threaded joint

Is this homework?!

Incorrect tightening would be my guess.

(H. T. threaded ( of high hardness)???... tensile strength isn't the same as hardness.)

Wrong torque specifications on the HT bolt. You probably would need to increase the thread count from a coarse thread to a medium thread or to a more fine thread. Thus giving you more surface bite area for the nut. And you might need to increase the depth of your threads to increase the actual bite on the bolt. Not going over 10% of diameter for the bite. HT bolts only have a specific tenesile strength and stretch coeifient value. If it is over torqued, it is then placed in a more stressed condition and your molecular alignment is now disrupted. The more you stretch metal, the more heat it generates, which lessens its' ability to perform its specific design. Thus dropping the shear value of the HT bolt.

Maximo

I a&m sorry but yesterday i had not the time to ask you several questions:

- are you aware that fine threads have a higher stress on the flanks and a higher notch effect at the thread root in the bolt?

- fine threads have a stiffer thread than coarse? thus an increase of the thread number does not make the stress distribution better? may be even worse?

- what do you understand by the bite on the bolt and the 10%? although i am quite familiar with bolting i have not yet met the term.

- do you really believe that over stressed bolts do change their "molecular" structure? do you think in fact about crystals?

- have you ever computed -for estimations only- the amount of heat generated by stretching and the possible rise in temperature? I have the feeling that in fact you think about the heat generated by friction during tightening which is considerable and limits in fact the rpm a bolting can afford without loosing its quality.

Please be kind to give me the answers since according to my opinion you are not on right path, but i accept the idea to be wrong so that i would appreciate to have answers to my questions.

describe the joint and the failure so that you can get a correct answer.

According to you how high are the loads ? what means HT i terms of steel mechanical quakity ( strength)?

Was the load variable or not ? What kind of application?

Try to give the answers as good as you can and if you can make a photo of the failed section and send it. If I can see the failure kind I can give you the reasons

Improper application.

Or....one of these.

So this is the 4th of your ME questions today. Do you have an exam tomorrow?

I think Maxomo said it all. However, we have had reports of HighTensile bolts failing which has made us a bit wary of where we buy them from.

Whislt looking in to the purchase of bolts and stud we have found that unless you are really carefull you can be buying something that the material test certificate describes as conforming to specification but doesn't. Without risking a bit of slander I can so no more.

Research your suppliers and the original manufacturing mill.

Since most of the standard reasons have been given I will go for the esoteric.

A little know cause of bolt failure is lubricating (with oil or anti-seize products) the underside of the bolt head.

The lubricant reduces the coefficient of friction between the head and the washer which permits additional tightening with the same torque reading, this can and has led to premature bolt failure.