Max torque on radial screws in single shear? 1

[kevingeneer]

Hello,

I am trying to attach a round nylon part to a hollow steel tube using radial screws. I have attached an image showing the set up. There is a torque applied to the round plastic part, and the screws keep it attached to the steel cylinder and prevent the plastic part from spinning. To make sure the screw can handle the torque, I calculated the shear in the screw. I am unsure about how to calculate the stress in the plastic threads to make sure they are ok. The guides that I have read for screws in plastic talk about calculating the pull out force. Would I have to consider the screw bending which causes a tensile and compressive axial load on either side of the threads?

Thank you!

 

[rb1957]

1st off, the sketch shows threads on the shear plane ... not the best thing to do.

2nd, the fasteners are bearing in the plastic center disc ... probably not critical, but i'd want a large margin in a simple bearing calc to account for the threads.

maybe put a threaded insert in the plastic, so the fastener threads bear into metal and the plastic bears against a nice smooth surface?

Quando Omni Flunkus Moritati

 

[kevingeneer]

Thanks for the prompt reply! This is helpful.

 

[tbuelna]

kevingeneer-

First off, thanks for providing a sketch. That's way more information than many people posting questions usually provide.

As rb1957 noted, putting load bearing screw threads directly in a plastic substrate is generally not a good idea. For your mechanical attachment between the steel tube and nylon shaft, did you consider using a shear pin with some sort of retaining feature? A shear pin would also address the problem of having the threaded portion of the screws loaded in shear, which as rb1957 notes, is usually frowned upon.

With plastic materials subject to sustained loads, one primary concern is always creep. When you analyze your connection be sure to consider creep in the plastic part.

Good luck.
Terry