Downhole drilling tool

1. A downhole drilling tool comprising a drillstring and a drilling head, wherein a freewheel for transferring drive from the drillstring to the drilling head is positioned between the drillstring and the drilling head, the freewheel comprising:

a first race;

a second race; and

a locking element interposed between said first and second races;

one of said races having a wedge-shaped depression upon which the locking element moves between a disengaged position in which the first race is disengaged from the second race and an engaged position in which the locking element locks the first and second races together, and the other of said races having a depression formed in a surface thereof, the depression being shaped to receive at least a portion of the locking element when the locking element is in the engaged position;

wherein the wedge-shaped depression is formed in an end face of the first race, and the depression is formed in an end face of the second race.

2. A downhole drilling tool as claimed in claim 1 , wherein said other of said races has a plurality of depressions formed in the surface thereof.

3. A downhole drilling tool as claimed in claim 2 , wherein said one of said races has a plurality of wedge-shaped depressions, and the freewheel further comprises a plurality of locking elements interposed between the first and second races.

4. A downhole drilling tool as claimed in claim 1 , wherein the locking element is a ball bearing.

5. A downhole drilling tool as claimed in claim 1 , wherein the depression is a groove that extends in a generally circumferential direction relative to the axis of rotation of the driven member.

6. A downhole drilling tool as claimed in claim 1 , wherein the wedge-shaped depression is curved in a generally circumferential direction relative to the axis of rotation of the driving member.

7. A downhole drilling tool as claimed in claim 1 , wherein the depression is a hole.

8. A downhole drilling tool as claimed in claim 1 , wherein the depression is a recess.

9. A downhole drilling tool as claimed in claim 3 , wherein the number of depressions is greater than the number of locking elements.

10. A downhole drilling tool as claimed in claim 1 , wherein the driven member is able to rotate freely when the locking element is in a disengaged position, and the driving and driven members are engaged when the locking element is in an engaged position in which the locking element is jammed into the depression by the wedge-shaped depression.