Bolt Design 3

[brickwoman]

I am having a problem with M12 bolts not yeilding at their specified torque. I understand that friction can have a large effect on torque. Can anyone help me understand bolt behavior?

 

[TVP]

Two potential causes:

1. Friction - coatings? lubricants? contamination? corrosion? All of these will affect friction which is why a portion (over 75%) of the Torque is not being converted into a tensile force on the fastener.

2. Higher than expected yield strength of the material. Are you ordering the fasteners to an industry standard? ISO 898, SAE J429, etc.? Do you know what the yield strength is? Or do you only have hardness and/or ultimate tensile strength data? Don't confuse "proof" strength with yield strength-- proof strength in fastener terminology is usually ~ 90 % of the yield strength.

 

[brickwoman]

Thanks for the help TVP.

The bolt material is very well controlled.

The bolts yeild in test conditions at the calculated torque it is only when it is assembled onto the cylinder head that we see the problem.

I am interested to hear that friction plays such a significant role, is it possible to calculate losses due to friction if you can eliminate contamination?

Would this be friction under the head of the bolt or on the threads themselves?

 

[CoryPad]

How are you determining that the fasteners are not yielding during component assembly? Are you using a torque + angle tightening strategy?

It is possible to determine the friction forces during tightening. Look at the two fastening forums at this website, especially the last few posts. There are links to previous posts and other websites that have information that can help you.

The friction sources are under the head (bearing) and in the threads. Typically, bearing friction is ~ 50% of input torque, and thread friction is ~ 40%. That leaves about 10% of the input torque left as the pitch torque, which develops the joint preload.

Friction is highly variable, with many factors that can change the friction between lab tests and assembly line use. Tool speed, joint alignment, contamination, etc. may be involved. If a torque + angle strategy is used, you can avoid friction variation altogether.

If you need professional assistance, feel free to contact me.

Cory
cpadfield@omnimetalslab.com

 

[Patton]

Find a whay to measure your preload or bolt enlongation.

 

[diamondjim]

What material is the bolt compressing
standard steel with thick cross sections?
Are you bolting thru minimum webs?
Are the parts gasketed? Something is amiss.
i.e. is the bolts compressing an cross sectional
area at least 2 times the bolt cross sectional
area? I assume you are using hardened washers
under the bolts.

 

[Kenneth]

All good advice above. A good place to start reading/learning about the role of friction, etc. on bolted joints is MIL-HDBK-60, available for free at

http://astimage.daps.dla.mil/docimages/0000/22/90/70455.PD9.

 

[poetix99]

One ad hoc approach is to "calibrate" the torquing to what is in fact required to achieve yield in your particular configuration. In other words, if 200 ft-lb (sorry about customary units) should, in theory, yield the fastener, and 500 ft-lb is required in practice, you might at least have a better idea of where you are...

One problem is that you will find so much variation from one screw to another that this won't pin it down.

Understanding the entire clamped system, as "diamondjim" suggests is also helpful - not so much for solving the friction problem, but for better identifying clamping requirements and understanding what is actually achieved with a given bolting config.

You claimed that the bolts yield under test conditions: are you actually torquing them "under test conditions", or are you applying straight tensile force that is reckoned to be equivalent to the applied torque?

What about the (remote) possibility of material incompatibility, vis-a-vis galling?

 

[Mav]

Go to

http://www.boltscience.com

-read the tutorial