What is the difference between static and dynamic friction in torque tightening?

What is the difference between static and dynamic friction in torque tightening?

• A. Dynamic friction higher
• B. They are identical
• C. Friction only matters at high speeds
• D. Static friction is higher; dynamic friction lower once movement starts; affects torque vs clamp load

Answer: (D)

In torque tightening, how friction behaves at the contact surfaces directly shapes how much clamping force you get from a given torque. Static friction resists the initial start of motion and is higher than dynamic (kinetic) friction, so you need more torque to break static friction and begin turning the bolt. Once movement starts, friction drops to the dynamic value, so the same applied torque causes more bolt elongation and a greater clamping load. This means the torque–clamp load relationship changes after movement begins: the initial torque goes into overcoming higher static friction, and after rotation starts, the lower dynamic friction makes torque more effective at increasing clamp force. The statement that static friction is higher and dynamic friction is lower once movement starts best captures this behavior and its effect on the torque-to-clamp-load conversion. The other ideas don’t fit how friction actually behaves during tightening.

In torque tightening, how friction behaves at the contact surfaces directly shapes how much clamping force you get from a given torque. Static friction resists the initial start of motion and is higher than dynamic (kinetic) friction, so you need more torque to break static friction and begin turning the bolt. Once movement starts, friction drops to the dynamic value, so the same applied torque causes more bolt elongation and a greater clamping load. This means the torque–clamp load relationship changes after movement begins: the initial torque goes into overcoming higher static friction, and after rotation starts, the lower dynamic friction makes torque more effective at increasing clamp force. The statement that static friction is higher and dynamic friction is lower once movement starts best captures this behavior and its effect on the torque-to-clamp-load conversion. The other ideas don’t fit how friction actually behaves during tightening.