Rock bolt expansion anchor having windened expansion range

Claim 1

An expansion anchor assembly for insertion in a rock formation drill hole having a diameter which may vary between relatively wide dimensional limits, said assembly being effective to expand into engagement with the surrounding wall of said drill hole and provide at least a minimum desired holding force over the entire range of dimensional limits of said drill hole diameter, said assembly comprising: (a) a hollow expansion shell having upper and lower ends and a plurality of portions arranged concentrically about a central axis; (b) a nut having upper and lower ends and an internally threaded, central opening extending therethrough; (c) each of said portions having an internal surface facing said central axis and tapered from said upper end of said shell toward said central axis at a first angle for a first axial portion of its length and at a second angle for a second axial portion of its length; (d) said nut having an external surface tapered from said lower end thereof away from the axis of said opening at a third angle for a first axial portion of its length, substantially equal to the length of said first axial portion of said shell portions, and at a fourth angle for a second axial portion of its length; (e) said first angle being greater than said second angle, and said third angle being greater than said fourth angle; and (f) means retaining said shell portions and nut in assembled relation prior to expansion with said lower end of said nut inserted into said upper end of said shell by a distance substantially equal to said first axial portion of the length of each of said shell and nut, the surfaces of said shell and nut in said first axial portions of each being in contact; (g) the relative lengths of said first and second axial portions of said shell portions and said nut, and the values of said first, second, third and fourth angles permitting expansion of said assembly from an initial diameter prior to expansion, to a minimum expanded diameter in engagement with the surrounding wall of a drill hole of a first diameter, and to a maximum expanded diameter in engagement with the surrounding wall of a drill hole of a second diameter, the difference between said minimum and maximum diameters being at least 1/8".