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.
Nuts are most commonly hexagonal in shape. It is important to use the correct sized spanners to avoid, ‘rounding off,’ the nut. Otherwise, if using an adjustable spanner you must ensure it is full tightened over the flat faces. Other specialized shapes exist for certain needs, such as wingnuts for finger adjustment and captive nuts (eg cage nuts) for hard to reach areas.
UNITS OF MEASUREMENT FOR THREAD TYPES
Metric CoarseandMetric Fineare in Millimetre (mm) units and were developed to simplify the imperial systems. Europe moved to this system but the Americans chooseImperialas their default. Acoarsethreadhaslesshelical coils per mm and a fine one has more.Coarseis used forheavierloads andfineis used forlighterloads prone tovibrations.
British Association (BA)screw threads, named after the British Association for Advancement of Science, were devised in 1884 and standardised in 1903. Screws were described as "2BA", "4BA" etc., BA threads are specified by British Standard BS 93:1951 "Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A." They are associated with metric units of measurement.
British Standards Brass (BSB) is a specialist thread form based upon the Whitworth thread and consisting of 26 threads per inch (imperial) whatever the thread diameter.
British Standard Cycle Thread (BSC) thread has extra fine threads 26 TPI originally for use on bicycles and motorcycles. The thread runs at a 60 degree rather than a 55 degree angle.
British Standard Threads are very rare and hard to source, hence, some of them are very expensive to replenish stocks.
British Standard Fine (BSF)thread form is based upon the British Standard Whitworth form but with a finer thread (more threads per inch) and has the same thread angle as the BSW and smaller thread depth.
British Standard Whitworth (BSW) isa thread form developed by Sir Joseph Whitworth in 1841. The thread form has rounded roots and crests, a thread angle of 55 degrees, the thread form is specified in BS 84: 1956
UNIFIED THREAD STANDARDS
The basic American standards for fastening screw threads as agreed upon by standard bodies of Canada, the United Kingdom, and the United States. They are a complete and integrated system of threads for fastening purposes. Their outstanding characteristic is general interchangeability of threads achieved through the standardization of thread form, diameter-pitch combinations, and limits of size
Unified National Coarse (UNC)is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a larger thread pitch than an equivalent diameter UNF thread. The unified thread is based on inch sizes and was first standardised in 1948 unifying the Whitworth and American standard thread forms
Unified National Extra Fine (UNEF)is a Unified thread form with a very fine (small) pitch and are typically used on instruments and parts requiring a fine adjustment.
Unified National Fine (UNF)is a thread form with a 60 degree flank angle, rounded roots and flat crests. For a given diameter it has a smaller thread pitch than an equivalent diameter UNC thread.
Unified National (UN)thread form with a rounded root contour, applies only to external threads. (The UN thread form has a flat, or optionally, a rounded root contour.) The majority of fasteners with a Unified thread form (UNC UNF) have a rounded root contour.