Machinery's Handbook, 31st Edition
British Fasteners
1767
to 1 1 ∕ 4 times the nominal diameter of the shank. With rolled threads, the lead formed at the end of the bolt by the thread rolling operation may be regarded as providing the necesssary chamfer to the end; the end being reasonably square with the center line of the shank. Screw Thread Form: The form of thread and diameters and associated pitches of stan dard ISO metric bolts, screws, and nuts are in accordance with BS 3643:Part 1:1981 (2004), “Principles and Basic Data.” The screw threads are made to the tolerances for the medium class of fit (6 H /6 g ) as specified in BS 3643:Part 2:1981 (1998), “Specification for Selected Limits of Size.” Length of Thread on Bolts: The length of thread on bolts is the distance from the end of the bolt (including any chamfer or radius) to the leading face of a screw ring gage which has been screwed as far as possible onto the bolt by hand. Standard thread lengths of bolts are 2 d + 6 mm for a nominal length of bolt up to and including 125 mm, 2 d + 12 mm for a nominal bolt length over 125 mm up to and including 200 mm, and 2 d + 25 mm for a nominal bolt length over 200 mm. Bolts that are too short for minimum thread lengths are threaded as screws and designated as screws. The tolerance on bolt thread lengths are plus two pitches for all diameters. Length of Thread on Screws: Screws are threaded to permit a screw ring gage being screwed by hand to within a distance from the underside of the head not exceeding two and a half times the pitch for diameters up to and including 52 mm and three and a half times the pitch for diameters over 52 mm. Angularity and Eccentricity of Bolts, Screws and Nuts: The axis of the thread of the nut is square to the face of the nut subject to the “squareness tolerance” given in Table 5. In gaging, the nut is screwed by hand onto a gage, having a truncated taper thread, until the thread of the nut is tight on the thread of the gage. A sleeve sliding on a parallel exten sion of the gage, which has a face of diameter equal to the minimum distance across the flats of the nut and exactly at 90 degrees to the axis of the gage, is brought into contact with the leading face of the nut. With the sleeve in this position, it should not be possible for a feeler gage of thickness equal to the “squareness tolerance” to enter anywhere between the leading nut face and sleeve face. The hexagon flats of bolts, screws and nuts are square to the bearing face, and the angularity of the head is within the limits of 90 degrees, plus or minus 1 degree. The eccentricity of the hexagon flats of nuts relative to the thread diameter should not exceed the values given in Table 5 and the eccentricity of the head relative to the width across flats and eccentricity between the shank and thread of bolts and screws should not exceed the values given in Table 4. Chamfering, Washer Facing and Countersinking: Bolt and screw heads have a chamfer of approximately 30 degrees on their upper faces and, at the option of the manufacturer, a washer face or full bearing face on the underside. Nuts are countersunk at an included angle of 120 degrees plus or minus 10 degrees at both ends of the thread. The diameter of the countersink should not exceed the nominal major diameter of the thread plus 0.13 mm up to and including 12 mm diameter, and plus 0.25 mm above 12 mm diameter. This stipu lation does not apply to slotted, castle or thin nuts. Strength Grade Designation System for Steel Bolts and Screws: This Standard includes a strength grade designation system consisting of two figures. The first figure is one tenth of the minimum tensile strength in kgf/mm 2 , and the second figure is one tenth of the ratio between the minimum yield stress (or stress at permanent set limit, R 0.2 ) and the minimum tensile strength, expressed as a percentage. For example with the strength designation grade 8.8, the first figure 8 represents 1 ∕ 10 the minimum tensile strength of 80 kgf/mm 2 and the second figure 8 represents 1 ∕ 10 the ratio
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