screw and fixed trough
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.
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).
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.
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.