New method of fixing the sun gear to the shaft

 Hi,

this is going to be just a short log about new way of fixing the sun gear to the aluminium shaft mounted to the rotor. During some high speed tests of the actuator I noticed that the sun gear slipped on the aluminium axis when I tried to stop the output 3d-printed lever. I was kind of expecting it as I haven't made a strong interference fit and used just a regular epoxy adhesive and paid no extra attention to that part. I decided to try some new methods. 

The first idea was to make a hole in the aluminium stock before milling, and then mill the motor shaft part so that the shaft has this long groove. A similar groove was milled in the sun gear as well. As you might expect it was supposed to be a keyway connection. The key was simply a piece of mild steel screw shaped to fit the empty space. 

before pressing in the key

After pressing the key

The connection looked ok, but was a bit springy when I tried to break it using pliers. I succeeded eventually and saw the shaft was weakened by the groove. 

broken connection

The second idea was to make something like a shape connection (I do not know the professional name) where the shaft and gear are milled with a matching shape and then just pressed onto each other. This connection does not require additional elements, however it is inevitable to avoid the rounded edges when milling the shaft "tab". The rounded corners favor breaking the gear by pushing on its walls outwards. 

"shape fit"

This kind of connection weakened the gear and when I tried to break it I just broke the gear. It could be influenced by the additional force exerted by the pliers as well, but I did not want to take that chance. 

The last idea was to make two small tabs on the shaft (making it more rigid as well) and two grooves of the same shape in the gear. However the tabs were substantial smaller, and they reached only half of gear's length. The rest was just a normal shaft, though slightly larger in order to make a strong connection when the gear is press fitted onto it. 

Two small tabs milled with 6mm endmill

Two small grooves made with 1,5mm endmill

The two parts were cleaned with alcohol and epoxy adhesive was applied on the contact surfaces. I used a vise and slowly pressed the gear onto the shaft. I haven't really tried to break this part manually, however I mounted it in the motor module and till now it survived a few hard shocks and about 0.45Nm of continuous torque (on the motor shaft). 

If it fails I'll try adding loctite adhesive, but hopefully this connection will be strong enough ;)