Plastic Part Carrying Torque and Radial Load

[surfmase]

Hi Folks,

I'm at it again. I'm working on a plastic "carrier" component to transmit power from an output shaft. I wanted to use a round shaft with two opposed flat sides to do this. I've tested some 3D printed parts and it transmits the torque fine. My problem is that this part is always under a radial load. When I test this the shaft eats up the inside diameter. I've got about 70W at 200rpm with 200N radial force on a ø10mm shaft.

I've designed a similar plastic carrier in which two mini ball-bearings were pressed and the part was placed on a stationary shaft. This time the torque was transmitted via an additional geometry on the part. But the bearings were carrying the same radial load. At the end of the lifetime the bearings no longer have a press fit, but this has not been any real problem.

One problem I have is that the prototype material is PA12 and the final material will be POM acetal resin. So the properies are significantly different. I'm working on doing a realistic test, but I'm wondering if there are fundamentally better options such as a cross-bore and pin instead of the two flat sides. We have sintered steel carriers which mate to the same shaft using a woodruff key and the key is pretty worn at the end of expected lifetime. Trying to go to plastic due to more complex shapes for functionality.

Any tips are greatly appreciated.

Cheers,

[Kelly_Bramble]

Would you please post a picture or drawing?

[ebtn0]

Agreed. An image would help a lot with guesses/advice.

What 3D printer are you using?

[surfmase]

front view.jpg
iso view.png

Sorry; went on vacation then had to catch up with everything. So, in one view you can see the shaft with two flat sides. This shaft is driven by the gear in the back and should turn the black plastic part with the 3 "ears" This is the link to the load (radial and torque). Torque is no problem alone, but include radial load and the flimsy prototypes don't last. I don't know which device makes the parts, we buy them, and can't seem to find a viable solution to print POM. I have just ordered some simplified samples to be lathed from POM Bar. We'll see if that shows us anything.

Thanks

[Kelly_Bramble]

but I'm wondering if there are fundamentally better options such as a cross-bore and pin instead of the two flat sides. We have sintered steel carriers which mate to the same shaft using a woodruff key and the key is pretty worn at the end of expected lifetime.

Ok, I think the question is about shear loading and what feature is best to absorb that loading during operation.

This would depend on the torque and loading applied on the shaft and locking feature. Do you have any idea what the applied torque is?

[surfmase]

Yes that is essentially my question.

The torque as well as the radial load can vary by application and user. Typically torque is between 2.5 Nm and 4.0 Nm. There are fluctuations but not jolts or impacts. The radial load is typically between 150 N and 250 N. Radial load will remain constant for a given application, but is usually set higher with increasing torque.

Thanks

[Kelly_Bramble]

Yes that is essentially my question.

The torque as well as the radial load can vary by application and user. Typically torque is between 2.5 Nm and 4.0 Nm. There are fluctuations but not jolts or impacts. The radial load is typically between 150 N and 250 N. Radial load will remain constant for a given application, but is usually set higher with increasing torque.

Thanks

I think you should think in terms of bearing, shear area and ease of manufacturing. Do the math...

see: http://www.engineersedge.com/gears/d...key__13740.htm

[surfmase]

I think you should think in terms of bearing, shear area and ease of manufacturing. Do the math...

see: http://www.engineersedge.com/gears/d...key__13740.htm

Thanks for the tip. The current design with key is sufficient. Ease of manufacturing and cost are high on my list. This is why I am looking at alternatives to a key by using injection molded components. In the previous posts I described and shared an image of a shaft with two flat sides. There is no key in this design. How should I calculate the strength of the femal part? Is this shear along the arc where the material is removed when creating the flat spot? I suspect there is more to it than that. How would I go about calculating the strength of the female part for any geometry for that matter?

Thanks.