Another and excellent locking device for bolts or nuts, is the cotter shown in [Fig. 437], which is sometimes forged solid and sometimes split, as in the figure. By being made taper from a to b, it will take up the wear if driven farther in. Its width gives it strength in the direction in which it acts to lock, the overhanging head is to drive it out by, and the bevelled corner c is to enable its easy insertion, because if left sharp it would be liable to catch against the edge of the cotter-way and burr up. If made split, its ends are opened out after it is inserted, as shown at d. When closing the ends of either split cotters or split pins to extract them it is better to close one side first and bend it over a trifle too much, so that, when closing the other side, by the time the pin is straightened the two ends will be closed together, and extraction becomes easy.
Fig. 438.
A very safe method in the case of a single nut or bolt head is to provide a separate plate, as in [Fig. 438]. The plate p is provided with three sides, corresponding to the sides of the hexagon, as shown, and in the middle of these sides are cut the notches a b c, so that by giving the nut n one-twelfth of a turn its corners d e would be held by the notches b c, s being a small screw to hold p. It is obvious that a simple set screw passed through the walls of the nut would grip the bolt thread and serve to hold the nut, but this would damage the bolt thread, and, furthermore, that thread would under jar or vibration compress and let the set screw come loose.
A better plan than this is to provide a thick washer beneath the nut and let a set screw pass through the washer and grip the bolt, fastening or setting up the set screw after the nut is screwed home. This, however, makes the washer a gripping piece and in no wise serves to lock the nut. In addition to the washer a pin may project through the radial face of the washer and into the work surface, which will prevent, in connection with the set screw, both the bolt and the washer from turning.
When a bolt has no thread but is secured by a taper pin, set screw, cotter, or device other than a nut, it is termed a pin. So, likewise, a cylindrical piece serving as a pivot, or to hold two pieces together and having no head, is termed a pin.
The usual method of securing a pin is by a set screw or by a taper pin and a washer; and since the term pin applying to both may lead to misunderstanding, the term bolt will here be applied to the large and the term pin to the small or securing pin only.
