Patent ID: 6843482

Claim:
Apparatus for providing a seal between first and second relatively rotatable parts of a mechanism used in an environment comprising pulverulent material entrained in a fluid, the apparatus comprising: a first element and a second element between which an annular passage is defined when the first element is mounted on one said part of the mechanism and the second element is mounted on the other said part of the mechanism and the one part is rotated with respect to the other, wherein the annular passage is defined by interfacing surfaces of the respective elements between which there is an initial clearance of substantially greater size than that of pulverulent material entrained in the fluid and is such as to enable said pulverulent material to enter the annular passage when the apparatus is in use and to be deposited in layers which build up so that there is formed between the layers a labyrinthine passage having a width which is substantially smaller than the size of said pulverulent material so that the flow of fluid through the labyrinthine passage is substantially restricted and substantially no further said pulverulent material can flow through the annular passage, the ratio of the minimum value of the initial clearance between the interfacing surfaces to the minimum diameter of the annular passage being not substantially less than 1:150.