Resilient rotary coupling

A resilient, indexing, rotary marine drive coupling for a marine propellor drive having a propellor and a propellor drive shaft, the coupling having an axis of rotation and being adapted to couple the propellor and the propellor drive shaft of the marine drive, the coupling comprising:

• (a) an outer rotary member having an axially elongated inner surface of generally square cross-sectional configuration comprising a series of four of a circumferentially measured arc length chordally disposed flat areas of a circumferentially measured dimension and rounded corners, the flat areas being connected by the rounded corners, the rounded corners being disposed a greater radial distance from said axis than the radial distance of a bi-secting radius normal to said flat areas, the circumferentially measured arc length of each rounded corner being at least equal to the circumferentially measured dimension of each flat area,
• (b) an inner rotary member comprising a metallic bushing adapted to be received on and matingly engage a shaft, said inner member having an axially elongated periphery opposing the inner surface of said outer member, said periphery being of generally square cross-sectional configuration generally corresponding and complementary to the inner surface of said outer member and having corresponding flat areas and rounded corners being lesser radial dimensions than the flat areas and rounded corners, respectively, of the outer member such that the periphery of said inner member is spaced radially inwardly from the inner surface of said outer member when each member is centered on said axis and no load is imposed on the coupling, and
• (c) resilient elastomeric means of vulcanized rubber disposed between said outer and inner members and bonded to said inner member, said resilient means being approximately equally radially compressed throughout its circumferential and axial extent between said inner surface and said periphery and contacting completely in the circumferential direction of said coupling at least a portion of each of said inner surface and said periphery, the amount of compression being about 35 percent upon assembly of the coupling and about 60 percent when the inner member and outer member are angularly displaced such that the corners of the inner member are nearest the flat areas of the inner surface of the outer member,
• (d) in which the corresponding flat areas of the outer and inner members are generally parallel and symmetrically disposed about said axis when there is no load imposed on the coupling, and
• (e) in which the radial distance from the axis of the rounded corners of the inner member is less than the radial distance of the bi-secting radius of the outer member, the resiliency of said elastomeric means and the difference between said radial distances being such that permanent rotary displacement of the inner member relative to the outer member occurs to provide said indexing.