A self-tapping screw can pass incoming inspection and still fail on the line — not because the part is defective, but because the standard's breaking torque is a floor for the screw, while a driving torque is a setting for the process. This guide separates the three torques that govern every self-tapping joint, explains why the bolt formula does not transfer, and shows how to set a drive torque with a margin you can defend.

This article analyzes the issues of bolt loosening and fracture. It first points out that in the vast majority of cases, bolt fracture is not caused by insufficient tensile strength or fatigue strength, but by loosening—"if they don't loosen, they won't break; once they loosen, they break." Taking an M20×80 grade 8.8 high-strength bolt as an example, its minimum tensile load is as high as one hundred thousand times its own weight, while in actual use it only exerts one thousandth of its load-bearing capacity, so its strength margin is more than sufficient. Threaded connections are designed according to the self-locking condition and will not loosen by themselves under static load. However, under impact, vibration, variable load, and temperature changes, the friction force of the screw pair decreases or momentarily disappears, and if this occurs repeatedly, the bolt will gradually loosen. The enormous kinetic energy generated after loosening acts directly on the fastener and the equipment, causing consequences such as thread damage, the bolt being pulled apart or sheared off, and damage to the bolt hole. Finally, the article points out that selecting a threaded anti-loosening method with excellent anti-loosening effect is the fundamental solution to the problem, while simply increasing the bolt diameter and strength grade is not a professional approach, with large investment and small benefit. Common anti-loosening methods are divided into three categories: friction anti-loosening, mechanical anti-loosening, and permanent anti-loosening, among which the first two are removable anti-loosening, while the latter is non-removable anti-loosening.

This article explains that after a bolt is tightened, torque decay occurs as preload decreases over time, which in turn leads to loosening, abnormal noise, and clearance shift. This phenomenon is independent of material and strength grade; both 304 stainless steel and carbon steel bolts can experience it. The core causes include metal creep, contact surface indentation and plastic settlement, vibration and alternating loads, and temperature and environmental stress changes. Torque decay is not simply a product quality problem, and even qualified GB- and DIN-standard bolts cannot completely avoid it. On site, decay can be slowed by using spring washers together with flat washers, precisely controlling tightening torque, adopting anti-loosening solutions such as lock nuts or threadlocker, and rechecking and retightening 24 hours after a new machine starts running, thereby ensuring long-term stable operation of equipment.

This article compiles 60 basics of mechanical design, systematically covering machine composition, connecting parts, shafts and couplings, bearings, springs, kinematic pairs and mechanisms, gear transmission, part failure and design criteria, commonly used materials and heat treatment, and other commonly used mechanisms. It focuses on the classification, failure modes, design criteria, advantages and disadvantages, and selection principles of various components, aiming to help readers review the old to learn the new, consolidate their fundamental knowledge of mechanical design, and build an overall knowledge framework.

This article introduces ten fundamental mechanisms that must be mastered in the design of non‑standard equipment, covering both linear‑motion and rotary‑motion categories. The linear‑motion mechanisms include ball screws (high precision and stability), rack‑and‑pinion (heavy load, unlimited travel), cylinders (two‑point positioning, various types), timing belts (low cost, moderate precision), and chain lifts (heavy load, low cost). The rotary‑motion mechanisms include direct motor drive (requires a gearbox and coupling), rotary actuators (light load, significant vibration), cylinder with connecting rod (cost‑saving, low precision), motor with gears (speed reduction and torque increase, large footprint), and cam indexers (high‑precision intermittent indexing, widely used in turntables). The article summarizes the key characteristics, typical applications, and advantages and disadvantages of each mechanism, serving as a reference for mechanism selection in mechanical design.

When a torque wrench fails to emit a "click" at the set torque, it does not mean the fastener is properly tightened—it signals a loss of accuracy and a safety hazard. Based on real workshop experience, this article highlights four commonly overlooked operational mistakes: setting the torque below the wrench's minimum range, failing to lock the adjustment ring, applying force too abruptly, and using the wrench for loosening fasteners. If these are ruled out and the wrench still doesn't click, the issue is likely internal—spring fatigue, contamination by oil or metal chips, or worn parts. For budget wrenches, repair is not cost‑effective; replacement is recommended. For high‑end imported units, disassembly, cleaning, spring replacement, and recalibration can restore factory accuracy. The article concludes with a maintenance mantra: reset to zero after use, never loosen with it, apply torque smoothly, and avoid dropping or knocking. Proper care extends tool life for years, while abuse can scrap it in three months.