thread loading when nut/bolt are dissimilar materials

[nomorenames]

The basic rule for the design of threaded joints is that six turns of thread engagement provides very nearly the maximum strength possible, as described here:

http://www.gizmology.net/nutsbolts.htm

Presumably this applies when the materials for the male and female thread have similar elasticities, e.g. steel bolt + steel nut. But what about when the materials have very different elasticities, e.g. when threading a steel bolt into a plastic or aluminum part? The relatively deformable plastic or aluminum would readily conform to the degree of stretch exhibited by the bolt, meaning that more turns of thread engagement could allow one to create higher tensile loads in the bolt before stripping out the plastic/aluminum.

Is there an analysis out there somewhere that suggests how many turns of thread one could/should use when threading a steel bolt into an aluminum part or a plastic part?

 

[Dougt115]

We (meaning I) use the standard equations and analyze the nut and bolt using the weaker material for both parts.

The weaker material will give first every time. So by analyzing using both parts out of the weaker material I will have my answer as to when, in your example, the nut will fail.

 

[ctopher]

When threading steel into alum, I always use an insert in the alum.
We have had always good luck with minimum 3 thds.
I'm not sure of any analysis or stds for this.

Chris, CSWA
SolidWorks 14
SolidWorks Legion

 

[Screwman1]

When both parts are the same strength the failure will be simultaneous shear thru the pitch line. When one or the other component has a differing strength (not elasticity) then you have to calculate the cylindrical shear area of the threads of the weaker material and compare that to the strength of the tensile stress area of the male threaded part. The length required to develop full thread strength in a weaker nut material can become quite large if a high strength bolt is used to it's full capability and for plastics with a low flex modulus it can become impractical to try to reach full strength.

 

[desertfox]

Hi

You can find formulae here for assessing the strength of threads for a combination of materials-

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html.

A word of caution though, the formula suggests that you can keep on increasing the thread engagement and reducing the stress, however in practice once your thread engagement exceeds one or two times thread diameter then there is little benefit in increasing the length anymore,it's probably better to increase thread diameter at this point and recalculate the stresses.

 

[CoryPad]

Thread engagement usually is based on shear strength of the mating threads. One standard that covers this is VDI 2230. Here are some past Eng-Tips threads that cover this topic:

thread725-35222

thread404-174165

thread725-65572

 

[inline6]

I did some testing of gr12.9 metric m12 screws into various aluminum alloys to look at thread stripping and some elasto plastic axisymmetric FEA as correlation some time ago.
What I found was that due to the lower elasticity of the aluminum the loading of threads was highly nonlinear. At small loads the first couple threads took the vast majority of load but once the bolt tension approached the strength of the bolt then the threads become very uniformly loaded before subsequent shearing of the threads. This effect is more pronounced than in steel. The thread stripping resistance was basically linear with engagement length in the typical range of engagement lengths used.
On a fine thread I could not get the bolt to snap in any of the alloys the threads always stripped no matter the length of engagement, I did get quite close though. However with the coarse thread with 6061-t6 and 2011-t3 extruded bars it was easily possible. My tests were done in uniaxial tension so no torsion in the bolts or possibility of galling was accounted for so one would need to investigate additional safety factor if the threaded joint need to be reassembled several times. The tolerance class of the threads is important but requires further study
For stripping resistance you either need to look at UTS or even better correlation comes from the true ultimate strength, yield strength is not relevant to stripping.

 

[Tmoose]

SPS Unbrako discusses empirically derived strength of some pretty cheesy materials starting on 1996 document page 64 here.

http://www.dalecompany.com/pdf/UNBRAKO.pdf

If my cyphering did not predict similar results I would look at my assumptions again.