Bolts
ASME B18.2.1 defines a bolt as "an externally threaded fastener designed for insertion
through the holes in assembled parts, and is normally intended to be tightened or
released by torquing a nut". Using this definition to determine whether a particular
threaded fastener is a screw or a bolt requires that an assumption be made about
the intended purpose of the threaded fastener and as a practical matter doesn't
seem to be followed by most threaded fastener manufacturers. It also conflicts with
common usage such as the term, "head bolt", which is a threaded fastener that mates
with a tapped hole in an engine block and is not intended to mate with a nut.
For critical high-tensile-strength applications, low-grade bolts may fail, resulting
in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is
impressed on the heads to allow inspection and validation of the strength of the
bolt. However, low-cost counterfeit fasteners may be found with actual strength
far less than indicated by the markings. Such inferior fasteners are a danger to
life and property when used in aircraft, automobiles, heavy trucks, and similar
critical applications.
Critical applications of screws and bolts will specify a torque that must be applied
when driving it. The main concept is to tension the bolt, and compress parts being
held together, creating a spring-like assembly. The stress thus introduced to the
bolt is called a preload. When external forces try to separate the parts, the bolt
experiences no strain unless the preload force is exceeded. As long as the preload
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 (for example,
a steel bolt threaded into aluminum, then a thread-locking adhesive or insert may
be used.
High-strength steel bolts usually have a hexagonal head with an ISO strength rating
(called property class) stamped on the head. The property classes most often used
are 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength
in MPa divided by 100. The number after the point is 10 times the ratio of tensile
yield strength to tensile ultimate strength. For example, a property class 5.8 bolt
has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield
strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa. Tensile ultimate
strength is the stress at which the bolt fails (breaks in half). Tensile yield strength
is the stress at which the bolt will receive a permanent set (an elongation from
which it will not recover when the force is removed) of 0.2 % offset strain. When
elongating a fastener prior to reaching the yield point, the fastener is said to
be operating in the elastic region; whereas elongation beyond the yield point is
referred to as operating in the plastic region, since the fastener has suffered
permanent plastic deformation.
A nut is a type of hardware fastener with a threaded hole. Nuts are almost always
used opposite a mating bolt to fasten a stack of parts together. The two partners
are kept together by a combination of their threads' friction, a slight stretch
of the bolt, and compression of the parts. In applications where vibration or rotation
may work a nut loose, adhesives, safety pins, and other tricks are used to prevent
fastener rotation. The most common shape is hexagonal, for similar reasons as the
bolt head - 6 sides give a good granularity of angles for a tool to approach from
(good in tight spots), but more (and smaller) corners would be vulnerable to stripping/rounding.
Other specialized shapes exist for certain needs, such as wing nuts for finger adjustment
and captive nuts for inaccessible areas.
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