Calculating Pretension Force in Bolt
Calculating Pretension Force in Bolt
(OP)
Good day,
I need a bolt to be pretensioned to about 560N/mm^2 and need to relate it to the turn of the nut pretension method. I want to calculate how much tensile stress is put into a bolt when it is turned 1/3 of the way after being snug tight.
I originally assumed that I could use Hooks law and the pitch of the bolt as follows:
For a A325 UNC, 20mm diameter, 80mm long bolt with 10 threads per inch:
= 1 thread per 2.54mm
one third of that is 0.846mm
Assuming that the extension of the steel is therefore 0.846mm
E = stress/strain
Strain = change in length/original length
Strain = 0.846/80 =0.0106
Taking E as 205 GPa
E = stress/strain
205x10^3 = stress/0.0106
stress = 2173 N/mm^2 ... which seems too high.
Can anyone let me know where I am going wrong with this calculation and logic?
I need a bolt to be pretensioned to about 560N/mm^2 and need to relate it to the turn of the nut pretension method. I want to calculate how much tensile stress is put into a bolt when it is turned 1/3 of the way after being snug tight.
I originally assumed that I could use Hooks law and the pitch of the bolt as follows:
For a A325 UNC, 20mm diameter, 80mm long bolt with 10 threads per inch:
= 1 thread per 2.54mm
one third of that is 0.846mm
Assuming that the extension of the steel is therefore 0.846mm
E = stress/strain
Strain = change in length/original length
Strain = 0.846/80 =0.0106
Taking E as 205 GPa
E = stress/strain
205x10^3 = stress/0.0106
stress = 2173 N/mm^2 ... which seems too high.
Can anyone let me know where I am going wrong with this calculation and logic?






RE: Calculating Pretension Force in Bolt
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Calculating Pretension Force in Bolt
Bolt dia. = 20mm = 0.787” say ¾” dia.
Per AISC 14th Ed., Table 7-2, Allowable Bolt Tension for an A325 3/4" bolt = 19.9 kips / 0.442 in^2 = 45 ksi
20mm bolt area = 0.487 in^2
L = 80mm = 3.15”
Assume a 0.5” (15/32") tall nut for a 3/4" bolt
L – 0.5” nut = 2.65” stretchable bolt length
10 threads per inch
1 turn - 1/10" stretch
1/3 turn = stretch of 1/30” = 0.0333”
PL/AE = 0.0333”
E = 29000 ksi
0.0333” = (P x 2.65”) / (0.487in^2 x 29000 ksi)
P = 177.5 kips
Tension stress = 177.5 kips / 0.487in^2 = 364.5 ksi (much too great)
1/3 turn is too much if the bolt is totally snugged up before turning.
PL / AE = allowable stretch = (19.9 kips x 2.65”) / (0.442 in^2 x 29000 ksi) = 0.004114”
0.004114” x (10 turns /1 in) = 0.04114 turns maximum < 1/24 turn, not 1/3 turn
www.PeirceEngineering.com
RE: Calculating Pretension Force in Bolt
RE: Calculating Pretension Force in Bolt
RE: Calculating Pretension Force in Bolt
PL/AE assumes elastic deformation, no?
I know many engine head bolt fasteners use TTY (torque to yeild) fasteners and their tensioning is often phased in a specific pattern and based upon rotation.
RE: Calculating Pretension Force in Bolt
another day in paradise, or is paradise one day closer ?