Expansion element

An expansion mechanism comprising n (where n is an integer greater than 2) substantially identical hook-shaped or spiral-shaped elements disposed in a circular array about a central axis so that each element has first and second adjacent elements, and each element having first, second and third portions each having an inner surface and an outer surface, the inner surfaces of said first portions respectively defining a regular n-sided polygonal chamber, the center of which lies on said central axis, the outer surfaces of said second portions respectively being sliding surfaces, and the inner surface of said third portion of each element being a sliding surface which is in sliding contact with the outer sliding surface of the second portion of said first adjacent element, whereby upon linear displacement of one of said elements in a direction along that side of the polygon which is defined by the inner surface of said first portion of that element, sliding movement of the outer surface of the second portion of that element along the inner surface of the third portion of the second adjacent element, and of the inner surface of the third portion of that element along the outer surface of the first adjacent element, causes said adjacent elements to be displaced correspondingly, between a first condition in which the outer surfaces of the third portions respectively are relatively close to said central axis and a second condition in which the outer surfaces of the third portions respectively are relatively distant from said central axis.