Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations JAE on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Torque chart 9
- Thread starter patsand99
- Start date
-
2
- #2
-
1
- Thread starter
- #3
-
2
- #5
If you really want to understand fasteners and learn how to calculate the proper torque for a particular fastener, here is a good starting point:
-
1
- #6
You mean like this?
Or this?
Ted
Or this?
Ted
There is more to it than just having a chart. It depends on the fastener being used (i.e. bolt grade) and whether or not it is lubricated.
Here is another link to how to calculate the proper torque for a particular fastener:
Here is another link to how to calculate the proper torque for a particular fastener:
-
1
- #9
IceStationZebra
Mechanical
This is an eternal question on many engineering sites. Like CGilman said there is more too it.
In a nut shell, the only true way to determine the proper torque value is to do a study and measure the fastener clamp load or stretch with your parts in your installation.
ISZ
In a nut shell, the only true way to determine the proper torque value is to do a study and measure the fastener clamp load or stretch with your parts in your installation.
ISZ
cgilman - please explain to me how the bolt grade affects the torque calculation. I completely understand how lubrication affects it, but the equation I have do not have any effect for bolt grade. Young's modulus - yes, but yield strength (i.e. bolt grade) - no.
-
2
- #11
Oh - I get that. That's a no-brainer.
What I want to know is how the grade affects the actual calculation itself - of the torque. For example, if I want a target bolt stress of X psi, then how does the grade of bolt affect the calculation?
Sure, the grade affects what the value of X can be. But I asked how it affects the calculation of the resultant torque.
What I want to know is how the grade affects the actual calculation itself - of the torque. For example, if I want a target bolt stress of X psi, then how does the grade of bolt affect the calculation?
Sure, the grade affects what the value of X can be. But I asked how it affects the calculation of the resultant torque.
TGS4 - I suggest that you READ the article I sent the link on. It has a good explanation as to why different grade bolts are torqued differently.
It has to do with the yield strength of the bolt. Here is another handy reference: This link gives yield strength for different grade bolts. Once you know the yield strength, then you can calculate the tensile load (P).
Then given the formula: T= K x D x P you now have the information you need to calculate the proper torque.
Where:
T = Target tighten torque (the result of this formula is in inch pounds, dividing by 12 yields foot pounds
K = Coefficient of friction (nut factor), always an estimation in this formula
D = Bolts nominal diameter in inches
P = Bolt's desired tensile load in pounds (generally 75% of yield strength)
It has to do with the yield strength of the bolt. Here is another handy reference: This link gives yield strength for different grade bolts. Once you know the yield strength, then you can calculate the tensile load (P).
Then given the formula: T= K x D x P you now have the information you need to calculate the proper torque.
Where:
T = Target tighten torque (the result of this formula is in inch pounds, dividing by 12 yields foot pounds
K = Coefficient of friction (nut factor), always an estimation in this formula
D = Bolts nominal diameter in inches
P = Bolt's desired tensile load in pounds (generally 75% of yield strength)
cgilman - I read the reference. The grade is important ONLY for determining the target assembly bolt stress.
Now, I may be taking a bit of leap of faith here, but given that the OP requested information regarding ASME grade bolts, I am going to assume a pressure vessel or piping use. Now, the determination of an appropriate target assembly bolt stress (or load, whatever your fancy) is extremely important, and takes into consideration, among other things, the gasket in the joint.
Now, once you have determined your target assembly bolt stress, you need to calculate a torque. Going from stress (or load) to torque doe NOT involve the grade of bolt. Rather, the grade of bolt was determined from the desired stress. Basically - the bolt doesn't know what grade it is, but with the application of a torque on the nut, it achieves a particular stress.
BTW, my formula for torque is:
T=P*(p_t/2*pi+(mu*d_b)/2*cos(alpha)+mu_n*d_n/2)
where:
T is the torque
P is the bolt's desired tensile load
p_t is the thread pitch
mu is the coefficient of friction between the nut and the bolt (often lubricated)
d_b is the nominal (basic) diameter
alpha is the thread half angle
mu_n is the coefficient of friction between the nut and the flange surface (often not lubricated)
d_n is the mean contact diameter under the nut head (average of the diameter across the flats and across the corners)
Neither in your formula nor mine is there a place for E, or yield strength, or anything that relates to the bolt grade.
Now, I may be taking a bit of leap of faith here, but given that the OP requested information regarding ASME grade bolts, I am going to assume a pressure vessel or piping use. Now, the determination of an appropriate target assembly bolt stress (or load, whatever your fancy) is extremely important, and takes into consideration, among other things, the gasket in the joint.
Now, once you have determined your target assembly bolt stress, you need to calculate a torque. Going from stress (or load) to torque doe NOT involve the grade of bolt. Rather, the grade of bolt was determined from the desired stress. Basically - the bolt doesn't know what grade it is, but with the application of a torque on the nut, it achieves a particular stress.
