Fig. 412.
Circular nuts are employed where, on account of their rotating at high speed, it is necessary that they be balanced as nearly as possible so as not to generate unbalanced centrifugal force. [Fig. 410] represents a nut of this kind: two diametrically opposite flat sides, as a, affording a hold for the wrench. Other forms of circular nuts are shown in [Figs. 411] and [412]. These are employed where the nuts are not subject to great strain, and where lightness is an object.
That in [Fig. 411] is pierced around its circumference with cylindrical holes, as a, b, c, to receive a round lever or rod or a wrench, such as shown in [Fig. 459].
That shown in [Fig. 412] has slots instead of holes in its circumference, and the form of its wrench is shown in [Fig. 461].
Fig. 413.
When nuts are employed upon bolts in which the strain of the duty is longitudinal to the bolt, and especially if the direction of motion is periodically reversed, and also when a bolt is subject to shocks or vibrations, a single nut is liable to become loose upon the bolt, and a second nut, termed a check nut, jamb nut, or safety nut, becomes necessary, because it is found that if two nuts be employed, as in [Fig. 413], and the second nut be screwed firmly home against the first, they are much less liable to come loose on the bolt.
Considerable difference of practice exists in relation to the thickness of the two nuts when a check nut is employed. The first or ordinary nut is screwed home, and the second or check nut is then screwed home. If the second nut is screwed home as firmly as the first, it is obvious that the strain will fall mainly on the second. If it be screwed home more firmly than the first, the latter may be theoretically considered to be relieved entirely of the strain, while if it be screwed less firmly home, the first will be relieved to a proportionate degree of the strain. It is usual to screw the second home with the same force as applied to the first, and it would, therefore, appear that the first nut, being relieved of strain, need not be so thick as the first, but it is to be considered that, practically, the first nut will always have some contact with the bolt threads, because from the imperfections in the threads of ordinary bolts the area and the force of contact is not usually the same nor in the same direction in both nuts, unless both nuts were tapped with the same tap and at about the same time.
When, for example, a tap is put into the tapping machine, it is at its normal temperature, and of a diameter due to that temperature, but as its work proceeds its temperature increases, notwithstanding that it may be freely supplied with oil, because the oil cannot, over the limited area of the tap, carry off all the heat generated by the cutting of a tap rotated at the speeds usually employed in practice. As a result of this increase of temperature, we have a corresponding increase in the diameter of the tap, and a variation in the diameter of the threads in the nuts. The variation in the nuts, however, is less than that in the tap diameter, because as the heated tap passes through the nut it imparts some of its heat to the nut, causing it also to expand, and hence to contract in cooling after it has been tapped, and, therefore, when cold, to be of a diameter nearer to that of the tap.