Bolt Length

[jbknudsen]

Greetings to all,

Is there a rule of thumb when considering the length of a bolt. For instance, I would not want to use a 3/8" dia bolt that was 14" long. I am aware that the thread engagement should be 1:1 in respect to the dia. of the fastener. At what point will the bolt be so long that torsion is a concern with respect to the tightening ability. I know that material selection will play a part in the formula, I am curious if there is a "rule of thumb" for this type of issue.

Thanks
Jay B. Knudsen

 

[CoryPad]

A rule of thumb is look at shank torsion if the bolt length to diameter ratio exceeds ~ 10 or 12. These are long bolts, and high friction leads to high torsional stress. Using angle controlled tightening and lubricated fasteners removes the problems since there is low torsional strain and accurate rotation measurement, which translates into accurate axial strain/force calculation. I have seen fasteners with l:d = 20, but they had problems (e.g. shank straightness, head perpendicularity, stress concentration at thread runout...).

 

[ornerynorsk]

I guess that I'll take the conflicting side in this case. (After all, I'm usually more comfortable in conflict than in concurring with everyone LOL ) I have seen bolts with 32 to 1 and greater L/D ratio used with great success in injection mold tooling, as well as other applications. It all depends (as corypad mentioned) on what torque they NEED to be tightened to.

 

[jbknudsen]

Thanks to both. I know that in some applications, such as hydraulic cylinder end caps, the bolt L/D can be great. The bolt L/D ratio for this application is 10/1 and will not be in a high stress/load environment.

Thanks for the advice.

-Jay