Common Terminology for Mechanical Properties of Fasteners

　　Torque

　　Torque is a force that causes rotation. Below are some of the most common examples of torque:

　　1. Wind the watch 2. Unscrew the bottle cap 3. Turn the doorknob 4. Screw in the screw.

　　Torque needs to be given careful consideration in most applications.

　　The following four types of torque differ slightly from one another.

　　1. Driving Torque

　　2. Seating Torque

　　3. Break-away Torque

　　4. Prevailing Torque

　　All of these torques arise in practical applications, but their relative significance varies depending on the specific operating conditions.

　　1. Driving torque — the rotational force that must be applied to bring components together.

　　2. Tightening torque — the force required to achieve a predetermined level of tightness in an assembled mechanical component.

　　3. Loosening torque — the rotational force required to separate assembled components.

　　4. Preload Torque — A characteristic of fasteners that enables them, when engaged with a mating threaded component, to resist loosening by generating frictional resistance. The driving torque required to overcome this frictional torque is referred to as the preload torque.

　　Driving Torque: In applications involving screw cutting, thread rolling, and self-locking fasteners, driving torque is a primary consideration. As the maximum force required by rotating components, it is an essential parameter. Excessive driving torque can lead to rotational failure or stripping, which in turn increases fastener costs; therefore, minimizing driving torque is crucial. This requirement gives rise to another engineering metric: the “drive‑to‑pullout ratio.” This ratio represents the relationship between the torque needed to drive the fastener and the torque required to resist pullout or failure of the mating internal thread. A wider range for this ratio helps reduce assembly defects, minimize reassembly, and lower associated costs, making the fastener more suitable for its intended application.

　　For thread‑rolling self‑tapping screws, the required drive‑to‑strip ratio is 1:3, meaning that for every unit of driving torque, the thread strength must be capable of withstanding three units of pull‑out resistance.

　　Tightening Torque: Tightening torque is the force required to turn a fastener to the specified degree of tightness or to achieve a given torque‑tension value. It is expressed as a maximum value. This means that, when the applied torque remains below the specified maximum, the desired level of tightness or clamping force can be attained—this is critically important in all cases. Fasteners must be tightened adequately to ensure proper assembly, yet excessive torque should be avoided. Over‑torquing can lead to drive‑system failure, increased labor intensity, and joint‑fit failure, all of which ultimately raise fastener‑related costs.

　　Loosening Torque: Loosening torque refers to the rotational force required to release a fastener from its tightened state. This is particularly relevant in situations where fasteners are prone to loosening. Loosening torque is typically associated with self-locking fasteners—though it applies not only to such fasteners but also to anti-loosening fasteners—and is specified as a minimum value, meaning the fastener cannot be disassembled from the assembly with a torque lower than the loosening torque.

　　Preset Torque: Technically speaking, preset torque is a resistance to rotational motion. It typically refers to the torque required to unscrew a fastener that is securely seated but not yet fully tightened. Like loosening torque, preset torque is used in applications where loosening is likely; its specified value is also a minimum. Under a torque below the prescribed level, the fastener must remain securely seated—though not necessarily fully tightened—and should not rotate. Each fastener requires a certain amount of torque to initiate driving; this is the “driving torque” of the preset torque. Furthermore, each preset‑torque fastener demands a higher torque to achieve full tightening, ensuring proper secure seating; this is the “tightening torque.” Additionally, every fastener needs a specific torque to release it from its locked condition; this is the “loosening torque.” Finally, each fastener requires a certain torque to remove it from a state that is neither fully tightened nor completely loose; this is the preset torque itself. The unit of torque measurement is pound‑inches (in‑lb) or pound‑feet (ft‑lb). One pound‑inch is the force of one pound acting at a perpendicular distance of one inch from the axis of rotation. Two pound‑inches correspond to either two pounds acting at a perpendicular distance of one inch from the axis, or one pound acting at a distance of two inches. From this, we can conclude…

　　Torque = Perpendicular distance from the axis to the force × Force

　　Dividing pound-inches by 12 yields pound-feet; conversely, multiplying pound-inches by 12 gives pound-feet. The choice of unit depends on the magnitude of the value. For example, we typically use 100 pound-feet rather than 1,200 pound-inches.

　　Note: Both pound-inches and pound-feet are technically correct units. However, we often see them written as inch-pounds and foot-pounds. Torque is the force required to produce rotation; it is the force applied at the end of a lever arm that causes an object to rotate.

　　Answer the question (do not look back at the previous content):

　　1. What is torque?

　　2. What are the four different types of torque?

　　3. What is the unit of torque?

　　Review the preceding materials to check your answers, and if anything is missing, write down the correct answer.

