Bolt preload is the tensile force intentionally created in a fastener when a joint is tightened. This cheat sheet covers how tightening torque produces preload, why friction dominates the torque demand, and how engineers estimate safe target clamp loads. Students need it because bolted joint design depends on keeping enough clamping force without yielding the bolt or losing joint integrity.
The core relationship is T = K F d, where T is tightening torque, F is preload, d is nominal bolt diameter, and K is the nut factor. More detailed models split torque into thread torque and bearing torque, both strongly affected by friction coefficients and geometry. Design checks compare target preload with proof load, yield strength, joint separation limits, slip resistance, and tightening method accuracy.
Key Facts
- The common torque-preload estimate is T = K F d, where T is torque, K is nut factor, F is bolt preload, and d is nominal bolt diameter.
- A typical nut factor for clean, lightly lubricated steel fasteners is about K = 0.18 to 0.22, but actual values can vary widely.
- Bolt tensile stress area is approximated for metric threads by As = pi/4 (d - 0.9382 p)^2, where d is nominal diameter and p is thread pitch.
- Bolt tensile stress from preload is sigma = F / As, where F is preload and As is tensile stress area.
- A common target preload for reusable structural joints is Fi = 0.70 Sp As, where Sp is proof strength and As is tensile stress area.
- Total tightening torque can be modeled as T = T_thread + T_bearing, where thread friction and under-head or nut-face friction both consume torque.
- Only a small fraction of applied torque becomes useful bolt stretch, and most tightening torque is lost to friction at the threads and bearing surface.
- For a slip-critical joint, the available friction resistance is R = mu_joint n Fi, where mu_joint is interface friction coefficient, n is the number of slip planes, and Fi is preload per bolt.
Vocabulary
- Preload
- The tensile force placed in a bolt when it is tightened before any external service load is applied.
- Clamp load
- The compressive force that the tightened bolt applies to the joined members.
- Nut factor
- An empirical coefficient K in T = K F d that combines the effects of thread friction, bearing friction, and geometry.
- Proof load
- The maximum tensile load a fastener can support without permanent deformation under a specified proof test.
- Tensile stress area
- The effective cross-sectional area of a threaded bolt used to calculate tensile stress from axial load.
- Torque scatter
- The variation in achieved preload for the same applied torque due to friction, lubrication, surface condition, and tool accuracy.
Common Mistakes to Avoid
- Using T = F d without the nut factor is wrong because it ignores friction, which accounts for most of the applied tightening torque.
- Assuming the nut factor is always K = 0.20 is wrong because lubrication, coatings, surface roughness, and washer condition can change preload dramatically.
- Using nominal shank area instead of tensile stress area for threaded sections is wrong because the thread root reduces the effective load-carrying area.
- Setting preload equal to yield load is wrong for most designs because tightening scatter and service loads can push the bolt into permanent deformation.
- Treating applied torque as a direct measurement of preload is wrong because torque control is indirect and can have large preload uncertainty.
Practice Questions
- 1 A bolt has nominal diameter d = 12 mm, nut factor K = 0.20, and target preload F = 30 kN. What tightening torque T is required using T = K F d?
- 2 An M10 x 1.5 bolt has proof strength Sp = 600 MPa. Using As = pi/4 (d - 0.9382 p)^2, estimate As and the target preload Fi = 0.70 Sp As.
- 3 A slip-critical joint has 4 bolts, preload Fi = 25 kN per bolt, joint friction coefficient mu_joint = 0.35, and one slip plane. What total slip resistance is available?
- 4 Explain why two bolts tightened to the same torque can have different preload values even when they have the same diameter and grade.