Movable barrier operator

1. A movable barrier operator comprising:

a transmission housing;

a stop in the transmission housing;

a motor supported by the transmission housing and including a rotatable drive shaft;

a rotatable drive supported by the transmission housing and configured to be connected to an elongate driven member;

a shaft supported by the transmission housing;

a compound gear mounted on the shaft and rotatable in the transmission housing around a first axis, the compound gear including a first gear operably coupled to the motor drive shaft and a second gear operably coupled to the rotatable drive, the first and second gears connected such that turning of the first gear around the first axis causes turning of the second gear around the first axis;

portions of the first and second gears configured to cammingly engage and shift the first and second gears axially apart on the shaft in response to turning of the first and second gears relative to one another caused by a torque applied to the rotatable drive exceeding a predetermined threshold torque; and

wherein one of the first and second gears is positioned to engage the stop upon the first and second gears having shifted axially apart so that the stop resists turning of the one of the first and second gears.

2. The movable barrier operator of claim 1 wherein the compound gear includes at least one snap fit connection between the first and second gears.

3. The movable barrier operator of claim 1 wherein the first and second gears include mating features that transfer rotation between the first gear and the second gear.

4. The movable barrier operator of claim 1 wherein the first gear is a worm wheel and the second gear is a spur gear.

5. The movable barrier operator of claim 1 wherein the first gear includes a first number of teeth, and the second gear includes a second number of teeth different than the first number of teeth.

6. The movable barrier operator of claim 1 wherein the first gear is made of a plastic material and the second gear is made of a metallic material.

7. The movable barrier operator of claim 1 wherein the portions of the first and second gears include at least one ramp portion configured to shift the one of the first and second gears into position to engage the stop upon turning of the first and second gears relative to one another.

8. The movable barrier operator of claim 7 wherein the at least one ramp portion is configured to shift the first gear along the shaft away from the second gear or to shift the second gear along the shaft from the first gear.

9. The movable barrier operator of claim 1 wherein the portions of the first and second gears include ramp portions of raised ridges of the compound gear.

10. The movable barrier operator of claim 9 wherein the compound gear includes a plurality of recesses that receive the raised ridges and a pair of walls associated with each recess, wherein one of the ramp portions of each of the raised ridges is configured to cammingly engage one of the pair of walls with turning of the first and second gears relative to one another.

11. The movable barrier operator of claim 1 wherein the shaft is fixed relative to the transmission housing, and the compound gear is rotatably mounted to the shaft.

12. The movable barrier operator of claim 1 wherein the rotatable drive shaft includes a worm gear engaging the first gear of the compound gear.

13. The movable barrier operator of claim 1 wherein the rotatable drive is rotatable about a second axis and the rotatable drive shaft is rotatable about a third axis; and

wherein the second and third axes extend transversely relative to one another and do not intersect.

14. The movable barrier operator of claim 13 wherein the second axis is parallel to the first axis and wherein the third axis does not intersect the first axis.

15. The movable barrier operator of claim 1 wherein the one of the first and second gears has teeth; and

wherein the stop includes a protrusion sized to fit between the teeth of the one of the first and second gears upon the first and second gears having shifted axially apart.

16. The movable barrier operator of claim 1 wherein the stop is spaced from the first axis a distance less than half of a maximum outer diameter of the second gear, the second gear positioned to engage the stop upon the first and second gears shifting axially apart.

17. The movable barrier operator of claim 1 wherein the rotatable drive includes a spur gear engaged with the second gear, the spur gear having a through bore;

wherein the transmission housing includes a base wall having a through opening to receive an output shaft that extends into the through bore of the spur gear; and

wherein the stop includes a protrusion upstanding from the base wall of the transmission housing.