# Blog

## Calculation Method of Bolt Pretightening, Bolt Tension, Torque Value and Required Bolt Protrusion

When purchasing tools such as hydraulic wrenches and hydraulic bolt tensioners, you need to provide some information about bolts, such as bolt pretightening, bolt tension, torque value, etc., to help manufacturers recommend suitable hydraulic tools to us. In addition, pressure, temperature, vibration or some external force will cause the bolts to loosen, so for the safety and efficiency of field equipment, most users will use tools such as hydraulic wrenches and hydraulic bolt tensioners to pre-tighten the bolts. This has also become a widely used bolt loosening detection method.

Therefore, in many cases, we need to calculate data such as bolt pretightening, bolt tension, torque value and required bolt protrusion, or get this information by referring to the technical sheet. Now, TorcStark will tell you how to calculate these values.

### 1. Knowing the bolt tension, the calculation method of converting the torque value

(1) Known: bolt tension F, bolt diameter M

Calculate the torque value N.m required for the bolt: torque value (Nm)=F(KN)*M(mm)*factor 0.16

Assumption: the bolts require a pulling force of 70 tons, and the bolts are M52

(2) Calculation: (70*9.8) KN*52mm*0.16 (factor)=5707.5Nm

### 2. Calculation of bolt torque value converted into tensile force

(1) Known: the torque value of the bolt Nm, the diameter of the bolt M, and the required tensile force F of the bolt.

Tensile force F (KN) = torque value N.m/{M(mm)}*factor (0.16)

Assumption: the torque required for the bolt is 7134.4Nm, and the diameter of the bolt is M52

(2)Calculation: tensile force=7134.4/(52*0.16)=857.5KN

### 3. The bolt pretightening is converted into the torque value required by the bolt

Known: bolt pre-tightening force (ie bolt tension) F, bolt diameter M

E.g.: bolt M56, pretightening force (minimum) 810,000 N, calculation: 810000N*0.056 (bolt diameter: unit m)*0.16 (factor)=7257.6=7258Nm

### 4. Calculation method of required bolt protrusion

Determination of the applicable minimum length of bolts protrusion (when using hydraulic bolt tensioners). Generally, the minimum length of the hydraulic bolt tensioner on the bolt must be more than 80% of the bolt diameter.

Example: M76*6 bolts, the known exposed length is 42-45mm. Q: Can a hydraulic bolt tensioner be used?

Calculate the minimum length of the bolts that need to be buckled: 76*0.8=60.8=61mm

After calculation and comparison, the length needs to be 61mm, and the actual minimum exposed length of the bolt is only 42mm, which is not available.

Conclusion: The bolts with exposed length of 42-45mm and M76*6 cannot be operated normally with the hydraulic bolt tensioner.

The above is the calculation method of bolt pretightening, bolt tension, torque value and required protrusion of bolt. Of course, you can refer to the bolt parameter table, hydraulic wrench parameter table, bolt tensioner parameter table, etc.

### For example

“Recommendation Table of Bolt Pre-tightening Force “: It is a reference table of recommended bolt pre-tightening force based on bolts of different materials, different strength levels and different specifications. Before referring to this table, the user needs to know the material, specification and strength of the bolt, and then check the table to find the required torque. Or when the bolts are loaded and unloaded on site, if there is no exact torque requirement, you can also refer to this table to get a reference value. After checking the “Recommendation Table of Bolt Pre-tightening Force ” to determine the pre-tightening force, it is necessary to adjust the pressure of the pump according to the “Hydraulic Wrench Torque conversion Chart”.

“Hydraulic Wrench Torque Conversion Chart”: It is to find out the corresponding hydraulic pump pressure value according to the output torque value of the purchased hydraulic wrench model. The premise is that the user needs to know the pre-tightening force of the required bolts, but does not know how much pressure the hydraulic pump should be adjusted to. However, due to the different models of hydraulic wrenches of various brands, the “Hydraulic Wrench Torque Conversion Chart” is also different, so it should depend on the model of hydraulic wrench purchased.

Check the TorcStark catalog to determine the model of hydraulic wrench or bolt tensioner you need. You can view the TorcStark product catalog on the TorcStark Resources Downloads page.

## Summarize

Before purchasing hydraulic tools such as hydraulic wrenches or hydraulic bolt tensioners, be sure to calculate the required pretightening force, tension, torque and required protrusion of the bolts, which will help you find the right tool. Then purchase the appropriate hydraulic tool by contacting the factory.

## 2 comments on “Calculation Method of Bolt Pretightening, Bolt Tension, Torque Value and Required Bolt Protrusion”

## Lapid

Nice read, I just passed this onto a colleague who was doing a little research on that. And he just bought me lunch as I found it for him smile So let me rephrase that: Thank you for lunch!

## Xiomara

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