What is the optimal profile for a self reversing screw?

[George Zucas]

Hello everyone,

I am trying to design a self reversing screw. What I am doing is, I am sketching a rectangle and using extruded cut option in Solidworks, cut it along a helical curve. One more and this time in the opposite direction. It comes out nicely. Now what I don't know is, should I use a rectangle or a trapezoid for the profile? I examined a self reversing screw and I am sure it was a rectangle. Though I've found a video on youtube and the guy there uses a trapezoid. Does anyone know this? Trapezoidal thread seems more logical but then I wonder why the one I've seen is rectangle. I'd like to get as much information as possible about self reversing screws, especially the relationships between the screw dimensions and the small part that is going around (sorry I don't know its name). Google doesn't help much. Here is the video:



(Skip to sixth minute)

Thank you.

 

[Dr.D]

The answer to your question is in the kinematics of the rolling contact. Tangential speed on the rolling cam varies with the radius and the cam follower contacts the wall of the groove over a range of radii. Ideally, it should roll on the groove wall without slipping, and this requires that the radius of the follower must vary as well. If a cylindrical follower is used, there must necessarily be slipping, and slipping means additional wear.

 

[George Zucas]

The answer to your question is in the kinematics of the rolling contact. Tangential speed on the rolling cam varies with the radius and the cam follower contacts the wall of the groove over a range of radii. Ideally, it should roll on the groove wall without slipping, and this requires that the radius of the follower must vary as well. If a cylindrical follower is used, there must necessarily be slipping, and slipping means additional wear.

Yeah after some more investigating I got it. It is logical but the screw I've seen confused me a bit. Thanks.

 

[George Zucas]

Thanks Felmon. I've found that in one of the previous projects company manufactured such a screw. They told me to make one similar to that so I copied it. It was a trapezoidal section and the angle between the side lines were 60 degrees. It has been in use for 15 years so I think the wear is not a problem (or it is not by much even if it isn't the best design possible). I made the follower similar to this:

http://www.abssac.co.uk/uploads/site/products/p_ogw2f/img_selfreversing.jpg