　　Tensile Strength

　　Tensile strength refers to a material’s ability to resist fracture under tensile loading. Have you ever stretched a rubber strip until it broke? If so, you were testing the rubber strip’s tensile strength. The same principle applies to fasteners: tensile strength is the maximum tensile force that a fastener can withstand without fracturing.

　　Tensile strength is one of the most common physical properties of fasteners. It represents the fastener’s ultimate strength and serves as the fundamental metric for assessing its load-bearing capacity in service.

　　Tensile strength is expressed in pounds per square inch (PSI). It refers to the force that can be applied uniformly across the fastener’s minimum‑diameter cross‑section. For example, if a fastener has a breaking tensile load of 100,000 pounds and its minimum‑diameter cross‑sectional area is 1 square inch, then the fastener’s tensile strength is 100,000 PSI. In other words:

　　Tensile strength = Force/Area = 100,000 pounds / 1 square inch = 100,000 PSI

　　Tensile strength is the ability of a fastener to resist axial tensile forces. It indicates the fastener’s capacity to withstand axial tensile loads. Tensile strength typically refers to the ultimate tensile strength (UTS). Since a fastener’s yield strength and proof load are related to its tensile strength, we will discuss these aspects in greater detail later.

　　Answer the question (do not look back at the previous content):

　　1. What is tensile strength?

　　2. What is the unit of tensile strength?

　　Review the preceding materials to check your answers, and if anything is missing, write down the correct answer.

　　Yield Strength

　　In theory, every axial tensile force causes the fastener to elongate to varying degrees. Since all metals possess a specific modulus of elasticity—also known as Young’s modulus—removing the load typically allows the fastener to return to its original length. When the fastener can no longer recover its original length, the corresponding load value is its yield point. The yield point marks the threshold at which the fastener begins to undergo plastic deformation under axial loading.

　　In the fastener industry, we strive to operate fasteners within their elastic limit to ensure joint integrity, rather than pushing them beyond the yield point. Doing so would compromise the fastener’s effectiveness. Once a fastener is stretched past its yield point, it can no longer return to its original length. This loss of recovery provides the effective clamping force that secures the joint. We can readily visualize how a fastener locks and functions: imagine a fastener as a tightly coiled spring. Consider a door held closed by such a spring; as long as the spring remains below its yield point, it can reliably keep the door shut. However, if the spring is overstretched and cannot regain its original length, it will fail to hold the door closed. Similarly, when a spring is overstrained, it reaches its yield point, at which point it loses its ability to exert clamping force, and the spring’s original tension is gone. The same principle applies to fasteners: once they are overstretched, they lose their intended clamping force.

　　In general, the yield strength is equal to 25% of the ultimate tensile strength. For fasteners, the yield point refers to the stress at which the material undergoes permanent elongation under axial loading.

　　Answer the question (do not refer back to previous content):

　　1. What is the yield strength of a fastener?

　　2. What is the relationship between the tensile strength and the yield strength of fasteners?

　　Review the preceding materials to check your answers, and 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, for a given 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 and must therefore be annealed; at that point, both yield and tensile strengths are reduced, with their ratio falling to approximately 45–50%. In contrast, Grade 5.6 bolts subjected to cold forging without further treatment exhibit a yield‑to‑tensile strength ratio of about 35–40%. As for Grade 8.8 bolts, cold forging followed by quenching and tempering yields a yield‑to‑tensile strength ratio of roughly 20–25%; while Grades 10.9 and 12.9 bolts, also cold‑forged and then quenched and tempered, show a yield‑to‑tensile strength ratio of approximately 10–20%.

　　Proof Load

　　The proof load is the maximum axial tensile force that a fastener can withstand without undergoing permanent elongation. Taking a spring as an example, if we consider the fastener to be a spring, we can imagine stretching it to its maximum length without causing permanent deformation—meaning that, once the load is removed, the fastener will return to its original length.

　　This demonstrates that the yield point and the proof load of a fastener are closely related. In theory, if there are two adjacent points within a certain range—one slightly lower than the other—then the lower value represents the proof load, while the higher value corresponds to the yield point. Because these two points are so close together, in practical applications they are often treated as equivalent. For carbon‑steel fasteners, the proof load is 75% of their ultimate tensile strength. For example, if a carbon‑steel fastener has an ultimate tensile strength of 100,000 PSI, its proof load—i.e., its yield point—would be 75,000 PSI.

