Abstract: A mechanism that automatically locks a movable storage unit against closure of an aisle between the unit and a blocking object such as a facing movable or stationary storage unit or wall, while permitting continued opening of the aisle, without any electrical controls. A ratcheting device, such as a pawl and a toothed wheel, is shiftable between a ratcheting state permitting movement of the movable storage unit only away from the facing storage unit or wall, and a non-ratcheting state permitting movement of the unit in either direction. The mechanism includes a bar biased toward the facing storage unit or wall, and a drive shaft rotated by the bar and driving a cam block to shift the pawl from its non-ratcheting state to its ratcheting state due to the movable storage unit having moved away from the facing storage unit or wall by a predetermined distance.