BS 3692

ISO METRIC PRECISION HEXAGON BOLTS, SCREWS AND NUTS — SPECIFICATION

Product grades A and B

This British Standard gives the general dimensions and tolerances of precision hexagon bolts, screws and nuts with ISO metric threads in diameters from 1.6 mm to 68 mm inclusive. Mechanical properties are given only in respect of carbon or alloy steel bolts, screws and nuts, which are not to be used for special applications, such as those requiring weldability, corrosion resistance or ability to withstand temperatures above 300 ºC or below 50 ºC.

Finish

Steel precision bolts, screws and nuts are supplied with the following finishes:

a)  Heat-treated bolts and screws. Components heat-treated after manufacture are customarily dull black, although the manufacturer may machine some of the surfaces of the larger size bolts and screws after heat treatment.
b)  Bright finished bolts and screws. This term is used to describe bolts and screws which are finished bright on all surfaces or which have a finish on the hexagon, produced by bright drawing.
c)  Nuts. These may be bright on all surfaces or dull black when heat-treated.
d)  Other finishes. If the purchaser requires bolts, screws or nuts of steel (or other material) to be coated, they should state the type of coating required in their enquiry and order and it is recommended that reference be made to BS EN ISO 4042, in this respect.

If steel bolts, screws or nuts are required finished bright all over, the purchaser should state this in their enquiry and order

Length of bolts and screws
Nominal length

The nominal length of a bolt or screw is the distance from the underside of the head to the extreme end of the shank including any chamfer or radius. The standard nominal lengths shall be as given in Table 8.

Tolerance on length

The tolerance on the nominal length shall be as given in Table 8.

Ends of bolts and screws

The ends of bolts and screws may, at the option of the manufacturer, be finished with either a 45º chamfer to a depth slightly exceeding the depths of thread or a radius approximately equal to one and a quarter times the nominal diameter of the shank. When bolts and screws are made with rolled threads, the lead formed at the end of the bolt by the rolling operation may be regarded as providing the necessary chamfer to the end with no other machining operation being necessary, and the end shall be reasonably square with the centre line of the shank. (See Figure 5 and Figure 6.)

Screw threads
General

The form of thread and diameters and associated pitches of standard ISO metric bolts , screws and nuts shall be in accordance with BS 3643-1.

Tolerances

The screw threads shall be made to the tolerances for the medium class of fit (6H/6g) as specified in BS 3643-2, unless a purchaser requires other classes of fit or fine pitch threads when this shall be the subject of agreement between the purchaser and the manufacturer.

Length of thread on bolts and screws
Bolts

The length of thread on bolts shall be the distance from the end of the bolt (including any chamfer or radius) to the leading face of a screw ring gauge, which has been screwed as far as possible on to the bolt by hand.

The standard thread lengths, as given in Table D.2, are based on the formulae given in Table 1.

Table 1 — Basis for standard thread lengths

Nominal length of bolt l

Length of threadb

Up to and including 125 mm

2d + 6 mm

Over 125 mm up to and including 200 mm

2d + 12 mm

Over 200 mm

2d + 25 mm

The length of thread run out shall not exceed the values given in column 3 of Table 5.

Bolts that are too short for minimum thread lengths shall be threaded as screws and shall be designated screws. Guidance in this respect is given in Annex D

Tolerances

The tolerances on bolt thread lengths shall be plus two pitches for all diameters, as given in Table D.2.

The eccentricity of the thread relative to the shank of bolts shall not exceed the values given in column 19 of Table 5.

Screws

Screws shall be threaded to permit a screw ring gauge 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, in accordance with column 3 of Table 5.

Angularity and eccentricity of bolts, screws and nuts

The axis of the thread of the nut shall be square to the face of the nut subject to the “squareness tolerance” given In column 9 of Table 6, which is equivalent to an out of squareness of ±1º.

The nut shall be screwed by hand on to a gauge having a truncated taper thread until the thread of the nut is tight on the thread of the gauge. A sleeve sliding on a parallel extension of the gauge, and which has a face of diameter equal to the minimum distance across flats of the nut and at exactly 90º to the axis of the gauge, shall be brought into contact with the leading face of the nut.

The sleeve shall be in such a position so that it shall not be possible for a feeler gauge of thickness equal to the squareness tolerance to enter anywhere between the leading face of the nut and the face of the sleeve (see Annex A).