　　Understanding and the significance of proof load are crucial for field personnel, as they are often required to specify the tensile strength and allowable load that a fastener can safely withstand in actual service. A general rule to keep in mind is: applying 75% of the proof load yields optimal performance. This principle applies universally to fasteners when assessing their tensile capacity. For example, consider the fastener we just discussed, which has a tensile strength of 100,000 PSI. Since the proof load equals 75% of the tensile strength, its proof load is 75,000 PSI. If a customer asks, “What tensile load can this bolt safely handle?” you should respond: “75% of the proof load, or (25,000 × 75%), which amounts to 56,250 PSI.” The proof load represents the maximum force a fastener can sustain without undergoing plastic deformation. Remember these three key principles:

　　1. The guaranteed working load of carbon steel is 75% of its tensile strength.

　　2. Tightening the fastener to 75% of its proof load will ensure it performs at its maximum efficiency.

　　3. Generally, fasteners should be tightened to 50%–60% of their tensile strength to ensure optimal performance.

　　Translator’s note: The proof load is a fixed value specified according to the nominal dimensions and product grade; when subjected to this prescribed load, the fastener must not undergo any deformation that could compromise assembly. Similarly, the relationship between proof load and tensile strength is not a rigid 25%; see the annotation in the preceding paragraph for further clarification.

　　Answer the question (do not refer back to previous content):

　　1. What is the guaranteed load?

　　2. What is the relationship between guaranteed load and tensile strength?

　　3. To what percentage of the rated load should fasteners typically be tightened?

　　4. For a fastener with a strength of 100,000 PSI, to what tensile force would you tighten it (please provide the calculation formula)?

　　Review the preceding materials to check your answers, and if anything is missing, write down the correct answer.

　　Torque-Tension

　　Torque–Elongation Relationship: The torque–elongation relationship refers to the fact that applying torque to a fastener results in both elongation and an opposing resistance. This relationship is critically important in practical applications. As mentioned earlier, field engineers often advise customers to tighten fasteners to 75% of the specified proof load. Customers then typically ask, “What torque is required to achieve this tensile force?” Before answering, it’s essential to understand why this question is reasonable for many customers. When using fasteners, our primary concern is applying the appropriate tensile force. Given this, why do customers still inquire about torque? Because fasteners are tightened by applying torque during installation; therefore, measuring torque is far more convenient than directly determining the actual tensile force.

　　Now you understand why this is such a meaningful question, don’t you? You can start thinking about the answer. First of all, torque and resistance have a distinct relationship. Below are some factors that influence this relationship:

　　1. Surface condition of the fastener (natural finish or electroplated)

　　2. Surface Condition of Threaded Joints

　　3. Surface Condition

　　4. Thread Grade

　　5. Thread Type

　　6. Strength of Fasteners

　　7. Strength of the accompanying materials

　　8. Lubrication Condition

　　All of the aforementioned differences will affect the relationship between torque and resistance during actual operation.

　　Sales personnel should be careful not to speculate about the relationship between torque and clamping force on behalf of customers. Youhui has already acquired specialized equipment for determining this relationship (Note: torque‑tension gauges, torque‑friction testing, and torque‑clamping‑load testing). Clearly, such a complex issue is difficult for sales staff to resolve on their own. The role of sales personnel is to understand the actual operating conditions of fasteners and provide feedback so that we can best address our customers’ needs. If you encounter questions related to torque, you should be aware of:

　　1. How to use fasteners.

　　2. What material are fasteners made of?

　　3. What type of fastener will you use?

　　4. What tensile (clamping) force do you need?

　　5. What type of surface coating is used?

　　6. What type of lubricant do you plan to use?

　　7. Other usage conditions you consider important.

　　If possible, obtain samples of the fasteners used in the assembly. A general understanding of torque‑tension is essential. Torque‑tension refers to the tensile force that develops when torque is applied to a fastener. Another related concept is clamping force, which will be discussed later.

　　Answer the question (do not refer back to previous content):

　　1. What is the relationship between torque and tensile force?

　　2. Sales personnel should explain to customers the relationship between torque and tensile force under their specific operating conditions.

　　Review the preceding materials to check your answers, and if anything is missing, write down the correct answer.

　　Clamp Force

　　Clamping force is the force exerted by a fastener on the clamped surface (i.e., the bearing surface) when securing an object.

　　In practice, the magnitude of the force acting on the clamped surface is exactly equal to the tensile force experienced by the fastener; in fact, the relationship between torque and clamping force in the assembly is identical to the ratio of torque to stress in the fastener.

　　Torque is critical in most applications because adequate tensile force in fasteners ensures proper assembly. Keep in mind: developing torque–stress relationship charts is not the sales team’s responsibility. However, this does not mean sales personnel can sidestep these issues; rather, they should gather the necessary information and pass it on to the relevant departments so that the best possible solutions can be provided.

