Mine roof expansion anchor

Claim 1

A radially expansible shell assembly for use with a tapered camming plug to anchor an elongated bolt in a bore hole in a rock formation to support and reinforce said formation, said assembly comprising: a) a pair of unitary shell elements each including two leaf members having upper and lower ends, inner and outer surfaces and first and second side edges, and a bridge portion integrally joining the respective leaf members of each pair adjacent said lower ends thereof, said bridge portion of each of said elements having upper and lower ends, and inner and outer surfaces; b) bail means including leg portions extending axially of said leaf members and having respective terminal ends fixedly attached to said bridge portions with said terminal ends superposed with said bridge portion outer surfaces, said bail means holding said pair of elements in assembled relation symmetrically arranged about a central axis; c) said inner surfaces of said leaf members adjacent said lower ends thereof being contiguous with said said inner surfaces of said bridge portions at a substantially uniform distance from said central axis; and d) a relieved area extending into said inner surface of each of said bridge portions from said upper ends of said bridge portions for at least a portion of the distance to the lower ends thereof, said relieved areas being laterally centered with respect to said bridge portions, whereby said relieved areas provide a clearance for said camming plug when the latter travels downwardly between said shell elements.