Joint Stiffness Ratio 1

[MintJulep]

It seems that all of the recognized bolted joint gurus are willing to publish a statement along the lines of:

"It is important that the stiffness of the members be large compared to the stiffness of the bolt."

However none seem willing to actually say how much larger.

Is there a stiffness ratio above which you just don't have to worry, and don't need to bother with the joint separation calculation?

 

[CoryPad]

I don't understand your question. When you say joint separation calculation, do you mean joint opening such that there is zero compression in the joint and all external force is carried by the screw? If so, then a stiff joint/compliant bolt is worse in this case, since more external force goes into relieving the joint compression.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MintJulep]

Cory,

Isn't it desirable that the majority of the external force goes towards relieving the joint compression? Providing of course that the compression does not go to zero.

If the external force gets applied to the fasteners then the fasteners tend to fail in fatigue.

 

[CoryPad]

MintJulep,

Yes, it is desirable to have the external force taken by the joint rather than the screw. Because of this, the conservative approach (i.e. you don't need to bother with the joint separation calculation) means all the external force relieves the initial joint compression. This is independent of the joint stiffness ratio - just create a large enough pretension/precompression that the external forces are small in comparison. High pretension comes from high strength fasteners, or large diameter fasteners. The joint then will need adequate bearing area to resist the high precompression force.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[threaded]

Going back to MintJulep's original question:

"It is important that the stiffness of the members be large compared to the stiffness of the bolt."

This refers to the risk of relaxation and embedment, from my experience unavoidable, could be up to 10% of clamp load for the first 24h, a further 15% after the first thermal cycle (if applicable of course).

A good rule of thumb is the Gauge Length Ratio calculation:
Free length of the bolt divided by diameter of the bolt

Your should target a number above 1, preferably above 2.

 

[israelkk]

MintJulep:

As higher the stiffness of the members be large compared to the stiffness of the bolts means exactly that.

When a preloaded joint is loaded to relieve the compression force caused by the bolt the bolt is extended by the amount the members released which is directly related to the menber stiffness.

For example, if the member stiffness is 10 times higher than the bolt stiffness then, when one bolt is torqued to a load of 5000lb and extend let say by 0.04", the menbers are comressed only by 0.004" (10 times smaller than the bolt).

Therefore, when the members are pulled away from each other by a force of 5000lb the bolt extends by 0.004", meaning the load in the bolt is now 5500lb (10% higher than before). But, if the members stiffness was 2 times than the bolt stiffeness the members compression was 0.02" instead of 0.004". Therefore, when the members are pulled away from each other by a force of 5000lb the bolt extends by 0.02" meaning the load in the bolt is now 7500lb (50% more than before).

This is exactly the reason why the members stiffness should be as high a possible compared to the bolt stiffness.

 

[CoryPad]

threaded,

Joint separation calculation is not a direct function of embedment and relaxation. Embedment and relaxation are controlled mainly by surface finish and creep, although the magnitude of force loss is dependent on the screw and joint stiffnesses. This force loss does affect separation calculation. I agree that the l/d ratio should be above 1 - in fact, something more like 5 is desirable as this lessens fatigue concerns (high l/d ratio allows for a compliant screw whose elastic deformation can help damp high cycle vibration).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.