and 10 a rivet joint 68 is shown as an alternate embodiment of rivet joint 36 wherein rivet shoulder 52 of rivet 40 is received within a spring shoulder bushing 70 as best seen in fig1 spring shoulder bushing 70 is of unitary construction and includes a bush portion 71 having a bushing inner surface 72 and a bushing outer surface 74 and a washer portion 75 having a washer inner surface 76 and a washer outer surface 78 the washer portion 75 of spring shoulder busing 70 is dished with washer inner surface 76 forming a concavity spring shoulder bushing 70 is preferably constructed of a self lubricating plastic and more preferably of nylon spring shoulder bushing 70 is received within a larger linkage connection hole 46 of a connecting link 37 where bushing outer surface 74 and at least a portion of washer inner surface 76 are in contact with connecting link 37 rivet tenon 54 is received within a thrust washer 80 which is interposed between connecting links 37 thrust washer 80 is preferably constructed of bronze or zinc plated steel to reduce rotational friction between connecting links 37 and provide an electrical circuit connection between the connecting links 37 tenon 54 is also received within smaller linkage connection hole 48 of a connecting link 37 a conventional riveting peening process is used to assemble rivet joint 68 when rivet joint 68 is fully assembled thrust washer 80 is preferably in contact.