Final answer:
The inquiry relates to mechanical engineering and discusses specifics of a bolt's preload based on its proof strength, with the mention of UNC standard, bolt class, and thread formation. The question seems incomplete as it refers to a torque coefficient without fully explaining what is being asked about it.
Step-by-step explanation:
The subject of the question pertains to a scenario in mechanical engineering, related to the preload applied to a bolt. The mention of "UNC", which stands for Unified National Coarse, indicates that the question revolves around a standard thread specification for bolts and screws in North America. Additionally, class 7 refers to the tolerance class for the bolt, and "rolled threads" suggests a method by which the threads have been formed.
The question appears to be incomplete as the torque coefficient 'k' is mentioned without providing its value or asking a specific question about it. Typically, in engineering, the torque coefficient is a dimensionless constant that represents the fraction of input torque that is converted into tension within the bolt. Preloading a bolt to 80% of its proof strength is a common practice to ensure that it operates safely within its elastic limit, preventing any permanent deformation.