A locking device for a threaded fastener comprises a pair of washers each having a plurality of wedge cam sets that include a base-cam and a raised tooth-cam. When the washers are mated together in use, the wedge cam sets of each washer are placed into confronting engagement with those of the other washer, with the raised tooth-cams of each washer engaging the base-cams of the other washer. The base-cam surfaces are ramped so that, as the tooth-cams slide along them during relative rotational movement, the combined axial height of the two washers changes--decreasing during tightening and increasing during loosening so that unintended partial loosening of the fastener is offset by expansion of the washers to thereby maintain bolt tension. Raised shoulders at each end of the tooth-cams are used to inhibit the amount of relative rotational movement of the two washers in both directions.