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Bolt Torque โ€” T = K ร— D ร— F

Calculate proper torque for bolt tightening based on size, grade, and friction. Overtorque strips threads; undertorque causes loose joints. K factor varies with lubrication.

Concept Fundamentals
T = K ร— D ร— F
Torque
0.2 dry, 0.15 oiled
K Factor
~85 ksi proof
Grade 5
~120 ksi proof
Grade 8
Calculate Bolt TorqueSelect bolt size, grade, and friction condition.

Why This Construction Metric Matters

Why: Proper torque achieves design clamp force without stripping threads or fatiguing the bolt. Friction absorbs 90% of applied torque; only 10% creates clamp.

How: T = K ร— D ร— F. K from thread geometry and friction. Clamp load from proof strength or target. Stress = F / stress area.

  • โ—Dry K โ‰ˆ 0.2; lubricated 0.1โ€“0.15. Same torque, different clamp.
  • โ—75% of proof load typical for clamp; don't exceed proof.
  • โ—Crisscross pattern for multi-bolt; torque in stages.
  • โ—Replace stretch bolts; don't reuse.
Sources:ASTM F568 Bolt StandardsASME B18.2.1

Bolt Torque Calculator

T = Kร—Dร—F โ€ข Clamp force โ€ข Grade comparison

Quick Examples โ€” Click to Load

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1/4-20 Grade 5

Common automotive bolt

๐Ÿ—๏ธ

3/8-16 Grade 8

Heavy duty structural

โš™๏ธ

M8x1.25 Class 8.8

Standard metric bolt

๐Ÿ’ช

M12x1.75 Class 10.9

High strength metric

๐Ÿš—

Wheel Lug Nut

1/2-20 Grade 5

๐Ÿข

Structural Bolt

3/4-10 ASTM A325

Major Diameter: 0.25 in
Pitch: 20 TPI
Stress Area: 0.0318 inยฒ
๐Ÿ”ฉ

SAE Grade 5

Medium carbon steel, quenched and tempered

Proof: 85 ksiYield: 92 ksi

๐Ÿ”ง Friction & Lubrication

Output
Torque: 2 ft-lbs | Clamp: 2,027.25 lbs

๐Ÿ“Š Results

Torque

2.00 ft-lbs

2.72 N-m

Clamp Force

2027.25 lbs

9017.65 N

Thread Stress

63750.00 psi

439.54 MPa

Safety Factor

1.33

SAFE โœ“

๐Ÿ”ฉ Load Limits

Proof Load:2703.00 lbs
Yield Load:2925.60 lbs
Utilization:75.00%

โš™๏ธ Torque Factor

K Factor:0.0474
Friction:Dry (Unlubricated)
Lubricant:None

Step-by-Step Calculation

=== Bolt Torque Calculation ===
Bolt Size: 1/4-20
Bolt Grade: SAE Grade 5
Friction Condition: Dry (Unlubricated)
Lubricant: None

=== Torque Factor (K) Calculation ===
Major Diameter: 0.25 inches
Pitch: 20 TPI
Minor Diameter: 0.2062 inches
Stress Area: 0.0318 inยฒ
K Factor: 0.0474

=== Calculated Clamp Load ===
Proof Strength: 85.0 ksi
Stress Area: 0.0318 inยฒ
Proof Load: 2703.0 lbs
Typical Clamp Load (75%): 2027.3 lbs

=== Torque Calculation ===
Formula: T = K ร— D ร— F
K Factor: 0.0474
Diameter: 0.25 inches
Clamp Load: 2027.3 lbs
Torque: 2.00 ft-lbs
Torque: 2.72 N-m

=== Stress Calculation ===
Thread Stress = Clamp Load / Stress Area
Thread Stress: 63750.0 psi
Thread Stress: 439.5 MPa

=== Load Limits ===
Proof Strength: 85.0 ksi
Yield Strength: 92.0 ksi
Proof Load: 2703.0 lbs
Yield Load: 2925.6 lbs

=== Safety Analysis ===
Safety Factor: 1.33
Utilization: 75.0% of proof load
Status: SAFE โœ“

๐Ÿ“ˆ Visualizations

๐Ÿ“Š Bolt Grade Comparison

Planning estimates only. Verify with a licensed engineer or contractor before construction.

