Torque vs Tension: What’s the Difference?

Torque vs. Tension What's the Difference

Last Updated on February 27, 2024 by Ossian Muscad

When tightening a bolt, you want to ensure that it is secured in place and will not loosen over time. One question that often arises when it comes to tightening bolts is: What is the difference between torque vs. tension? What exactly do these terms mean, and how do they affect the tightness of a bolt? Also, which one should you be using, and when?

This article will discuss the difference between torque and tension and show you how to apply them correctly. That way, you can make sure all your bolts are fastened correctly and avoid any potential issues down the line.

 

Tightened Bolt Behavior

Bolts, when tightened to the correct specification, exploit the inherent elastic properties of the material they are made from. Just as a spring stretches under load and returns to its original shape when the load is removed, a bolt will do much the same. When a torque wrench or tensioner is used to apply a load, the bolt elongates within its elastic limit. Upon releasing the application tool, the bolt attempts to return to its original length. This phenomenon creates what is known as a clamping force, which is essential for holding the components together securely.

The clamping force elicited by the stretched bolt interacts with the friction present between the bolt’s threads and the clamping surfaces. This friction, along with the tension produced, creates a tight joint that resists loosening. This is particularly critical in creating a seal in applications where preventing gasses or liquids from leaking is paramount. As such, bolts are an integral component in creating airtight and liquid-tight seals in various machinery and infrastructure. The interplay between the tension force and the friction across the threads is what ultimately ensures a bolted joint can withstand operational stresses and environmental factors.

 

Difference Between Torque and Tension

Torque and tension, while often used interchangeably in the context of bolt fastening, refer to distinct physical concepts. Now that we know how a tightened bolt behaves let’s compare torque with tension and see how they differ from each other:

Torque

Torque is the rotational equivalent of linear force, often conceptualized as the twist or the rotational force that causes an object to turn around an axis. It is what you apply when using a tool, such as a torque wrench, to tighten a bolt. This twisting force is crucial as it determines the clamp load indirectly applied to the bolt through turning.

The unit used to measure torque is the Newton-meter (Nm), representing the force of one Newton acting on a one-meter lever arm. A higher torque value typically means more force is applied to initiate rotation, which is essential for understanding how tight a bolted connection will be.

In practical applications, when you use a torque wrench to fasten a bolt, you’re essentially gauging the force you exert to cause the bolt to rotate into its seat. The precise amount of torque required can vary depending on the bolt’s size, the materials it’s connecting, and the conditions under which they’re being fastened.

To ensure uniformity and accuracy in bolted assemblies, specific torque values are calculated and provided as a part of engineering specifications, ensuring that each fastened bolt provides the intended clamp force without being under or over-tightened.

Tension

Tension refers to the force that is directly related to the stretch, or elongation, of a bolt and is responsible for the clamping force that holds a joint together. Unlike torque, which results in rotational motion, tension leads to linear motion, essentially pulling the bolt along its axis.

As a bolt is tensioned, it stretches slightly; this elongation, which occurs within the bolt’s elastic limit, is what generates the clamping force that binds the facing materials together securely. Failure to achieve proper tension could result in a joint that is vulnerable to slippage or one that is overly stressed, potentially leading to bolt fatigue and breakage over time.

When using a tensioner to tighten a bolt, the objective is to measure and apply the precise force necessary to achieve the desired stretch, thereby exerting the correct clamping pressure between components. This method is widely considered a direct measure of the clamping force created in the bolted assembly.

The unit of measurement for tension is Newton (N), which quantifies the amount of force required to produce the tension. Ensuring the correct tension is achieved is crucial, as it determines the integrity and longevity of the bolted joint, especially in critical applications where component failure can lead to significant consequences.

 

Torquing and Hydraulic Torque Wrenches

Torque wrenches are precision tools designed to apply a specific amount of force to a fastener, such as a bolt or nut. This force follows the principle articulated in Newton’s third law, which states that for every action, there is an equal and opposite reaction. When torque is applied using a torque wrench, it creates a reactive force that stretches the bolt, generating the clamping force. The application of the torque wrench ensures the appropriate degree of this force to suit the requirements of the bolted joint, and this adjustment is a critical aspect of ensuring the structural integrity of the assembled components.

The capacity of a torque wrench to deliver the correct amount of force lies in its mechanical design, which leverages the length of the wrench and the exerted manual force. The longer the handle of the torque wrench, the greater the torque that can be applied to the fastener with the same amount of force. This is because torque is fundamentally the product of the force applied and the perpendicular distance from the axis of rotation – the longer the distance (wrench handle), the higher the torque for the same force. Manufacturers calibrate torque wrenches for accuracy, ensuring that the applied force matches the desired torque settings, taking into account both the length of the wrench and the force applied by the user.

