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== Mechanics ==
A '''screw''' is often considered to be a specialized application of the [[wedge]] or inclined plane. In this view, the screw is a wedge wound around an interior cylinder or shaft. The technical analysis (see also [[statics]], [[dynamics]]) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. See also [[simple machine]].
Screws and bolts are in tension when properly fitted. In most applications they are not designed to bear large shear [[force]]s. When, for example, two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough that the [[friction]] between the two bars can overcome the longitudinal force. If the bars slip then the bolt may be sheared in half. For this type of application, high-tensile [[steel]] bolts are used. These are usually in the form of hexagonal cap screws with a strength rating stamped on the head. The strength rating most often seen stamped on cap screws is "8.8", which is roughly four times stronger than mild steel with a rating of 4.6.
== Types of Screw ==
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