter” of a thread is understood to mean its diameter at the top of the thread and measured at a right angle to the axis of the bolt. When the diameter of the bottom or root of the thread is referred to it is usually specified as diameter at the bottom or at the root of the thread.
The depth of a thread is the vertical height of the thread upon the bolt, measured at a right angle to the bolt axis and not along the side of the thread.
A true thread is one that winds around the bolt in a continuous and even spiral and is not waved or drunken as is the thread in [Fig. 253]. An outside or male thread is one upon an external surface as upon a bolt; an internal or female thread is one produced in a bore or hole as in a nut.
The Whitworth or English standard thread, shown in [Fig. 248], is that employed in Great Britain and her colonies, and to a small extent in the United States. The V-thread [fig. 246] is that in most common use in the United States, but it is being displaced by the United States standard thread. The reasons for the adoption of the latter by the Franklin Institute are set forth in the report of a committee appointed by that Institute to consider the matter. From that report the following extracts are made.