Bolted joint- bolt preload

[christophersrikanth]

I modeleld a bolted joint in NX and applied external loads.The bhaviour was as expected and I am happy with the results so far.I have plotted applied load vs bolt preload as in the figure attached.This is at ambient temperature.

Now, in NX there is an option to apply a temperarature preload.

Mind you they call it "Temperature PRELOAD" and imagine I apply a temperature PRELOAD wherein the temperatuire is higher than the ambient temperature.

Which would physically mean the bolt is pre-loaded at at a high trmperature.

My question is:

1) What would be the effect of high temperature on the applied load vs bolt tension graph compared to the one at ambient temperature?

I just want to undestand the prob first as I'm into Thermals at all.

 

[dhengr]

Chris:
What causes the preload in the bolt and how do you achieve/apply that? What does that do to the bolt in the way of stress and elongation, at whatever temp. you start out at? What does that do to the several plies that are being joined, and what are their mechanical properties and temps? What is the coef. of thermal expansion for the bolt material and for the materials in the plies? What does a 100̊ change in temp. do to the bolt length and to the sum of ply thicknesses? Take a look at AISC manual and RCSC Spec. and literature on the subject. Maybe take a look at your Strength of Materials and Machine Design texts on the subject. Come back with those answers and a little thought on the matter and you will likely have answered your own question.

 

[dinjin]

Is the temperature preload similar to using heated rivets and when they
cool provide the preload? I have read that in some cases, they heat the
bolts to a certain temperature, apply the nut, and when the bolt reaches
normal temperature, it provides the preload in the joint.

 

[SnTMan]

I think I'd try to find out what the software is doing. Would make the physical interpretation soooo much easier.

Regards,

Mike

 

[MintJulep]

Mind you they call it "Temperature PRELOAD" and imagine...

Imagination is good for designers and artists.

Not so good for engineering analysis.

Figure out what "Temperature Preload" means to NX and what it does when you change the value.

 

[dhengr]

Dinjin:
The plies have to be in contact when you drive the hot rivet. Then the rivet is expanded to fill the hole and any irregularities. Then as the rivet cools it does apply some added clamping force, but remember this strain/shortening is not very large in terms of inducing clamping force. Not usually equivalent to a half or full turn of a nut. And, yes the bolt could be heated, and the plies cooled, and then the nut turned to some tightness/preload; then the thermal movement of the parts would add some additional preload, and this can be fairly accurately calced. My questions to Chris were to try to make him think, a step at a time, through the various steps of the problem. I can’t see his sketch in that format, I would much sooner see it as a pdf file. A guy by the handle of Desertfox and I went through a similar discussion in the Mech. forum a couple years ago, maybe search for that.

The riveted connection usually doesn’t slip much because the rivets fill any hole misalignments and are immediately brought into bearing with the holes in the plates. While the typical bolted connection must move a bit and causes yielding in bearing around some of the bolts before they are all brought into play. With the use of high strength bolts we can develop preloads which make the joint slip resistant (slip-critical) through friction btwn. the faying surfaces.

 

[Screwman1]

Spend some time studying bolted joint theory and design and your questions will be answered. Don't rely on software to take the place of your knowledge of the subject. A quick joint diagram will take care of most of this. Read Bickford if you haven't already.

 

[desertfox]

Hi

I agree with screwman1 and ditch the software and get a feel for what's happening and along that line here is a thought experiment.
Take a steel nut and bolt and a aluminium tube, assembly the nut and screw through the tube and wind the nut down till it just touches the tube (ie no preload).
Place the assembly in a furnace and assume everything is heated uniformly.
Well the aluminium tube will want to expand further than the steel for a given temperature rise, however the steel will expand also but will not want to go as far as the tube, so equilibrium must be obeyed, so the tube and screw must have the same force acting on them but in opposite directions.
The result will be that the screw will go into tension due to being forced beyond its free expansion limit and the tube will go into compression because the screw will limit the tube expansion below its normal free expansion limit.

Hope this helps

This link gives an example

http://www.kaplanaecengineering.com/kaplanaecengineering/documents/irgs/fundeng18_ch09.pdf

 

[rb1957]

1) applying preload using temperature is an olde school fudge ... it isn't physical

2) if you do have temperature and mixed materials, well that's a different problem.

3) can you explain your model a little ... do you have three bolts sharing the "applied load" ? i'm surprised that the bolts are not fully gapped at your maximum load (160k) given a preload of some 30k ... for an applied load of 160k your bolts are either
a) reacting not enough load ... for 150k applied, bolt 1 reacts 110k, or
b) reacting too much load (if all three share the applied load) ... for 150k applied, the bolts react 110+95+95 = 300k ??

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