Heat treatment is the core of high-strength bolts.

Mercedes-Benz is recalling seven S-Class vehicles due to improper torque on the mounting bolts.

Recently, Mercedes-Benz (China) Automobile Sales Co., Ltd., in accordance with the requirements of the Regulations on the Administration of Recalls of Defective Automotive Products and the Measures for the Implementation of the Regulations on the Administration of Recalls of Defective Automotive Products, filed a recall plan with the State Administration for Market Regulation. Effective immediately, the company is recalling certain imported S-Class vehicles manufactured between June 6, 2018, and August 21, 2018, totaling seven units. In some vehicles within the scope of this recall, the torque of the mounting bolts securing the trim panels on the A‑pillar, B‑pillar, and C‑pillar may not meet specifications. If the vehicle experiences a collision that triggers the side curtain airbags, improperly installed trim panels could detach from the vehicle body and enter the passenger compartment, increasing the risk of injury to occupants. Furthermore, such panels may prevent the side curtain airbags from providing the intended level of protection, posing a safety hazard. Mercedes-Benz (China) will inspect the installation condition of the A‑pillar, B‑pillar, and C‑pillar trim panels on all affected vehicles at no charge and, if necessary, perform any required repairs to ensure that the trim panels are properly installed and comply with the specified requirements, thereby eliminating the identified safety risks.

Why can Japan’s never-loosening nuts not be replicated even after their principles have been made public?

There’s a saying that strikes a nerve with many Chinese: “Even given the principles and blueprints, you still can’t make Japan’s ‘never‑loosening nut’!” However, I can confidently assure you that if the exact drawings were available, Chinese engineers could certainly produce such a nut. On the internet, most self‑proclaimed media outlets, in their quest for attention, resort to sensationalist headlines—“If it doesn’t shock, it’s not worth publishing”—to undermine public confidence. Yet this particular nut isn’t just beyond the reach of Chinese manufacturers; even the Japanese themselves, or Germans—who also boast deep manufacturing expertise—cannot replicate it. So what secret lies behind Hardlock nuts, making them so difficult to copy? In 1961, a Japanese company employee named Katsuhiko Wakabayashi attended a trade show and brought back several samples. Among them, a special anti‑rotation nut caught his eye. Made from stainless‑steel wire clips to prevent loosening, this nut featured a complex structure and came at a high price. Drawing on the quintessential Japanese spirit of craftsmanship, Wakabayashi pondered: “Couldn’t we make something simpler and cheaper?” Soon after, he designed an anti‑rotation nut, which he called the “U‑nut.” The U‑nut’s anti‑loosening mechanism relies on an elastic, high‑pressure contact between a friction plate integrated into the nut and the threads of the bolt, effectively preventing relative rotation and thus securing the joint. To commercialize his invention, Wakabayashi founded Fuji Precision Works, and the new nut quickly gained market traction. Confident in his design, he boldly advertised it as “the nut that never loosens.” However, some customers complained that the U‑nut began to loosen when used in excavators and pile drivers. Determined to uphold his claim of “never‑loosening,” Wakabayashi left his own company and devoted himself to refining the design. In 1973, while passing by the Sumiyoshi Taisha Shrine near his home in Osaka’s Sumiyoshi Ward, he gazed up at the shrine’s towering gate and suddenly had an epiphany: “If I drive in a wedge like that, it won’t come loose.” Inspired by this insight, he devised a novel structure integrating a hammer, a wedge, and the nut itself, based on the principle of wedging.

