I’ve watched the very well explained Tutorial from “FE Analyst” about the loading of a moment in Prepomax, but still had a couple of questions about my approach:
i have constructed a rigid body constraint to the top of the small cylinder, and ankered the small cylinder in the large one via surface interaction (friction)
after that i applied a moment to the Reference point on top of the small cylinder (same one than for the rigid body)
Now to my questions:
i still dont quite understand what the rigid body constraint does. When i only apply it to the top surface and then apply a moment to only one reference point on that surface, isnt my torque twisting mainly the rigid body surface and only the rest of the cylinder by extension?
is this the best way to simulate (with a real simple model) the moment on a screw in a cylinder?
Thank you very much in advanced for your help!
Greetings from Vienna.
This is highly simplified because the screw transfers the moment through the thread. If you don’t model the thread and just define frictional contact between both parts, it will act like a pin rotating in a socket. If you use tie constraint instead of contact (give it a try) then the load will be transferred directly but it won’t be very realistic either.
Hi,
thank you for your response. It’s highly simplified because it only should show the difference of the same moment applied from different angles to the screw. It doesnt need to represent the correct stress distribution of the screws thread in the cylinder. I’ve tried the tie constraint and it works. Thank you for the tipp.
What i still don’t understand is, how the rigid body constraint works. why is it only on one surface of the cylinder? What does it do if I set it like that? If only the one surface of the cylinder is rigid, arent i rotating it against the rest of the part?
A picture of the stresses with tie constraints and 1 Nm of torque:
It’s best visible on a simple model of a bar fixed on one end and twisted via rigid body constraint applied to the other end. The rigid body constraint replaces a part (handle, screwdriver, spanner or something else like this) that would transmit torque to the bar/screw. Of course, you can select a segment of a cylindrical surface instead of just the circular base. You can’t select the whole cylinder though as it would make the entire part rigid.
In your simulation, the screw is essentially welded/glued/bonded to the large cylinder with no possibility of separation or sliding. So it can’t twist inside. It can only rotate the whole large cylinder if it’s free to move. It will be hard to represent this properly without the thread. Screw tightening is usually simulated with equivalent pre-tension force.
The large cylinder isnt supposed to move. If i tie constraint the small one to the big one then it cant move inside and the stress distribution is only distributed on the non enclosed part of the small cylinder. Thats why i tried to define the connection with friction. I know that it is not accurate. But even with the friction connection between the to parts, the stresses contain themself to the visible area. (with friction connection the part should be able to move in the large cylinder shouldnt it?)
Any idea on how to fix that without using a more detailed model?
Yes, with frictional contact the small cylinder should rotate inside the hole causing some shear stresses. Keep in mind that you should have Nlgeom enabled and deformation type in results set to true scale to see realistic deformation.
The large cylinder should move to balance moments. You don’t see it probably because your big cylinder is massive and much more stiff but look at this when you make it thinner:
