Bolt Torquing Calculation
Here are the details for calculating the bolt torquing moment for piping flanges. In industrial piping systems, achieving the correct bolt torque is crucial for ensuring the integrity of connections. Behind this seemingly straightforward task are several factors to consider, including material properties, gasket characteristics, flange design, and operating conditions. But we will provide a basic calculation method that will give you a rough idea about the level of torquing moment value. These formulas can be used for structural connections and also non-standard flange torquing.
Practical Bolt Tightening Torque (Moment) Calculation
The practical formula is below for the bolt torquing moment. We should know the pre-load for assembly (Fa) and the diameter of the bolt.
Equation-1
Mt=0,2 x Fa x d2 » 0,17 x Fa x d (Practical Formula for Bolt Torquing Moment)
d= Nominal diameter, mm
d2=Average Diameter, mm
Fa=Pre-Load (for Assembly), N
Pre-load (Fa) during assembly can be calculated by the formula below prior to the bolt torquing moment calculation.
Equation-2
Pre-Load, N
Gy=Yield Strength of Bolt Material, N/mm2
A=Bolt Section Under Tensile Stress, mm2
P=Pitch, mm
d2= Average Bolt Diameter, mm
d3= Bottom of Bolt Diameter, mm
n= Friction Coefficient (Generally 0.12 being applied but may vary depending of the lubricant used).
Bolt Section Under Tensile Stress, mm2
Bolt Torquing Moment (Mt)
Equation-3
Bolt Tightening Torque (Moment), Nmm
This equation is the moment calculation for bolt torquing.
Let’s see an example.
Example:
Metric hexagon bolt M10 size, DIN 931 standard. Material Quality is 8.8. Now, we should find the tightening torque (moment).
d=10mm
d2=9.026mm
d3=8.16mm
P=1.5mm
n=0.12
d0=8.593mm
A=57.9935mm2
All these size information can be found in the table below.
Gy=8*8*10=640N/mm2 (Material is 8.8)
By all these information we should calculate the pre-load (Fa) with the Equation-2.
Fa=27415N = 27.5 kN Pre-load during assembly, according to n=0.12 lubricant.
Now, we can calculate the moment value for the bolt torquing. For this, we should first calculate the dk value. dk is the bolt head friction force impact average diameter, see below sketch.
Nut or Bolt Head Sitting Surface Illustration
Bolt Torquing Impact Surface Radius
dG =Hole Diameter = 11mm
dD=Bolt Head Sitting Surface Outside Diameter = 17mm
dk value is (11+17)/2=14mm so, dk/2 is 7mm which is the friction force impact radius.
With all this information provided, we can now calculate the moment by Equation 3.
Mt=Fa*(0,159*P+n*(0,577*d2+dk/2))=27500*(0,159*1,5+0,12*(0,577*9,026+7))
Mt=45877Nmm = 46Nm
This is the maximum torque (moment) that can be applied.
It is important to show the Moment value in documents or drawings.
For this, we must find the range of the moment.
There is a tightening factor for various torquing methods.
1- Hand Torque a=4.0
2- Torque via impulse-controlled wrench. Min. 10 pieces of bolts or %10 of the bolts should be controlled by the wrench. a=2.5
3- Torquing with a special wrench that can see the moment value. The special wrench should control all the bolts. a=1.6
4- This method includes controlling turning angle or bolt length expansion. The torquing can be done either by hand or by wrench. a=1.0
We can accept that our bolt will be torqued by a special wrench so the a=2.5.
Mt(max)= a * Mt(min) =46 Nm
Mt(min)= 29 Nm
Mt(nom)= (46+29)/2=37.5 Nm
So, the torque value that should be sown on the document/drawing is 37.5 Nm ± 8.5 Nm
Reference: “CIVATALAR” by M.Guven Kutay
