be activated without mechanical or significant hydraulic changes to the apparatus application_number US 201414206548 A description fig1 shows a rod pushing pulling machine generally referred to herein as a thrust unit 10 intended for pipe bursting the thrust unit 10 is connected to a rod string 12 for pushing into an existing pipe 14 and pulling back a pipe splitter 16 and a new pipe 18 the rod string 12 may be threaded or may be hooked together from rod sections by turning or fitting pipe sections together one skilled in the art will appreciate that the process of pushing the rod 12 into the existing pipe 14 will require less thrust than pulling back the splitter 16 and new pipe 18 for pipe bursting purposes with reference now to fig2 a detailed view of the thrust unit 10 is shown with the rod string 12 removed for clarity the thrust unit 10 comprises a rail 26 a thrust frame 28 a gripper 30 two cylinders 32 two rams 34 a rod spinner 35 and a stationary frame 36 the thrust frame 28 is movable along the rail 26 relative to the stationary frame 36 the thrust frame 28 supports the gripper 30 the two cylinders 32.