Hydraulic torque wrenches take this a step further, employing hydraulic power to apply a predetermined torque to fasteners with incredible precision. They are especially useful in situations where high torque values are required, as they reduce the manual effort needed and enhance the uniformity and reliability of the bolting operation. As the hydraulic torque wrench operates, it magnifies the force applied, thereby allowing for a controlled and accurate application of torque. The accuracy of this tool is paramount in scenarios where bolts must handle significant loads, as it assures that each bolted connection will respond appropriately to operational stresses without becoming compromised.

 

Torquing and Friction

Friction serves as a critical factor when it comes to the act of torquing a bolt, primarily because the torque applied to a bolt is used in two major ways: to overcome the friction present and to stretch the bolt to achieve the desired clamping force. In essence, the coefficient of friction between the bolt’s threads and the nut, as well as the friction under the bolt’s head, has substantial influence over the amount of torque required. Intriguingly, the initial torque applied contributes only about 10 to 15% to the actual elongation or stretching of the bolt, which underlines the force that goes into overcoming friction instead of contributing to clamping force.

The majority of the torque, approximately 85 to 90%, is indeed spent on battling the resistance caused by friction between the contacting surfaces of the threads and under the nut and bolt head. This is where the coefficient of friction comes into play – a value that measures how much force is necessary to slide one object over another. It’s important to note that the lower this coefficient, the less force that’s needed to move one surface over the other. This implies that with a lower frictional coefficient, less torque is required to achieve the same amount of tension, which could be advantageous in assembly processes.

When tackling everyday tasks such as changing a car wheel or dismantling an engine, a manual or digital handheld torque wrench is typically sufficient. These tools are designed for the required torque ranges and precision needed in automotive applications and can be calibrated to match the specific torque specifications for different fasteners. They offer the convenience and the right amount of power suitable for these mechanical jobs where precision is important. Still, the forces involved are within a manageable scope for human operators.

In contrast, industrial applications often demand tools that can deliver significantly higher torque with consistent precision due to the larger scale and higher stakes associated with their assemblies. For instance, tightening pipe flanges within the oil and gas industry involves a greater torque output and engages a higher number of bolts than most manual operations. Hydraulically powered torque wrenches are invaluable in such settings, providing the necessary power and ensuring each fastener is sufficiently and evenly torqued, thereby enhancing the safety and reliability of these vital connections. Their use is crucial in minimizing the risk of bolt failure, which can be catastrophic, and in ensuring operational efficiency and long-term durability.

 

How Hydraulic Tensioning Works

Hydraulic tensioning is a sophisticated bolting method that utilizes hydraulic force to apply precise tension to fasteners. A hydraulic tensioner has two primary cylinders: the high-pressure cylinder and the low-pressure cylinder. The high-pressure cylinder houses a piston that directly engages with the bolt that requires tensioning. This is in contrast to the low-pressure cylinder, which is connected to a hydraulic fluid pump through a second piston.

Operation begins when force is exerted on the high-pressure cylinder’s piston, pushing it downwards as a simultaneous action pushes the piston in the low-pressure cylinder. This coordinated movement causes the hydraulic fluid to transition from the low-pressure cylinder to the high-pressure side of the system.

As the fluid moves, the pressure on the high-pressure side escalates. This surge in fluid pressure directly amplifies the force being applied by the piston within the high-pressure cylinder. The force, now intensified by the hydraulics, is then transferred unambiguously to the bolt. Due to this force, the bolt is stretched, creating the requisite tension needed for the joint.

This tension ensures that the assembled components are held together securely, achieving structural integrity without subjecting them to undue stress. The ability to apply a precise and controlled amount of stretch to bolts is a fundamental advantage of hydraulic tensioning, particularly for critical applications where exact clamping forces are paramount.

 

General Procedure for Tensioning

The tensioning process generally follows a standardized procedure, regardless of the specific hydraulic tensioner being used. Knowing and adhering to this procedure is crucial in achieving accurate tension, ensuring safety, and preventing equipment damage. A general tensioning procedure follows the steps outlined below:

Step 1: Fitting the Tensioner Over the Bolt Stud

The tensioning procedure begins with clean bolt threads to ensure accurate operation. The tensioner is positioned over the bolt stud carefully so that it aligns properly. The puller must be properly seated inside the tensioner; if it’s not, the hydraulic force could be misdirected, potentially damaging the bolt or the machinery. Proper alignment is crucial for even force distribution during tensioning.

Step 2: Applying Hydraulic Pressure to the Tensioner

Hydraulic pressure is then applied through a hand or electric pump connected to the tensioner. As the pressure increases, the hydraulic fluid is forced into the tensioner, pushing the piston against the stud. The effect is that the stud is elongated, creating tension. This pressure is closely monitored and controlled to stretch the stud to the specifications required for the particular application, ensuring the right clamping force is achieved without overstressing the bolt.

