Windmills. These mechanical wonders have helped harness the power of the wind for the betterment of mankind. The Dutch are famous for their windmills, amongst other things, and have exploited them for many things, for milling grains to make flour, to powering the sawmills that ensure a constant supply of ships for the Dutch East Indies company, to even relocating the ocean. Why? Because who really wants to live above sea level anyways? These windmills rely on an effective mechanical means for transmitting power called torsion. Torsional loading is characterized by the application of a twisting moment along the axis of an object. In our windmill example, the rotation of the blades is transmitted through rotating wooden gears and shafts to the base of the mill where the power can be exploited. Modern wind turbines utilize the same principle. However, rather than utilizing torsion to transmit power to where we need it, we utilize it to turn a generator that transforms the mechanical power into electrical power. The same principles are used in steam turbines used to produce electrical power from other energy sources, such as the burning of fossil fuels and nuclear fission. As an aerospace engineer, I am more interested in the reverse of this process. We can utilize various types of engines to generate power, transmitting it through torsion to a fan or turbine in order to make our own wind that allows us to take to the skies. For the more terrestrial inclined, torsion is equally important for helping make our mechanical creations mobile. Let's return for a moment to our definition of torsion as a twisting moment applied along the axis of a shaft. To annotate torsional loading on a given shaft, we can first draw its axis, and then the pair of twisting moments that react each other along that axis. As engineers like to talk in scalars rather than vectors, we need to establish some form of sign convention for these torsional moments. Enter the right-hand grip rule. Taking your right hand in a gripped thumbs-up position align your thumb with the axis of the shaft with the tip of your thumb pointing in the outward normal direction of the shaft section. With this alignment complete, your gripped fingers point in the direction of a positive torsional moment. Returning to our cylindrical shaft on the right, we see that according to this rule, the right most torsional moment is positive due to the outward normal shaft pointing to the right, and the torsional moment on the left is also positive due to the outward normal shaft pointing to the left. Creating three-dimensional representations of torsion moments in a two-dimensional drawing space can often lead to confusion. To prevent this, a simplified notation of a double-headed arrow is often used to denote a torsional moment. Here, the direction of the arrows align with the pointing direction of the thumb in the right-hand grip rule. Torsion. It is a form of loading that quite literally has a potential to propel us in life. It is a powerful loading for any engineer to master. And now that you are equipped with the notation and sign convention to describe it, we can dive deeper into understanding the effects of torsion on the deformation, performance, and failure of engineering structures. .