How does bolt length relate to stretch/preload?

[DDario]

I got asked a question today that stumped me,

We are fastening a 700HP drive motor to a new pedestal & the PM was concerned that less than 12" of bolt length would result in there "not being enough stretch" (1" bolts/studs)

I understand that a longer bolt would have more twist and stretch, but would have thought total bolt preload was set by the torque regardless of length.

Can someone here help set me straight on this?

 

[boo1]

Bolt stretching according Hook's Law:
dl = (F*L)/(E*A)

Torque = K x d x F to find F (preload:
F = T/(K x d)

You can see as the bolt length changes with constant F, the bolt stretch increases.
In the spring analogy, a bolt would be thought of as a complex spring. This results in a longer spring. Which increases fatigue resistance ,and decrease the forces the bolt sees.

 

[desertfox]

Hi

The total bolt preload is set by the torque you use to tighten the joint with has you correctly stated, the elongation of the bolt will depending on the length of the bolt involved in the actual clamping of the component parts.
The longer the bolt length acting within the joint affects it's stiffeners IE longer = more deflection and obviously shorter = less deflection of the bolt.
So when an external load is applied to the joint, then a longer bolt stretches more than a shorter bolt and the share out of the external load is proportional to the stiffness of the joint and bolt.

See this link

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

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Tmoose]

bolts with longer grip length are less vulnerable to preload loss (which is a major source of loosening) due to embedment, thermal expansion, etc.

I'm guessing they also provide a tightening spec of some kind.
Edit - I'm guessing your company provides a torque spec, too.