Washers refer to the parts that are padded between the connected parts and the nut. Generally flat metal ring, used to protect the surface of the connected parts from the nut abrasion, dispersing the pressure of the nut on the connected parts.
Materials
Washers can be fabricated from a variety of materials including, but not limited to:
Steel – Carbon steel, spring steel, A2 (304) stainless steel, and A4 (316/316L) stainless steel
Non-ferrous metal – Copper, brass, aluminium, titanium, iron, bronze, and zinc
Alloy – Silicon bronze, Inconel, Monel, and Hastelloy
Plastic – Thermoplastics and thermosetting polymers such as polyethylene, PTFE (Teflon)
Nylon – Nylon 6, Nylon 66, Nylatron, and Tecamid MDS
Specialty – Fibers, ceramics, rubber, felt, leather, bimetals, and mica
Phenolic – The material has good electrical insulation, is lightweight, tough, has low moisture absorption, is heat resistant, and is resistant to chemicals and corrosion. Phenolic washers are substitutes for flat metallic washers in cases where electrical insulation is required. Phenolic washers are stamped out of large sheets of the phenolic material. The term "phenolic washer" is sometimes used for stamped washers from laminated materials such as paper, canvas, and Mylar.
Type and form
The American National Standards Institute (ANSI) provides standards for general use flat washers. Type A is a series of steel washers at broad tolerances, where precision is not critical. Type B is a series of flat washers with tighter tolerances where outside diameters are categorized as "narrow", "regular" or "wide" for specific bolt sizes.
Form A: Normal diameter, normal thickness
Form B: Normal diameter, light thickness
Form C: Large diameter, normal thickness
Form D: Large diameter, light thickness
Form E: Normal diameter, normal thickness
Form F: Large diameter, normal thickness
Form G: Largest diameter, larger thickness.
Washer 'form' when comparing different washer material types is used quite freely by stockists. In relation to BS4320 specifically, washer forms 'A' to 'D' inclusive are designated 'bright metal' washers and are supplied self-finished in various metals including: steel alloys, brass, copper, etc. Whereas, BS4320 washer forms 'E' to 'G' inclusive are designated 'black' (uncoated) mild steel washers, which normally are specified with a supplementary protective coating supply condition.
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