Power tools with reversible, self-shifting transmission

1. A power tool comprising:

a motor having a motor shaft;

an output shaft configured to be coupled to a fastener to rotate the fastener; and

a transmission coupled between the motor shaft and the output shaft to transmit rotation from the motor shaft to the output shaft in a low-speed, high-torque mode and in a high-speed, low-torque mode, wherein the transmission is configured to self-shift between the low-speed, high-torque mode and the high-speed, low-torque mode in response to a change in a torque required to rotate the output shaft in both a clockwise direction and a counterclockwise direction.

2. The power tool of claim 1 , wherein the transmission comprises a speed adjuster coupled to the motor shaft, a speed-sensitive clutch coupled to the motor shaft, and a bi-directional overrunning clutch coupled between the speed adjuster and the output shaft.

3. The power tool of claim 2 , wherein the bi-directional overrunning clutch is also coupled between the speed-sensitive clutch and the output shaft.

4. The power tool of claim 2 , wherein the transmission is configured such that, when the transmission is in the high-speed, low-torque mode, the speed-sensitive clutch couples to the bi-directional overrunning clutch to transfer rotation from the motor shaft to the bi-directional overrunning clutch for rotation therewith.

5. The power tool of claim 2 , wherein the speed-sensitive clutch is a centrifugal clutch.

6. The power tool of claim 5 , wherein the centrifugal clutch is configured to overcome a biasing force when rotating above a threshold speed to couple to the bi-directional overrunning clutch.

7. The power tool of claim 2 , wherein the speed adjuster is configured to reduce a speed of rotation transferred from the motor shaft to the bi-directional overrunning clutch.

8. The power tool of claim 7 , wherein the speed adjuster is a speed-reducing planetary gear set.

9. The power tool of claim 2 , wherein the bi-directional overrunning clutch comprises an outer race coupled to the speed adjuster, an inner race coupled to the speed-sensitive clutch, and an engagement assembly located in a space defined radially between the outer race and the inner race.

10. The power tool of claim 9 , wherein the engagement assembly is configured to selectively (i) connect the inner race to the outer race for rotation therewith or (ii) disconnect the inner race from the outer race so that the inner race can overrun the outer race when the inner race rotates faster than the outer race.

11. The power tool of claim 9 , wherein the output shaft is coupled to the inner race for rotation therewith.

12. The power tool of claim 9 , wherein the outer race is formed to define an outer race cavity that houses the speed-sensitive clutch.

13. The power tool of claim 9 , wherein the motor shaft, the outer race, the inner race, and the output shaft each rotate about a common axis.

14. A power tool transmission comprising:

an output shaft;

a centrifugal clutch;

a speed-reduction gear set; and

a bi-directional overrunning clutch coupled between the centrifugal clutch and the output shaft and coupled between the speed-reduction gear set and the output shaft.

15. The power tool transmission of claim 14 , wherein the bi-directional overrunning clutch comprises an outer race coupled to an output of the speed-reduction gear set, an inner race coupled to an output of the centrifugal clutch, and an engagement assembly located in a space defined radially between the outer race and the inner race.

16. The power tool transmission of claim 15 , wherein the engagement assembly is movable between an unlocked position, in which the inner race is free to move relative to the outer race in both a clockwise direction and a counterclockwise direction, and a locked position, in which the engagement assembly is wedged between the outer race and the inner race to couple the outer race to the inner race for rotation therewith in both the clockwise direction and the counterclockwise direction.

17. The power tool transmission of claim 15 , wherein the output shaft is coupled to the inner race for rotation therewith.

18. The power tool transmission of claim 15 , wherein the engagement assembly comprises a first biasing shoe frictionally engaged with the outer race and a first plurality of locking members configured to move with the first biasing shoe within the space between an unlocked position, in which the first plurality of locking members are free to move relative to the outer race and the inner race, and a locked position, in which the first plurality of locking members are wedged between the outer race and the inner race to couple the outer race to the inner race for rotation therewith.

19. The power tool transmission of claim 18 , wherein the engagement assembly further comprises a second biasing shoe frictionally engaged with the outer race and a second plurality of locking members configured to move with the second biasing shoe within the space between an unlocked position, in which the second plurality of locking members are free to move relative to the outer race and the inner race, and a locked position, in which the second plurality of locking members are wedged between the outer race and the inner race to couple the outer race to the inner race for rotation therewith.

20. The power tool transmission of claim 18 , wherein the first plurality of locking members is a plurality of rollers.