๐Ÿ“ Construction Industry Facts

๐Ÿ”ฉ

T = K ร— D ร— F. K โ‰ˆ 0.2 dry, 0.15 oiled.

โ€” ASME

โš™๏ธ

90% of torque goes to friction; 10% to clamp.

โ€” Industry

๐Ÿ“Š

Grade 5: 85 ksi proof. Grade 8: 120 ksi.

โ€” ASTM

๐Ÿ’ช

Clamp load typically 75% of proof load.

โ€” Best practice

๐Ÿ“‹ Key Takeaways

  • โ€ข T = K ร— D ร— F (torque = factor ร— diameter ร— clamp force)
  • โ€ข K โ‰ˆ 0.2 (dry) to 0.15 (lubricated)
  • โ€ข Grade 5 โ‰ˆ 120 ksi; Grade 8 โ‰ˆ 150 ksi
  • โ€ข Overtorque strips threads; undertorque = loose joint

What is a Bolt Torque Calculator?

A bolt torque calculator helps you determine the correct torque value for tightening bolts based on their size, grade, thread specifications, and friction conditions. Proper torque is critical for achieving the desired clamp force while avoiding bolt failure, thread stripping, or joint failure. This calculator considers bolt grade strength, thread geometry, friction coefficients, and lubrication to provide accurate torque recommendations.

๐Ÿ”ฉ

Torque Calculation

Calculate proper torque for bolt tightening

โš–๏ธ

Clamp Force

Determine clamp force from torque

๐Ÿ“Š

Stress Analysis

Calculate thread and joint stress

๐Ÿ›ก๏ธ

Safety Factor

Safety margin analysis

How to Calculate Bolt Torque

Calculation Formula

T = K ร— D ร— F

Where:
T = Torque
K = Torque factor (friction coefficient)
D = Bolt diameter
F = Clamp force

Quick Reference

1/4-20 Grade 5: ~7-10 ft-lbs (dry)
3/8-16 Grade 5: ~20-30 ft-lbs (dry)
1/2-13 Grade 5: ~50-75 ft-lbs (dry)
M8x1.25 Class 8.8: ~20-30 N-m (dry)

Bolt Grade Reference

SAE Grade 2

Low carbon steel, general purpose

Proof:55 ksi
Yield:57 ksi

Light duty, Non-critical, General assembly

SAE Grade 5

Medium carbon steel, quenched and tempered

Proof:85 ksi
Yield:92 ksi

Automotive, Machinery, Structural

SAE Grade 8

Medium carbon alloy steel, quenched and tempered

Proof:120 ksi
Yield:130 ksi

Heavy duty, High stress, Critical applications

Metric 4.6

Low carbon steel, general purpose

Proof:225 MPa
Yield:240 MPa

Light duty, Non-critical

Metric 8.8

Medium carbon steel, quenched and tempered

Proof:600 MPa
Yield:640 MPa

Automotive, Machinery, General construction

Metric 10.9

Medium carbon alloy steel, quenched and tempered

Proof:830 MPa
Yield:940 MPa

Heavy duty, High stress, Automotive

Torque Formulas

Torque: T = K ร— D ร— F
Clamp force: F = T / (K ร— D)

Friction Coefficient Guide

ConditionCoefficientDescriptionApplications
Dry (Unlubricated)0.2No lubrication, standard dry conditionGeneral assembly, Standard conditions
Oiled0.15Light oil or machine oilAutomotive, Machinery
Lubricated (Anti-seize)0.1Anti-seize compound or moly greaseHigh temperature, Corrosive environments
Moly Disulfide0.08Molybdenum disulfide coatingHigh performance, Precision applications
Teflon Coated0.06PTFE coating for low frictionSpecial applications, Low friction required

Bolt Grade Summary

Grade 2 < 5 < 8. Metric: 4.8 < 8.8 < 10.9 < 12.9.

Frequently Asked Questions

Lubricated vs dry?

Lubricated reduces friction (lower K), so same torque gives higher clamp force. Use manufacturer specs when available.

Tightening Tips

  • โ€ข Use crisscross pattern for multi-bolt joints
  • โ€ข Torque in stages (e.g., 50%, 75%, 100%)
  • โ€ข Replace stretch bolts; don't reuse
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