Sealing washers have a synthetic rubber seal bonded to the steel section that provides a tighter fit and prevents the washer from being over-tightened. The rubber acts like a cushion, preventing leakage from oils, water, gases and other materials. In a residential setting, sealing washers are often used in plumbing.
The washers are almost like an open-faced sandwich. The rubber seal is on the bottom. This is the part that comes in contact with the attached piece. The metal backing, which compresses the seal when tightened, covers the rubber and remains face-up when used. The fastener, usually a six-sided screw, fits in the middle.
Once tightened, sealing washers provide an effective seal on both smooth and rough surfaces. They work equally well on glass, porcelain, ceramic, plastic and fiberglass. Sealing washers used on painted surfaces don't cause chipping, scratching or other surface damage.
The rubber seal is quite elastic, making these washers ideal for machinery that constantly vibrates. In the automotive industry, for example, sealing washers are used on engine block heaters and emission control equipment. Both of these units experience almost constant vibration when a vehicle is on the road. These seals are also used on motorcycles, aircraft, metal buildings and even for attaching aiming sights to archery bows.