Bolt Preload vs. Bolt Stress

[jondon]

I have a question regarding bolt preload and the resulting bolt "load factor".

I am designing a bolted connection for a cargo crane foundation on a large marine vessel. We are limited to a 1" bolt diameter by the crane vendor's bolting flange design, and we would prefer to use stainless steel bolts for the connection because of the marine environment. My calculations show that, under the applied crane proof test load, our chosen 1" 316-SH stainless bolts (proof stress 63,000 psi, yield stress 70,000 psi) have a bolt "load factor" of 0.79 when torqued to the typically recommended value of 75% of yield. Most of my references indicate that the designer should shoot for a "load factor" of 1.0 or greater to ensure that the bolt load does not exceed the proof load.

My main question is this: given that bolt size and material grade are fixed, can we back off on the specified bolt torque to reduce the bolt stress below the proof stress, so long as the final bolt load is adequate to prevent separation of the bolted flanges?

Many of the references I have come across suggest that bolts be torqued to 90% of yield, or even to the yield point, to create an efficient tension joint, but this would seem to contradict the general design practice of keeping the bolt "load factor" below 1.0. Can someone help clarify?

For reference, I am calculating the bolt "load factor" as n=(Sp*At-Fi)/(C*P), where:
n = load factor (to be greater than 1.0)
Sp = bolt proof strength
At = bolt tensile-stress area
Fi = bolt preload
C = joint stiffness constant
P = external tensile load

 

[desertfox]

Hi Jondon

Yes provided the joint cannot separate and the bolt remains below its proof stress when the external load is applied the lower bolt torque joint is acceptable.

I can I just point out though that in your last paragraph you talk about the bolt load factor being less than unity as standard practice, however you state it should be greater than unity everywhere else.

 

[jondon]

Thanks for the response, and thanks for catching the goof in the last paragraph. My references indicate that the load factor should, in general, be greater than 1.0 to keep stress below proof strength.

With that said, why then do some specs suggest tightening the bolts to 90% of yield or greater? Doesn't this conflict with the "load factor" design practice? A bolt that has been tightened to yield would be at a load factor of 1.0 or less even before an external load is applied. I know that the bolts will still have some load carrying capacity beyond the yield point, but what would be the advantage of applying such a large preload? Are larger preloads generally only specified for permanent joints, where the bolts may deform, and do not need to be reused?

 

[paddingtongreen]

There is a difference between loading a bolt that is an exact fit for capacity, and one that is over capacity. It is force that you should be looking at here. Either way, if the preload exceeds the design load by a decent margin, have a nice day.

How do you set the tension in the bolt? it looks like you are torquing them, in my experience, that is not a very accurate way unless you test some in a Skidmore first.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[desertfox]

Hi

Joints that are torqued to 90% of proof load should not be subject to high dynamic tensile loads,because depending on the stiffness ratio's of the joint and bolt materials one or the other might yield.
Most fastener tables give the amount of maximum preload that can be taken by the bolt material and not what the materials being clamp can actually withstand.
I have seen a nut tightened to its maximum,unfortunately the clamped material wasn't as strong and the nut embedded itself into the said material.

 

[desertfox]

Hi again

Have a look at this link

http://www.engr.sjsu.edu/youssefi/me154/notes/Threaded Fasteners.pdf