Glow plug

Abstract

A glow plug ( 1 ) including a heater ( 2 ), a metallic shell ( 4 ) assuming the form of a tube having an axial bore, a center shaft ( 3 ) and a contact member ( 7 ). The contact member ( 7 ) is configured such that a contour ( 70 ) of a cross section ( 75 ) includes a second contour segment ( 71 ) in the form of a straight line, a first contour segment ( 72 ) in the form of a curve swelling radially, and third contour segments ( 73 ) in the form of curves connecting the second contour segment ( 71 ) and the first contour segment ( 72 ).

1 . A glow plug comprising:
 a heater having, in a forward end portion, a heat-generating resistor which generates heat through energization;   a metallic shell assuming the form of a tube having an axial bore extending along a first axis which is the axis of the metallic shell, and holding the heater directly or indirectly at a forward end portion;   a center shaft assuming a rodlike form, disposed in the axial bore of the metallic shell with a gap formed between the same and an inner circumferential surface of the axial bore, having one end portion connected to a rear end portion of the heater, and having the other end portion projecting from a rear end of the metallic shell; and   a contact member formed from an electrically insulative elastic member, assuming an annular form, inserted into the axial bore at a rear end portion of the axial bore, and disposed in such a manner as to be in contact with the inner circumferential surface of the axial bore and with the center shaft;   the glow plug being characterized in that:   a contour of one of two cross sections resulting from cutting the contact member in a state prior to attachment to the glow plug by a plane which contains a second axis which is the axis of the contact member has   a first contour segment assuming the form of a curve extending along the second axis and swelling radially outward with a radius R 1  of curvature and   a second contour segment assuming the form of a straight line extending along the second axis or the form of a curve extending along the second axis and swelling radially inward with a radius R 2  of curvature which satisfies a relational expression R 1 <R 2 .   
     
     
         2 . A glow plug as claimed in  claim 1 , wherein the contact member is such that, as viewed on the one cross section, a length along an extending direction of the second axis is longer than a length along a direction orthogonal to the second axis. 
     
     
         3 . A glow plug as claimed in  claim 1 , wherein:
 the center shaft further comprises
 a rear trunk portion disposed at such a position with respect to the extending direction of the first axis of the metallic shell as to face the rear end portion of the inner circumferential surface of the axial bore, and having a diameter greater than that of the other end portion and 
 a shoulder portion connecting the rear trunk portion and the other end portion in a tapered manner; 
   the metallic shell further comprises a taper portion expanding in a tapered manner at the rear end portion of the axial bore from a position located forward of a forward end of the shoulder portion toward the rear end located rearward of the position with respect to the extending direction of the first axis; and   the contact member is disposed such that the second contour segment is in contact with the center shaft and such that the first contour segment is in contact with an inner circumferential surface of the axial bore located forward of the taper portion.   
     
     
         4 . A glow plug as claimed in  claim 3 , wherein the contour of the contact member further comprises a third contour segment which is connected, at one end, to the second contour segment; which extends radially outward from the one end toward the other end thereof while extending along the second axis; which is connected, at the other end, to the first contour segment; and whose point of connection to the first contour segment is an end of the contact member with respect to the extending direction of the second axis. 
     
     
         5 . A glow plug as claimed in  claim 4 , wherein the contour forms a mirror image with respect to a center position in the extending direction of the second axis.