A coupling, comprising: a first coupling element, a second coupling element, a pivot pin connecting the first and second coupling elements for pivotal movement relative to each other, and a cam to vary the amount of relative pivotal movement between the first and second coupling elements, wherein the first coupling element is generally U-shaped and includes a pair of arms (113, 114) and a bight portion connecting the arms (113, 114), wherein the second coupling element includes a coupling portion disposed between the arms (113, 114), the pivot pin extending through the coupling portion and into the arms (113, 114), the cam being received by the first coupling element and acting on the second coupling element to define a selected stop position for the first and second coupling elements relative to each other, wherein the cam comprises a pair of relatively movable cam elements (138, 139) received in aligned holes in the first coupling element, wherein the coupling further includes a screw acting on the cam elements (138, 139) to cause the cam elements (138, 139) to move selectively toward or away from each other, wherein the screw extends into both cam elements (138, 139), wherein the arms (113, 114) have said aligned holes to slidably receive the cam elements (138, 139), wherein the screw is received in the cam elements (138, 139), and wherein rotation of the screw in one direction causes the cam elements (138, 139) to move toward each other and rotation of the screw in the opposite direction enables the cam elements (138, 139) to move away from each other, and wherein the second coupling element includes a cam surface cooperable with the cam elements (138, 139). a first coupling element, a second coupling element, a pivot pin connecting the first and second coupling elements for pivotal movement relative to each other, and a cam to vary the amount of relative pivotal movement between the first and second coupling elements, wherein the first coupling element is generally U-shaped and includes a pair of arms (113, 114) and a bight portion connecting the arms (113, 114), wherein the second coupling element includes a coupling portion disposed between the arms (113, 114), the pivot pin extending through the coupling portion and into the arms (113, 114), the cam being received by the first coupling element and acting on the second coupling element to define a selected stop position for the first and second coupling elements relative to each other, wherein the cam comprises a pair of relatively movable cam elements (138, 139) received in aligned holes in the first coupling element, wherein the coupling further includes a screw acting on the cam elements (138, 139) to cause the cam elements (138, 139) to move selectively toward or away from each other, wherein the screw extends into both cam elements (138, 139), wherein the arms (113, 114) have said aligned holes to slidably receive the cam elements (138, 139), wherein the screw is received in the cam elements (138, 139), and wherein rotation of the screw in one direction causes the cam elements (138, 139) to move toward each other and rotation of the screw in the opposite direction enables the cam elements (138, 139) to move away from each other, and wherein the second coupling element includes a cam surface cooperable with the cam elements (138, 139).