ABSTRACT
We applied torque, and the nut turned, stretching the bolt and creating preload. We have seen
that control through torque or turn, or both, did not give us ideal control of preload. We need
something better for critical applications. In this chapter we’re going to look at several other
ways to control bolt preload. These include ‘‘stretch control’’ in which the change of length of
the fastener is monitored, and a variety of control means or systems called ‘‘direct preload
control.’’ Some of these actually come close to being that; others don’t, as we’ll see. Most of
these other ways to control preload control it more accurately than torque or torque-turn
control, but most are also more expensive to apply. To complete the survey we’re going to
look at ultrasonic control of stretch and preload.
