the greatest force so that a significant feature for proper operation of the adjustment device is a high base friction or braking of the tube 2 in its guide the brake force and hence the dimensioning of the brake device 4 are therefore designed for proper operation of the adjustment device this depends essentially on the dimensions of the mutually engaged areas of the brake cone 41 and of the brake bearing 42 and on the coefficient of friction of the brake surfaces and hence on the material of the brake cone 41 and brake bearding 42 or on the material of the surfaces of the brake cone 41 and brake bearing 42 because small dimensions of the brake device 4 are also desired for minimizing the installation space of the adjustment device the effective area of the brake cone 41 and brake bearing 42 is limited so that the effect of the brake device 4 depends significantly on the coefficient of friction the coefficient of friction can be increased by means of a suitable surface design of the brake cone 41 and brake bearing 42 e g by means of a suitable surface structure or by the use of materials with high coefficients of friction thus e g a coating of the surfaces of the brake cone 41 and brake bearing 42 or the use of suitable inserts of a soft bearing material such as e g rubber can cause a high coefficient of friction if at the same time it.