Question 1

What is the torque formula for bolt tightening?

Accepted Answer

The simplified nut-factor formula is: T = K Ã— F Ã— d, where T = tightening torque (NÂ·m), K = nut factor (dimensionless, typically 0.15â€“0.25), F = desired clamp/bolt load (N), d = nominal bolt diameter (m). This formula accounts for thread and bearing face friction combined into the K factor.

Question 2

What is the nut factor (K factor) for bolts?

Accepted Answer

The nut factor K combines the effect of thread pitch, thread friction, and bearing friction into a single coefficient. Typical values: dry/as-received steel â‰ˆ 0.20, lightly oiled â‰ˆ 0.17, heavy grease/MoSâ‚‚ â‰ˆ 0.13, zinc plated â‰ˆ 0.15, hot-dip galvanised â‰ˆ 0.25. K varies significantly with surface condition â€” always verify with fastener manufacturer data for critical joints.

Question 3

What percentage of proof load should I tighten bolts to?

Accepted Answer

Standard practice for non-critical joints: 65â€“75% of proof load. For structural and critical joints: 75â€“85% of proof load. ISO/VDI 2230 recommends 70% of yield strength for friction-grip bolted connections. Never exceed 90% of proof load â€” this risks bolt failure or permanent elongation.

Question 4

What is proof load for a bolt?

Accepted Answer

Proof load is the maximum tensile force a bolt can sustain without permanent deformation. It is approximately 90% of the yield strength times the tensile stress area. Proof load (N) = proof strength (MPa) Ã— tensile stress area (mmÂ²). Standard bolt grades (e.g., 8.8, 10.9) have defined proof strengths specified in ISO 898-1.

Question 5

What is the difference between ISO 8.8 and 10.9 bolts?

Accepted Answer

ISO 8.8: proof strength 600 MPa, yield 660 MPa, UTS 800 MPa. Most common high-strength grade for general and structural applications. ISO 10.9: proof strength 830 MPa, yield 940 MPa, UTS 1040 MPa. Used in heavy machinery, automotive and high-fatigue joints. Grade 10.9 bolts require more controlled tightening and are more susceptible to hydrogen embrittlement.

Question 6

How does lubrication affect bolt tightening torque?

Accepted Answer

Lubrication significantly reduces the torque needed to achieve the same clamp load. Example for M16 bolt to 80% proof load: Dry: â‰ˆ243 NÂ·m. Lightly oiled: â‰ˆ207 NÂ·m. MoSâ‚‚ grease: â‰ˆ158 NÂ·m. Galvanised (dry): â‰ˆ303 NÂ·m. Always define the lubrication condition in torque specifications â€” an error in K factor of Â±0.05 causes Â±25% error in clamp load.

Question 7

What tool do I use to achieve accurate bolt torque?

Accepted Answer

Reliable torque application methods in order of accuracy: 1) Torque wrench (click or beam type) â€” Â±4%, 2) Torque angle method â€” Â±3%, 3) Stretch measurement (ultrasonic or elongation gauges) â€” Â±1%, 4) Hydraulic tensioners â€” Â±5%. Standard torque wrenches (Â±4%) are acceptable for most joints. Critical joints (pressure vessels, structural) may require angle or elongation methods.

Question 8

What bolt grades are used for pressure vessels in South Africa?

Accepted Answer

South African pressure vessel bolting (governed by Pressure Equipment Regulations, SANS 347, and ASME standards): Flange bolting typically uses ASTM A193 Grade B7 studs (1040 MPa UTS) with A194 Grade 2H heavy hex nuts. For SS flanges: A193 B8M (316 SS). For temperatures above 400Â°C, use A193 B16. All pressure equipment bolting must be traceable and certified to the appropriate material standard.

Grade	Proof Str. (MPa)	Yield Str. (MPa)	UTS (MPa)	Application
4.6	225	240	400	M5â€“M100. General purpose, low carbon steel.
4.8	310	340	420	M1.6â€“M16. General purpose, no heat treat.
5.6	380	300	500	M5â€“M100. Medium grade.
6.8	440	480	600	M1.6â€“M16. Medium carbon steel.
8.8	600	660	800	M16â€“M72. Structural & general use. Most common high-strength bolt.
10.9	830	940	1040	M5â€“M100. High strength, medium carbon alloy steel, quenched & tempered.
12.9	970	1100	1220	M1.6â€“M100. Very high strength. Alloy steel, Q&T. Avoid hydrogen embrittlement risk.
A2-70	450	450	700	304 SS. Corrosion resistant, moderate strength.
A2-80	600	600	800	304 SS â€” higher strength version.
A4-70	450	450	700	316 SS. Marine and chemical environments.
A4-80	600	600	800	316 SS â€” higher strength version.

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