BOLT PRE-TENSION ANALYSIS 1

[StressMan2506]

Fellow stress/structural/mechanical engineers:

I am investigating a fitting which is secured with pre-tensioned bolts. I have access to Niu's stress book and he presents a pre-separation expression:-

Ptot = Pp + k*Papp

where Ptot is the total bolt load, Pp is the pre-tension, Papp is the applied load. k depends on the materials. Niu presents a chart of k for steel bolts, nuts & washers of varying diameters joining aluminium plies.

Niu also presents an expression which relates pre-tension to torque and also involves a coefficient (beta). There is a table of values of beta, but again, only steel bolts and nuts are covered.

My case is nickel alloy bolts, inconel nuts & steel washers joining aluminium & titanium plies. Can anyone direct me to (or supply me with) references and/or data to suit my case?

Thanks in anticipation...

 

[rb1957]

i take it you've the stress version of Niu (i've got the design version, which doesn't cover these technical details). k is obviously the joint stiffness, and i guess beta is the thread friction. Other than substituting the correct E for your materials, I'd think that friction shouldn't be significantly different.

a couple of points ...
remember it's ok generally for the joint to gap at ultimate, so that all the load is going thru the bolt, and it makes the calcs easier ! then check the joint doesn't gap at limit ... the easy way is to assume the joint is essentially perfectly elastic/plastic (bolt preload is constant until the applied load = preload so that the joint gaps and bolt load = applied)
generally the only time to worry about bolt loads in this detail is for fatigue loading, then simpler conservative solutions can also be applied.
worry about temperature effects (with your dissimiliar materials)
worry about preload scatter (how are you measuring preload ? PLI washers, torque wrench, a number on a drawing?) if th ejoint is so sensitive as to need this detailed approach

good luck

 

[StressMan2506]

Thanks rb1957; I'll run some numbers based on your suggestions

A simple approach gives me an RF = 0.99 (nut pull-off) so I am looking into a more sophisticated analysis. Our drawing shows max. permissible torque and I am looking at a couple of formulae for estimating pre-load from torque. So far I have wide variation...

 

[tbuelna]

StressMan2506,

What seems like a simple, straight forward analysis such as your fastener load case, is in reality a very complex situation if you want to achieve an optimized result. Most analysts take the easy way out and simply apply a large FoS to their analysis. This is because there are many variables involved in assessing a bolted joint design, and those variables can be difficult to control at assembly, resulting in widely varying amounts of fastener stress/strain and joint preload.

The most reliable approach to achieving accurate preload in a bolted joints is to use strain gauges. Unfortunately, this is usually cost prohibitive.

The next best way is to measure strain in the fastener dimensionally. This can be done with a micrometer, PLI washers or with a degree wheel type torque wrench.

The least accurate way is to use a conventional torque wrench. The friction in the screw thread and at the nut/washer face can vary widely. And if you're using a self locking nut, the prevailing locking torque can add a large amount of variation. I've seen small diameter (1/4") self locking nuts that had a higher prevailing torque at installation than the specified seating torque value. And don't forget to add the torsional stress that the fastener sees at installation to the preload tension forces when you do your analysis.

As rb1957 mentioned, be aware of CTE mismatch between your fasteners and flanges.

Also be careful to evaluate for any increase in individual fastener load due to moments at the joint. If this joint must maintain a seal, all fastener preloads must exceed the max working loads, so that the joint does not separate. A thorough FEA will expose any flange bending issues.

Finally, you state that your joint is composed of many different type of metals. Be careful of any galvanic corrosion issues that may arise when the joint is exposed to the elements.

You may want to download the NASA fastener design manual. It's an excellent reference. Go here:

http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?1990/RP-1228.html

Good luck!

 

[StressMan2506]

Thanks Tbuelna. I've downloaded the NASA manual and will be studying it to se how it applies to my case.