ABSTRACT
We have tried to use torque as a means of controlling the preload in the fastener and have
found problems. Even perfect input torque can give us a +25%–30% variation in preload. But when we apply torque, the nut turns. Can we use the angle through which the nut
turns, instead of torque, to control preload? At first glance this looks very promising. After
all, when we turn the nut on a machine-tool lead screw by 3608, the screw advances or retracts with a linear displacement equal to exactly one pitch of the threads. Won’t a bolt stretch by
this amount when we rotate the nut one turn? If so, we could use the lead screw equation to
relate bolt stretch to turn of the nut, or
DL ¼ P uR 360
(8:1)
where DL is bolt stretch (in., mm), P is the pitch of the threads (in., mm), and uR is the angle of nut rotation (degree) with respect to the bolt. We could then get bolt preload very easily,
assuming that we knew the spring constant or stiffness of the bolt, from
FP ¼ KB DL ¼ KBP uR 360
(8:2)
where KB is bolt stiffness (lb=in., N=mm) and FP is preload (lb, N). But life isn’t this simple. As illustrated in Figure 8.1, the lead screw moves a distance of
one pitch when we turn the nut only if the nut is rigidly restrained and the lead screw is
perfectly free. If the nut were free and the screw restrained, of course, it is the nut which would
move, as it does when we’re running a free nut down against a joint. We still have relative
motion, between the bolt and nut, equal to one pitch; but turning a loose nut obviously
produces no preload whatsoever.
