Power tool

1. A power tool, comprising:

an output shaft for outputting torque;

a transmission mechanism for driving the output shaft;

a motor for driving the transmission mechanism; and

a housing for containing the motor,

wherein the transmission mechanism comprises:

a planet gear system comprising a first ring gear being capable of rotating relative to the housing;

locking pins for limiting the rotation of the first ring gear;

a locking ring for limiting the locking pins to rotate with the first ring gear; and

biasing elements for biasing the locking pins so as to move toward the locking ring,

wherein the planet gear system comprises:

a sun gear driven by the motor; and

planet gears being capable of engaging with the sun gear and the first ring gear;

wherein the first ring gear is formed with apertures going through the first ring gear in an axial direction of the output shaft and the locking pins pass through the first ring gear through the apertures and contact with the locking ring,

wherein the locking ring is formed with locking protuberances for contacting and engaging with the locking pins and the locking protuberances protrude towards the locking pins along the axial direction of the output shaft, and

wherein the transmission mechanism further comprises a second ring gear disposed between the locking ring and the motor, the second ring gear has an inner surface and an outer surface, the inner surface is formed with inner teeth, the outer surface is formed with protrusions, the locking ring is formed with lug bosses for engaging with the protrusions so as to lock or release the circumferential rotation of the second ring gear, and the lug bosses are extended from one side of the locking ring close to the second ring gear toward the second ring gear along an axial direction.

2. The power tool of claim 1 , wherein the transmission mechanism comprises an adjusting element for adjusting a biasing force of the biasing elements acting on the locking pins, the power tool further comprises a gearbox for containing the transmission mechanism, and the adjusting element is capable of moving relative to the gearbox.

3. The power tool of claim 2 , wherein the adjusting element is capable of sliding relative to the gearbox and the locking pins are arranged between the adjusting element and the locking ring in the axial direction of the output shaft.

4. The power tool of claim 2 , wherein the biasing elements are coil springs, the adjusting element is connected or formed with locating pins, and the coil springs are mounted on the locating pins respectively.

5. The power tool of claim 1 , wherein the transmission mechanism comprises a washer being arranged between the locking pins and the biasing elements.

6. The power tool of claim 5 , wherein the first ring gear is surrounded by the washer.

7. The power tool of claim 1 , wherein each locking pin comprises two ends, one end of each locking pin contacting with the locking ring has a spherical surface and the other end has a flat surface.

8. The power tool of claim 1 , wherein the planet gear system comprises a planet carrier for rotatably mounting the planet gears and the planet carrier is rotated with the output shaft synchronously.

9. The power tool of claim 1 , wherein the first ring gear comprises a body for containing the planet gears and a flange for forming the apertures and wherein the flange is protruded out of the body in a radial direction.

10. The power tool of claim 9 , wherein the apertures are arranged on an edge of the flange and formed with gaps on the edge of the flange.