Bolt Torque

Max. tightening torque and preloads for ISO metric and imperial UNC/UNF threads

Available for metric and imperial units.


For calculation of tightening torque of screws you need to know the geometry and material data as well as the condition of friction of thread and head. The friction coefficents are in such a big range that the calculation should always be done for the min. and max. friction coefficient. Simplified the friction coefficient for thread and head can be united to one totale friction coefficient. In the app you can choose between different conditions to get these simplified  coefficients. The max. tightening torque is given by the min. friction coefficient and stresses during torque up to 90% of the yield point of the screw material. The min preload is given by the max. friction coefficient.


After that you can also input any tightening torque and calculate the preloads  and stresses for min. and max. friction condition.  So you get all relevante values you need to choose a good tightening torque. 


This app is very usefull for all who need to know tightening torque and preloads  for many screws in a quick and very easy way. So the app is a "must have" for all engineers, mechanical designers and for workshops and hobbyists.   


Why do I get 2 values for stresses and preload forces?

Because the coefficient of the friction condition is in a very big range and you get the values for min. and max. friciton coefficient. The max preload is given by min friction, the min preload is given by max friction.

Which friction coefficient do I choose for a new black srew?

New black screws are a lightly oiled. So you can take:

"Black (lightly oiled)"

Which material data are in the app?

In literature you can find different values. This app takes the following data for yield limit Re:


1. For iso metric screws: 

Material     4.6   5.6     6.8   8.8     9.8   10.9     12.9  
Re [MPa]   240   300   480   640   720    900  1080
Re [kpsi]   35   44   70   93  104   130   157

2. For imperial screws with SAE grades: 

Material       Gr. 2     Gr. 2  Gr 5 Gr. 5      Gr. 5   Gr. 5.2     Gr. 7    Gr. 8 
Sizes   1/4 - 3/4   over 3/4   1/4 - 1.0  -  1 .5  over 1.5       all    all     all
Re [MPa]      393     248    634   558      400    634   793   896
Re [kpsi]      57      36     92    81       58     92   115  130




The friction coefficients, screw and material data are from literature and the results can be used as a good recommendation. But valid are only the specifications of the suppliers. Therefore all data are given without guarantee, the use of this app is at your own risk. In case of high stressed bolts please contact engineering departements.