Mine roof support assemblies

A mine roof support assembly comprising three adjacent mine roof support units and apparatus for guiding and aligning said units, the units each comprising a floor sill and telescopic prop means for bracing against the roof of a mine excavation, the units being provided with shifting ram means for displacing the units, the guiding and aligning apparatus comprising first and second pairs of telescopic guide rods, the first pair of guide rods being pivotally connected, by joints, to the units of one pair of adjacent units, and the second pair of guide rods being pivotally connected by joints, to the units of the other pair of adjacent units, wherein each guide rod of the first pair of guide rods is constituted by a springless, non-hydraulic piston-and-cylinder device and a first of the guide rods of the second pair of guide rods is constituted by a springless non-hydraulic piston-and-cylinder device and the second guide rod of the second pair of guide rods is constituted by a double-acting hydraulic piston-and-cylinder device, and wherein the guide rods of each pair are inclined to one another at an acute angle, the guide rods being so arranged that, when the units of a given pair of adjacent units are positioned side-by-side, one of the guide rods of the associated pair of guide rods is completely retracted with its axis perpendicular to the direction in which the units are to be displaced and, when one of the units of that pair of units has been displaced by the shifting ram means relative to the other unit of that pair of units, the other guide rod of the associated pair of guide rods is completely retracted.