Bolt Stretch Versus Clamping Force 1

[kander]

All,

To achieve a certain clamping force between two friction faces (or bolted connections), is there a requirement of a certain bolt length to provide enough bolt stretch to hold the connection together.

i.e. For 1" bolt connecting two faces, do you require a minimum nut seperation length to provide sufficient bolt stretch?

 

[vims]

Yes, everything depends on how big you need the clamping force to be. That is why you use washers, to increase the thickness of the clamped material.

 

[rb1957]

actaully, isn't it more a function of bolt diameter ... Preload Torque = 0.2*(preload)*(bolt dia)

certainly you can control bolt preload by measuring bolt strain (= stretch).

like vims, you start with your applied load, determine the preload you need, then determine the bolt you need.

 

[MintJulep]

Well, no - at least no for the question posed.

You can achieve whatever preload with whatever bolt grip length.

However, in general terms longer is better. Factors to consider include: Joint:bolt stiffness ratio and potential loss of preload due to embeddment and relaxation.

There are entire books devoted to the subject.

 

[desertfox]

Hi kander

Have a look at this site:-

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

regards

desertfox

 

[vims]

Good site, desertfox.
Agreeing with mintjulep, the preload is only one factor in determining clamping force.
Clampingforce = preload*(1+(stffness of bolt/ stiffness of clamped material))

 

[jedward]

for some environments clamping force is not the only criteria.
"Stretching" the bolts may be necessary to prevent failure under a viration environment if the stack of materials is not compliant.
some "spring" seems to be desirable for this environment.

 

[byrdj]

In my industry, stud spacers are used to allow fow the tollerance in the application of the desired stretch.

For example a 3" dia stud loaded to 30Kpsi with a 7500ft-lb torque. one having an active lenght of 4" the other 8"
measuring the stretch to ensure loading, a 0.002 error in a short stud that is stretch to 0.004 is significant. But not so for the longer stud

 

[Gymmeh]

If your company has a Bolt suppyier check if the bolt manufacture has that info on had.

I had to check something like this a last week... someone thought a preload on 4 bolts we use was to high for ANSI

I just called the bolt manufacture which had a preload chart with all the info I needed and more and then I just figured the load on the joint, required saftey factor, etc... and found N number of bolts were required to get the clamp force required. Our design is great and then everyone was happy.

If the information is out there, its faster and easier to not re-invent the wheel.

 

[Gymmeh]

If i can spell...Supplier has the information needed on hand.

 

[cbrn]

Hi,
if you can, retrieve and carefully read VDI-2230. It's extremely rigorous and informative.

Regards

 

[Tmoose]

MITCalc mentions its algorithms are based on VDI-2230, plus a fewe dozen others