Fig. 390.
In some cases the end of the set screw is tapped through the enveloping piece (as a hub) and its end projects into a plain hole in the internal piece of the work, and in this case the end of the thread is turned off for a distance of two or three threads, as at a in [Fig. 390]. Similarly, when the head of the screw is to act or bear upon the work, the thread may be turned off as at b in the figure.
Fig. 391.
When a bolt has no head, but is intended to screw into the work at one end, and receive a nut at the other, it is termed a stud or standing bolt. The simplest form of standing bolt is that in which it is parallel from end to end with a thread at each end, and an unthreaded part in the middle, but since standing bolts or studs require to remain fixed in the work, it is necessary to screw them tightly into their places, and therefore firmly home. This induces the difficulty that some studs may screw a trifle farther into the work than others, so that some of the stud ends may project farther through the nuts than others, giving an appearance that the studs have been made of different lengths. The causes of this may be slight variations in the tapping of the holes and the threading of the studs. If those that appear longest are taken out and reduced to the lengths of the others, it will be found sometimes that the stud on the second insertion will pass farther into the work than at the first, and the stud will project less through the nut than the others. To avoid this those protruding most may be worked backward and forward with the wrench and thus induced to screw home to the required distance, but it is better to provide to the stud a shoulder against which it may screw firmly home; thus in [Fig. 391] is a stud, whose end a is to screw into the work, part b is to enter the hole in the work (the thread in the hole being cut away at the mouth to receive b). In this case the shoulder between b and c screwing firmly against the face of the work, all the studs being made of equal length from this shoulder to end e, then the thickness of the flange or work secured by the nut being equal, the nuts will pass an equal distance on end d, and e will project equally through all the nuts. The length of the plain part c is always made slightly less than the thickness of the flange or foot of the work to be bolted up, so that the nut shall not meet c before gripping the flange surface.
There are, however, other considerations in determining the shape and size of the parts a and c of studs.
Thus, suppose a stud to have been in place some time, the nut on end e being screwed firmly home on the work, and perhaps somewhat corroded on e. Then the wrench pressure applied to the nut will be in a direction to unscrew the stud out of the work, and if there be less friction between a and the thread in the work than there is between d and the thread in the nut, the stud and not the nut will unscrew. It is for this purpose that the end a requires firmly screwing into the work. But in the case of much corrosion this is not always sufficient, and the thread a is therefore sometimes made of a larger diameter than the thread at d. In this case the question at once arises, What shall be the diameter of the plain part c?
