Question 1

What is bolt preload and why is it important?

Accepted Answer

Preload is the initial tension in a bolt created by tightening. Proper preload ensures the joint stays clamped under service loads, prevents fatigue failure by reducing cyclic stress range, and maintains joint integrity. Insufficient preload can lead to joint loosening, while excessive preload risks bolt failure.

Question 2

What is the nut factor (K) and how does it affect torque?

Accepted Answer

The nut factor K is a dimensionless coefficient that relates applied torque to achieved preload. It accounts for friction in threads and under the bolt head/nut. Typical K values range from 0.10 (well-lubricated) to 0.20 (dry). Higher K means more torque is needed for the same preload.

Question 3

Should I use coarse or fine threads?

Accepted Answer

Coarse threads are more common, easier to assemble, and more tolerant of damage. Fine threads have higher tensile strength (larger stress area), better resistance to loosening, and allow finer adjustment. Use fine threads for precision applications and where vibration loosening is a concern.

Question 4

What percentage of proof load should I target for preload?

Accepted Answer

For most applications, target 70-75% of proof load for initial preload. This provides adequate clamping force while leaving margin for variations and preventing yield. Critical applications may use 80-90% with careful control. Never exceed proof load as the bolt may yield and lose preload.

Question 5

Why does lubrication affect required torque so much?

Accepted Answer

Only about 10-15% of applied torque creates preload; the rest overcomes friction. Lubrication reduces friction, so less torque is needed for the same preload. Using dry torque values with lubricated bolts will result in excessive preload. Always match torque specifications to actual lubrication conditions.

Question 6

What is the difference between Class 8.8 and Class 10.9 bolts?

Accepted Answer

The numbers indicate tensile strength: Class 8.8 has 800 MPa tensile and 640 MPa yield (80% of tensile). Class 10.9 has 1040 MPa tensile and 940 MPa yield. Class 10.9 bolts are stronger but also more brittle and sensitive to hydrogen embrittlement. Match bolt grade to application requirements.

Question 7

How accurate is torque-controlled tightening?

Accepted Answer

Torque-controlled tightening typically achieves ±25-30% preload variation for dry bolts and ±15-20% for lubricated bolts. For critical joints, consider angle-controlled (torque-angle) or stretch-controlled methods which achieve ±10-15% accuracy. Ultrasonic bolt measurement provides the highest accuracy.