Bolt Stretch Versus Clamping Force
Bolt Stretch Versus Clamping Force
(OP)
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?
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?





RE: Bolt Stretch Versus Clamping Force
RE: Bolt Stretch Versus Clamping Force
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.
RE: Bolt Stretch Versus Clamping Force
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.
RE: Bolt Stretch Versus Clamping Force
Have a look at this site:-
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regards
desertfox
RE: Bolt Stretch Versus Clamping Force
Agreeing with mintjulep, the preload is only one factor in determining clamping force.
Clampingforce = preload*(1+(stffness of bolt/ stiffness of clamped material))
RE: Bolt Stretch Versus Clamping Force
"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.
RE: Bolt Stretch Versus Clamping Force
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
RE: Bolt Stretch Versus Clamping Force
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.
RE: Bolt Stretch Versus Clamping Force
RE: Bolt Stretch Versus Clamping Force
if you can, retrieve and carefully read VDI-2230. It's extremely rigorous and informative.
Regards
RE: Bolt Stretch Versus Clamping Force