titanium Maximum bolt torque 2

[UMDpru]

Hello,

I am trying to determine the maximum allowable torque to be applied to te a 3/8-16UNC-2A Bolt made of the following materials:

Ti CP1(Grade4) Yield=70ksi
Ti CP4(Grade1) Yield=25ksi
Ti 6A1-V4 Yield= 120ksi

I have found some formulas that relate to steel in the Machinery's handbook 23rd edition page 1279 formulas (2a) and (2b). My first question is why do they list two different formulas which can give vastly different stress areas based upon the material yield Strength? Shouldnt the stress area be based upon the bolt geometry and not any material properties?

Using the two different formulas yields really different numbers for maximum torque.

Secondly, it states in the handbook that these are for steels. can they be used with titanium or other bolt materials? If they can not be used, can someone please point in the direction where I can find the correct formulas?

I have used the following formulas to determine maximum allowable torque.

Fmax=Maximum tensile load
Fmax=.8Yield*At where At=Stress Area

Tmax=Maximium Torque
Tmax=.2*D*Fmax where .2=assumed coefficient of dynamic friction, D=nominal thread diameter

Ti CP1 Tmax=95.33 ft-lbs
Ti CP4 Tmax=34 ft-lbs
Ti 6A1-4V Tmax= 163.5 ft-lbs

These numbers were generated using formula 2a for calculating At for all.

If I use formula 2a for Ti CP1, and Ti CP4 which have yields below 100ksi and use Formula 2b for Ti 6A1-4V which has a yield of 120ksi, it turns out that the Ti CP1 can be torqued to twice as much as the Ti 6A1-4V? that doesnt make sense to me.

Please help.

 

[CoryPad]

I have no idea why two equations are listed. You are not the first person to ask this:

thread177-33175

The stress area is based on geometry. FED-STD-H28/2B, ASME B1.1, ASME B1.13 and ISO 898-1 have standardized on the equivalent of Machinery's Handbook's equation 2a - this is the average of the root and pitch diameters. This equation is used for materials with strengths up to 180,000 psi (1200 MPa).



Regards,

Cory

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

 

[UMDpru]

From "An Introduction to the Design and Behavior of Bolted Joints (Mechanical Engineering (Marcell Dekker))"

There are multiple formulas used for stress area because each one has a different factor of safety- one is more conservative than the other.

I found it this morning )

Oh and also, equation 2a states that it is valid up to 100,000 psi not 180,000 psi?

I just used equation 2a to get a ball park figure.

Thanks for the help.

 

[CoryPad]

Machinery's Handbook states that equation 2a is valid to 100,000 psi. The consensus standards I listed show validity up to 180,000 psi or 1200 MPa. Since they are sources used for actual design, procurement, disputes, etc., they trump Machinery's.

Regards,

Cory

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

 

[UMDpru]

Ok,

Thanks for clearing that up.

I appreciate the help.

Adam

 

[CoryPad]

I have Bickford's book, so I opened it to review the safety factor info you mentioned. I haven't been on page 30 (of 951!) of his book for a long time - if I had, I would have been able to answer this question 5 years ago. Bickford has the same equation that Machinery's calls 2b, and he attributes it to:

Srinivas, A.R., Determination of Torque Values for Metric Fasteners - an Approach, Space Payload Group, Space Applications Center, Indian Space Research Organization, Ahmedabad, India, Document 380 053, May 1992

That is quite the obscure reference!

I won't be using it, but this does help explain why there is a second equation floating around.

Regards,

Cory

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

 

[UMDpru]

HA, i didnt even look at the reference for the equation but you sure are right, that is one heck of an obscure reference. I challenge someone to get a hold of it haha.

I probably wont be using the second equation as it is less conservative and it appears that the use of the stress area formula can be hazardous if caution isnt used.

Adam