
Determining the correct screw size torque specifications is essential for achieving optimal performance and safety in various applications. While torque charts and guidelines offer starting points and estimates, it's important to recognize that several variables, such as human error, surface texture, lubrication, and joint conditions, can impact the torque-tension relationship. Therefore, experimentation and evaluation of each specific application are necessary to establish the most suitable torque values for screw sizes.
What You'll Learn
Suggested tightening torques
When tightening screws and bolts, it is important to ensure proper assembly and maintain the integrity of the joint. Torque is a measure of the rotational force applied to an object and is often expressed in units like Newton-meters (Nm) or foot-pounds (ft-lb).
The recommended tightening torques provided below are a great starting point for determining the required torque. However, it is important to note that adjustments may be necessary depending on the specific connection and the amount of lubrication used on the fasteners.
For coarse-threaded metric steel bolts with lightly oiled threads and tension at 85% of proof load, the maximum recommended tightening torques are as follows:
- M6: 14 Nm
- M8: 34 Nm
- M10: 61 Nm
- M12: 100 Nm
- M14: 160 Nm
- M16: 226 Nm
- M20: 423 Nm
- M24: 705 Nm
These values are only offered as a guide, and the correct torque value can only be determined through experimentation under actual joint and assembly conditions due to the many variables that affect the torque-tension relationship.
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Torque-tension relationship variables
The torque-tension relationship is influenced by several variables, and it is challenging to determine the precise range of conditions a fastener will experience. Torque is the twisting force required to spin a nut along a bolt's threads, while tension is the elongation of a bolt that provides the clamping force of a joint. This relationship is linear, but factors like surface texture, lubrication, rust, oil, debris, thread series, and material type can impact it. Human error is also a significant variable.
The torque coefficient (K) is a critical dimensionless factor that accounts for these variables, particularly friction, the most influential factor. K values can range from 0.10 for a well-lubricated or waxed assembly to over 0.30 for a dirty or rusty one. The formula T = KDP, where T = Torque, D = Nominal Diameter, and P = Bolt Clamp Load, is used to estimate the torque-tension relationship. However, these values are only estimates, and actual assembly conditions may vary.
To determine the correct torque, experimentation under real joint and assembly conditions is necessary. Calibrated torque wrenches and load-indicating devices can help equate torque to the desired tension. Direct tension indicators (DTIs) are another accurate method to ensure proper tension. DTIs compress at the proper tension, ensuring the desired clamp load is achieved.
The relationship between torque, tension, and friction is often misunderstood. Traditional equations are correct, but misinterpretation leads to misconceptions. For example, reducing bearing surface friction may not directly increase bolt tension. A system model that considers both the tool and joint portions of the tightening system is necessary for a complete understanding.
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Lubrication and torque
The type and amount of lubrication used can significantly impact the torque required to tighten a screw or bolt. When a screw or bolt is lubricated, less torque is needed to achieve the desired tension compared to a dry screw or bolt. For example, a slightly lubricated 1" Grade 5 coarse bolt may have a maximum tightening torque of 483 lbf ft, while a dry bolt of the same specifications may require approximately 30% more torque, resulting in a torque value of 628 lbf ft.
It is important to note that the torque values for lubricated screws and bolts cannot be directly applied to dry screws and bolts. Using torque values specified for dry or slightly oiled screws or bolts on a lubricated fastener may result in overload and potential breakage. Therefore, it is crucial to determine the appropriate torque values based on the specific lubrication used.
The selection of the appropriate lubricant is also essential. Different lubricants have varying effects on the Nut Factor, which is an experimental value related to the coefficient of friction. The Nut Factor influences the tightening torque and bolt tension. Some lubricants, such as nickel-based lubricants, can cause galling, a form of wear caused by adhesion between sliding surfaces. On the other hand, molybdenum disulfide (moly) lubricants are effective in preventing galling and are suitable for most environments.
Proper lubrication practices involve ensuring consistent frictional properties, adequately lubricated fasteners, lower breakout torques, physical and chemical stability of the lubricant, and ease of application. By following these practices, assemblers can achieve the desired torque, clamping force, and gasket stress while minimizing variations in torque levels among fasteners.
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Bolt preload tension
Bolt preload is the tension created in a fastener when it is tightened. This tension is essential to the functionality of the bolt. When a bolt is tightened, it stretches like a stiff spring, creating tension and simultaneously compressing the components together, which results in what is known as clamp force. This clamp force is crucial to the effectiveness of the bolted joint.
The clamp force in an unloaded bolted joint is assumed to be equal and opposite to the preload. If the preload and, therefore, the clamp force is insufficient or not maintained, the bolted joint is likely to experience issues such as fatigue failure, joint separation, or self-loosening due to vibration. This can ultimately lead to joint failure.
Bolt preload is particularly important because it enables a fully tightened bolt to withstand applications that would cause an untightened or loose bolt to fail rapidly. When a bolt is tightened, the joint acts as a conduit, allowing the force to flow through it and into the assembly materials. Consequently, the bolt assembly is subjected to only a fraction of the force exerted by the working load.
The preload tension in a bolt can be determined through various methods, including the use of torque wrenches, preload indicating washers, direct tension indicating washers, and tightening the bolt to a specific degree. While these methods may not provide an exact measurement of preload tension, they offer a reasonable estimation.
In summary, bolt preload tension is critical to the effectiveness and reliability of bolted joints. It allows bolts to distribute the working load across the joint, reducing the likelihood of failure. By understanding and applying the correct preload tension, engineers can enhance the performance and longevity of bolted assemblies.
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Bolt elongation
When it comes to screw size torque specifications, understanding bolt elongation is crucial. Bolt elongation refers to the increase in the length of a bolt when it is subjected to tensile stress or axial stress. This phenomenon is essential to consider when tightening bolts to ensure the required clamping force is achieved without exceeding the bolt's yield strength.
The elongation of a bolt is directly related to the amount of axial stress applied, as long as the stress remains within the elastic range of the bolt material. This means that as tensile stress is applied, the bolt will elongate proportionally. However, if the stress exceeds the material's yield strength, the bolt may deform or fracture.
To calculate bolt elongation, several factors need to be considered. Firstly, the effective bolt length (Leff) is important. This includes the contribution of the bolt's length, as well as the height of the bolt head and nut. The applied tensile load (F) and the tensile stress area of the bolt (A) also play a role in elongation. Additionally, the Young's Modulus of Elasticity (E) or the bolt modulus of elasticity is required for the calculation.
By using the formula δ = Ft × LB ÷ E, the elongation δ in inches or millimetres can be determined. This formula takes into account the required axial stress (Ft), the effective bolt length (LB), and the bolt modulus of elasticity (E). It is important to note that the elongation is directly proportional to the applied axial stress, as indicated by the formula.
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Frequently asked questions
The torque required depends on several factors, including screw size, lubrication, surface texture, and human error. The best way to determine the correct torque is through experimentation under the specific joint and assembly conditions.
A torque chart provides estimated torque calculations for different screw and bolt sizes. These values are only a guide and should be used as a starting point. The specific torque required may vary depending on factors such as lubrication and joint type.
Lubrication can significantly impact torque values. For lubricated fasteners, the torque values should be reduced by 25-30% from the recommended ratings for dry fasteners. This reduction ensures optimal performance without exceeding the fastener's capacity.