How do you calculate fastener torque from preload?

Fastener torque is calculated using the equation T = K × d × P, where T is the applied torque, K is the nut factor (typically 0.20 for dry steel on steel), d is the nominal fastener diameter, and P is the preload force. The preload force is calculated as the preload stress multiplied by the tensile stress area of the thread (per ASME B1.1 for inch fasteners or ISO 724 for metric). This is the standard method per Shigley's Mechanical Engineering Design and MIL-HDBK-60.

What torque should I use for screws into plastic or FR4?

When fastening into soft materials like ABS plastic, polycarbonate, or FR4 fiberglass PCB, the torque must be limited by the bearing stress capacity of the material under the screw head, not the fastener yield strength. ABS typically allows 6,500 psi (45 MPa) bearing stress, FR4 allows about 8,700 psi (60 MPa), and polycarbonate allows about 8,700 psi (60 MPa). Always use large-flange screws or washers to distribute the load, and consider thread inserts (Heli-Coil or Keensert) for repeated assembly.

What is prevailing torque and how does it affect bolt torque?

Prevailing torque (also called running torque) is the additional torque required to spin a locking fastener before it seats, caused by the locking mechanism such as a nylon insert (Nyloc) or deformed thread. Per ASME B18.16.6 and IFI-100, this torque must be added to the seating torque to get the total applied torque. Importantly, prevailing torque does not contribute to clamp load — it is purely friction in the locking feature.

What is the nut factor K in torque calculations?

The nut factor K (also called the torque coefficient) accounts for friction between mating surfaces and thread geometry. For dry steel on steel, K is approximately 0.20. With lubrication (cadmium plating or oil), K drops to about 0.15. With moly disulfide (MoS2), K is about 0.13. Stainless steel on stainless steel has a higher K of about 0.28 due to galling tendency. The nut factor is defined in MIL-HDBK-60 and Shigley's Mechanical Engineering Design.

What is the tensile stress area of a bolt thread?

The tensile stress area is the effective cross-sectional area of the threaded section that resists tension. For inch fasteners it is calculated per ASME B1.1 as At = (π/4) × [D − 0.9743/n]², where D is nominal diameter and n is threads per inch. For metric fasteners it is calculated per ISO 724 as At = (π/4) × [(d2+d3)/2]², where d2 is the pitch diameter and d3 is the minor diameter.

What is MMPDS and how does it relate to fastener torque?

MMPDS (Metallic Materials Properties Development and Standardization) is the FAA-managed replacement for MIL-HDBK-5, and is the primary source of statistically based design allowables for metallic materials and fastened joints used in aerospace structures. MMPDS Chapter 8 provides Ftu and Fty allowables for aerospace fastener materials like titanium 6Al-4V, A-286 CRES, Inconel 718, and 4340 steel. These allowables are used to calculate maximum preload force, which then determines the maximum torque via T = K × d × P.

Fastener Torque Calculator | MechanicalAI Engineering

TorqueCalc

Free online bolt & screw torque calculator — ASME B1.1 · ISO 724 · MIL-HDBK-5 · MMPDS

Torque from Preload Equation:

T = K × d × P

Where: K = Nut Factor (~0.20 dry steel)

d = Nominal Screw Diameter

P = Preload Force (lbf or N)

P = σ_preload × A_stress

Ref: Shigley's MED, MIL-HDBK-60, Bickford

Unit System

Fastener Parameters

Screw Size / Thread (select from table)

Nominal Diameter (in)

Stress Area (in²)

Screw Material

Material Proof Str (psi)

Yield Str (psi)

UTS (psi)

Ref. Basis

Stress Area Formula (ASME B1.1 / ISO 724): Select screw size to see computed Aₜ
Inch: Aₜ = π/4 × [D − 0.9743/n]² | Metric: Aₜ = π/4 × [(d₂+d₃)/2]² | These formulas are used by MMPDS Ch.8 to compute tensile allowable loads.

% Preload of Selected Strength

Industry default: 65–75% of Proof Stress (IFI-100, MIL-HDBK-60). This matches published torque tables. Using yield strength as the basis produces significantly higher torques — only appropriate when maximum preload is required. Never exceed 90% yield.
Note: Proof Stress ≈ 85–92% of Yield for most steel fasteners.

Joint & Friction Parameters

Nut Factor K (friction / geometry)

Active K Factor

0.20

Nut
Factor

Custom K (if selected above)

Prevailing / Running Torque

Prevailing torque is the drag torque to spin a locking fastener before seating — per ASME B18.16.6 & IFI-100 it must be added to seating torque. It does not contribute to clamp load. Sources: nylon-insert (Nyloc), all-metal distorted thread, serrated flange nuts.

Prevailing Torque Type

Prevailing Torque Range

Min – Max (from size table)

— select screw size —

Use Value

Clamped Materials

Clamp Material 1 (under screw head side)

Clamp Material 2 (nut side / opposite)

Application / Notes

Fastener Lookup Table

Stress areas per ASME B1.1 (UNC/UNF) and ISO 724 / ISO 898-1 for metric. Root diameters and stress areas are computed from thread geometry.

Size	Designation	Nom. Dia	TPI / Pitch	Root Dia	Stress Area	Prevailing Torque Range	Standard

⚠ Engineering Disclaimer: This tool provides torque estimates based on standard fastener preload equations. Results are for reference only. Always consult a licensed structural or mechanical engineer for safety-critical applications. Fastener performance is affected by surface finish, lubrication, temperature, fatigue, and installation method. Reference: ASME B1.1, MIL-HDBK-60, NASA-STD-5020, Shigley's Mechanical Engineering Design.

References

ASME B1.1 — Unified Inch Screw Threads (UN/UNR)
ISO 724 — Metric screw threads: basic dimensions
ISO 898-1 — Mechanical properties of fasteners (metric)
NASA-STD-5020 — Requirements for Threaded Fastening Systems
MIL-HDBK-60 — Threaded Fasteners — Tightening to Proper Tension
Shigley, J.E. & Mischke, C.R. — Mechanical Engineering Design, 10th Ed. (McGraw-Hill)
Bolt Science — Tightening and Preload Fundamentals
Fastenal Technical Reference Guide — Torque & Clamp Load Data

Free Fastener Torque Calculator — Bolt Preload, Bearing Stress & Engineering Drawing Notes | MechanicalAI Engineering

TorqueCalc

How Torque Is Calculated

Bearing Stress & Soft Materials

MMPDS & MIL-HDBK-5 Material Data

Prevailing Torque for Locking Fasteners

Common Use Cases