Engineering Guide · 5 min read
Bolt Torque Values: How to Choose the Right Number
Choose bolt torque from preload and friction, not size alone. Compare published M10 reference values and calculate how the same setting can change bolt tension.
The short answer: choose a bolt torque that produces enough preload to hold the joint together while staying within the load capacity of the bolt, threads, and clamped parts. Check that result at both ends of the expected friction range. For an existing machine, the equipment’s assembly specification supplies that decision; for a new joint, the torque follows from the joint design.
A torque wrench can reach its setting accurately while the bolt develops the wrong preload. To understand why, start with what tightening is meant to achieve.
Start with the force holding the joint together
Tightening stretches the bolt and compresses the parts beneath its head and nut. Before an external load is applied, the bolt’s tension balances that compression. When a service load tries to separate the parts, it relieves some of the compression and adds load to the bolt; their relative stiffness affects how the load is shared.[1]
The design therefore needs a lower preload limit that keeps the joint functioning and an upper limit that avoids overload. Too little remaining clamp force can allow separation or, in a friction joint, slip. Too much preload can overload the bolt, strip the mating thread, or indent the bearing surface. Increasing the bolt’s strength class addresses only part of that problem. Use Fastener Grades Explained to identify its mechanical properties.
Read the assumptions behind a torque chart
Bossard’s January 2025 guide gives the following reference values for the same M10 coarse-thread, class 8.8 screw. Here, μ is the coefficient of friction at each contact: the table assumes equal values in the threads and under the rotating head or nut.[2]
| Thread and bearing friction, μ | Maximum reference preload | Maximum reference torque |
|---|---|---|
| 0.10 | 30.3 kN | 43 N·m |
| 0.14 | 28.8 kN | 54 N·m |
These are published upper reference values, based on 90% utilization of yield strength, suitable head strength and bearing area, and medium clearance holes. No safety factor is included. They are not two interchangeable assembly recommendations.[2]
Notice that the higher torque accompanies a lower preload. Thread and bearing friction consume torque, and the bolt also carries torsional stress during tightening. A torque limit based on bolt strength is therefore different from a pure tensile-load limit.[1][3]
Hold preload constant to see the effect of friction
Assume an initial preload of 20.0 kN solely for this calculation. Use the same M10 × 1.5, class 8.8 reference geometry, with ordinary non-self-locking hardware. The model omits tool error, friction scatter, settling, temperature changes, and service loads; 20.0 kN is an illustrative input, not a recommended preload.
A compact approximation is T ≈ K × F × d: torque T in N·m, preload F in N, nominal diameter d in meters, and dimensionless nut factor K. K is not the friction coefficient μ. It combines the effects of thread geometry and contact friction.[1][3]
Bossard’s M10 conversion factors, X = 1.42 mm and 1.89 mm, give K = X/d = 0.142 and 0.189.[2] With the same assumed preload:
- μ = 0.10: T ≈ 0.142 × 20,000 × 0.010 = 28.4 N·m.
- μ = 0.14: T ≈ 0.189 × 20,000 × 0.010 = 37.8 N·m.
Now keep the old torque after friction changes. At 37.8 N·m, changing μ from 0.14 to 0.10 gives F ≈ 37.8/(0.142 × 0.010) = 26.6 kN, about 33% above the assumed target. In the reverse case, retaining 28.4 N·m as μ rises to 0.14 gives only 15.0 kN, about 25% below target.
The two μ values do not stand for universal “dry” and “lubricated” conditions. A coating, oil, or anti-seize compound needs friction data for the actual mating surfaces. The calculation shows why adding lubricant while retaining an old torque setting can materially change bolt tension; it does not provide a universal percentage reduction for lubrication.[3]
Check a preload range, not just a nominal result
A production joint adds variation to that calculation. A calibrated wrench controls applied torque; it does not eliminate variation in thread and bearing friction. NASA-STD-5020B bases nominal preload on supporting tests and explicitly accounts for preload variation in its fastening systems.[4]
For a simple initial screening with positive, bounded K values, fixed diameter, and no running torque, the equation gives:
- Lowest predicted preload: Fmin ≈ Tmin/(Kmax × d).
- Highest predicted preload: Fmax ≈ Tmax/(Kmin × d).
Use the lower initial preload to calculate the remaining clamp force after settling, temperature effects, and service-load unloading, then check that it meets the joint’s requirement. Use the upper initial preload to check the bolt under combined loading, the mating threads, and the bearing surfaces. Select a torque setting only if its entire predicted initial-preload range fits within the limits established by that joint analysis.
If no torque setting can meet both limits across the expected variation, the design, friction control, or tightening method must change. VDI 2230 Part 1:2015-11 provides a systematic calculation framework for highly stressed single-bolt joints.[1][5]
What makes a torque specification usable
The assembly instruction needs the hardware and washer arrangement, finish and lubricant with application locations, required preload range, and a torque range tied to the tightening method. State whether the bolt head or nut is turned. Complete the fastener callout with How to Specify a Bolt.
Where suitable torque–preload data are missing, request testing of representative hardware and surfaces. ISO 16047:2005, with Amendment 1:2012, specifies torque/clamp-force test conditions within its scope; it does not assign a universal installation torque. Fasteners with additional self-locking features are excluded from that scope.[6] Where a locking feature produces running torque, state whether the specified setting is total torque or torque above that running torque. The running component does not generate preload.[4]
A useful bolt torque value is backed by a preload range and controlled assembly conditions. Matching the bolt size and grade is only the start.
References
- NASA. Preloaded Joint Analysis Methodology for Space Flight Systems, NASA TM-106943, 1995, preload mechanics, torque coefficient, preload changes and joint-failure analysis. General mechanics are used here; its aerospace design defaults are not prescribed for other equipment.
- Bossard. Preload and tightening torques, January 2025, printed pp. 60–62. Manufacturer reference values and conversion factors; the comparison calculations and chart are HexFastener’s worked example.
- Fastenal. Bolted Joint Design, revision March 4, 2009, PDF pp. 9–11: torque-controlled tightening and nut-factor variability.
- NASA. NASA-STD-5020B, 2021, Appendix A.2 and installation documentation requirements. A spaceflight-hardware standard, cited for preload variation and running-torque treatment.
- VDI. VDI 2230 Part 1:2015-11, official scope: systematic calculation of highly stressed joints with one cylindrical bolt.
- ISO. ISO 16047:2005, Fasteners — Torque/clamp force testing, with Amendment 1:2012; official public scope and exclusions.