Torque effects on screw stress 4

[coolbreeze]

I understand that putting torque on a fastener helps distribute external loads, but that doesn't seem to come across in my calculations for the stresses. When I calculate a bolt pre-load, I add it to the external tensile loads and can't seem to get a reasonable safety factor. How do you calculate what percentage of the external loads are taken up by the members being joined? At what point is it necessary to determine if a pre-load is necessary?

 

[MarkRPflow]

Try <A HREF="

http://www.boltscience.com"

TARGET="_new">

http://www.boltscience.com</A>.

There are some downloads that may help you.

 

[Anthonyr]

The bolt pre-load is applied when there is no additional load applied at the joint. When the external load is now applied, ideally, the compression at the pre-loaded surface is reduced by the amount of the external load. If the initial bolt pre-load is higher than the external load, then the bolt should not see any alternating stress when the external load is applied. This should produce a good factor of safety. Typically, the bolt pre-load should be about 150% to 200% of the anticipated external load.

 

[Stress]

If I am interpreting your question correctly, all of the extermal load does not go through the bolt/fastener once a preload is applied. Once the joint is preloaded, and the external load applied the load in the bolt should follow a different slope since the stiffness of the joint now comes into play.

 

[PriamEngineering]

In a bolted joint to which no external force is applied, the force in the bolts is produced by the preload in the joint. The bolt extends and the joint compresses. When an external load is applied, for example in tension, the load in the bolt becomes preload + a percentage of the external load. This percentage of external load is known as the load factor and is the ratio of the stiffness of the bolt over the stiffness of the clamped parts. This maintains intil the joint seperates at which point the preload is lost and the load on the bolt is the applied load only.Unfortunately it is a common misconception that if preload is not overcome then the bolt see no load increase. If this were the case then you wouldn't need a bolt to keep the joint together. Look at a specialist textbook by Bickford or german standard VDI2230

 

Bold preload is necessary when:
- the two joined surfaces must remain attached (no gap shall occur), or
- load fluctiation (fatigue) is significant

This because the load on the bold does not increase (so it does not elongates) until the external load reaches the bolt preload

 

[swertel]

Since your post was January of 2000 and it is now 2001, it seems that any extra information is unnecessary. But, I would be happy to give you specific on finding the percentage of external load carried by the bolt and clamped material. In order to do this, since I don't want to write a textbook, I need to know more about the problem.

Is it a single bolt with similar materials clamped? Is it a bolt pattern? A confined gasket or unconfined gasket? What are the materials being clampled? What bolt are you using?

With the above information I can give you the equations and coefficients that apply to your problem.

--Scott Wertel
scottw@interfaceforce.com

 

[delta769]

Adding to Deeko's comment:
Preload is tensile in the bolt and compressive in the clamped flanges. When external load is applied, the ext load is carried by both, proportioned by the relative stifnesses. Upon joint separation, there is zero compressive load in the flanges and all of the preload + external load will then be carried by the bolt. See both Bickford (still the standard work on bolted joints) and the fastening and joining chap in Shigley's Machine Design.