Question 1

Why is lubrication so important for bolt torquing?

Accepted Answer

Without lubrication, about 50% of torque is lost to underhead friction and 40% to thread friction, leaving only 10% for preload. Lubrication reduces friction losses, resulting in higher and more consistent preload for the same torque. However, using a lubricant with lower K-factor than specified can over-stress the bolt. Always match the K-factor to the actual lubrication condition used during assembly.

Question 2

What is the difference between torque, torque-angle, and stretch control?

Accepted Answer

Torque control (±25% preload accuracy) is simplest but least accurate due to friction variation. Torque-angle (±15%) applies a snug torque then turns a specified angle — less sensitive to friction. Ultrasonic stretch measurement (±5%) directly measures bolt elongation and is the most accurate. Use torque for general assembly, torque-angle for engine head bolts and structural joints, and stretch measurement for critical turbine and reactor bolts.