Torque specifications for castings 2

[ajs83]

I have a 7/16 Grade 8 standard steel bolt with a tensile strength of 150,000 psi which threads into a casting made of either:

Ductile iron type 65-42-12 -> tensile strength = 65,000 psi
Gray iron type G4000 -> tensile strength = 40,000 psi

I am looking for the torque specs of both castings for a 7/16 bolt and additional standard sizes. Any feedback would be appreciated.

 

[drawoh]

ajs83,

How deep can your tapped holes go?

JHG

 

[CoryPad]

What is the coating on the bolt? That will tell you what the friction coefficient is.

You can calculate a torque specification using this: faq725-536

You can calculate the required axial thread engagement using this: thread725-35222

Regards,

Cory

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

 

[ajs83]

The depth of the tap is .94 inches and the depth of the drill is 1 inch. The plating on the bolt is yellow dichromate.

 

[MintJulep]

You are going about this out of sequence however. You should:

Calculate the loads on the joint and determine what fastener clamping load you need.

Determine how much variation in that load your assembly can tolerate.

Iteratively, select fastener size and number such that you properly make use of the fastener's design strength while not exceeding the pull-out strength of the casting.

Calculate the nominal fastener torque necessary to achieve the preload determined above.

Calculate the possible variation in preload that torque control will give and compare to the allowable variation determined above. Decide if torque control is sufficient for the application, or select a better method of preload control.

 

[ajs83]

What is a good rule of thumb to determine the clamping force required with respect to the preload.

 

[CoryPad]

A rule of thumb is for the initial preload to be five times the maximum applied force to the screw.

Regards,

Cory

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

 

[MintJulep]

And a better rule of thumb is:

Don't use rules of thumb in place of proper engineering.

 

[CoryPad]

No argument with that.

 

[drawoh]

Bolt strength and torque is written up in quite a lot of detail in the Machinery's Handbook. Is there any good reason to not read this and follow the instructions?

JHG

 

[ajs83]

The casting has a tensile strenght of 40 Ksi and the bolt has a tensile strengh of 150 Ksi. Thus, shouldn't the threads on the casting fail before the bolt. I have an older version of the engineering handbook but I did not see anything on the internal thread strength of iron castings.

 

[CoryPad]

You can design your part to have the casting threads shear before the screw fractures, or you can design for the opposite. The variable involved is the axial thread engagement.

If you are just looking for a simple calculator that doesn't involve detailed engineering, try the bolt torque calculator here:

http://www.futek.com/boltcalc.aspx

Regards,

Cory

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

 

[ajs83]

Does anyone know the yield stress of:

G4000 gray iron casting with a tensile strength of 40 ksi

65-45-12 ductile iron casting with a tensile strength of 65 ksi

 

[CoryPad]

Gray iron usually does not have a yield stress - it is elastic until fracture.

The 45 in the ductile iron is the yield strength - 45 ksi.

Regards,

Cory

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