Draw works for drilling rig

Claim 1

Earth drilling apparatus comprising a mast, a drill head arranged for travel longitudinally along the mast, said drill head being adapted for attachment to the upper end of a string of drill pipe to support the latter within the mast during a drilling operation, hoisting means including a hoisting cable system having a hoisting cable operatively connected with the drill head to enable lifting of the drill string to be effected and drawdown means including a drawdown cable system having a drawdown cable operatively connected with the drill head to enable drawdown of the drill string to be effected, the hoisting and drawdown means further including a single winch drum and drive means for rotating same in both directions, said hoisting and drawdown cable systems being arranged such and being operatively connected to said drill head and said single winch drum in such a fashion that when the drum is rotated one of said cables is tensioned and wraps on said drum in a single layer while the other cable unwraps from the drum to apply lifting or drawdown forces, as the case may be, to the drill head so that the latter is caused to travel along the mast in a direction determined by the direction of rotation of the drum, said single winch drum being dimensioned such that not more than one layer of cable need wrap on said drum regardless of the position of the drill head within the mast.