60 b in the vertical direction each hoist assembly 55 a 55 b in combination with a boom 51 a 51 b can function in a manner similar to a crane by extending and retracting a cable or wire to move the associated top drive 60 a 60 b vertically for example the vertical position of each top drive 60 a 60 b can be controlled by winding and unwinding cable drum not shown or a spool which can be rotated by a motor not shown to control the height of the top drive 60 a 60 b relative to the opening 21 any type of motor or other motive source e g a hydraulically or electrically powered source as well as any other known method for extending or retracting cable of sufficient force in this application and resulting in vertical movement of the drive assembly can be used likewise moving the drive assembly in the vertical direction can be accomplished by any mechanism capable of providing sufficient force as one example the mechanism can include an internal rack and pinion mechanism whereby a pinion which depending on the size and mass of the drive assembly and its application can be powered by an electrical motor or other motive and or power source engages teeth along the length of the mast booms not shown causing movement in the vertical direction as another example the lifting assembly can incorporate a traveling block with a series of sheaves and cables powered by a winch in this example the winch can be manually operated or powered by a motor the lifting assembly can include a hydraulic ram connected to the top drive top drive assemblies usable with the embodiments depicted in fig1 a and 2b are shown.