Calculation of required fastener tightening torque depends on two variables. Which pair correctly identifies these variables?

• A. Material strength and thickness
• B. Desired preload and fastener diameter
• C. Number and size of fasteners
• D. Length of fastener and thread type

Answer: (B)

The main idea is that tightening torque is used to produce a specific clamp force in the bolt, called preload. To reach a desired preload, you control two things: how much clamping force you want (the target preload) and the size of the bolt you’re using (the fastener diameter). In practice, tightening torque roughly scales with these two factors, often summarized as T ≈ k · F · d, where F is the preload, d is the bolt diameter, and k represents friction and thread geometry. So, increasing the target preload requires more torque, and using a larger diameter bolt also increases the torque needed to achieve that preload.

The other factors matter, but they don’t define the primary inputs for calculating torque to reach a given clamp. Materials and thickness affect how much preload you can safely apply, or whether the joint yields, but not the basic torque-to-preload relationship. The number and size of fasteners affect joint load distribution rather than the torque required per fastener to achieve a specific clamp force. Length and thread type influence friction and engagement, which change the torque constant k, but the two guiding variables you choose to reach the needed preload are the desired preload and the fastener diameter.