Dual thrust anchor shell assembly

An anchor shell assembly for anchoring a bolt in a roof bolt hole, said bolt having a longitudinal axis, a lower head for forcing a roof plate against a portion of a mine roof surrounding the mouth of said roof bolt hole, and a shaft with an upper threaded portion said anchor shell assembly comprising in combination:

• (a) a first upper expansion member disposed pivotally relative to said axis, said first upper expansion member including an upper portion having first and second upper camming surfaces that are sloped relative to said axis, and an elongated lower portion, said elongated lower portion having a lower camming surface that is sloped relative to said axis;
• (b) a second upper expansion member disposed pivotally relative to said axis and adjacent to said first upper expansion member, said second upper expansion member having an elongated lower portion disposed on an opposite side of said shaft from said lower elongated portion of said first upper expansion member, said lower elongated portion of said second upper expansion member having a lower camming surface that is sloped relative to said axis;
• (c) a first lower expansion member disposed pivotally relative to said axis, said first lower expansion member having a lower portion having first and second lower camming surfaces that are sloped relative to said axis, and an elongated upper portion having an upper camming surface that is sloped relative to said axis;
• (d) a second lower expansion member disposed, pivotally relative to said axis and adjacent to said first lower expansion member, said second lower expansion member having an elongated upper portion disposed on the opposite side of said shaft from said elongated upper portion of said first lower expansion member, said upper portion of said second lower expansion member having an upper camming surface that is sloped relative to said axis;
• (e) threaded nut means disposed on said threaded portion of said shaft for forcing said first and second upper expansion members toward said first and second lower expansion members in response to tightening of said bolt;
• and
• (f) retaining means for retaining said first and second lower expansion members in fixed longitudinal relationship to said bolt during tightening of said bolt,
• said lower camming surfaces of said first and second upper expansion members engaging said first and second lower camming surfaces of said first and second lower expansion members during said tightening of said bolt, forcing said lower portions of said first and second upper expansion members outward into roof strata surrounding the portion of said bolt adjacent to said anchor shell assembly to prevent further downward movement of said first and second upper expansion members in said roof bolt hole, an initial distance between said lower camming surfaces of said first upper expansion member and one of said first and second lower expansion members being substantially less than a corresponding initial distance between said upper camming surfaces of said first upper expansion member and one of said first and second lower expansion members, whereby further tightening of said bolt forces said roof plate tightly upward against the portion of the mine roof surrounding the mouth of said roof bolt hole before said upper camming surface of one of said first and second upper expansion members engages said upper camming surface of one of said first and second lower expansion members, respectively.