Lip seal is a type of sealing arrangement used on ships at the stern tube arrangement. The lip seal forms a part of stern tube bearing and sealing arrangement and is the most frequently used sealing arrangements on ships nowadays
The lip seals are made up of nitrile rubber having elastic property for gripping the rubbing surface in order to have an effective sealing. If the temperature of the seal material exceeds 110°C, the rubber will lose its elasticity property.
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There is different position where the lip seal is positioned. The outer seals dissipate the heat to outboard sea water and the inward seals dissipate heat to the oil by the method of convection. The oil is circulated with the help of a pump from a small header tank.
The outboard gland middle seal keeps the water out and the aft seal protects the middle one from dirt and debris.
The lip seals create grooving over the chrome liner due to friction which can be overcome by using ceramic filler for the grooves or by using a distance piece with some allowance to compensate shaft and stern tube expansion.
Some new models have two void spaces in each gland seals, which are circulated by oil being changed several times by a circulation pump. This oil pressure is slightly less than that of sea water pressure to avoid oil pollution of sea. It also eliminates the problem of high temperature failure of seals.