　　Shear Strength

　　Shear strength is the ability of a fastener to resist deformation when subjected to a load perpendicular to its axis.

　　You may have broken a golf club head yourself while swinging, or at least seen someone else do so. You might have noticed that when the club strikes the ball and the ball flies off, the club head often hits the ground as well—people often say the head has been “sheared off” by the club. In fact, you’ve unwittingly performed a shear‑strength test, allowing you to see whether the club head can withstand forces applied perpendicular to its axis. For this reason, golf club heads are manufactured with a shear strength low enough not to compromise performance during impact.

　　When fasteners are subjected to loads perpendicular to their axis, their shear strength must be taken into account. Unlike a golf club head, fasteners must withstand such loads to perform their intended function.

　　Shear strength is typically estimated to be two-thirds of the tensile strength; thus, if a fastener has a tensile strength of 180,000 psi, its shear strength would be 120,000 psi. In other words, the fastener should be able to withstand a load of 120,000 psi applied perpendicular to its axis.

　　Shear forces typically occur when a fastener is driven perpendicularly into two overlapping, parallel materials, with the two materials subjected to forces in opposite directions.

　　The fastener will not fail only when its shear strength exceeds this force.

　　Fatigue Strength

　　Fatigue strength is the ability of a fastener to resist fatigue failure under cyclic variations in stress. In other words, it is the fastener’s capacity to withstand fluctuating loads arising from various causes.

　　In general, the fatigue strength is significantly lower than its ultimate tensile strength.

　　There are several types of fatigue, but vibration is the most common.

　　A typical example of “vibration” fatigue is the head bolt used in diesel engines: when the piston moves downward within the cylinder, the head bolt experiences no load; however, as the piston rises again, the pressure inside the cylinder gradually increases until it drops back to zero upon the next downward stroke. Throughout this cycle, the variation and periodicity of the applied load are closely linked to the engine speed.

　　To mitigate and ensure against fatigue failure under vibrational cyclic stresses, fasteners shall be capable of withstanding stresses exceeding the maximum stress encountered in the expected service cycle.

　　Fatigue failure is a factor that must be considered for fasteners subjected to shear or tension.

　　Ductility

　　Ductility is the ability of a material to undergo permanent deformation without fracturing.

　　Ductility is mentioned in various contexts throughout the fastener industry, yet the underlying concept remains consistent. First and foremost, ductility refers to the material used in fastener forming: the greater a material’s ductility, the better it can deform without cracking. To illustrate ductility, we’ll use clay as an example, even though it is not a fastener material.

　　You can mold a sphere of clay into a flat shape, and its surface will never crack or fracture—this is ductility. By contrast, if you perform the same test with stone, even the slightest deformation will cause cracking and brittle fracture. These two extreme examples illustrate that materials can exhibit vastly different properties. Some materials, such as aluminum, are highly ductile, capable of undergoing substantial deformation without cracking, while others, like hard steel, will crack with only minor deformation. Material ductility is a critical factor in determining the forming processes used to manufacture products.

　　The ductility of the material used to form threads must also be taken into account, since thread formation involves deforming the material around the fastener rather than removing excess material by cutting. Furthermore, if one were to roll threads into clay and stone, the process would proceed as follows: first, drill a hole of the same size in each material, then insert a screw and turn it in. In the case of clay, the surrounding material will shift or deform without any actual cutting; however, when the same screw is driven into stone, the material will crack. This demonstrates the material’s brittleness. Of course, no one would actually drive a screw into clay or stone, but the materials used to manufacture screws should possess comparable ductility—meaning they can deform without cracking or fracturing.

　　The ductility requirements for fasteners ultimately depend on their intended application. In some cases, fasteners are subjected to impact or heavy hammering. Generally, such fasteners should deform before fracturing, which is crucial. If a fastener is brittle, it will crack under impact; if it exhibits good ductility, it will merely deform without breaking.

　　Consider clay and stone once more. If you have two cylinders of similar size and shape—one made of clay and the other of stone—and apply the same force perpendicular to their axes, the clay will bend, while the stone will crack. Fastener materials exhibit a similar behavior: their degrees of ductility vary.

　　Only by defining all the requirements for fastener applications can Youhui and its customers jointly develop the most suitable fasteners for each specific application.

　　Ductility is the ability of a material to undergo permanent deformation without fracturing.

