Discussion on friction performance requirements of automotive thread fasteners
Wake up to block||||||, the friction coefficient P is a physical concept in the usual sense, and is the ratio of friction and positive pressure. In the threaded connection, the friction can be divided into two parts: the thread pair friction and the end face friction. The friction conditions of the two parts are often different, so the friction coefficient of the thread pair and the friction coefficient of the end face friction coefficient are different due to the material, the surface condition and the lubrication condition. And different. ! ! ! ); d is the nominal diameter of the thread (mm); F is the axial preload of the bolt (kN).
The torque factor is a parameter determined by the friction coefficient and the shape of the thread. For a specific ideal threaded coupling pair, the torque coefficient scale is determined when the friction coefficient is determined. If yu=/i=0.15, the torque coefficient K of ordinary coarse thread and fine thread can be obtained. It should be specially pointed out that the friction coefficient has a clear physical meaning, which can be understood as a material constant when the friction surface After the material, surface condition and lubrication conditions are determined, the coefficient of friction is determined. (Strictly speaking, the coefficient of friction between metals decreases with the relative sliding speed or temperature: the bolt can be obtained by (3) The relationship between the allowable axial (s) force and the monotonic tensile allowable strength when tightening CT/CT, and the torque coefficient (see). It is visually shown that the torque coefficient is selected after the bolt strength is selected. Large, the smaller the axial force is allowed.
The equivalent stress of the thread is 1.28, that is, the allowable strength of the bolt when tightening is about 78% of its monotonic tensile allowable strength (guaranteed stress or yield strength). Of course, this proportional relationship will vary with the thread pair friction condition. And change.
::: Group 雉 Wipe itch 毪 When tightening with torque control, threaded fasteners are usually required to meet certain friction performance requirements. In 1963, Nippon Minda Co., Ltd. and Nippon Oil & Fat Co., Ltd. jointly developed the Bu Li-CA-H water-based friction coefficient stabilizer, which is widely used in the Japanese Toyota Motor Corporation's threaded fasteners. Recently, Mingdao Co., Ltd. developed abundance. The force-B-2 type friction coefficient stabilizer further stabilizes the friction performance. The frictional properties of the existing film-forming friction coefficient stabilizers in Japan are shown in Table 1.
Table 1 film formation type friction coefficient stabilizer friction coefficient stabilizer thinner friction coefficient standard deviation coefficient of variation Bu / force a CA-H water solubility In order to stabilize the friction performance, Volkswagen requires the surface of the threaded fastener to be coated with a water-dispersible synthetic polymer slip agent (the slip agent is not allowed to contain silicon, such as the Torque'nTensionControlFluidBrach-Sal from MacDermid Co., Ltd.), which is dried to produce a A waxy sliding film. Table 2 is the result of the torque coefficient of two batches of bolts measured by Volkswagen. Table 2 Volkswagen crepe fastener torque coefficient measured results bolt average standard deviation coefficient of friction friction coefficient article I1 Dak "镞缬 囿 囿 囿 囿Electroplating! Dacromet electroplating; Dak 9; electroplating plating! Dacromet lubrication no water coating rotor oil coating oil I no setting agent Bu Ba Li-CA-H+ Cai Ba-B 2 friction coefficient stabilizer And the influence of friction coefficient. French PSA Group bolt fasteners have proposed friction performance requirements, requiring the common friction coefficient of ordinary friction parts (surface galvanized strengthening passivation) between 12 and 0.18 (average 0.15, standard The difference is less than 0.01, the coefficient of variation is less than 0.067), and the weak friction is between 6 and 0.09 (the coefficient of variation is less than .067).
The above analysis shows that the international automotive industry generally requires threaded fasteners to meet certain friction performance requirements. A common measure is to apply a friction performance stabilizer to the surface of the bolt.
In recent years, with the introduction and digestion of China's automotive products and related process technologies, people's understanding of threaded fastening has gradually deepened, and new requirements for thread fastener quality and tightening torque control are constantly put forward. In order to finally have a place in the world automobile industry, China's automobile complete vehicles and engines, axles and other host companies must improve the quality and reliability of their products, which will inevitably promote and deepen the assembly torque control. Friction performance requirements for threaded fasteners.
Under the torque control tightening condition, the axial force, tightening torque and torque coefficient of the bolt are related to each other. According to the formula (1) and the error analysis theory, the relative error between the axial force of the bolt and the relative error of the tightening torque and the relative error of the torque coefficient have the following relationship: Ef=The above formula indicates that the torque must be effectively implemented for the axial force control. Control and K value control are two-pronged. If the X value is strictly controlled, on the one hand, the requirements for torque control accuracy can be relaxed, and the investment in the assembly process can be saved; on the other hand, the axial force of the bolt can be improved and the bolt strength utilization and connection reliability can be improved. Therefore, value control has greater economic value and technological progress than torque control.
China's torque control tightening tools mainly rely on imports and are expensive. Dongfeng Motor Corporation equipment manufacturing plant tightening tool factory introduced German DGD company technology, formed a full range of tightening torque control tool production capacity, its 200Nm tightening tool control precision and price comparison see Table 3 (only).
Table 3 tightening tools cost-effective tightening tools single-axis calibration box actual control twist box hemp price f million) electric combination tightening machine (sensor control bubble) 18 (two-axis) electric combination screw machine i (current control type) 12 (double Axis) Pneumatic combination Twisting and tightening machine Hydraulic pulse wrench Man-made torsion wrench Impact wrench does not control the twist According to the working experience of Dongfeng Motor Co., the actual control torque of the tightening tool is 10% ~ 15% in the assembly of the whole vehicle and the main assembly. It is more economical and reasonable. Considering that there is a certain process control capability, in order to facilitate the following analysis and calculation, the actual control of the tightening tool (down to page 35) carbide has a very bad influence on the tool. It is recommended that the carbide size should be controlled below 5im.
The tap tends to form a stress concentration during the manufacturing process. The structure of the tap determines that the stress concentration may be preferentially formed at the position indicated by the middle arrow. There are large particles of carbides or non-metallic inclusions in the material, or their densely distributed state, which has intrinsic conditions for forming internal stress concentration during heat treatment. When these conditions appear near the position indicated by the middle arrow, Different degrees of stress concentration will occur, causing the tap to have a degree of brittleness such that it will crack during the grinding process or early breakage during use.
The tap breaks during the grinding process, mainly due to the presence of large particles of carbides or non-metallic inclusions in the tap material and the dense strip distribution of the carbide. The structure of the tap is conducive to the formation of high stress concentration and exacerbates the tendency of cracking. . In order to avoid the above phenomenon, according to the previous analysis, the quality of the tap material should be strictly controlled first. Secondly, the tap structure should be appropriately changed to reduce the stress concentration.
Low-E Insulated Glass Unit,Energy Saving Insulated Glass,Insulated Low E Glass,Single Pane Insulated Glass
Shanghai Lead Glass Co.,Ltd , https://www.leadglaze.com