Torque is a force that causes an object to rotate, while tension is a force that causes an object to stretch or elongate. A closer look at the common units of torque, ft·lb (N·m), reveals that torque consists of a force multiplied by a distance. But it is not just any distance; it is the distance perpendicular to the direction of the force between the object’s axis of rotation and the point at which such force is applied.
HOW ARE BOLT TORQUE AND TENSION RELATED?
Torque is a method of creating tension in the bolt by stretching it. To visualize the rotational force causing stretch in the bolt, consider that the bolt’s threads act as a wedge or an inclined plane wrapped around the bolt’s body. As torque is applied to the bolt and the bolt rotates relative to the nut or to a threaded hole, the wedging action between the mating threads causes the bolt to stretch, compressing the clamped members. This wedging action is analogous to the principle behind one of the earliest forms of a fastener, the unthreaded bolt known as the forelock bolt circa the Roman Empire. As illustrated in Figure 1b, a forelock or wedge was hammered into a slot at one end of this antiquated bolt to create tension.
Torque is the most commonly used method both in industrial settings and at home to create bolt tension, and its counterpart, clamp load, in the joint. In most applications, it is an extremely viable and cost-effective method of bolted joint assembly. Hand wrenches and manual torque wrenches are mainly used for smaller bolt diameters (≤1 in. [≤25 mm]), whereas for diameters greater than 1 in. (25 mm), impact wrenches and hydraulic torque wrenches are preferred.
IS TORQUE ACCURATE IN CREATING BOLT TENSION?
To appropriately answer this question, it helps to first consider that only approximately 10-15% of the input torque actually stretches the bolt; the remaining 85-90% of input torque is primarily used to overcome friction between the mating threads and friction under the bolt head or nut. Friction is dependent on a number of variables, including but not limited to lubricant type and condition, surface roughness, plating, and material, so it can vary significantly, even among bolts in the same assembly. Additionally, because friction dominates the torque-tension relationship, even a slight variation in friction can lead to significant variation in bolt tension.
It is not surprising then that torque, the most common method for creating tension in a bolt, is also the least accurate method—the general consensus is that variation in bolt tension due to torque control is ±25% to ±35%. A variation as high as ±35% means that at exactly the same measured torque, the tension in the one bolt could be less than 50% of the tension in another bolt. After factoring in any tolerance in the acceptable range of torque values, variation can be even higher. Therefore, accurately torqued bolted joints are not equivalent to accurately tensioned bolted joints.
ALTERNATIVE METHODS OF CREATING BOLT TENSION
In critical bolted joints, torque control alone is often not accurate enough to provide a safe and effective bolted joint; other methods of creating bolt tension are often more suitable, or supplemental methods of verifying bolt tension are needed. Furthermore, as the diameter of bolts increases and torque becomes less physically feasible or too time-consuming, alternative methods are necessary.
Hydraulic tensioning, either in the form of bolt tensioners or hydraulic nuts, uses hydraulic pressure to stretch threaded fasteners. Because hydraulic pressure can be easily and accurately measured, and because the pressure area of the hydraulic cylinder remains constant, hydraulic tensioning offers far better accuracy (< ±10% variation in bolt tension) than torque control methods. In both hydraulic tensioning methods, it is important to note that the high rotational forces of torque and their associated friction are not present. This lack of sliding surfaces in contact under high loads minimizes the risk of thread galling, another benefit of hydraulic tensioning.
Bolt tensioners are tools that grab and pull the threads protruding beyond a nut, as shown in Figures 3 and 4. Hydraulic nuts, shown in Figure 5, employ the same principle of using hydraulic pressure to stretch bolts, but function as more than just a tensioning tool. By replacing standard nuts in the application, hydraulic nuts become part of the structural bolted joint. After the appropriate pressure is reached, the hydraulic nut’s locking ring mechanically retains the bolt’s tension.
A distinguishing feature of hydraulic nuts is that it is possible to simultaneously tension all bolts in a bolted joint. By eliminating the need for a tightening pattern or sequence, assembly and disassembly time can be reduced. Also, cross-talk or load sharing between neighboring bolts is minimized, which results in more uniform tension among bolts.
Bolt heating is another method of creating bolt tension, but is typically only used in very large diameter bolt applications. In this time-consuming method, heat is used to stretch the bolt or stud while simple hand tools are used to turn and tighten the nut. The stretch of the bolt or the stud has to be measured directly, but such measurement has to occur after the bolt or stud has cooled. This iterative process of heating, cooling, and measuring is repeated until the stretch is within specified limits.
SUPPLEMENTAL METHODS OF INDICATING BOLT TENSION
There are supplemental methods or tools that can be used in conjunction with any method of torqueing or tensioning to more directly indicate bolt tension and reduce the variation in bolt tension in a bolted joint. Such methods include actually measuring the bolt stretch, or using load indicating devices.
If both ends of the bolt are accessible, caliper micrometers can be used to measure the actual bolt stretch. In blind hole applications, or in larger bolt diameter applications, depth micrometers can be used with a datum rod placed in an axially drilled hole in the bolt to measure stretch. Equipment in which an ultrasonic pulse’s time-of-flight is measured offers yet another way of determining bolt stretch. In all of these measuring methods, before and after measurements are required to definitively determine the amount of stretch.
Load indicating devices, such as load indicating fasteners or direct tension indicating washers, are also available. Load indicating fasteners essentially have built-in datum rods, allowing for tension to be directly measured with a ±5% accuracy. Unlike the method of measuring the stretch of standard fasteners, load indicating fasteners do not require measurements before the fastener is stretched. They also offer easy in-situ and real-time tension indication.
Torque rightly has its prevalence in most applications, and can be effective in achieving adequate bolt tension. But in more critical bolting applications where precise bolt tension is required, where torque becomes impractical or where torque poses safety risks, other methods to create tension, such as hydraulic tensioning, or other methods to verify tension after torqueing, such as load indicating fasteners, are proven solutions.
Regardless of the method used to create bolt tension, obtaining the proper bolt tension during installation is imperative for the integrity of the bolted joint; excessive tension during installation can overstress the bolts or cause damage to the clamped members, but insufficient tension during installation can lead to premature joint failure due to joint separation and/or fatigue.
Remember: an accurately torqued bolted joint is not equivalent to an accurately tensioned bolted joint.