Torque reduction for castings- Rule of thumb

[jchinyut]

For example:
A 1 1/2" SAE grade bolt has a torque value of 1910 ft-lbs when using the Machinery handbook formula:
T= 10* where * is b+mlogd m= 2.965, b= 2.759 and d= 1.5

What would be the reduction of torque values if used to bolt two casted pieces one BS 1452 Grade 17 and the other BS 4360 Grade 43A?

Is there a rule of thumb? I have seen a reduction of approximately 50% for bolting a metal to non metallic parts.

James Chin Yut

 

[CoryPad]

This is a poor way to design bolted joints. I recommend you search this site (including FAQ) to educate yourself. One place to start is faq725-600.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jchinyut]

Cory,
Thanks for the info.
My intention was not to design a bolted joint but to find out what criteria was used in reducing torque values when castings were employed.
Like I stated earlier there is a reduction when metals are bolted to non metals what criteria did they use? Obviously it is not 90% of the yield strength of the stud or bolt that is employed in calaculation of the torque.



James Chin Yut

 

[plasgears]

Bolting a dissimilar metal with a bolt invites either loosening or tightening under working temp conditions. Suggest you show the bolt material on a stress-strain plot. The difference of strain over the clamped length between associated parts will shift the bolt strain either up or down.

For example:

1. If the net strain under temp is positive, the bolt will strain the additional amount. If you are uncomfortably close to yield, you may break the bolt if it is brittle in nature.

2. If the net strain under temp is negative, the bolt will relax that amount. That's OK if the preload is not violated. If preload is unduly relaxed, allow more tighenting during assy.

 

[MintJulep]

James,

If you have two parts being held together with a threaded fastener, and you are trying to determine the required bolt preload, then you are designed a bolted joint whether you intended to or not.

You should never ever use torque as a design input when designing a bolted joint. There is a necessary preload that must be introducted to the fastener. Design the joint in terms of fastener preload.

Torque is one method (a poor one) of controlling the preload introduced into a bolt at assembly.

 

[GregLocock]

We have curves showing required thread engagement for a given bolt spec, for an internal thread of a given yield stress, if you are thinking of the thread stresses in a tapped casting.

There is also the standard bolt diagram that takes into account the stiffness of the parts being joined.



Cheers

Greg Locock

 

[jchinyut]

GregLocock,
Thanks for your response, you are closest to what I was after. I was not clear with my initial post and thanks to all the previous responses.
I am aware that preload of the fasteners is required to keep any bolted joint together and torque is an indirect means of setting that preload (+/- 25% accurate). Although frequently used to set the preloads.
In the case of castings where there a stud is employed in tapped casting. The yield strength of the stud may not the limiting factor on the level of preload that can be employed but the stressses of the threads in the casting. The threads may fracture before you get to 80% of the yield strength of the stud.
It is clearly not the same situation when studs are employed to keep two 150# flanges together.
Could you post those curves that you have mentioned and any other information you have on studs in castings- both cast iron and cast steel?
Regards,

James Chin Yut

 

[GregLocock]

No, I can't. They are proprietary.

However

http://www.boltscience.com/pages/boltcalctraining.htm

discusses this very topic

Cheers

Greg Locock

 

[CoryPad]

This topic has been discussed extensively on this site. Perform a search and you will find the equations you need.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.