c-g-designs

(EN) METRIC SCREWS

Questions are often asked on worldwide Forums, mainly americans on subject replacement screws for european machanisms containing the said screws.

The screws  usually found in the local shops shelves are  almost always of inferior quality and does not correspond  exactly to the original model. Only highly sprcialised Internet vendors (McMaster-Carr in USA..) can supply screws corresponding to the desired characteristics.

We will mention here only the socket screws with hexagonal  hollow drive (Allen). It exist an other inprint (TORX) actually much more confidential.

A screw is characterised by :

1°) Its form, corresponding to its DIN denomination                      

DIN 912= Socket head capscrew , high (standard) head.

 DIN 913= Socket set screw, flat head.                                

 DIN914=Socketsetscrew, pointed                                                                   

DIN915= Socket set screw,dogpoint .                                                                                

DIN 916= Socket set screw, hollow head.                                                                              

DIN 7884= Socket head capscrew, low head.                             

DIN 7991= Socket head capscrew, flat head.          

2°) Its dimensions ; diameter, pitch and lenght, expressed in millimetres : M3, M4, M5,M6 and the lenght.

‘M’ alone designate a screw wirh coarse threads : M3x0,50 , M4x0,70 , M5x0,80 , M6x1.

‘M’ followed by ‘F’ conventionally identify a fine thread of which the value is always indicated : MF5x0,50 , MF6x0,75 as it can exist more than one fine thread for a given diameter.

  3°) The material quality (cold hammered screws, heat treared or not).

The material quality is defined for the screws of concern  from their mechanical propertires by two numbers : 8.8 , 10.9 , 12.9 , 14.9.

  -The firstnumber multiplied by 100 give the traction resistance expressed in MegaPascals.

-The second multiplied by the first represent the 1/10th of the elastic limit expressed in MPa.

If the headless (set) screws are always class 12.9 or 14.9 , it is preferable to use DIN912 or DI N7884 screws of class  12 .9 (1080MPa) preferably to 8 .8 (640MPa).

The stainless screws A2 (304) and A4 (316) have machanical properties close the bottom of the scale.

The electro-zinc plated screws have an aspect close to those of the stainless screws, with better mechanical properties .

4°) Exemples :

Vis DIN 912 M6x20, class 8.8.

Vis DIN912 MF6-0,75x15 class 12.9

Vis DIN 7991 M5x25,class 10.9.

R.G.C

12-2018