Serrated Washer - Internal
Shakeproof washers are an anti-vibration type of locking washer. The surface of the washer features special teeth that lock when tightened, making them resistant to vibration. They are sometimes referred to as lock tooth washers.
On an internal shakeproof washer the teeth are found on the internal rim of the washer. They are generally neater in appearance than external shakeproof washers when installed.
Shakeproof washers are most commonly used in the electrical industry.