A lock washer is similar to a standard washer in that it is a metal disc with a hole in the center intended to be placed between a fastener (such as a bolt or a screw) and the work surface. However, where the standard washer is a smooth disc, the lock washer has sharp edges which bite into the fastener head and work surface to prevent them from turning once they have been tightened down. This prevents loosening of the fastener due to vibration or other mechanical forces. When used in a nut and bolt configuration, the lock washer is typically used with the nut side. When used with a tapped hole, there is no nut and the lock washer is placed under the bolt head.

Because the lock washer bites into the work surface, it is also used for electrical grounding purposes. The bite ensures an excellent conductive connection between the fastener or crimped ring terminal and the work surface because it penetrates corrosion, paint, or other defects which could degrade the connection.

There are four basic types of lock washer, in addition to less commonly seen types used for specific applications such as high-torque fastening or countersunk holes.

  • Internal Tooth - The teeth are located on the inside of the ring and ensure a good connection with small fastener heads.

  • External Tooth - The teeth are located on the outside of the ring, giving them more surface area to provide a better connection with large fastener heads. This style is useless with small fastener heads because the fastener head must completely cover the teeth to be effective.

  • External Internal Combination Tooth - This style has teeth on both the inside and outside of the ring to provide the best possible connection with large fastener heads. It is also useful for oversized holes because the internal teeth can bite into the fastener head while the external teeth bite into the work surface.

  • Split - This style, also called a helical spring washer, looks like a standard washer that has been cut on one side and bent slightly outward. The split bites in only one direction to prevent the fastener from turning in the counterclockwise direction (which is the loosening direction on a standard right-handed fastener). Additionally, the split lock washer is compressed as the fastener is tightened down, providing extra holding force.

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