Common Terminology for Mechanical Properties of Fasteners

Torque is a force that produces rotation. Here are some common examples of torque: 1. Winding a watch; 2. Unscrewing a bottle cap; 3. Turning a door handle; 4. Screwing in a screw. Torque must be carefully considered in most applications. There are four types of torque, each with subtle differences: 1. Driving torque; 2. Seating torque; 3. Breakaway torque; 4. Prevailing torque. All these torques arise in practical use, but their relative importance varies depending on the specific application. 1. Driving torque is the rotational force required to assemble components together. 2. Seating torque is the force needed to achieve a predetermined level of tightness when assembling parts. 3. Breakaway torque is the rotational force necessary to separate assembled components. 4. Prevailing torque refers to a characteristic of fasteners that allows them to resist loosening by creating frictional resistance at the mating threads. The driving torque required to overcome this friction is called prevailing torque. Driving torque: In applications involving screw cutting, screw rolling, and self-locking fasteners, driving torque is a primary concern. As the maximum force required for rotating mechanical components, it must be carefully managed. Excessive driving torque can lead to rotational failure, increasing costs associated with fasteners. Therefore, minimizing driving torque is essential, which brings us to another engineering requirement known as the "drive-to‑strip ratio." This ratio compares the torque needed to drive a fastener into place with the torque required to prevent stripping or damage to the mating internal threads. A higher drive-to‑strip ratio helps reduce assembly errors, rework, and associated costs, making the fastener more suitable for its intended purpose. For example, in thread-rolling self-tapping screws, the recommended drive-to‑strip ratio is 1:3—meaning one unit of driving torque should correspond to three units of resistance against stripping. Seating torque: Seating torque represents the force applied to tighten a fastener to a specified degree of tightness or torque value. It is expressed as a maximum allowable value. This means that if the applied torque falls short of the specified maximum, the desired tightness or clamping force may not be achieved—a critical consideration in all cases. Fasteners must securely lock to ensure proper assembly without imposing excessive torque, which could lead to system failure, increased labor intensity, and assembly defects—all of which raise fastener costs. Breakaway torque: Breakaway torque refers to the rotational force required to loosen a fastener from its locked state. This is particularly relevant in situations where fasteners are prone to loosening. Breakaway torque is typically associated with self-locking fasteners (and other anti-loosening fasteners), and it is defined as a minimum threshold: a fastener cannot be removed unless the applied torque exceeds this breakaway value. Prevailing torque: Technically speaking, prevailing torque is a counteracting moment that resists rotation. It usually applies when attempting to unscrew a fastener that has been secured but not yet fully tightened. Like breakaway torque, prevailing torque is used in contexts where loosening is likely, and its prescribed value is also minimal. If the applied torque remains below the prevailing torque threshold, the fastener will remain locked (though not necessarily fully tightened) and unable to rotate. Each fastener requires a certain amount of torque to initiate movement—this is the "driving torque" of prevailing torque. Additionally, every fastener requiring prevailing torque must have sufficient torque to secure it properly, ensuring an appropriate locking state—this is the "seating torque." Finally, each fastener needs a specific torque to release it from its locked condition—this is the "breakaway torque." Torque is measured in pound-inches (in-lbs) or pound-feet (ft-lbs). One pound-inch equals the force of one pound acting perpendicularly at a distance of one inch from the axis of rotation. Two pound-inches equal twice the force acting at one inch from the axis, or one pound acting at two inches. From this, we derive the formula: Torque = perpendicular distance from the axis × applied force. Converting pound-inches to pound-feet involves dividing by 12; conversely, multiplying pound-inches by 12 yields pound-feet. The choice of unit depends on the magnitude of the value—for instance, we typically use 100 pound-feet rather than 1,200 pound-inches. Note: Both pound-inches and pound-feet are technically correct units, though they are often mistakenly written as "inch-pounds" or "foot-pounds." Torque is the force necessary to produce rotation—it acts at the end of an arm to cause an object to spin. Answer the following questions (without referring back to earlier material): 1. What is torque? 2. What are the four distinct types of torque? 