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washers 
Ringlike parts placed between a nut or a bolt and a part to be tightened; they distribute the stress.
flat washer
Placed underneath a screw or the head of a bolt, it distributes the pressure while protecting the work surface.
lock washer
Slightly spiral-shaped washer, which acts as a spring to prevent screws from loosening.
internal tooth lock washer
Washer having protuberances on the inside so that a screw tightens securely and will not loosen.
external tooth lock washer
Washer having protuberances on the outside so that a screw tightens securely and will not loosen.
