holes 30 bears against bearing surface 56 there is a functional relationship between the pressure bearing on bearing surface 56 and the pressure transferred across sealing surface 52 the greater the area of bearing surface 56 the greater the pressure transferred across sealing surface 52 however one cannot simply maximize the area of bearing surface 56 to produce the maximum sealing pressure on sealing surface 52 the drill pipe 26 passing through stripper 10 and particularly a tool joint 28 tends to tear seal 36 along or adjacent to sealing surface 52 often at the intersection of sealing surface 52 and upper transition surface 50 excess sealing pressure exacerbates the problem because sealing surface 52 tends to deform into the region between the drill pipe 26 and shoulder 44 or the drill pipe 26 and lower retainer 38 during those periods in which drill pipe 26 is rapidly removed or inserted tripping in or tripping out the frictional force between the drill pipe 26 and sealing surface 52 can cause sealing surface 52 to heat up and weaken as the tool joint 28 passes by it tends to lop off the extruded portion ruining the sealing surface 52 transition surfaces 50 54 are designed to assist the passage of the drill pipe 26 particularly the tool joints 28 by allowing the tool joints 28 to impinge on a tapered surface giving seal 36 an opportunity to deform out of the path of the drill pipe 26 and tool joints 28 as they pass through stripper 10 cavity 48 provides a chamber into which seal 36 can deform when pressure is applied to it by deforming into cavity 48 seal 36 is less likely to deform into the region between the.