Rotating screw and fixed trough
A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around an interior cylinder or shaft, that either fits into a corresponding plane in a nut, or forms a corresponding plane in the wood or metal as it is inserted. 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. Wedges are discussed in the article on simple machines.
Critical applications of screws and bolts will specify a torque that must be applied when tightening. The main concept is to stretch the bolt, and compress the parts being held together, creating a spring like assembly. The stretch introduced to the bolt is called a pre-load. When external forces try to separate the parts, the bolt sees no strain unless the pre-load force is exceeded (this takes some effort to imagine).
As long as the pre-load is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (eg. a steel bolt into aluminum threads) then a thread locking adhesive may be used.
If the pre-load is exceeded during normal use the joint will eventually fail. The pre-load is calculated as a percentage of the bolt's yield tensile strength, or the strength of the threads it goes into, whichever is less.