What is the purpose of applying preload before rotational tightening in some fasteners?

• A. To lubricate surfaces
• B. To establish the initial clamping force prior to rotation
• C. To stretch the bolt to failure
• D. To gauge temperature

Answer: (B)

Preload before final tightening is used to establish the initial clamping force in the joint. By pulling the bolt to create tension first, the parts are drawn together and any gaps or clearances are taken up, ensuring full contact of the surfaces. When the final tightening torque is applied, that already-established clamp is enhanced without excessive seating or settlement, leading to more even load distribution, better resistance to loosening under vibration, and improved fatigue life. This approach isn’t about lubricating surfaces, stretching the bolt to failure, or measuring temperature, but about locking the joint together with a reliable initial clamp before the final torque is added.