3. What are the units of torque? Check your answers against the preceding text, and write down any missing information. Tensile Strength: Tensile strength refers to a material's ability to resist breaking under tensile stress. Have you ever stretched a rubber band until it snapped? If so, you were testing its tensile strength. Similarly, the tensile strength of a fastener indicates the maximum tensile load it can withstand without fracturing. Tensile strength is one of the most fundamental physical properties of fasteners. It represents the ultimate strength of the component and serves as a key indicator of its load-bearing capacity during use. Tensile strength is expressed in pounds per square inch (PSI). It reflects the average force distributed across the smallest cross-sectional area of the fastener. For example, if a fastener has a breaking tensile load of 100,000 pounds and its smallest cross-section measures one square inch, then its tensile strength is 100,000 PSI. In other words: Tensile Strength = Force / Area = 100,000 lbs / 1 sq in = 100,000 PSI. Tensile strength measures a fastener's ability to resist axial tension. It indicates how well the fastener can endure axial tensile loads. Typically, tensile strength refers to the ultimate tensile strength (UTS), since the yield strength and proof load of a fastener are closely related to its tensile capacity. We will discuss these concepts further later. Answer the following questions (without referring back to earlier material): 1. What is tensile strength? 2. What are the units of tensile strength? Verify your answers against the preceding text, and note any omissions. Yield Strength: In theory, every axial tensile load causes a fastener to elongate to varying degrees. Since metals possess a specific modulus of elasticity (also known as Young’s modulus), removing the load generally allows the fastener to return to its original length. When a fastener fails to regain its original length despite the removal of the load, that point marks its yield point. The yield point is where the fastener begins to undergo plastic deformation under axial tension. In the fastener industry, we strive to keep operations within the elastic limit to ensure safe connections, avoiding stretching beyond the yield point. Doing so reduces the effectiveness of the fastener: once a fastener reaches its yield point, it loses its ability to contract back to its original length. Such contraction provides the effective locking force during assembly. We can clearly observe how a fastener locks and functions. Imagine a fastener as a tightly coiled spring. Picture a door held closed by a spring: while the spring remains below its yield point, it effectively keeps the door shut. However, if the spring is overstretched and cannot return to its original length, it becomes ineffective and fails to hold the door closed. Similarly, when a fastener is stretched past its yield point, it loses its original tensile strength. Generally, the yield strength is approximately 25% of the ultimate tensile strength. The yield point of a fastener is the moment when axial loading results in permanent elongation. Answer the following questions (without referring back to earlier material): 1. What is the yield strength of a fastener? 2. How are a fastener's tensile strength and yield strength related?Review the preceding materials to check your answers; if anything is missing, write down the correct answer. Translator’s note: The relationship between yield strength and tensile strength is not a fixed 25%. Generally speaking, for the same material, the higher the tensile strength—whether due to work hardening or heat treatment—the greater the ratio of yield strength to tensile strength, while ductility decreases. For example, Grade 4.6 bolts, after cold forging, cannot meet the required 30% elongation, so they must undergo annealing. During this process, both yield strength and tensile strength decrease, with the ratio typically around 45–50%. After cold forging, Grade 5.6 bolts, left untreated, exhibit a yield-to-tensile strength ratio of approximately 35–40%. As for Grade 8.8 bolts, after cold forging, they undergo quenching and tempering, resulting in a yield-to-tensile strength ratio of roughly 20–25%. Similarly, Grades 10.9 and 12.9 bolts, following cold forging, are subjected to quenching and tempering, yielding a yield-to-tensile strength ratio of about 10–20%. Proof Load Proof load refers to the maximum axial tensile force that a fastener can withstand without undergoing permanent elongation. Taking a spring as an example, imagine stretching the spring to its maximum length without causing permanent deformation—this means that once the load is removed, the fastener returns to its original length. This clearly demonstrates the close relationship between a fastener’s yield point and its proof load. In theory, if there are two adjacent points within a range—one slightly smaller than the other—the smaller value represents the proof load, while the larger one corresponds to the yield point. Because these two values are very close, in practical applications they are often treated as equivalent. For carbon steel fasteners, the proof load equals 75% of their maximum tensile strength. For instance, if