A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.
Washers are usually metal or plastic. High-quality bolted joints require hardened steel washers to prevent the loss of pre-load due to Brinelling after the torque is applied.
Rubber or fiber gaskets used in taps (or faucets, or valves) to stop the flow of water are sometimes referred to colloquially as washers; but, while they may look similar, washers and gaskets are usually designed for different functions and made differently.
Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces.
The origin of the word is unknown; the first recorded use of the word was in 1346, however, the first time its definition was recorded was in 1611.
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Type and form
Washers can be categorised into three types;
- Plain washers, which spread a load, and prevent damage to the surface being fixed, or provide some sort of insulation such as electrical
- Spring washers, which have axial flexibility and are used to prevent fastening or loosening due to vibrations
- Locking washers, which prevent fastening or loosening by preventing unscrewing rotation of the fastening device; locking washers are usually also spring washers.
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.
"Type" is not to be confused with "form" (but often is). The British Standard for Metric Series Metal Washers (BS4320), written in 1968, coined the term "form". The forms go from A to G and dictate the outside diameter and thickness of the flat washers.
- 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: Larger diameter, normal thickness
- Form F: Larger diameter, normal thickness
- Form G: Largest diameter, larger thickness.
Plain washers
Spring and locking washers
Lock washers, locknuts, jam nuts, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.
Gaskets
The term washer is often applied to various gasket types such as those used to seal the control valve in taps. Crush washers are made of a soft metal such as aluminium or copper, and are used to seal fluid or gas connections such as those found in an internal combustion engine.
Specialised types
Materials
Washers can be fabricated from a variety of materials including, but not limited to:
- Steel - Carbon steel, spring steel, 304 stainless steel, and 316/316A stainless steel
- 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
Corrosion resistance
A number of techniques are used to enhance the corrosion resistant properties of certain washer materials:
- Metallic coatings - Typical coatings used to produce corrosion resistant washers are zinc, cadmium, and nickel. Zinc coating acts as a sacrificial surface layer that falls victim to corrosive materials before the washer's material can be harmed. Cadmium produces a high-quality protective surface but is toxic, both biologically and environmentally. Nickel coatings add protection from corrosion only when the finish is dense and non-porous.
- Electroplating - This method involves coating the washer by electrolytic deposition using metals such as chromium or silver.
- Phosphating - A resilient, but abrasive surface is achieved by incorporating a zinc-phosphate layer and corrosion-protective oil.
- Browning or bluing - Exposing the washer (typically steel) to a chemical compound or alkali salt solution causes an oxidizing chemical reaction, which results in the creation of a corrosion-resistant, colored surface. The integrity of the coating can improved by treating the finished product with a water-displacing oil.
- Chemical plating - This technique utilizes a nickel-phosphor alloy that is precipitated onto the washer surface, creating an extremely corrosion- and abrasive-resistant surface.
Standard metric flat washers sizes
Washers of standard metric sizes are listed in the table below. Measurements in the table refer to the dimensions of the washers as described by the drawing.
Source of the article : Wikipedia
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