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.

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.

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

"Black (lightly oiled)"

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.