a carbon steel fastener has a tensile strength of 100,000 PSI, its proof load—which coincides with its yield point—would be 75,000 PSI. Knowledge of proof load and its significance is crucial for sales personnel, as they may sometimes be asked to specify the allowable tensile forces and loads that a fastener can sustain under actual service conditions. Remember the general principle: applying 75% of the proof load yields optimal performance. This is a standard guideline regarding tensile forces when using fasteners. For example, consider the previously mentioned fastener with a tensile strength of 100,000 PSI. Since the proof load equals 75% of the tensile strength, it amounts to 75,000 PSI. If a customer asks, “What tensile force can this bolt withstand?” you should respond, “75% of the proof load, which is (25,000 × 75%), or 56,250 PSI.” The proof load is the maximum force a fastener can endure without undergoing plastic deformation. Keep these three key principles in mind: 1. For carbon steel, the proof load equals 75% of the tensile strength. 2. Tightening a fastener to 75% of its proof load maximizes its effectiveness. 3. Typically, fasteners should be tightened to 50–60% of their tensile strength to ensure proper performance. Translator’s note: According to regulations, the proof load is a fixed value determined by nominal size and product grade. When a fastener is subjected to a load up to this specified limit, no deformation that could compromise assembly integrity should occur. Likewise, the relationship between proof load and tensile strength is not a rigid 25%; this follows the same reasoning as noted in the previous paragraph. Answer the questions (without referring back to earlier content): 1. What is proof load? 2. How are proof load and tensile strength related? 3. To what percentage of the proof load should fasteners generally be tightened? 4. For a fastener with a tensile strength of 100,000 PSI, to what tensile force would you tighten it (provide the calculation formula)? Review the preceding materials to verify your answers; if anything is missing, write down the correct answer. Torque–Tension Relationship The torque–tension relationship describes how applying torque to a fastener results in elongation and resistance. This relationship is highly significant in practical applications. As mentioned earlier, sales representatives often advise customers to tighten fasteners to 75% of the proof load. Customers then frequently ask, “How much torque is needed to achieve this tensile force?” Before answering, it’s essential to understand why such questions are reasonable. When using fasteners, our primary concern is applying appropriate tensile force. So why do customers also inquire about torque? Because during installation, torque is used to secure the fastener, making torque measurement more convenient than directly measuring tensile force. Now you can see why this question is meaningful. You may begin formulating your response. First, note that torque and resistance have distinct relationships. Several factors influence their interplay: 1. Surface condition of the fastener (plain finish or plated) 2. Condition of the threaded mating surface 3. Condition of the bearing surface 4. Thread class 5. Type of thread 6. Strength of the fastener itself 7. Strength of the mating material 8. Lubrication condition All these variables affect the actual torque–resistance relationship during use. Sales personnel must avoid speculating on torque–resistance relationships for customers. Youhui has acquired specialized equipment designed specifically to determine torque–resistance dynamics (e.g., torque–extension meters, torque–friction testers, and torque–clamping-load testers). Clearly, addressing such complex issues is challenging for sales staff. Their responsibility lies in understanding the real-world operating conditions of fasteners and providing feedback so we can best resolve customer concerns. If encountering torque-related queries, you should gather the following information: 1. How the fastener will be used. 2. What materials were used to manufacture the fastener. 3. Which type of fastener you plan to employ. 4. The required tensile (clamping) force. 5. The type of surface coating applied. 6. The lubricant you intend to use. 7. Any other operational conditions you deem important. Whenever possible, obtain samples of the assembled fasteners being used. A general understanding of torque–tensile relationships is vital. Torque–tensile refers to the tensile force generated when torque is applied to a fastener. Another closely related concept is clamping force, which will be discussed later. Answer the questions (without referring back to earlier content): 1. What is the torque–tensile relationship? 2. What kind of torque–tensile relationship should sales personnel communicate to customers based on their specific usage conditions? Review the preceding materials to check…

Warm congratulations on the launch of the website for Jingjiang Yongyou Standard Parts Co., Ltd.!

Warm congratulations on the launch of the website for Jingjiang Yongyou Standard Parts Co., Ltd.!