ASME B18.3 Hexagon Socket Head Shoulder Screw

ASME B18.3 Dimensions of Hexagon Socket Head Shoulder Screw

GENERAL NOTE:
For additional requirements, refer to Notes (5) through (16) and section 2, General Data, of this Standard.
NOTES:
(1) Screw Point Chamfer. The point shall be flat or slightly concave, and chamfered. The plane of the point shall be approximately normal to the axis of the screw. The chamfer shall extend slightly below the root of the thread, and the edge between flat and chamfer may be slightly rounded. The included angle of the point should be approximately 90 deg.
(2) Edge of Shoulder. The edge of the shoulder may be broken. The radius or chamfer shall not exceed 0.005 in. for shoulders to 0.373 in. diameter and 0.008 in. for larger diameters. (See Detail Z.)
(3) Head Diameter. The head may be plain or knurled at the option of the manufacturer.
(4) Neck Under Shoulder. The neck under the shoulder shall allow the shoulder to seat against the face of a standard basic GO thread ring gage.
(5) Shoulder. Shoulder refers to the enlarged unthreaded portion of the screw, the diameter of which serves as the basis for derivation of the nominal size.
(6) Length. The length of the shoulder screw shall be measured, parallel to the axis of the screw, from the plane of the bearing surface under the head to the plane of the shoulder at the threaded end. The basic length dimension on the product shall be the nominal length expressed as a three-placed decimal.
(7) Standard Lengths. The difference between consecutive lengths of standard shoulder screws shall be as designated in the following tabulation.

(8) Neck and Fillet Under Head. Screws may be necked under the head at the option of the manufacturer. The fillet extension above D, at the intersection of the head bearing surface and neck or shoulder, shall be controlled by maximum dimension, M, and minimum radius of curvature, R. (See Detail Y.)
(9) See Table 18 for hexagon socket dimensions and Mandatory Appendix I for gaging of hexagon sockets.
(10) Threads. Threads shall be Unified external thread: Class 3A, UNC Series. Acceptability is to be based on System 22, ASME B1.3.Class 3A does not provide a plating allowance. When plated products are required, it is recommended that they be procured from the manufacturer (see para. 1.10).
(11) Head Chamfer. The top of the head shall be flat. The intersection of the top of the head and the side of the head may be chamfered or radiused within the limits of C, at the manufacturer’s option.
(12) Thread Length Tolerance. Tolerance on thread length E shall be −0.020 in. for screw thread sizes up to 3⁄8 in., inclusive, and −0.030 in. for screw thread sizes larger than 3⁄8 in.
(13) Bearing Surface. The plane of the bearing surface shall be perpendicular to the axis of the shank, within 2 deg obtained by holding the shoulder screw on the shoulder or major thread diameter within 1 diameter of the bearing surface of the head, rotating 360 deg, and inspecting on an optical comparator, or comparable inspection equipment.

Runout
(a) The runout of the head with the axis of the shoulder shall be within 2% of the nominal size, or 0.006 in., whichever is greater.
Runout for (a) above is defined as the full indicator movement (FIM) obtained by holding the shoulder screw on the shoulder within 1 diameter of the bearing surface of the head, but beyond the maximum length of F, rotating the shoulder screw 360 deg and indicating on the outside diameter of the head.
(b) The runout of the socket with the axis of the shank of the shoulder screw shall be within 3% of the maximum shoulder diameter, D, or 0.005 in., whichever is greater for nominal sizes through 1⁄2-in. diameter and 6% for nominal sizes above 1⁄2 diameter.
Runout for (b) above is defined as the full indicator movement (FIM) obtained by holding the shoulder screw on the shoulder or major screw thread diameter within 1 diameter of the bearing surface of the head, but beyond the maximum length of F, rotating 360 deg and indicating on each of the six hexagon flats.
Runout between the screw thread and shoulder shall be checked by firmly seating the screw thread in a threaded bushing. The thread of the bushing must be basic size and the bushing O.D. must be concentric with its axis, and the ends must be square with its axis.
The runout between the screw thread and shoulder shall be a maximum of 0.004 in. full indicator movement (FIM) when checked on the shoulder diameter at a distance of 0.188 in. from the shoulder at the threaded end, and at within 0.005 in. full indicator movement (FIM) per inch of shoulder length, with a maximum of 0.025 in., when checked on the shoulder a distance of 2F from the underside of the head. The bushing is to be rotated, in a V block or equivalent, 360 deg, with the indicator riding on the major diameter of the shoulder in the appropriate locations.
The squareness of the shoulder shall be perpendicular to the axis of the shoulder screw, within 2 deg obtained by holding the shoulder screw on the shoulder diameter within 1 diameter of the thread end of the shoulder, rotating 360 deg, and inspecting on an optical comparator or comparable inspection equipment.

(15) Material. Shoulder screws shall be fabricated from an alloy steel having one or more of the following alloying elements: chromium, nickel, molybdenum, or vanadium, in sufficient quantity to ensure that the specified hardness range of 32 HRC to 43 HRC at the surface is met when hardened by quenching from the austenitizing temperature and tempered at not lower than 343°C (650°F). Decarburization and carburization limits shall be the same as those specified for socket head cap screws in ASTM A574.
Shoulder screws shall meet the following mechanical property requirements: the hardness shall be 32 HRC to 43 HRC anywhere in the section.
There are no other specific mechanical tests called out, but for design considerations, these shoulder screws should develop the following:
(a) 140,000 psi minimum tensile strength based on the minimum thread neck area
(b) 84,000 psi minimum shear strength in the thread neck area, based on the minimum thread neck area
(c) 84,000 psi minimum shear strength in the shoulder, based on the minimum shoulder area
(16) Designation. To promote uniformity and understanding in communications relating to products conforming to this Standard, it is recommended that Hexagon Socket Head Shoulder Screws be designated in accordance with the following data, preferably in the sequence shown:
(a) product name
(b) designation of the standard
(c) nominal size (number, fractional or decimal equivalent)
(d) nominal length (fractional or decimal equivalent)
(e) material.
(f) protective finish, if required (for plated products, see para. 1.7)
EXAMPLES:
Hexagon Socket Head Shoulder Screws, ASME B18.3, 1⁄4  11⁄4 , Alloy Steel
Hexagon Socket Head Shoulder Screws, ASME B18.3, 0.250  1.250, Alloy Steel, Phosphate Coated
For the recommended B18 part identifying numbering system (PIN), see ASME B18.24.