The importance of correct bolt torque in power transmission-MRO Magazine

2021-11-16 11:37:56 By : Ms. Dam Lin

Threaded fasteners are one of the most common parts in machine design. Such fasteners come in a variety of shapes, sizes and materials, and can be used in various applications in almost all industries in the world. In all cases, the performance of bolted connections depends to a large extent on the torque applied during installation. Below, the basic design and function of bolted connections are explained, best practices for tightening threaded fasteners are provided, and the importance of designing and applying appropriate torque specifications around any bolt or screw is emphasized.

Although the bolt joint looks quite rigid, the bolt acts like a coil spring and generates force when stretched or compressed. When torque is applied to the bolt, the thread pulls the end of the bolt away from the head, stretching it and creating tension in the bolt. This principle also applies to screws, where the fastener is screwed directly into one of the connections instead of using a nut. The tension on the bolt is called the preload because it occurs before any external load is applied to the joint.

Figure 1: Simple bolt connection diagram. Image source: ABB Motors and Mechanical Inc.

The pre-tightening force on the bolt creates an opposing compression force between the head and the nut, clamping the joint together. This compression is called the clamping force and is used to analyze the strength of the bolt connection. However, not all applied torque is converted into clamping force. Most of the torque is usually used to overcome the friction under the head of the bolt itself and the friction between the threads.

Bolt torque specifications Any standard inch or metric bolt has an industry standard or manufacturer's recommended torque value based on size and grade. These torque values ​​are usually designed to stretch the bolt to its guaranteed load. The guaranteed load of the bolt is the maximum tension that the bolt can bear without plastic deformation. Bolts of higher grade or larger diameter will have greater tensile strength, higher guaranteed load, and therefore higher recommended torque.

In addition to size and grade, torque specifications are usually given in the form of "dry" or "wet" threads, indicating whether the threads are lubricated. Both the dry torque value and the wet torque value should reach the standard load of the bolt, but the wet torque value will be significantly reduced. This is because a large part of the applied torque will be lost due to thread friction, so lubricating the thread will reduce the friction and the torque required to reach a given preload value.

Similarly, lubricating threads will reach a dry torque preload that is much higher than that of dry threads (which may overload the bolt or connection and cause failure). Even thread-locking compounds can provide significant lubrication to threads, reduce friction and increase preload for a given torque value.

Therefore, it is always important to use a torque value suitable for the bolt size, grade, and thread condition. Unless you are using a wet torque valve, you should not use lubricants or locking compounds on threaded fasteners. In some applications, equipment manufacturers may provide torque specifications that differ from industry standards based on the requirements of specific applications. Where applicable, it is important to refer to the torque values ​​provided by the equipment manufacturer. If none is available, the hardware manufacturer or design manual should provide an appropriate torque value according to industry standards.

Bolts in power transmission The goal of any bolt connection is to use the clamping force of the bolts to fix the mating parts together. For industrial power transmission components, these joints usually take the form of round bolt flanges. This arrangement is very common on couplings, as shown in Figure 2.

Figure 2: Tire coupling equipped with radial bolt style. Image source: ABB Motors and Mechanical Inc.

As with any coupling, when torque is applied to a hub, the torque is transmitted through the element to rotate the opposite shaft. In this design, when the bolt is tightened correctly, the bolt itself does not transmit any applied torque. Instead, the friction between the compressed clamping ring and the hub holds them together and transfers torque from one shaft to another. The bolt itself will not be subjected to stress exceeding its tensile proof load.

The tapered bushing also demonstrates this concept, where the bushing is compressed into a tapered hole, as shown in Figure 3.

Figure 3: Image source of tapered bushing assembly: ABB Motors and Mechanical Inc.

The same principle applies to the past. The bolt itself is only affected by the preload during installation, while the torque load is transmitted through the friction between the bushing and the hole.

In some applications, such as torque couplings, bolts have a dual role, transmitting torque through friction between the clamping flanges, and supporting the overhanging load of the motor and gearbox at the same time.

Due to the safety risks and costs of cantilever drive components, the bolt flanges in torque couplings are designed to be higher than most power transmission components. The design of the bolt ring must ensure that the bending moment of the supported driver never exceeds the fatigue limit of the coupling flange or the bolt itself, and at the same time provide sufficient clamping force to transmit the torque of the high-horsepower conveyor driver. If the dimensions and installation are correct, the service life of the torque coupling is at least as long as the shaft itself. Applications like this highlight the importance of properly designing and installing bolted joints.

Effect of incorrect bolt torque If the correct installation torque is not applied to the correct bolt, the joint will not function as designed and may fail or break completely. For rotating equipment, failures like this can disperse debris hundreds of feet, posing a serious safety risk to nearby personnel and equipment. Proper bolt torque is not only a reliability issue, but also a safety issue.

Figure 4: Example of torque coupling drive. Image source: ABB Motors and Mechanical Inc.

If the torque is insufficient, the design load may overcome the clamping force on the joint, causing the components to separate or slide from each other. This will expose the bolts to bending and shear stresses that their design cannot withstand, which may cause them to fail. Insufficient torque also reduces the preload on the bolt, reduces friction between the threads and loosens the fastener over time.

Excessive torque may be equally harmful, because bolts and connectors may not be able to withstand the resulting pre-tightening and clamping forces, resulting in thread loss or component failure. To prevent such problems, a torque wrench should always be used to ensure that the bolts are properly preloaded.

Even if the torque specification is correct, the bolt itself may not be suitable for the application. If the bolt is insufficient (too small or too short, or low grade), the tensile strength of the bolt may not meet the required pre-tightening force of the joint, causing the joint to loosen or break during or shortly after installation.

Figure 5: Examples of bolt head markings for common ASTM hardware grades (left) and ISO hardware grades (right). Image source: ABB Motors and Mechanical Inc.

This can happen when replacing parts whose original hardware specifications are unknown. Always check the size and grade of bolts required for the application. Hexagon head bolts have a mark on the head to indicate ANSI or ISO grade (Figure 5). For other types of bolts, you should check the tensile strength and torque rating information with the supplier.

Integrating them together threaded fasteners is the cornerstone of machine design in many industries. Although they may be small, nuts and bolts can make or destroy the largest machines. Understanding and respecting the importance of correct bolt torque is the key to ensuring the safety and reliability of equipment. MRO ___________ Michael Dunn is a product development engineer for Dodge Mounted Bearings at ABB Motors and Mechanical, Inc. He has more than seven years of engineering experience. He holds a Bachelor of Science degree in Mechanical Engineering from Clemson University.

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