Torque Technical Information A guide to specifying torque values for fasteners continued... TABLE 1 MECHANICAL PROPERTIES OF FASTENERS The designation system for property classes (strength grades) of metric fasteners, consists of a symbol comprising two figures. The first figure indicates 1/100 of the nominal tensile strength in newtons per square millimetre. The second figure indicates 10 times the ratio between the nominal yield stress and the nominal tensile strength. Multiplication of the two figures will give 1/10 of the nominal yield stress in newtons per square millimetre. Hence a grade 10.9 fastener has a nominal tensile strength of 1000 N/mm2 (the first figure multiplied by 100) and a nominal yield strength of 900 N/mm2 (both figures multiplied together and by 10). The minimum yield stress and the minimum tensile strength are equal to or greater than the nominal values. Strength Grade 3.6 4.6 4.8 5.6 5.8 6.8 8.8 9.8 10.9 12.9 *Yield Stress N/mm2 180 240 320 300 400 480 640** 720 900 1080 * Nominal values quoted  ** For grades 8.8 and above a proof stress is specified because of problems measuring yield. BS6104 Pt. 1 TABLE 2 THREAD COEFFICENT OF FRICTION Presented in this table are guideline values for the coefficient of friction of the threads for various surface finish conditions. Upper and lower normal limits are shown in the table. Care must be taken to ensure that the values are valid for your application. There are a number of effects (such as grease on the threads) which can result in the true thread coefficient of friction being outside the range quoted.                                                                          INTERNAL THREADS                                                           Steel Threads EXTERNAL THREADS Self Finish Zinc Plated Cast Iron Aluminium STEEL 0.10 to 0.16 0.12 to 0.18 0.10 to 0.16 0.10 to 0.20  DRY 0.08 to 0.16 0.10 to 0.18 0.08 to 0.18 0.10 to 0.18 SELF FINISH Or   PHOSPHATE TREATED  OILED 0.12 to 0.20 0.12 to 0.22 0.10 to 0.17 0.12 to 0.20 ZINC                                     DRY 0.10 to 0.18 0.10 to 0.18 0.10 to 0.16 0.10 to 0.18 PLATED                          OILLED 0.18 to 0.24 0.18 to 0.24 0.18 to 0.24 0.18 to 0.24 THREAD ADHESIVE Certain thread adhesives can generate very high thread friction values. Check manufacturer's data. The values are for guidance only, for critical applications the scatter in the cosfficient of friction must be determined experimentally. Specialist thread lubricants, such a molybdenum disulphide can reduce thread friction and also frictional scatter, guidance should be sought from manufacturer - for example Molykote. With some materials, such as certain types of stainless steel, cold welding can occur resulting in very high friction values. In such circumstances, specialist thread lubricants may be essential. TABLE 3 UNDERHEAD COEFFICIENTOF FRICTION As with thread friction the values are for guidance only and care should be excersied in their use. There are a number of effects (such as grease or oil on the surface) which can result in the true value of underhead friction being outside the range quoted. CONDITION OF THE BOLT HEAD OR NUT Zinc Plated Self Finish Cast Iron Aluminium ZINC PLATED               DRY 0.16 to 0.22 0.12 to 0.20 0.10 to 0.20 FINISH          Slight Oil Applied 0.10 to 0.18 0.10 to 0.18 0.10 to 0.18 SELF FINISH OR           DRY 0.10 to 0.18 0.10 to 0.18 0.08 to 0.16 PHOSPHATE OR BLACK OXIDE FINISH                        Slight Oil Applied  0.10 to 0.18 0.10 to 0.18 0.12 to 0.20 0.08 to 0.20 Gaps in table indicate a lack of available published data. There has been little information published on the underhead friction coefficient on fasteners being used on painted surfaces. One study reported that the mean value of underhead friction on a painted surface was 0.21 with an extreme range of 0.10 to 0.32.                                                                                                       Contd.....