Bolted joints

Bolted joints are common in steelwork , and care should be taken to ensure that the correct bolts are used. Standard “black” bolts are used for normal connections, and generally carry loads in shear, but are generally specified by their tensile strength. Care should be taken that the specified bolts (and nuts) are not mixed with ungraded bolts that may be on site for other purposes. High strength friction grip bolts (and nuts) are made from higher-grade steel, and are torqued to transfer loads by friction between the steel members.

Overview of Bolted Joints and Mechanical Properties

The ISO metric bolted joints and their mechanical properties are presented in this section as background information. Figure 12.1shows the nomenclature for ISO metric threads. The ISO threads can be described in the form of M d × P and a coarse pitch is assumed when P is omitted in the specification. The following parameters are used to define the ISO metric thread geometry:

H = height of fundamental triangle in mm

P = pitch in mm

D = major diameter of internal threads (nut) in mm

D1 = minor diameter of internal threads in mm

D2 = pitch diameter of internal threads in mm

d = major (nominal) diameter of external threads (bolt) in mm

d2 = pitch diameter of external threads in mm = d − 0.649519 P

d3 = minor diameter of external threads in mm = d − 1.226869 P

α = thread (flank) angle

A through-bolted joint and a tapped thread joint (also termed a screw joint) are the commonly used joint assemblies in the automotiveindustry. The through-bolted joint and the tapped thread joint are abbreviated to DSV and ESV in VDI 2230, respectively. Figures  depict the sections of these joints subjected to an external axial load and their theoretical deformation (pressure) cone shapes.

The following variables are used to define these bolted joints:

DA = diameter of the interface surface of circular clamped plates

DK = diameter of the deformation cone at the interface surface of circular clamped plates

Dkm = effective diameter of bolt head or nut-bearing area

dw = diameter of a washer head

dh = clearance hole diameter

lK = total clamping length

h = thickness of a clamped plate

φD = deformation cone angle for a through-bolted (DSV) joint

φE = deformation cone angle for a tapped thread (ESV) joint

z = distance from the washer (bearing) surface

dz = distance increment from the washer (bearing) surface

The induced internal forces and externally applied loads to a bolted joint are introduced here. The induced bolt preload, clamp loads, balanced thread torque, and under-head torque in a bolted joint due to an assembly tightening torque are shown in Figure 12.4, and the applied loads to a through-bolted joint are illustrated in Figure 12.5. The following symbols are used:

FM = bolt preload load due to MA

FA = axial force on the bolted axis

FQ = transverse force normal to the bolt axis

MA = applied assembly torque

MG = thread torque

MK = under-head torque

MB = bending moment at the bolting point

MT = torque (twist moment) at the bolt position at the interface

Induced bolt preload, clamp loads, balanced thread torque, and under-head torque in a bolted joint due to an assembly tightening torque

Some design guidelines for hex cap screws, coefficients of friction on various contact surfaces, and mechanical properties are presented. The recommended clearance hole diameter and the minimum bearing diameter of hex cap screws for a specific ISO metric thread are shown in Tables 12.1 and 12.2.

