Learning Forum

[Muhammad Qasim]

Hi, i am working on a very simple design of power screw (Lead screw with tripozoidal threads) having threaded Bolt rod and partial Threaded Nut Rod. In Mechanical Model, The Frictional Contact was made between threaded Surfaces. Cylindrical Joint was made (Body to Body) so that Nut side Rod Rotates and Translates. The Meshing elemnet size was 3mm as shown in picture.  

In Static Structure analysis, The Force of ramped 6400N was applied on Nut Rod end surface. Bolt Rod side was fixed with fixed support and joint load with rotation of 9 degree ramped was applied on Nut Rod. and also Joint load with displacement of ramped 0.01mm was applied on NUT SIDE, (i selected those values with little variation with actual thread pitch which was 4mm per rotation. Those Two values on joint can be an issue as if i apply them according to the nut bolt pitch, the stresses on threads dont appeare, may be due to No contact between surfaces, so i have to varry them to make contact) (kindly give me a better solution for this issue).

With large diflections ON, and analytical setting of initial time step of 0.05 and max step of 0.2. The solution took too much time. The stresses on threads are not symmetric. First thread surfaces of Nut bolts are having large stresses on bolt one first thread making the stresses to go on plastic regime (which would not actually be like this, if all threads carry equal stresses) as shown in Figure. 

Kindly suggest me to overcome this issue. More, the stresses highly depend on the Joint Loads (Rotational and Displacement) and solution doesnt converge if i varry them too much or change the direction agaisnt the pitch thread. (More, this solution is independent of applied Force as i changed the force to 1000N but got same stresses). (One More thing, if i bring this set up in Transient structural, the solution goes till step equal to 1 and then become non-converged, means there is an issue in applying conditions). & The file is not attaching due to some issue. 

 

[peteroznewman]

A nut and a lead screw are usually purchased components. The nut and screw each have a rated load. Design a machine and calculate the load that goes through the nut. Do a simple analysis where the screw is beam elements. The load that the nut drives can be connected to the beam with a simple joint such as a spherical joint. Probe the Joint to measure the load going through the nut. Select the appropraite nut and screw size once you know the load.

The first thread on a nut takes the most load and each thread after that takes less and less. After about one diameter of length, there is almost no load on the threads. This is normal.

[Sean Harvey]

Hi Muhammad,

What Peter suggests is simple and great suggestion.  Also, please don't expect uniform load transfer across threaded joints.  This phenomenon is not only for threaded joints but almost all structural joints that are not optimized to shear in the loading, so as bonded joints, brazed joints, etc.

One other option is to use simplified thread modeling which is detailed here. This can capture the stiffness response, but won't get you the thread root stress, which the mesh in your model can't get anyway.

37.2. Problem Description (ansys.com)

Finally, you can convert a cylindrical joint into a screw joint with a few APDL commands in a command objecy, so you get the twist extension relationship without modeling any threads.

Best of luck.

Regards,
Sean