The Differences Between Torque & Tension￼
From skyscrapers, to automobiles, and everything in between, bolts are paramount in keeping together the structures that make up our world. As a result, installing them correctly is a crucial part of keeping people in society safe. Joints are typically sealed using torque and tension, and it is important to understand what this refers to in the context of manufacturing and construction.
At American Bolt Corporation, we get asked a lot about the differences between torque and tension. Best practices for securing bolts via torque and tension is not black and white, and it depends heavily on a variety of factors including specific manufacturing conditions, application, surface texture, and lubrication to name a few. Before we dive into specifics on the difference between torque and tension, you need to have a firm grasp on the meaning of each concept. The experts at American Bolt are here to help you out.
Torque is a measurement of the twisting force required to turn a nut along a bolt, and tension refers to the stretching, or elongation of a bolt that provides the clamping forces of a joint. Securing bolts through torquing is often less expensive than using tensioners, but it’s important to consider that this may not hold up as well over time as investing in tensioners. Tensioners offer more accuracy and are faster to install then torquing bolts, but it will require more of an investment. American Bolt can help you plan out the logistics of your project to ensure that you are getting the most out of your budget, while also designing outcomes that will last.
We calculate torque at American Bolt using the following formula:
T = KDP
“T” refers to the torque needed to turn a bolt, “K” refers to the indication of lubricity, and “D” is the nominal diameter of the bolt. “P” refers to the desired clamp load and can be calculated using the formula below. It’s important to know that these formulas are meant to serve as a reference point, and you should consult with your engineer before putting these into practice.
P = Pounds of Clamp Force / Preload (75% of Yield Strength (PSI) x Tensile Stress Area (TSA))
Tensile stress is the resistance of a material breaking under tension. Proof load refers to the maximum tensile force that can be applied to a bolt that will not result in deformation. It’s crucial to not exceed the maximum tensile strength when torquing bolts, and keep in mind that torque is only an indirect indication of tension, and that many factors, including surface texture, rust, oil, and debris can impact this relationship. American Bolt offers products that help ensure proper tension on bolts. One example is direct tensioning washers. These are designed with protrusions that collapse as the nut is tightened. Another example is tension control bolts, designed with a spline that shears off at the neck when installed and tightened properly. We sell many of these to structural steel fabricators.
Clamp load, also known as preload or initial load in tension on a bolt, helps you understand the usable bolt strength you’re working with. We get asked a lot why bolts loosen. Spontaneous loosening of bolts happens when a bolt rotates loose due to shock, vibration or loads, and even a small rotation can cause a bolted joint to lose its preload. Fatigue in bolted connections is best prevented by good design, as well as the tensile capacity of bolts. To maintain clamping load throughout the duration of a bolt’s lifespan, there are methods offered by American Bolt that can help.
Locknuts with thread locking designs such as all metal lock nut, nylon insert lock nuts, serrated flange, and double nut can prevent loosening. American Bolt also offers chemical adhesives and/or a patch applied to the fastener that becomes activated upon install. This can work as a lubricant during tightening and is heavily dependent on the cleanliness of threaded surfaces, materials, and chemical properties of the adhesive, to name a few. This solution can work well to reduce the friction when tensioning a bolt.
Other bolting solutions from American Bolt to maintain clamping load through the life of the fastened joint include thread lockers, or liquid form applied to the threads of the fastener, wedge locking washers, and locking wire, also known as safety wire, that is run through the hole in the bolt head. American Bolt can support you with drilling the hole to your application.
Another issue that comes up when torquing bolts is thread galling. This refers to the abrasive wear that can occur when metals rub together with poor lubrication. It typically happens with stainless bolts during high-speed metal to metal installation and leads to pressure breaking down the oxide coating of fastener surfaces, which increase the chances of galling. American Bolt recommends using anti-seize lubricant, and we carry various options with compounds of copper, silver, or nickel to help. Decreasing the RPM when installing down to 150 RPM’s or below can also help reduce the chances of thread galling during installation.
You should have a better understanding now of torque and tension, the relationship between the two, and solutions available to make securing your bolted connections easier. Do you have more questions on this topic that you’d like to run by our team of experts? Get in touch with us through our website contact form and we can assist you.