Socket head cap screw

01

Choose the head, material and finish

Haiyan Bolt, a fastener maker linked to this page, describes the hex socket bolt (socket head cap screw) as having a hexagonal socket in a head that it says "is usually cylindrical in shape", for a hex key or Allen wrench, a shank with a threaded portion for a nut or tapped hole, and coarse or fine threads to national and international standards. It lists steel that can be quenched and tempered, stainless steel for corrosion resistance, alloy steels containing chromium, molybdenum or vanadium, and, in special cases, aluminium or brass, and notes that the compact cylindrical head suits confined spaces. [2]

NASA's Fastener Design Manual (Reference Publication 1228) cautions that high-strength fasteners above 180 ksi bring problems such as brittleness, critical flaws, forged heads, cold rolling of threads and the need for stringent quality control, and that it is preferable to use more fasteners of ordinary strength than a few high-strength ones. Because, in NASA's words, "most plating processes are the electrolytic bath type" and so release free hydrogen, it notes that plating specifications require plated carbon steel fasteners to be baked for not less than 23 hours at 375 ± 25 °F within 2 hours after plating, and that hydrogen embrittlement problems increase with fastener strength. [1]

Editorial note: 180 ksi is about 1,240 MPa (180 × 6.895); when you order high-strength plated socket head cap screws, ask the supplier for the plating process and the embrittlement-relief baking record.

A specialist maker's catalogue gives the property values to compare. Unbrako's US product guide lists its 1960-series alloy-steel socket head cap screws (ASME B18.3) at Rockwell C39–43 with tensile strength of 190 ksi up to ½ in and 180 ksi above, yield strength of 170 and 162 ksi and shear strength of 114 and 108 ksi, a working temperature of −50 to +300 °C, and describes rolling threads after heat treatment as adding fatigue life. It states that its recommended seating torques are guidelines only, calculated to VDI 2230 with a 0.2% yield of 155 ksi and a friction coefficient of 0.125 for plain finish, and that the induced loads may vary by as much as ±25% depending on mating material, lubrication, surface finish, hardness and joint compliance. [3]

Give the supplier the standard (ISO, DIN or ASME/ASTM designation), diameter, pitch and length, thread length, property class or grade, material, finish or plating and any baking requirement, head and socket dimensions if non-standard, inspection and test certificates, quantity and packing.

02

Tighten to preload with the right torque coefficient

The NASA manual gives the quick formula T = KFd (torque = torque coefficient × axial load × bolt diameter). It says "the commonly assumed value for K is 0.2, but this value should not be used blindly", and proposes 0.15 as a more realistic value for steel on steel, and its table gives K from 0.074 to 0.250 for friction coefficients of 0.05 to 0.20. Friction coefficients can range from 0.04 to 1.10 depending on materials and lubricants, so torque tables must be adjusted to the user's friction. Proof load, it says, "is usually 75 percent of theoretical yield", to prevent inadvertent yielding through torque inaccuracies, and torquing close to yield is not advisable. [1]

Editorial calculation: for an M10 screw (d = 0.010 m) and a target preload of 20 kN, T = 0.15 × 20,000 × 0.010 = 30 N·m with K = 0.15, but 0.2 × 20,000 × 0.010 = 40 N·m with K = 0.2. If the true K were 0.15 and 40 N·m were applied, the preload would be 40 / (0.15 × 0.010) ≈ 26.7 kN, a third higher than intended. Size and preload are assumed examples; take preload limits from the screw's property class and confirm K for your finish and lubricant.

Editorial calculation: Unbrako's ±25% preload band means a joint torqued for 20 kN may hold anywhere from 15 to 25 kN, so if the minimum clamp is critical, specify the torque for the low end or use a preload-controlled method; and 190 ksi is about 1,310 MPa (190 × 6.895), above NASA's 180 ksi caution threshold. [3] [1]

03

Check head, socket and stress-area dimensions for metric ISO 4762 screws

OPAC's product page for its ISO 4762 (DIN 912) socket head cap screw lists heat-treated alloy steel in property class 12.9, with mechanical properties to ISO 898-1, and gives a dimension table by thread size. For M2 to M12 it lists the maximum head diameter D1, maximum head height K, nominal socket size S and the usable (stress) section in mm². Its order code takes the form H912-diameter-length, for example H912-6-20 for an M6 × 20 screw. [4]

Two points follow for an order. First, the head height K equals the nominal diameter in every size listed, from 2 mm for M2 to 12 mm for M12, so the counterbore depth for a flush head is set by the screw size; check D1 against the counterbore diameter as well. Second, the socket size S sets the hex key or bit, from 1.5 mm for M2 to 10 mm for M12, so name the size range when ordering tools with the screws. [4]

Editorial calculation: the tensile stress in the screw is preload divided by the stress area. For the 20 kN preload used in the torque example above, an M10 screw with 58 mm² carries about 20,000 ÷ 58 ≈ 345 MPa, an M12 with 84.3 mm² about 237 MPa and an M8 with 36.6 mm² about 546 MPa. Compare that stress with the proof stress of the property class you specify under ISO 898-1, and ask the supplier for test certificates showing the class was met. [4]

04

Compare what suppliers offer

Suppliers linked to this page list socket head cap screws; each card links to the page reviewed.

Listings include hex socket bolts with other bolts and screws, and socket head bolts from a fastener maker's range. Ask each for standard, grade, finish, baking practice and test certificates.

05

Trace the screw features

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your joint

A socket head cap screw quotation needs values for your joint. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the joint, screw and finish data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Screw: [standard; d × pitch × length; thread length]
Grade: [property class; material]
Finish: [plating; baking; corrosion test]
Joint: [materials; preload; tightening method]
Documents: [certificates; inspection]
Order: [quantity; packing]

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