Abstract: Disclosed is a torque transmitting and blocking device which includes a cylindrical drum, a pair of arcuate shoes positioned within the drum, and a spanner bar or ring extending between the shoes to one side of the axis of the drum. On the other side of the drum axis the shoes are displaceable toward each other, and may conveniently be lightly biased apart. The shoes have surfaces, which may include holes, lateral or transverse projections, or the ends of the shoes, through which rotative forces may be transmitted from sources external of the drum and through which the shoes may retransmit such forces. Application of a rotational force through such a surface into a shoe in a direction tending to displace the shoes toward each other results in rotation of the shoes and bar within the drum, and consequent transmission of torque, if desired, out of the drum through another shoe surface.