Thread Diameter	Clearance Hole dh (mm)
M3	3.4
M4	4.5
M5	5.5
M6	6.6
M8	9.0
M10	11.0
M12	13.5
M14	15.5
M16	17.5
M18	20.0
M20	22.0
M22	24.0
M24	26.0
M27	30.0
M30	33.0 Thread Diameter Width across the Flats (mm) Bearing Diameter dw,min (mm) M4 7 5.9 M5 8 6.9 M6 10 8.9 M8 13 11.6 M10 16 14.6 M10 17 15.9 M12 18 16.6 M12 19 17.4 M14 21 19.6 M14 22 20.5 M16 24 22.5 M18 27 25.3 M20 30 28.2 M22 32 30.0 M22 34 31.7 M24 36 33.6 M27 41 38.0 M30 46 42.7 Moreover, Tables 12.3 through 12.5 summarize the coefficients of friction in the thread, in the bolt- or nut-bearing area, and between the clamp plate interfaces, where μG = coefficient of friction in the thread μK = coefficient of friction in the bolt or nut-bearing area μT = coefficient of friction between the clamp plate interfaces μG External Thread (Bolt) Steel Black Oxide or Phosphated Zinc Plated Cad. Plated Adhesive Rolled Machined Machined or Rolled Dry Oiled MoS2 Oiled Dry Oiled Dry Oiled Dry Internal Threads (Nut) Steel Plain Machined Dry 0.12 to 0.18 0.10 to 0.16 0.08 to 0.12 0.10 to 0.16 — 0.10 to 0.18 — 0.08 to 0.14 0.16 to 0.25 Zinc Pl. 0.10 to 0.16 — — — 0.12 to 0.20 0.10 to 0.18 — — 0.14 to 0.25 Cad. Pl. 0.08 to 0.14 — — — — — 0.12 to 0.18 0.12 to 0.14 — Cast Iron Plain — 0.10 to 0.18 — 0.10 to 0.18 — 0.10 to 0.18 — 0.08 to 0.16 — Al-Alloy Plain — 0.08 to 0.20 — — — — — — — Note: MoS2 = molybdenum disulfide (Moly-lub) lubricated. Source: Adapted from Marbacher (1998).) (μK) Bolt Head Steel Black Oxide or Phosphated Zinc Plated Cad. Plated Cold Headed Machined Ground Cold Headed Dry Oiled MoS2 Oiled MoS2 Oiled Dry Oiled Dry Oiled Material of Joint Members Steel Plain Ground Dry — 0.16 to 0.22 — 0.1 to 0.18 — 0.16 to 0.22 0.10 to 0.18 — 0.08 to 0.16 — Machined 0.12 to 0.18 0.10 to 0.18 0.08 to 0.12 0.10 to 0.18 0.08 to 0.12 — 0.10 to 0.18 0.10 to 0.18 0.08 to 0.18 0.08 to 0.14 Zinc Pl. 0.10 to 0.16 0.10 to 0.16 — 0.10 to 0.16 — 0.10 to 0.18 0.16 to 0.20 0.10 to 0.18 — — Cad. Pl. 0.08 to 0.16 0.08 to 0.16 0.08 to 0.16 0.08 to 0.16 0.08 to 0.16 0.08 to 0.16 — — 0.12 to 0.20 0.12 to 0.14 Cast Iron Plain Ground — 0.10 to 0.18 — — — 0.10 to 0.18 0.10 to 0.18 0.10 to 0.18 0.06 to 0.16 — Machined — 0.14 to 0.20 — 0.10 to 0.18 — 0.14 to 0.22 0.10 to 0.18 0.10 to 0.16 0.08 to 0.16 — Al. Alloy — 0.08 to 0.20 0.08 to 0.20 0.08 to 0.20 0.08 to 0.20 — — — — — Note: MoS2 = molybdenum disulfide (Moly-lub) lubricated. Material Combination Static Friction Coefficient in the State Dry Lubricated Steel-steel/cast steel 0.1 to 0.23 0.07 to 0.12 Steel-grey cast iron 0.12 to 0.24 0.06 to 0.1 Grey cast iron-grey cast iron 0.15 to 0.3 0.2 Bronze-steel 0.12 to 0.28 0.18 Grey cast iron-bronze 0.28 0.15 to 0.2 Steel-copper alloy 0.07 Steel-aluminum alloy 0.1 to 0.28 0.05 to 0.18 Aluminum-aluminum 0.21 Source: Adapted from VDI 2230, published by Beuth Verlag GmbH, Berlin (2003). Table 12.6 shows the mechanical properties for metric steel bolts, screws, and studs, and Table 12.7 illustrates the ratio of ultimate shear strength to ultimate tensile strength for various materials, where RmS = ultimate tensile strength of the external threads (bolt) in N/mm2 RmM = ultimate tensile strength of the internal threads (nut) in N/mm2 RP0.2min = minimum 0.2% yield strength of the external threads in N/mm2 τBM = ultimate shear strength of the internal threads in N/mm2 τBS = ultimate shear strength of the external threads in N/mm2 Strength Grade RP0.2min (N/mm2) RmS (N/mm2) 4.6 240 400 4.8 340 420 5.8 420 520 8.8 660 830 9.8 720 900 10.9 940 1040 12.9 1100 1220 Materials τBM/RmM or τBS/RmS Annealing steel 0.60 Austenitic (solution heat treated) 0.80 Austenitic F60/90 0.65 Grey cast iron 0.90 Aluminum alloys 0.70 Titanium alloy (age-hardened) 0.60