Also known as Heavy Duty Seals, these seals are used in rotating applications. They function well in harsh environments where they have to withstand the ingress of rough and abrasive external media. Trelleborg Type DO is the most common form that uses an Trelleborg O-Ring as a secondary sealing element. Depending on seal type the following standard elastomers are used:- Type DO - Cast Iron and Acrylonitrile Butadiene Rubber (NBR)- Type DO - Bearing Steel (1.3505) Acrylonitrile Butadiene Rubber (NBR)