The ornamental design for a bellows joint, as shown and described.
FIG. 1 is a front perspective view of an embodiment of a bellows joint;
FIG. 2 is a front lower perspective view of the bellows joint of FIG. 1;
FIG. 3 is a front left perspective view of the bellows joint of FIG. 1;
FIG. 4 is rear left perspective view of the bellows joint of FIG. 1; and,
FIG. 5 is a rear elevation view of the bellows joint of FIG. 1.
The broken lines in drawing views are included for the purpose of illustrating portions of the double hinged joint that form no part of the claimed design.