Learning Forum

[Jerin]

I am doing an analysis to calculate the stress generated on a balls of bearing, i got a convergence under 5% with a maximum stress of 90Mpa how far should i go from this stress, is it safe to take this value for calculating Factor of safet??

I am fairly new to ansys, thanks for the help.

[peteroznewman]

Compare 5% with the uncertainty of other things in the model like the load, the diameter, the Yield Strength. Some of those probably have much larger amounts of uncertainty, so 5% in the numerical result may be good enough. 

[Jerin]

hi peteroznewman,

thanks for your quick response, i am worried whether or not i should reduce my allowable change to below 3% and run the simulation again with few more refinements. 

my meshes looks very well, have obtained the force convergence robustly however Is it safe to consider this stress value obtained currently?? even if i reduce the allowable change to 3% for next adaptive mesh refinement i get the following error as attached in image.

 

[peteroznewman]

Use a manual method of mesh refinement as described in this post.

[Jerin]

thanks peteroznewman,

the shared post was very useful to me.  I have another simulation running i am not able to decide which boundary condition i should use to restrict rigid body motion. I am trying to run a bending moment analysis on planetary gear train assembly as shown in the image. I just want to see the maximum stress generated on on planets. Planets are free to rotate and revolve in between two suns and outer ring. How can i restrict the rigid body motion. I am unable to obtain the force convergence. 

[Jerin]

[peteroznewman]

A half model doesn't make sense for a planetary gear model.

[Jerin]

actually i have provided symmetry loading conditions on half side, i forgot to add it in last workbench file but i have inserted the symmetry region and its still giving me rigid body motion, the aim is to see where maximum deformation occurs under bending moment. The planets are free rotate and revolve , and one of the sun is free to just rotate in its own place. I have added frictionless support in between planets and sun because of which there is a rigid body motion.

[peteroznewman]

A half model doesn't make sense for a planetary gear model even when you include a symmetry BC.

 

 

[Jerin]

but in my case the loading and geometry of the part both are symmetric,

we use symmetry condition in such a scenario i guess?? how will it effect the convergence??

[peteroznewman]

Yes, the geometry is symmetric, but the motion of the parts is not.

In the image below, the outer grey ring is fixed, the inner blue sun gear rotates in the direction of the black arrow, the orange planet gears rotate in the direction of the green arrows while also traveling in the direction of the magenta arrows.

Yes, the geometry is symmetric, but the motion of the bodies is not symmetric, and motion is what is important for using symmetry boundary conditions.

If you slice this geometry vertically, and apply symmetry, then the sun and planet gears are no longer free to rotate or move along the magenta arrows.

[Jerin]

sorry i think you have misunderstood the problem, i am not providing any rotary motion to the gears. I have fixed the outer ring and placed a remote point out of the plane trying to bend the sun gear making both the planets to be pushed away. Please refer to my attachecd image, there is an arrow red arrow ( pointing downward this arrow is my remote force at certain distance creating a bending moment about the center axis of sun. Shall i share u my workbench file??

[peteroznewman]

Are you also going to analyze the bending of the sun gear when the planet gears are 90 degrees away from this location?
That would seem to be a configuration a lot more flexible than this one.

[Jerin]

noo in actual condition, I will have 10 planets inside but currently, for simplicity, I have taken two planets 180 degrees apart which will take the maximum amount of bending load.

In my current model, the gears don't have teeth on there surface, they just have a cylindrical surface at pitch circle diameter. 

[peteroznewman]

I don't believe you can get meaningful stress information out of a planetary gear assembly using geometry with no gear teeth.

[Jerin]

I know the results won't be very accurate but for simplicity, i have removed the teeth of the gear, the aim is to see the amount of generated stress and deformation among all the components, planets, sun, ring, bearings and housing. 

[peteroznewman]

Since it is only a rough approximation, use Bonded Contact and there will be not be much of a problem with convergence.  Change that over to No Separation Contact to see if that still converges.

[Jerin]

thanks peter, i have got the convergence and results are pretty good through your some of the other posts/answers. Sorry for the late response.

I have one last question to ask, i got a warning at the end of the result - the matrix coefficient exceeds e-08...but the force convergence and areas where stresses are generated looks very clean and realistic. Should i consider this warning and try to improve my FEA model??

[peteroznewman]

It's good enough for a rough approximation.