the conveyor the face side end of each guide rod system 20 is connected to the part 22 by means of a respective head piece 21 and its goaf side end is slidably guided in slideways provided in the mutually facing side edges of the floor girders 10 of that unit each guide rod system 20 is completed by means of a hydraulic advance ram 24 which acts between the goaf side end of that system and a bracket 23 fixed to the floor girders 10 at their face side ends thus when the rams 24 are retracted the conveyor 18 is advanced in the direction of the arrow r that is to say towards the face and when the rams 24 are extended the roof support units a b and c are advanced in a follow up sequence each of the roof support units a b and c is fitted with the so called 34 lemniscate 34 guide system this system is fitted between the roof shield 12 and the floor girders 10 of each unit and consists in each case of two sets of links positioned one at each side of that unit each set of links has a main link one end of which is pivotally connected to its roof shield 12 by means of a pivot joint 26 and two guide links 27 and 28 each guide link 28 is pivotally connected at 32 to a bracket 29 fixed to the corresponding floor girder 10 and.