Calculation methods and requirements for bolt preload, tensile force, torque values, and the required length of bolt exposure

Thread Content

In the equipment installation and maintenance work in industrial and mining enterprises, we often encounter situations where the conditions and parameters of bolts are not clear; it is necessary to calculate the preload, tensile force, or torque of the bolts, as well as to determine whether the length of the bolts exposed allows for the use of hydraulic bolt tensioners, and so on. The calculation methods are described as follows:

I. Method for converting known bolt tension into torque value:
1. Given conditions: Bolt tension F, bolt diameter M. To calculate the required torque value in N.m: Torque value (N.m) = F (KN) * M (mm) * coefficient 0.16
Example: If the required bolt tension is 70 tons and the bolt diameter is M52.
2. Calculation: (70 * 9.8) KN * 52 mm * 0.16 (coefficient) = 5707.5 N.m

II. Method for converting bolt torque value into tension:
Given conditions: Bolt torque value in N.m, bolt diameter M. To calculate the required bolt tension F:
1. Tension F (KN) = Torque value (N.m) / M (mm) * coefficient (0.16)
Tension F (tons) = (Torque value (N.m) / (M (mm) * coefficient 0.16)) / 9.8
Example: If the required bolt torque is 7134.4 N.m and the bolt diameter is M52.
2. Calculation: Tension = 7134.4 / (52 * 0.16) = 857.5 KN; or 857.5 KN / 9.8 = 87.5 tons

III. Method for converting bolt preload into the required torque value:
Given conditions: Bolt preload (i.e., bolt tension) F, bolt diameter M
Example: For a M56 bolt with a minimum preload of 810,000 N.
Calculation: 810,000 N * 0.056 (bolt diameter in meters) * 0.16 (coefficient) = 7257.6 = 7258 N.m

IV. Determining the minimum length required for the bolt to be exposed (when using a hydraulic bolt tensioner):
Generally, the minimum length of the hydraulic bolt tensioner sleeve around the bolt should be at least 80% of the bolt’s diameter. Example: M76*6 bolt, with a known exposed length of 42~45 mm. Question: Can a stretcher be used? Calculate the minimum length required for the set screw: 76*0.8=60.8=61 mm. After calculation and comparison, a length of 61 mm is needed, whereas the actual minimum length of the bolt that remains exposed is only 42 mm, which is not sufficient. Conclusion: Bolts with a protruding length of 42–45 mm and a size of M76*6 cannot be properly operated using a hydraulic bolt tensioner