self gripping effect considering the substantial weight that the drill string can reach one can see that considerable radial forces are exerted on the drill stem so that the teeth 3 dig very strongly into the surface of the drill stem causing notches at the sharp edges that substantially reduce the fatigue resistance of the elements of the drill stem fig2 depicts part ii of fig1 modified according to the invention in fig2 we find plate 6 slips 2 as well as the element of drill stem 1 according to the invention a special profile is imparted to the external surface of the element of drill stem 1 which is intended to cooperate with slips 2 the internal surface of these slips that rest against the drill stem having a complementary shape the profile of the surface of the stem advantageously possesses an axial length greater than that of the slips in order to permit easy engagement fig3 shows on a larger scale the external profile of the surface of revolution made according to the invention on the drill stem drill stem 1 is again found in this figure where broken line 7 represents the external surface of the element of the drill stem before the profile according to the invention is imparted to it as is seen in fig3 the depth of the profile is distributed on both sides of line 7 which enables one to obtain a profile of given depth by reducing the thickness of the stem metal by only about half in the embodiment shown the profile has a toric shape 8 on the inside in which the radius of the generator circle which is as large as possible is between 0 2 and 2 mm for example the toric part 8 is extended on its.