Torsion (mechanics)
Torsion in mechanics refers to the twisting of an object resulting from equal and opposite torques. This phenomenon can be observed in everyday actions such as wringing out a towel or turning a key in a lock. Torsion is influenced by factors including the amount of applied torque, the material properties, and the object's geometric shape. Understanding torsion is crucial in various applications, particularly in engineering and manufacturing, as it can lead to material failure, which poses safety risks and can result in costly machinery breakdowns.
To assess how much torque a material can withstand before breaking, torsion tests are conducted, measuring the material's shearing point. These tests are essential for components used in machinery, such as drive shafts in vehicles or torsion bar suspensions, ensuring they possess adequate torsional stiffness and flexibility. Furthermore, torsion is a significant consideration in structural engineering; for example, bridge designs must account for potential torsional effects from wind forces, as demonstrated by the collapse of the Tacoma Narrows Bridge in 1940 due to excessive torsional vibrations. Overall, a sound understanding of torsion is fundamental for designing safe and effective mechanical systems.
Subject Terms
Torsion (mechanics)
Torque is the tendency of a force to rotate an object about an axis, a fulcrum, or a pivot. Turning a doorknob and fastening a bolt are examples of torque. When torque is applied to a fixed object that cannot turn, another torque is created by the resistance at the fixed point. Torsion is the twisting of an object caused by equal and opposite torques. Examples of torsion include wringing water out of a towel and turning a key in a lock. Torsion is determined by the torque, the material involved, and the object's shape.
Measuring Torsion
Measuring torsion is important because material failure is potentially destructive. Equipment breakage may cause costly production delays in industrial settings. Material failure also may cause injury or death to workers.
Torsion tests measure the strength of the material being used against the maximum torque, or twisting force, and must be performed to find out how much torque a material can take before it breaks. The breaking point is known as the shearing point. Metal beams and fasteners are tested before they are used in manufacturing, as are parts such as gear shafts.
Torsion tests, which include failure testing, proof testing, and operational testing, are measured in quarter-degree torque increments. Failure testing twists the material and measures the pressure under which it breaks. Proof testing applies a measure of torque load for a length of time to see if the material holds up. Operational testing checks the limits of products before they are sold.
Elasticity refers to a material's ability to return to its original shape and size when torque is removed. Elasticity is a characteristic of metal that has many useful applications in manufacturing. Ductile metals have high elastic limits, meaning they can take a great deal of torque before breaking. Brittle metals break under little strain and have low elastic limits.
Uses in Machinery
Torsional stiffness, or rigidity, is the resistance of a shaft to twisting forces. For example, when a drill bit is biting into wood, the bit (shaft) is meeting with resistance from the wood—torque. At the same time, the drill is exerting turning force on the top of the shaft. These opposing forces cause the shaft to twist. The shaft resists this twisting because it has torsional stiffness. This rigidity is measured in pounds per inch or newtons per meter. However, the flexibility of the shaft allows it to resist breaking.
Engineers commonly use torsionally loaded shafts in designing machines, particularly rotating machinery. For example, rear-wheel-drive vehicles generally have drive shafts, which use torsion to transmit rotation (torque) to the drive wheels.
Torsion beam suspensions are commonly used in front-wheel-drive vehicles. A coil spring works in combination with a shock absorber, allowing parts of the car to flex independently when under twisting stress. Other vehicles use torsion bar suspensions, in which a steel torsion bar acts as the spring, making the most of the bar's elasticity. The torsion bar is designed to be flexible and twist with the vehicle and is often used in off-road trucks as well as sports cars.
Helical springs, or torsion springs, can be used to apply torque or store rotational energy in a device. Some of the energy applied to the wire when coiling it is transferred and stored in the spring. The ends are fixed to components that rotate around the spring's center. As the components move, the spring tries to return them to their original points. A simple example of this action is a spring-loaded clothes pin; the spring ends push the two parts of the clothes pin together. Torsion springs are often mounted around a shaft and are used in couplings between motors and pump assemblies and in such devices as ratchets and garage doors.
Bridges
Engineers must consider torsion and shear stress when designing bridges. Bridges experience shear stress when two fastened structures move in opposite directions. Bridge designs must take factors such as wind into consideration.
High winds can cause a suspension bridge to twist and even break, as happened with the Tacoma Narrows Bridge in the state of Washington. From the day the bridge opened on July 1, 1940, it attracted visitors who paid to drive across it to experience the wave-like motion of the span. It was soon nicknamed Galloping Gertie for the way it moved and rolled in high winds, an experience that was compared to riding a roller coaster. The Tacoma Narrows stood for slightly more than four months. At about seven o'clock on the morning of November 7, 1940, the wind velocity was 40 to 45 miles per hour, probably the strongest wind the bridge had experienced since it was completed. By ten o'clock, torsional vibrations began in two segments of the span. The roadway twisted so far that when the sidewalks on one side rose, they were twenty-eight feet higher than the low opposite side before the two sides reversed positions. The bridge began to crack at about ten thirty. Just after eleven o'clock, the bridge came apart and collapsed. The torsional vibrations exceeded the elasticity of the structure.
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