Bolt Extension

[daparojo]

Firstly, it's a long time since I have calculated this, so I need a little refreshing.
I have a bolt M48 that is in a coupling hole 65 dia. Bolt is xxx mm long.
It is tensioned hydraulically at one end, nut tightened and then pressure released. The bolt has stretch xxx mm
The load is xxx kN

Am I right that the extension would be.

Extension = Force x Effective Lenth Of Bolt
-------------------------------
Youngs Modulus Elasticity x Tensile Stress Area Of Bolt

The bolt is threaded at both ends and from the undercut of thread, the diameter of the bolt increases in the coupling is 1mm less than the hole size i.e. bolt shank

What is the effective length of the bolt, is it the coupling width or the length of the bolt shank?

And what is the tensile stress area? the area of bolt shank, hole or thread undercut?

Thanks

Paul

 

[MintJulep]

DL = F((Lb/EAb) + (Ls/EAs))
Where:
DL = Total elongation of fastener (inch)
F = Design preload tension (lbs)
Lb = Bolt shank (unthreaded) length + ½ bolt head height (inch)
Ab = Cross sectional area of the unthreaded bolt shank (inch2)
Ls = Bolt effective thread length + ½ nut height (inch)
As = Cross sectional area of the threaded bolt shank (Stress area) (inch2)
E = Modulus of elasticity of bolt (psi)

 

[daparojo]

The bolt isn't that of a headed type. The bolt is threaded at both ends i.e. M48 - Thread Length 48mm. In between the thread there is a small under cut and the bolt diameter goes upto 64mm for more or less the coupling width.


-------------
-----/ 64mm \-----
48mm 48mm

(Profile)

The bolt is hydraulically load, with nuts on either end. As the bol streteched, the nut backs off slightly, at load, the pressure is held, and the nut tightened. Once tightened the pressure is released.

 

[MintJulep]

So it's a stud, not a bolt?

Modify the formula accordingly then.

 

[daparojo]

Yep, sorry for. the confusion.

 

[desertfox]

hi daparojo

Yes your formula is correct altough am I correct in assuming its the 65mm dia thats being stretched and not the 48mm and 65mm as thats a slightly different story.

desertfox

 

[daparojo]

Cheers for the help.
The tensioner is on one end and stretches the stud slighly at a predetermined load. Am I right in presuming that the 48mm thread diameter and 64mm diameter will stretch? Or should I base it on the shank i.e. undercut of the stud?

What happens if the bolt is fitted in the coupling
i.e.


Cplg Width
1----1---1
X /----\
---------- 65mm --------
I 48mm 48mm I

I---Y-----I



x=tensioned side

Is the effective length the same as the the coupling width or the length from the end of the stud to where it starts to become fitted i.e. Y?
Also is the stress area now the undercut of the stud?

 

[desertfox]

Hi daparojo

If the tensioner is gripping on the thread then both diameters will stretch in length.
The stud will stretch from the point at which it is anchored ie from the underside of the nut or washer at the point of first restraint.

desertfox

 

[desertfox]

Should of said in my last post, I would use the minor diameter of the thread for the elongation calculation.

desertfox

 

[daparojo]

Cheers fox, would you use the bolt length as the effective length or the distance that is gripped i.e. 'Y' in my rough sketch?

 

[desertfox]

Hi

I would ignore any bolt length that protrudes outside the nut.

desertfox

 

[daparojo]

The bolt ends are inside the nut 5mm either end, so I guess the bolt length is the effective length.
Cheers desertfox and others, it has refreshed my ageing mind.

 

[desertfox]

Hi daparojo

If I understand your last post correctly it is not good practice to leave the studs inside the nuts, the stud should have full thread engagement within the nut and protrude past the end of the nuts by a minimum of two threads.

desertfox

 

[MintJulep]

The bolt equation I presented earlier came from Bickford. It includes half the nut height and half the bolt head height in the "effective length".

For a stud I suggest half the nut height at each end.

 

[daparojo]

Cheers for your help people.