What Is Bolt Preload? Tension and Torque
Quick Answer: Bolt preload is the tensile force (clamping load) induced in a bolt when it is tightened. For flanged piping joints, the preload compresses the gasket to create a seal. The target preload for ASTM A193 B7 stud bolts is typically 40-60% of the bolt’s yield strength. Preload is achieved by applying controlled torque or by direct tensioning methods.
How Bolt Preload Works
When a nut is tightened on a stud bolt, the bolt stretches elastically. This elongation creates a tensile force (preload) in the bolt, which acts as a clamping force on the joint. The preload compresses the gasket between the flange faces. As long as the external service loads (pressure, thermal expansion, bending) do not exceed the preload, the joint remains sealed.
If external loads exceed the preload, the flange faces separate (gasket unloading), and the joint leaks. Under-preloaded bolts are the primary cause of flange leaks in operating piping systems.
Target Preload Values for A193 B7 Stud Bolts
The target preload is expressed as a percentage of the bolt’s proof load or yield strength. ASME PCC-1 recommends targeting approximately 50% of bolt yield for standard flanged joints.
| Bolt Diameter | Tensile Stress Area (in2) | Yield Strength B7 (psi) | Target Preload at 50% Yield (lbs) |
|---|---|---|---|
| 1/2”-13 UNC | 0.1419 | 105,000 | 7,450 |
| 5/8”-11 UNC | 0.2260 | 105,000 | 11,865 |
| 3/4”-10 UNC | 0.3340 | 105,000 | 17,535 |
| 7/8”-9 UNC | 0.4617 | 105,000 | 24,239 |
| 1”-8 UNC | 0.6057 | 105,000 | 31,799 |
| 1-1/8”-8 UN | 0.7900 | 105,000 | 41,475 |
| 1-1/4”-8 UN | 0.9691 | 105,000 | 50,878 |
| 1-3/8”-8 UN | 1.1553 | 105,000 | 60,653 |
| 1-1/2”-8 UN | 1.3546 | 105,000 | 71,117 |
Yield strength values are for B7 studs up to 2-1/2” diameter. For larger diameters, yield strength is reduced per ASTM A193 (95 ksi for over 2-1/2” to 4”, 75 ksi for over 4” to 7”).
Preload vs Torque Relationship
Preload and torque are related by the short-form equation:
F = T / (K x D)
| Variable | Definition |
|---|---|
| F | Bolt preload (lbs or N) |
| T | Applied torque (in-lbs or Nm) |
| K | Nut factor (dimensionless) |
| D | Nominal bolt diameter (in or mm) |
The nut factor K accounts for friction at the nut face and in the threads. Typical K values:
| Condition | K Factor | Effect on Preload |
|---|---|---|
| Unlubricated (dry) | 0.20 | Baseline |
| Light oil | 0.18 | +11% preload at same torque |
| Anti-seize (copper-based) | 0.15 | +33% preload at same torque |
| Moly-based lubricant | 0.10-0.12 | +67-100% preload at same torque |
Why Uniform Preload Matters
All bolts in a flanged joint must carry approximately equal preload to compress the gasket uniformly. Uneven preload causes:
- Gasket blowout on the under-loaded side
- Bolt yielding on the over-loaded side
- Flange warping from asymmetric stress distribution
- Leak paths at the gasket-flange interface
The ASME PCC-1 star-pattern tightening sequence and multi-pass approach are specifically designed to achieve uniform preload across all bolts. For critical joints (Class 900 and above), hydraulic bolt tensioners provide more accurate and uniform preload than torque wrenches.
Bolt preload is the foundation of every leak-free flanged connection. Without adequate and uniform preload, even the best gasket and flange face finish cannot prevent leakage.
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