modelling of thread
modelling of thread
(OP)
I need to make a fem model of a part where the properties and dimensions of the thread are of importance. I was thinking of modelling the contact of threads by gap elements. I am also considering using a slide line. However, i have little experience in this field. Anyone got any suggestions or useful links? Much aprreciated.
Using Femap 10 and NX-Nastran by the way.
Using Femap 10 and NX-Nastran by the way.





RE: modelling of thread
we know how strong threads are, though i suspect you've got something exotic/unusual in mind, there are so many real world things that are really hard to model that i suspect your conclusion will either be horribly conservative or unconservative (unfortunately you won't know which 'till its too late).
how difficult is it to test the component ?
how unrealistic is it to model the component as a "welded" assembly and analyze the thread interface by hand ?
RE: modelling of thread
The aplication is not to special but will need to be proven correct and up to a full spectrum of loads, including shock. There will be elaborate testing involved, to.
RE: modelling of thread
After doing the modeling you will have to analyze and use the results. From stress plots, you will have to determine if the strength is sufficient to resist the load. Furthermore, if you are involved with repeating impacts or load cycles will have to consider the fatigue of the joint. This will be the difficult portion of the analysis.
RE: modelling of thread
maybe using cyclindrical solid elements would give you what you're looking for ... load transfered per unit (thread?) thru the thickness ??
RE: modelling of thread
RE: modelling of thread
Here are some of the difficulties:
Bolts, especially those taking a big fatigue load are torqued to very high levels where local yielding just might occur.
Just getting a pre-load in the field that you assume in the analysis will be a huge achievement.
Many bolts have rolled threads. This means the local material properties at the thread root will be significantly different than the bulk material. Are you prepared to model that and have metallurgists tell you what kind of localized hardness and strength numbers you can use?
There can be big changes in stress gradient from small changes in geometry so you will likely have to run many anaylses to get a statistically meaningful set of data.
TOP
CSWP
BSSE
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RE: modelling of thread
Don't forget the bolt holes. These often see significant fatigue stresses if thermal loads and flange packs are involved. The threads are often the least of your worries.
Lastly, don't use Nastran for this - it's contact, friction, pre-load, and nonlinear capabilities are just not up to this sort of continuum problem.
gwolf2
RE: modelling of thread
RE: modelling of thread
OK, if you really want to model threads and get a half reasonable answer, don't use Nastran because it's contact and friction capabilities are not really up to it.
I also have a vague memory (Nastran experts help me here) that the only axisymmetric elements in Nastran are 6 node triangles which will give absolutely appalling contact stresses and must also be used with extreme caution for stress concentrations at the tooth roots.
I think it's a bad idea in general to lift heavy machinery with threaded holes if large dynamic loads are involved. Even if dynamic loads were not present I would want to see a static margin on thread pull-out of at least 5-10. Is it a through hole or a blind hole?
Try a pull-out hand calc assuming half the hole shear area is effective at the mean thread radius in combination with the known dynamic loads. What reserve factor on shear does that give you?
RE: modelling of thread
I will just give it a go with plain contact elements and see what rolls out the pc
RE: modelling of thread
Contact elements really won't work because of the required connectivity and the way they "load up". You need some type of surface contact algorithm that takes into account penetration of one surface through another and the resulting force required to make sure that it doesn't penetrate. Ideally, you would like it to account for friction, but this isn't an easy calculation.
2D-axisymmetric works pretty well to get the load path, but will likely over-predict stress by up to, in general, about 30% (please note in general...just a "ballpark").
The jump to 3-D is difficult and requires some serious computing power. It is easier if you are careful with your modeling approach. Depending on your software, you want to minimize how many surfaces you have to designate for contact and rotate your thread through a minimum distance if you are checking the fit-up stresses. If you are just looking at in-service and ignoring pre-stress (probably not a good assumption, but sometimes necessary) then you just need a lot of computer power.
RE: modelling of thread
To obtain a life estimate of typical bolt/nut arrangement and cut threads you can refer to the following ESDU papers where the life is evaluated as a function of the alternate nominal stress and the UTS of the material. There will be no finite elements required.
ESDU 84037 - Fatigue strength of external and internal steel screw threads under axial loading. (Standard forms not greater than 1.0 inch diameter.)
ESDU 80028 - Fatigue strength of large steel bolts and threaded connections under axial loading. (Thread diameter equal to or greater than 1.5 in)
For the preload, I recommend you to improve the design by having a good preload.
I wish you good success.
RE: modelling of thread
Consider this:
Positioning is the key word here. Think injection moulding press or the lead screw on a machining center or a lead screw used to extend landing gear. Preload is not possible. This is not a fastener analysis.
If the nut is moving then fatigue would likely be an issue in the nut, not the screw if the loading can happen anywhere in the travel. Lubrication would likely be assumed and a more consistent friction coefficient than in a fastening application.
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."