Best way to couple sun gear to rotor for a hollow shaft application

[nguyen_v]

Hello all,
I am currently designing a hollow shaft actuator using a custom compound planetary gearbox. The issue I am encountering is to find a way to couple the sun gear (15T, mod1) to the rotor, while still having the hollow shaft go through the sun gear. As the actuator is quite thin, the aun gear is limited in height (~12mm). I have seen a solution where some sort of custom gear coupler is machined to hold the sun gear.

Another solution that I've thought of is taking advantage of the radial exit of the gear cutter tool to get a larger base, where I can have room to add dowel pins for transmitting torque. However, this leaves a long section on the sun gear (highlighted in blue) where the planets cannot be engaged.

I feel like the first solution (machining a custom gear coupler, maybe having the sun and coupler with some profile shift) is currently the best way to go, but I've also seen designs with standard involute splines (something like ISO4156) directly embedded on the gear. In that case it would require sintered gears which is quite expensive for low volumes.

What are your thoughts?

 

[mfgenggear]

OP
Trying to follow your design
Most sun gears have through hole internal spline, with a counter bore
To locate in and it is the datum
Of both the gear and spline.
Planetary gears have to be precise
To control the backlash and runout.

 

[mfgenggear]

The external gear is machined fron the counter bore. The od of the gear will be precise for tooling to hold the gear to broach the internal spline. But not cheap. The entire gear train must be precise and controlled, with the correct backlash. The ring gear is actually the most difficult to manufacture.

 

[nguyen_v]

@mfgenggear
Thanks for your response. Let me clarify some details.

As you have pointed out, most sun gears use an internal spline. However in the design that I want to achieve, there is a hollow shaft going through the sun gear (first picture), which means that the sun needs to be coupled on its outer face.

This is why I am wondering if there is a good way to couple the outer face of the sun gear to the rotor.

 

[mfgenggear]

OP
The best design is the one advised.
However the issue will be a positive attachment to the hollow shaft.

Maybe have a raised step on the hollow shaft, a slight press fit to the sun.
Then electron beam weld it.
Then NDT. As long as the details are low carbon in those areas. The gear can be carburized.

 

[nguyen_v]

@mfgenggear

In my design, the hollow shaft tube does not rotate with the sun gear (the sun gear is coupled with the rotor, and the hollow shaft is coupled with the gearbox output). In this case I need to press fit the sun gear to a part rotating with the rotor. Sorry for the confusion

 

[mfgenggear]

OP
Please provide a picture of the coupling
and the sun gear. It may be possible to make one combined detail.
If it is low volume. The gear can be single flanked ground.