Surface traversing apparatus

A surface traversing apparatus adapted to be supported near a member for traversing the surface of the member comprising:

• first base means;
• second base means, said first and second base means being movable relative to one another alternately toward and away from the member to each of three relative positions including an intermediate position and an extreme position on each side of said intermediate position;
• first anchor means connected to said first base means;
• second anchor means movably connected to said second base means for movement transversely of said second base means and said first anchor means, said first and second anchor means being movable with the respective said first and second base means alternately to and away from the member for respective engagement with and disengagement from the member for respectively restricting and permitting relative lateral motion between the member and the respective said anchor means, both said first and second anchor means being positioned for said engagement with the member at said intermediate one of said three relative positions of said first and second base means and only an opposite one of said first and second anchor means being positioned for said engagement with the member at each opposite one of said two extremes of said three relative positions of said first and second base means;
• drive means operatively connected to said second base means and said second anchor means for selectively moving said second anchor means transversely of said second base means and said first anchor means;
• and
• actuator means connected to said first and said second base means for selectively moving said first and said second base means alternatively to each of said three relative positions to engage both said first and said second anchor means with the member at said intermediate one of said three relative positions and to disengage an alternate one of said first and second anchor means at either of said two extreme relative positions of said first and second base means while the other said anchor means remains engaged with the member so as to transport said second anchor means by said drive means transversely relative to said second base means and first anchor means thereby to move said first and second anchor means to new relative lateral positions.