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Preload in such cases is important for several reasons. First, a tightened bolt experiences only a small fraction of any external load that will be applied later, so that a fully tightened bolt can (depending on the exact application) sustain a much greater load than a loosely tightened bolt.
The applied torque causes the bolt to "climb" the thread causing a tensioning of the bolt and an equivalent compression in the components being fastened by the bolt. The preload developed in a bolt is due to the applied torque and is a function of the bolt diameter, the geometry of the threads, and the coefficients of friction that exist in the ...
bolt-strecth method utilizes a hydraulic ram that stretches the bolt by pulling on the threaded section of the bolt that protrudes through the nut. Nut is rotated into position with very small torque applied. Once the external stretching force is removed, the preload is established; heat tightening is based on stretching the bolt by heating it ...
In critical fastener joints, embedment can mean loss of preload. Flattening of a surface allows the strain of a screw to relax, which in turn correlates with a loss in tension and thus preload. In bolted joints with particularly short grip lengths, the loss of preload due to embedment can be especially significant, causing complete loss of preload.
It is often applied by some form of lever; for example a bolt is turned by a wrench whose handle functions as a lever. The mechanical advantage in this case can be calculated by using the length of the lever arm for r in the above equation. This extraneous factor r can be removed from the above equation by writing it in terms of torque:
It is therefore beneficial in high-frequency high-load situations with a higher risk of fatigue related failure, like a bolted down cylinder head, to use torque to yield bolts. [ citation needed ] The torque applied to the fastener must be determined such that it does not contribute to a service condition where the fastener enters a low-cycle ...
In these equations, is the torque applied to screw or nut, is the linear force applied, is the ball screw lead, and is the ball screw efficiency. Selection of the standard to be used is an agreement between the supplier and the user and has some significance in the design of the screw.
Multi-jackbolt tensioners (MJTs), registered under the trademark Superbolt or Supernut, are designed to decrease the torque required to tighten large bolted joints. One of the major problems associated with traditional bolt tightening methods is as the diameter of the bolt increases, the amount of torque required to tighten it increases in the third power of the diameter. [1]