WASHERS AND PRESSINGS
Plain Washers
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Flat Washer - A flat ring, often of metal, used to spread the load of a screwed fastening. Additionally, a plain washer may be used when the hole is a larger diameter than the fixing nut.
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Repair Washer - A flat washer with a particularly large outer diameter in proportion to its central hole. These are commonly used to spread the load on thin sheet metal, and are named after their use on automobile fenders. They can also be used to make a connection to a hole that has been enlarged by rust or wear.
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Torque Washer - Used in woodworking in combination with a carriage bolt; it has a square hole in the centre into which the carriage bolt square fits. Teeth or prongs on the washer bite into the wood, preventing the bolt from spinning freely when a nut is being tightened.
Spring & Locking Washers
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Belleville Washer - A washer with a slight conical shape, which provides an axial force when deformed. The surface may be serrated, which will have improved locking capabilities.
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Curved disc spring - Similar to a Belleville, except the washer is curved in only one direction, therefore there are only four points of contact. Unlike Belleville washers, they exert only light pressures.
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Split Washer - A ring split at one point and bent into a helical shape. This causes the washer to exert a spring force between the fastener's head and the substrate, which maintains the washer hard against the substrate and the bolt thread hard against the nut or substrate thread, creating more friction and resistance to rotation