[L1] 1 Overview [L2] 1) General Matters of Screw [L4] - α : Lead Angle = tan^-1(p/πd) [L4] - p : Pitch [L4] - Nominal Diameter (d) : Major diameter or screw standard [L4] - Minor Diameter (d1) : Bolt minimum diameter (Root diameter) [L4] - Pitch Diameter (d2) : Average diameter of major diameter and minor diameter (Effective diameter) [L4] - Screw Height (h) : Height between minor diameter and major diameter [L2] 2) Types of Screws [L3] ① Triangular Screw (V-Thread) [L4] - Metric Thread [L5] * Angle of thread is 60 degrees. [L5] cf. M3x0.5 M : Type of screw 3 : Nominal diameter (d) 0.5 : Pitch [L4] - Unified Thread [L5] * Angle of thread is 60 degrees. [L5] cf. 1/4-20UNC 1/4 : Diameter of screw 20 : Number of threads (per inch) [L5] UNC : Unified National Coarse (UNF : Fine thread) [L3] ② Square Screw [L1] 2 Specification of Screw [L2] 1) Definition of Screw Specification [L4] - Looking at the KS standard for screws, the specifications are defined by organizing three types of diameters below. [L4] - The reason for classifying screw specifications is that the required dimensions are different when calculating mechanical elements of screws. [L5] * D : Major Diameter (Nominal Dimension) [L5] * D1 : Minor Diameter [L5] * D2 : Pitch Diameter (Effective Diameter) [L2] 2) Details of Screw Specification [L3] ① D : Major Dimension (Nominal Diameter) [L4] - A dimension defined to designate the standard of the screw. [L4] - Refers to the dimension of the screw's outline (the peak of the thread). [L3] ② D1 : Minor Diameter [L4] - Necessary for calculating the tensile/compressive stress of the screw. [L3] ③ D2 : Pitch Diameter [L4] - Necessary for calculating the torque of the screw. [L4] - Pitch diameter is (D+D1)x1/2 only for square screws. [L2] 3) Precautions for Screw Design [L4] - When designing mechanical elements of screws, dimensions must be distinguished. [L5] cf. If tensile and torque are calculated with a nominal diameter of 10mm for an M10 bolt, the error is as follows (based on coarse thread). [L5] ▶ (0.785XD1^2)/(0.785XD^2)=0.70 [L5] ▷ (0.785XD2^2)/(0.785XD^2)=0.81 [L5] In other words, it becomes an incorrect calculation formula as it is 81% level for torque calculation and 70% level for tensile/compression calculation compared to the normal calculation value. [L5] When selecting screw specifications, D, D1, and D2 must be distinguished and used. [L1] 3 Mechanical Analysis of Screw [L2] 1) Square Screw [L3] ① Rotation Torque of Screw [L5] μ : Friction Coefficient ρ : Friction Angle [L3] ② Self-locking Condition of Screw [L4] - Related Formula (Loosening of Screw) [L5] * Analysis [L5] α=ρ, P'=0 : Stationary at arbitrary position [L5] α>ρ, P'<0 : Screw loosens [L5] α<ρ, P'>0 : Stationary (Force equivalent to the difference is required to loosen the screw) [L3] ③ Efficiency of Screw [L4] - Efficiency Defined by Torque [L4] - Efficiency Defined by Work [L2] 2) Triangular Screw [L3] ① Equivalent Friction Coefficient [L4] - Formula [L5] * β : Friction Angle (corresponds to 60 degrees) ρ : Equivalent Friction Angle [L3] ② Force to Tighten Screw [L3] ③ Efficiency of Screw [L1] 4 Prevention of Screw Loosening [L2] 1) Loosening Mechanics (Loosening Criteria) [L4] - Increase of T (Torque), P (Load) values is required. [L4] - Increase of contact surface friction force (Increase of μ value). [L4] - Increase of friction angle / friction coefficient (Increase of μ value). [L4] - Decrease of pitch (Decrease of p value). [L4] - Vibration suppression (Reduction of friction force during vibration). [L2] 2) Commercial Screw Loosening Prevention [L3] ① Friction Improvement [L4] - Increase friction force using adhesives (Loctite, etc.). [L4] - Increase surface friction force by roughening the washer surface. [L4] - Add friction force by inserting spring washers, etc. [L3] ② Physical Locking [L4] - Use of Locknut. [L4] - Use slotted nuts to prevent the nut from turning. [L4] - Use special washers to prevent bolts or nuts from turning. [L4] - Use set screws to fix the threaded part of the bolt. [L4] - Prevent loosening by drilling a hole in the bolt or nut and inserting a pin (Split Pin).