Tham khảo tài liệu 'modeling and simulation for material selection and mechanical design part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Wp for full plastified cross-section is used Wp pdo3 12 . Step 7 offers the lead angle of the thread profile j. Step 8 formulates the theory of maximum distortion energy for producing a material failure this is also called vMises theory of failure . This is the background-formula for step 1 to combine equivalent stress and axial stress. General information about theories of failure can be found in Ref. 3 . In general the friction coefficient m is defined as the ratio of normal force acting over produced tangential frictional force in a sliding motion of two bodies Fig. 17 . The frictional force is always directed against the direction of motion. For a screw the normal force is the preload Fp. The tangential force can be formulated as mt Fp in the thread contact zone and as mh Fp in the head support area. These tangential forces cause frictional torques because of the radii of thread and head contact zones due to screw axis diameters Deb resp. d2 see also Fig. 16 . Therefore the frictional Figure 17 Definition of friction coefficients mh and mt. Copyright 2004 by Marcel Dekker Inc. All Rights Reserved. coefficients define the part of preload which acts tangentially in the contact areas of a screw. Table 4 proposes classes of frictional coefficients valid for bolted joints based on the VDI 2230 guideline 70 and experience 17 . If no exact value is available one can select a value from this table which is valid for low surface roughness. But one must always remember that the friction coefficient depends on complex influences like materials surfaces lubrication incl. homogeneity hardness ratio of the two surfaces in contact local stress peaks or stress distribution in contact zone tolerances for contact geometry as well as tightening level and number of re tightenings. A selection table can only provide rough approximations. The supplier of screws can provide information related to friction behavior. In practice all parameters for calculations of Fig. 16 have .
