Fasteners

Bolts, screws and nuts hold machines together. Learn thread terminology, property classes, tensile stress area, preload and tightening torque, joint stiffness and load sharing, bolt groups in shear, and locking methods.

A car has thousands of fasteners; a wind turbine tower has hundreds of huge preloaded bolts. When bolts fail — by loosening, fatigue or overload — the consequences can be severe.

The key to a reliable bolted joint is correct preload: a properly tightened bolt clamps the joint so external loads mostly reduce clamp force rather than increasing bolt stress.

An analogy

A tightened bolt is a stretched spring holding two squeezed springs (the plates) together. As long as the plates stay squeezed, the bolt barely notices external loads.

Threads and grades

Metric ISO threads are specified as M12 × 1.75 (diameter × pitch). Property class 8.8 means S_u = 800 MPa and S_y = 0.8 × 800 = 640 MPa; 10.9 and 12.9 are stronger. Nuts must match (class 8, 10, 12). Stress area A_s (M12: 84.3 mm²) is used for tensile calculations.

Preload and torque

Most tightening torque (≈ 90 %) is lost to thread and under-head friction, so torque-controlled preload scatters by ±25 %. More precise methods: torque-angle, yield-controlled tightening, hydraulic tensioners, direct tension indicators and ultrasonic bolt elongation.

Joint load sharing

Clamped members are much stiffer than the bolt (C ≈ 0.2–0.3), so only a small fraction of an external load increases bolt tension, while the rest reduces clamp force. The joint must not separate: F_i > (1 − C)P with margin.

Fatigue and loosening

Bolts fail in fatigue at the first engaged thread; high preload reduces alternating stress. Rolled threads (after heat treatment) improve fatigue strength. Transverse slip causes self-loosening (Junker test) — prevent it with adequate preload, wedge-locking washers, thread-locking adhesive or prevailing-torque nuts rather than spring washers. VDI 2230 is the standard design method for highly stressed joints.

Formulas

• Recommended preload: F_i = 0.75 · S_p · A_s (reusable) … 0.9 (permanent)
• Tightening torque: T = K · F_i · d
• Bolt load with external tension P: F_b = F_i + C·P, F_m = F_i − (1 − C)·P
• Bolt group (direct shear): F_per bolt = V / n (+ torsional component for eccentric loads)

Worked example: Preload and torque

Given: M12 class 8.8: A_s = 84.3 mm², S_p = 600 MPa; K = 0.2

Find: Preload and torque.

• F_i = 0.75 × 600 × 84.3 = 37.9 kN
• T = 0.2 × 37 935 × 0.012 = 91 N·m

Answer: ≈ 38 kN preload, ≈ 91 N·m

Worked example: External load on a preloaded joint

Given: F_i = 37.9 kN, C = 0.25; P = 10 kN per bolt

Find: Bolt load and remaining clamp.

• F_b = 37.9 + 0.25 × 10 = 40.4 kN
• F_m = 37.9 − 0.75 × 10 = 30.4 kN (joint stays closed)

Answer: Bolt 40.4 kN; clamp 30.4 kN

Common mistakes

• Wrong: Using gross diameter area for bolt stress. — Right: Use the tensile stress area A_s.
• Wrong: Relying on spring washers against vibration loosening. — Right: Use proper preload plus effective locking (wedge washers, adhesive, prevailing-torque nuts).

Applications

• Engines: Cylinder-head and connecting-rod bolts tightened by torque-angle to yield.
• Structures & wind turbines: Pre-loaded high-strength bolts (HV/HRC) in flanges and slip-critical joints.
• Pressure equipment: Flange bolts preloaded to seal gaskets.

Key takeaways

• Use stress area A_s and property class.
• F_i ≈ 0.75 S_p A_s; T = K F_i d.
• Bolt sees only C × external load until separation.
• Preload is the key to fatigue and loosening resistance.

Going further

• VDI 2230: Systematic calculation of bolted joints including embedding loss, thermal effects, tightening scatter and fatigue.
• Eccentric bolt groups: Loads not through the group's centroid add torsional shear proportional to each bolt's distance.
• Ultrasonic preload measurement: Time-of-flight changes measure actual bolt stretch in critical joints.