You see lock washers all the time around your home. Lock washers are usually used when bolting two pieces of metal together. The lock washer grips into the surface below the screw or bolt head helping to prevent the threads from loosening over time. There are no set in stone standards for when to use lock washers or what type of lock washer to use in every situation. However general installation guidelines for lock washers will help you to choose what type of lock washer to use on your next project.
Place an external tooth lock washer over the threads of a bolt or screw larger than 1/2-inch diameter. The head of the fastener should cover the teeth on the external lock washer.
Slide the neck of a small fastener through the center of an internal tooth lock washer. Internal tooth lock washers ensure a good connection for fasteners 1/4-inch and less in diameter.
Install a combination internal and external tooth lock washer with over size holes and fasteners. The external teeth bite into the mating surface and the internal teeth lock into the fastener head.
Grab a split lock washer for most projects requiring a bolt or screw 3/8- to 9/16-inch in diameter. Split lock washers are more commonly found in most applications around the home.
Place split lock washers between the nut and bolt when using bolts to join pieces together. This prevents the nut from working loose.
Packages of assorted sizes and types of lock washers are available at home improvement centers containing most common sizes necessary for home projects.