Calculating Pretension Force in Bolt

[Redacted]

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?

 

[TehMightyEngineer]

I'm not 100% sure but I bet thread deformation will reduce the stress significantly.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[PEinc]

Converting from metric, here's my take on this:
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

http://www.PeirceEngineering.com

 

[Redacted]

Thank you for the responses so far. I find it strange that in the Canadian CSA code it specifies that for the turn of the nut method a 1/3 turn is required for 20 mm bolts. However, in the Eurocode it states that the pretension stress in the bolt should be 0.7*ultimate strength of bolt = 0.7*800 = 560 N/mm2.

 

[structSU10]

the bolt pretension is basically impossible to predict. stiffness of the components within the assembly affect the amount of pre-tension the bolt actually sees. If it is critical, the contract should use a whitmore test machine and calibrate torque wrenches to achieve the desired results. I have had a job where tension was critical, the contractor used some chart to calibrate the torque wrench, and testing on the bolts showed a greatly deficient pretension - the torque required was almost 30% more than what the chart told them. If pre-tension is not so critical, the rule of thumb, snug tight plus 1/3 turn, is generally used.

 

[azcats]

From the RCSC Commentary to section 8.2.1: "The turn-of-nut pretensioning method results in more uniform bolt pretensions than is generally provided with torque-controlled pretensioning methods. Strain-control that reaches the inelastic region (emphasis added) of bolt behavior is inherently more reliable than a method that is dependent upon torque control."

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.

 

[rb1957]

how much do the plates (the stuff being bolted) compress under this load ? This compression relieves the stretch in the bolt.

another day in paradise, or is paradise one day closer ?