BTW, my formula for torque is:
T=P*(p_t/2*pi+(mu*d_b)/2*cos(alpha)+mu_n*d_n/2)
where:
T is the torque
P is the bolt's desired tensile load
p_t is the thread pitch
mu is the coefficient of friction between the nut and the bolt (often lubricated)
d_b is the nominal (basic) diameter
alpha is the thread half angle
mu_n is the coefficient of friction between the nut and the flange surface (often not lubricated)
d_n is the mean contact diameter under the nut head (average of the diameter across the flats and across the corners)
Neither in your formula nor mine is there a place for E, or yield strength, or anything that relates to the bolt grade.
IceStationZebra
Mechanical
I think there are two real world reasons why it is only 0.75.
1. Since we all should know that K (Coefficient of friction) is only an estimation, if we are wrong and over torque to fastener it will be junk. Using 0.95 doesn't give much room for error.
2. Most mechanics like to give a little extra just to make sure it is tight.
ISZ
1. Since we all should know that K (Coefficient of friction) is only an estimation, if we are wrong and over torque to fastener it will be junk. Using 0.95 doesn't give much room for error.
2. Most mechanics like to give a little extra just to make sure it is tight.
ISZ
TSG4
Apart from the fact that torqing a bolt is subject to +/- 25% error when using a torque wrench, using 75% of the fastener yield strength is an accepted figure if the bolts are to be removed and re-used.
The 90% figure of fastener yield stress is used when the bolts are not going to be re-used but replaced.
That said it is the joint materials their strengths and function which should dictate how much pre-load is put on the bolt and not the maximum pre-load a particular bolt of a certain size and grade can withstand, sadly thats a mistake which often made these days.
desertfox
Apart from the fact that torqing a bolt is subject to +/- 25% error when using a torque wrench, using 75% of the fastener yield strength is an accepted figure if the bolts are to be removed and re-used.
The 90% figure of fastener yield stress is used when the bolts are not going to be re-used but replaced.
That said it is the joint materials their strengths and function which should dictate how much pre-load is put on the bolt and not the maximum pre-load a particular bolt of a certain size and grade can withstand, sadly thats a mistake which often made these days.
desertfox
desertfox - your second comment about the joint materials and function was exactly my point. And given that the OP asked about an SA- bolt, I assumed that the application was for ASME pressure equipment (piping or pressure vessel). PCC-1 is THE source for that information. Note that PCC-1 does NOT vary the assembly bolt torque depending on the grade of the bolt. Because for this application, it is not relevant. Which is why I asked the OP why he thought that the grade was relevant. And why I disagree with cgilman.
I question the "fact" that torquing a bolt is +/- 25% error. According to EN-1591, they recommend a scatter of 0.1+0.5*mu (where mu is the coefficient of friction between the nut and the bolt). You'd need a mu of 0.3 to get to +/-25% - which is a value that I would use for unlubricated only.
I question the "fact" that torquing a bolt is +/- 25% error. According to EN-1591, they recommend a scatter of 0.1+0.5*mu (where mu is the coefficient of friction between the nut and the bolt). You'd need a mu of 0.3 to get to +/-25% - which is a value that I would use for unlubricated only.
Hi TGS4
Firstly the 25% error comes from the Roy Mech site (link provided) I threw it in merely to show that using a torque setting for a bolt is not a very accurate method of making a joint.
go to the bottom of the page.
As regards what I said about pre-loads on joints then yes, one would normally workout the requirements of the joint for the pre-load, subsquent to that one would then select a a number of fasteners for the joint and ensure that the individual bolt could take the pre-load required.
I think that cgilman was actually saying the same thing and what he was getting at that whilst there are charts for bolt torques, the actual torque you can put on them will depend on the size and grade of the bolt that you use.
For instance we can design a joint and use say four large size bolts or eight smaller one's and in doing so we may have to change the grade and the torque setting to get the same joint pre-load.
I read your both posts as saying a similiar scenaro.
desertfox
Firstly the 25% error comes from the Roy Mech site (link provided) I threw it in merely to show that using a torque setting for a bolt is not a very accurate method of making a joint.
go to the bottom of the page.
As regards what I said about pre-loads on joints then yes, one would normally workout the requirements of the joint for the pre-load, subsquent to that one would then select a a number of fasteners for the joint and ensure that the individual bolt could take the pre-load required.
I think that cgilman was actually saying the same thing and what he was getting at that whilst there are charts for bolt torques, the actual torque you can put on them will depend on the size and grade of the bolt that you use.
For instance we can design a joint and use say four large size bolts or eight smaller one's and in doing so we may have to change the grade and the torque setting to get the same joint pre-load.
I read your both posts as saying a similiar scenaro.
desertfox
- Status
Similar threads
- Locked
- Question
- Locked
- Question
