What is Bolt Torquing
Proper bolt torquing and tensioning are critical for ensuring leak-proof, reliable flanged connections in piping systems, pressure vessels, and structural applications. Torquing applies rotational force to stretch bolts axially, creating preload (clamping force) that compresses the gasket and resists external loads like pressure or vibration. For flanged joints, industry standards (ASME PCC-1, EN 1591) recommend torquing bolts to 60–75% of yield strength to achieve optimal preload without overstressing the fastener. Techniques include torque control (common but friction-dependent), turn-of-nut (rotation-based for consistency), or hydraulic tensioning (most precise, using direct bolt elongation). Proper tightening sequences (e.g., star/cross patterns) ensure even gasket compression, while lubrication (K-factor) minimizes friction variability. For critical services (high-pressure/temperature), ultrasonic measurement or strain gauges verify preload accuracy, preventing joint failure or leaks. Always adhere to manufacturer specs and engineering standards for safe, compliant installations.
Critical Considerations
Friction Dominates Torque
50-90% of torque is lost to friction (thread + underhead).
Lubrication (K-factor) drastically affects torque accuracy.
Preload ≠ Applied Torque
Same torque can give different preloads if friction varies.
For critical joints, use direct preload methods (hydraulic tensioning, ultrasonic measurement).
Gasket Sealing
Preload must compress the gasket sufficiently (ASME PCC-1 guidelines).
Bolt Torque Calculator
Note: This calculator provides theoretical values. Actual torque requirements may vary based on specific conditions. For critical applications, consult engineering standards or perform testing.
Difference Between Recommended Torque and Preload per Bolt
Here’s a clear breakdown of the difference between Recommended Torque and Preload per Bolt, along with their engineering significance in bolted flange connections
Definition
Purpose
Key Formula
Units
Depends On
Engineering Goal
Recommended Torque
The twisting force (in Nm or lb-ft) applied to the bolt head/nut to achieve proper tightness.
Ensures the bolt is tightened to the correct rotational force.
T = K × d × Fₚ
(T: Torque, K: Friction factor, d: Bolt diameter, Fₙ: Preload)
Nm (metric) or lb-ft (imperial)
– Bolt diameter
– Lubrication (K-factor)
– Thread friction
Avoid under/over-tightening (prevents leaks or bolt failure).
Preload per Bolt
The axial clamping force (in kN or lbf) generated in the bolt shank when torqued.
Creates the clamping force that holds the joint together.
Fₚ = (Aₛ × σₙ × Preload%)
(Aₛ: Stress area, σₙ: Yield strength)
kN (metric) or lbf (imperial)
– Bolt material (grade)
– Stress area (Aₛ)
– % of yield strength
Ensure the joint can withstand operational loads (pressure, vibration, etc.).
