Patent ID: 8282107

Claim:
An oil seal for reciprocation motion comprising: a seal main body including a seal lip configured for sliding contact with a reciprocating shaft; a backup ring made of resin which is disposed so as to support the seal lip from an atmosphere side to ensure durability of the seal lip and in which at least a single annular groove is formed in an inner peripheral surface configured to oppose the reciprocating shaft, the annular groove capable of holding a lubricating material; a holding member that holds the backup ring between the holding member and the seal main body in a sandwiched-state, wherein: a plurality of annular projections each having an angled-cross section are arrayed on the inner peripheral surface of the backup ring along an axial direction of the reciprocating shaft, the annular groove is formed by a valley between two adjacent annular projections of said plurality of annular projections each having the angled-cross section, a virtual inner peripheral surface formed by a bus linearly connecting peak sections of the plurality of annular projections, each having the angled-cross section, along the axial direction forms a tapered inner peripheral surface that increases in diameter from a sealed side towards the atmosphere side in the axial direction, an angle of the taper inner peripheral surface is 1 to 15 degrees, peak angles of the annular projections are 120 to 160 degrees, a smallest diameter portion of the inner peripheral surface is a smallest diameter portion of the virtual inner peripheral surface and a size of an inner diameter of the smallest diameter portion of the virtual inner peripheral surface is larger than an outer diameter of the reciprocating shaft, and the oil seal for reciprocation motion is configured such that when the oil seal is used under a steady state in which a pressure from the sealed side is less than 2 MPa, there is a gap between the smallest diameter portion of the virtual inner peripheral surface and an outer surface of the reciprocating shaft, and when the oil seal is used under high pressure in which the pressure from the sealed side is 2 MPa to 9 MPa, at least an area of the virtual inner peripheral surface on the atmosphere side is maintained in a non-contact state with respect to the reciprocating shaft.