Step 3: Winding Down the Stud Nut Against the Joint Face

After the stud has been stretched and the correct tension has been achieved, the nut is then wound down against the joint face. This must be done while maintaining the hydraulic pressure to keep the bolt elongated. Once the nut is securely tightened against the joint face, it will hold the required tension in the bolt when the pressure is released. Careful control during this stage is critical to maintain the proper load.

Step 4: Releasing the Pressure

Finally, the hydraulic pressure is slowly and carefully released from the tensioner. This allows the piston to retract and the tensioned bolt to contract slightly, pulling the nut snugly against the joint face to maintain the clamping force. The tool is then removed cautiously, ensuring that the integrity of the tensioned joint is preserved. It’s important to verify that the final tension is still within the desired specifications after tool removal.

Once the tensioner is removed, the bolt will remain stretched, creating tension in the joint. This tension is what prevents the joint from coming apart under load. It’s important to note that the amount of tension in a joint can be affected by many factors, such as the bolt type, the number of bolts, the amount of torque applied to the bolt, and friction between the threads. 

In general, the more bolts that are used in a joint, the less tension each bolt needs to create. This is because the bolts are sharing the load, and each bolt only needs to create enough tension to hold its share of the load.

 

Torque Vs. Tension: Which is the Better Option?

The debate between torque and tension as the preferred method for securing joints is multifaceted, depending on various critical considerations. Each method has its unique merits and is suitable under different circumstances and for various applications.

  1. Critical Nature of Joints: Determining how vital the integrity of the joint is to the overall structure informs whether torque or tension is more appropriate. Safety-critical joints may necessitate the precision of tensioning methods.
  2. Accessibility of Joint Components: When deciding between torque and tension, one must consider the ease of access to joint components. In tight spaces, traditional torque tools might be challenging to use, thereby affecting the choice.
  3. Available Equipment: The selection between torque and tension may be influenced by the types of tools already available on-site, as acquiring new equipment can entail significant costs.
  4. Volume of Joints: The number of joints requiring fastening can impact the decision; for a large volume, torque may be more efficient, while tensioning might be better for fewer, more critical joints.
  5. Budgetary Considerations: Financial constraints play a pivotal role in this decision. The cost of implementing and maintaining the necessary equipment for either method can be a determining factor.
  6. Workforce Expertise: Lastly, the skills and experience of the technicians must be factored in. More complex tensioning techniques might require specialized training that personnel must possess to ensure proper application.

 

Frequently Asked Questions (FAQs)

Q1: How do I determine the correct tension to apply to a bolt?

The correct tension to apply to a bolt depends on the bolt’s size, grade, and the materials being clamped. Manufacturers often provide tensioning specifications, or it can be calculated based on the bolt’s yield strength and the target preload percentage desired. It’s crucial to follow engineering standards or consult with an engineer for accurate specifications.

Q2: Can weather conditions affect the choice between torque and tension?

Yes, weather conditions can affect the expansion and contraction of materials, impacting the effectiveness of the joint. For instance, high humidity or temperatures can cause materials to expand, potentially loosening joints secured by torque. Tension methods might offer more consistent load control under variable weather conditions.

Q3: How does vibration affect torque and tension reliability in joints?

Vibration can cause loosening in joints secured by torque, thanks to the phenomenon known as loosening or ‘self-loosening.’ Hydraulic tensioning, however, mitigates this risk by stretching the bolt and creating a more uniform force along the thread engagement, reducing the likelihood of vibration-induced loosening.

Q4: Is it possible to use both torque and tension methods on the same joint?

While it is generally recommended to use one method consistently for a given type of joint, hybrid methods that use both torque and tension are sometimes employed to achieve specific engineering objectives. Careful consideration and expert assessment are required to ensure integrity and reliability when combining methods.

Q5: What are the signs of improper bolt tensioning, and how can it be corrected?

Signs of improper bolt tensioning include misalignment of parts, uneven gaps in joint interfaces, unusual noises, and joint movement under load. To correct these, release the tension or torque and reapply it correctly, making sure that all procedures are following specifications and that the threads are clean and well-lubricated if necessary.

Q6: How often should tensions be checked on bolts in critical joints?

The frequency of tension checks depends on the critical nature of the joint, the operating environment, and the historical performance of the fastening system. Scheduled maintenance may include regular checks, but for highly critical joints, continual monitoring technologies such as load-indicating washers or bolt elongation measurements may be employed. Always consult maintenance and safety guidelines specific to your equipment.

 

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Conclusion

The meticulous application of torque and tension in bolting is paramount for the integrity and safety of mechanical assemblies. With a clear understanding of the variables at play, such as material properties, environmental conditions, and available equipment, technicians can make informed decisions to ensure each joint is secured to its optimal specification.

Regular maintenance, guided by industry best practices, ensures the long-term reliability of these critical connections. As innovations continue to emerge, staying up-to-date with technological advancements is key in the pursuit of precision and excellence in fastening techniques.

 

 

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