Levelling ram for roof-shield mine support systems

A roof-shield mine support system, comprising at least a pair of hydraulically operated levelling rams extendable between a shield frame unit and an associated gap-seal plate which is arranged slidably relative to the associated shield frame in a transverse direction relative to the direction of forepoling, each said ram comprising a piston and cylinder unit, opposite sides of said piston being connected to the hydraulic system of said piston and cylinder unit for extending and retracting said piston for aligning the associated shield frame unit relative to an adjacent shield frame unit, the piston rod of said piston having a longitudinal bore open at an outer end of said piston and closed at an end of said piston rod remote from said piston, and an elongated rod disposed within said bore, said rod being sealed relative to the interior of said bore, and the interior of said bore being connected in use to said hydraulic system for applying constant pressure between said piston and said cylinder for pressing the associated gap-seal plate tightly against the adjacent frame unit.