Importance of Proper Bolt Torque in Power Transmission

By Michael Dunn
Threaded fasteners are one of the most common components in machine design. Such fasteners come in a great variety of shapes, sizes, and materials that serve a wide variety of applications across virtually every industry in the world. In all cases, the performance of bolted joints depends heavily on the torque applied during installation. Below, the basic design and function of bolted joints is explained, best practices for tightening threaded fasteners are provided, and the importance of designing around and applying the proper torque spec to any bolt or screw is emphasized.
Bolted Joints
Although it appears rather rigid, a bolt acts much like a coiled spring, generating a force as it is stretched or compressed. When torque is applied to the bolt, the threads pull the end of the bolt away from the head, stretching it and generating tension in the bolt. This principle also applies to screws, where the fastener is threaded directly into one of the joined members rather than using a nut. The tensile force on the bolt is called the preload, as it occurs before any external loads are applied to the joint.

The preload on the bolt creates an opposing compressive force between the head and the nut, clamping the joint together. This compression is referred to as the clamping force and is used to analyze the strength of a bolted joint. However, not all the applied torque is converted into clamping force. The majority of the torque typically serves to overcome friction underneath the head of the bolt itself and the friction between the threads.
Bolt Torque Specifications
Any standard inch or metric bolt will have a recommended torque value from an industry standard or the manufacturer based on the size and grade. These torque values are generally designed to stretch the bolts to their proof load. The proof load of a bolt is the maximum tension the bolt can handle without plastic deformation. Higher grade or larger diameter bolts will have a greater tensile strength, a higher proof load, and thus a higher recommended torque.
In addition to size and grade, torque specifications are often given in terms of “dry” or “wet” threads, denoting whether or not the threads are lubricated. Both dry and wet torque values are meant to reach the bolt’s proof load, but the wet torque value will be significantly lower. This is because a large portion of the applied torque is lost to thread friction, so lubricating threads reduces that friction and the torque required to reach a given preload value.
Similarly, lubricated threads will reach a much higher preload with the dry torque value than dry threads (potentially overloading the bolt or the joined members and causing failure). Even thread-locking compounds can provide significant lubrication to threads, reducing friction and increasing the preload for a given torque value.
Therefore, it is always important to use a torque value appropriate for the bolt size, grade, and condition of the threads. Lubricants or locking compounds should not be used on threaded fasteners unless you use wet torque valves. In certain applications, the equipment manufacturer may provide a torque specification that differs from the industry standard, based on the requirements of the particular application. It is important to refer to torque values provided by the equipment manufacturer where applicable. If none are available, then a hardware manufacturer or design manual should provide appropriate torque values per industry standards.
Bolts in Power Transmission
The goal of any bolted joint is to use the clamping force of the bolt to hold the mated parts together. For industrial power transmission components, these joints normally take the form of a circular, bolted flange. This arrangement is commonly seen on couplings, as shown in figure 2.

As in any shaft coupling, when torque is applied to one hub, it is transferred through the element in order to turn the opposite shaft. In this design, the bolts themselves do not transfer any of the application’s torque when they are properly tightened. Instead, the friction between the compressed clamp rings and the hubs holds them together and transmits torque from one shaft to the other. The bolts themselves are never subjected to stress beyond their tensile proof load.
This concept is also demonstrated with tapered bushings, where the bushing is compressed into a tapered bore, as figure 3.

The same principle applies as before. The bolts themselves are subject only to the preload from installation while the torque load is transmitted by the friction between the bushing and the bore.
In some applications, such as a moment coupling, the bolts work double duty, transmitting torque through friction between the clamped flanges while supporting the overhung load of the motor and gearbox.
As a result of the safety risks and cost of the overhung drive package, the bolted flange in a moment coupling is more highly-engineered than most power transmission components. The bolt circle must be designed such that the bending moment of the supported drive never exceeds the fatigue limit of the coupling flange or the bolts themselves while also providing enough clamping force to transmit the torque of a high-horsepower conveyor drive. When properly sized and installed, a moment coupling will last at least as long as the shaft itself. Applications like this highlight the importance of proper design and installation of bolted joints.
Effects of Improper Bolt Torque
Without the proper installation torque applied to the proper bolts, the joint will not function as designed and may fail or break apart completely. With rotating equipment, failures like this can send debris hundreds of feet, posing a severe safety risk for nearby personnel and equipment. Proper bolt torque is not just a reliability concern, but a matter of safety as well.

With insufficient torque, the designed load may overcome the clamping force on the joint, allowing the members to separate or slide against one another. This subjects the bolts to bending and shear stresses that they are not designed to carry, potentially causing them to fail. Insufficient torque will also lower the preload on the bolts, reducing the friction between the threads and allowing the fasteners to loosen over time.
Excessive torque can be equally detrimental, as the bolt and joined members may not be rated for the preload and clamping force generated, leading to stripping of the threads or failure of components. To prevent such issues, a torque wrench should always be used to ensure that the bolts are properly preloaded.
Even with the correct torque spec, the bolts themselves may not be suitable for the application. With inadequate bolts (too small or short, or low grade), the bolts’ tensile strength may not meet the preload required for the joint, leaving the joint loose or breaking bolts duringor shortly after installation.

This can occur when replacing parts where the original hardware spec was unknown. Always check the size and grade of bolts required for an application. Hex head bolts have markings on the head to denote the ANSI or ISO grade (figure 5). For other styles of bolts, you should check with the supplier for tensile strength and torque rating information.
Holding It All Together
Threaded fasteners are a cornerstone of machine design in many industries. Though they may be small, nuts and bolts can make or break the largest machines. Understanding and respecting the importance of proper bolt torque is key to ensuring safe, reliable equipment. MRO
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Michael Dunn is a Product Development Engineer for Dodge Mounted Bearings at ABB Motors and Mechanical, Inc. He has over seven years of engineering experience. He has a Bachelor’s of Science in Mechanical Engineering from Clemson University.