　　Fasteners should take into account the three key factors of ductility:

　　1. The ductility of the fastener material itself.

　　2. Material ductility at the forming location.

　　3. Ductility requirements for fasteners in special applications.

　　Hardness

　　Hardness is a measure of a material’s resistance to abrasion, indentation, and bending.

　　The most important significance of fastener hardness is its ability to resist friction during assembly and/or in mechanical applications. For thread‑forming and thread‑cutting self‑tapping screws, since they rely on the extrusion or tapping of mating internal threads from the receiving hole, it is evident that their hardness must exceed that of the mating component. Otherwise, the self‑tapping threads will deform or be damaged during assembly, preventing proper screwing. Two materials with markedly different hardnesses can be readily distinguished—for example, clay and steel. If you attempt to drive a steel thread‑forming self‑tapping screw into clay, the soft, easily deformable clay will yield effortlessly. Conversely, if you try to drive a clay‑made thread‑forming self‑tapping screw into steel, it will not advance at all. Therefore, thread‑forming self‑tapping screws must be harder than the workpiece they engage with.

　　In certain applications, fasteners also rely on bearing surfaces. This is especially true for screws with shoulders, where hardness is critical only at a few key contact points yet plays a vital role. These bearing surfaces must be sufficiently hard to withstand the rapid, cumulative excess friction that builds up during assembly, thereby ensuring proper post‑assembly performance and reducing maintenance requirements. To illustrate, consider clay versus steel: since steel is harder than clay, steel fasteners perform better and last longer in mobile applications.

　　Another important point is that a material’s hardness is directly related to its tensile strength, shear strength, and ductility. As hardness increases:

　　1. Tensile strength increases.

　　2. Shear strength increases.

　　3. Reduced ductility.

　　Sales personnel should understand how to determine the appropriate hardness values for each application, taking into account the required strength level and ductility. Carefully inquire about these details and forward them to the technical department so that Youhui can recommend the most suitable fasteners for the customer.

　　There are two basic methods for hardening fasteners:

　　1. Cold working or work hardening.

　　2. Heat treatment.

　　When a fastener is deformed at room temperature, this process is referred to as cold working. Cold working applies stress to the deforming material, causing it to undergo plastic deformation and leaving residual stresses that harden the material. This phenomenon occurs in both cold heading and extrusion.

　　The hardness of a material also increases after undergoing several different heat treatments. These methods will be discussed in detail in the chapter on heat treatment.

　　The hardening of fasteners can be classified into three distinct methods, depending on their application:

　　1. Through Hardened.

　　2. Case Hardening.

　　3. Selectively Hardened.

　　These three types of fasteners are named according to the degree of hardening achieved throughout the material. Fully hardened fasteners have their entire cross-section subjected to hardening, with both the core and the surface hardened. Surface-hardened fasteners undergo hardening only in a very thin layer near the surface; as a result, the surface is significantly harder than the core. Selective hardening, on the other hand, involves hardening only specific areas of the fastener—typically the shank—and these selectively hardened regions are considerably harder than the unhardened portions.

　　After undergoing full hardening treatment, fasteners can achieve a higher strength grade; for example, bolts subjected to full hardening can withstand greater tensile forces without fracturing.

　　After surface hardening, fasteners primarily enhance their wear resistance during assembly. Most thread-forming and thread-cutting self-tapping screws undergo surface hardening because they must cut mating internal threads in the workpiece. If these screws were fully hardened to the same level as the surface-hardened layer, they would fracture, making such treatment impractical in most applications.

　　When full hardening or surface hardening of a fastener poses a safety risk, selective hardening can be employed. This process is particularly suited to thread-forming self-tapping screws. In certain applications, these screws are required to retain the strength and ductility associated with full hardening while also achieving sufficient hardness at the shank and thread roots to ensure proper engagement with the tapped hole. Selective hardening addresses this need by first hardening the entire fastener to the desired strength level, followed by surface hardening of the shank and the last 4–5 threads at the tip to preserve self‑tapping performance.

　　Selective hardening is far more costly than full hardening or surface hardening. However, when such treatment is indispensable for the intended application, it remains highly valuable.

　　As for the methods of performing heat treatment, refer to the heat treatment chapter of this manual.

　　Hardness is a measure of a material’s resistance to abrasion, indentation, and bending.

　　Hardness affects friction during fastener assembly.

　　Hardness testing should be performed on a machine that can apply a specified load to the material and measure the resulting indentation depth. The most commonly used instruments are the Rockwell hardness tester and the Brinell hardness tester (or Vickers hardness tester), each offering multiple scales for measuring hardness under various conditions.

　　Hardness is related to strength and ductility, and can be enhanced through work hardening or heat treatment.