The hexagon flats of bolts, screws and nuts shall be square to the bearing face, and the angularity of the head shall be within the limits of 90º ± 1º (see Annex E).

The eccentricity of the hexagon flats of nuts relative to the thread diameter shall not exceed the values given in column 10 of Table 6.

The eccentricity of the head relative to the width across flats and eccentricity between the shank and thread of bolts and screws shall not exceed the values given in columns 18 and 19, respectively, of Table 5.

Chamfering, washer facing and countersinking
Bolts and screws

Bolt and screw heads shall have a chamfer of approximately 30º on their upper faces and, at the option of the manufacturer, a washer face or full bearing face on the underside unless the purchaser in his enquiry or order specifically states that he requires one of the alternatives available.

Nuts

Nuts shall have a chamfer of approximately 30º on both faces.

Nuts shall be countersunk at an included angle of 120º ± 10º at both ends of the thread.The diameter of the countersink shall conform to the requirements of ISO/FDIS 4759-1:2000 Figure 69 as follows:

d ≤ 5 mm:damax = 1.15d
5 mm < d ≤ 8 mm:damax = d + 0.75
d> 8 mm:damax = 1.08d
for all sizes:damin = d

Requirements apply to both sides of symmetrical parts.

Strength grade designation system for steel bolts and screws

Being in accordance with BS EN ISO 898-1, the strength grade designation system (property class symbol) for steel bolts and screws consists of two figures. The first is one hundredth of the minimum tensile strength in N/mm2, and the second is one hundredth of the ratio between the minimum yield stress (or stress at permanent set limit,R0.2) and the minimum tensile strength, expressed as a percentage, as illustrated below, for strength grade designation 8.8, in accordance with Table 2.

Table 2 — Strength grade designations of steel bolts and screws

Strength grade designation

4.6

4.8

5.6

5.8

6.8

8.8

10.9

12.9

Tensile strength Rm min. N/mm2

400

400

500

500

600

800

1000

1200

Yield stress Re min. N/mm2

240

320

300

400

480

-

-

-

Stress at permanent set limit R0.2 N/mm2

-

-

-

-

-

640

900

1080

Material and manufacture of steel bolts and screws
Method of production

Steel bolts and screws may be produced by cold forging, hot forging or by turning from bar. The choice of method shall be left to the manufacturer.

Chemical composition

The chemical compositions of steels, given in Table 2 of BS EN ISO 898-1, are for guidance only. The choice of steels shall be at the discretion of the manufacturer. The steel used shall be such that the finished product possesses the mechanical properties appropriate to the strength grade quoted.

Heat treatment

Grades designated 8.8, 10.9 and 12.9 shall be heat-treated to give the mechanical properties given in BS EN ISO 898-1.

Other grades may be heat-treated if this is necessary to obtain the mechanical properties given in BS EN ISO 898-1.

Decarburization

Decarburization of the thread surface shall not be greater than stipulated below for bolts and screws of grade designations 8.8, 10.9, and 12.9.

The depth of the non-decarburized zone shall be not less than two thirds of the depth of the basic thread. In the root of the thread, the decarburization shall not exceed one tenth of the depth of the basic thread.

The method of measuring the amount of decarburization shall be as specified in BS EN ISO 898-1.




Rolling laps

NOTE - When threads are produced by rolling, small laps are commonly present at the crests and are generally of a magnitude which is not detrimental to the performance of the bolt.

A lap formed at the crest of the thread perpendicular to the axis of the bolt shall not be considered a cause for rejection if the depth of the lap does not exceed 33 % of the depth of the thread.

Bolts of grades designated 10.9 and 12.9 shall be free from laps in the thread flank below the effective diameter. Laps in the flanks above the effective diameter shall not have a depth greater than 33 % of the thread depth.

Mechanical properties of steel bolts and screws

Steel bolts and screws shall meet the requirements for mechanical properties given in BS EN ISO 898-1.

Strength grade designation system for steel nuts

The strength grade designation system for steel nuts shall be a number that is one hundredth of the specified proof load stress in N/mm2. The proof load stress corresponds to the minimum tensile strength of the highest grade of bolt or screw with which the nut can be used (see Table 3).

Table 3 — Strength grade designations of steel nuts
Strength grade designation 4 5 6 8 10 12
Proof load stress N/mm2 400 500 600 800 1000 1200

The mechanical properties of steel nuts are given in Table 9.

It is recommended that the grades of nut to be used with each grade of bolt and screw should be as shown in Table 4.

Table 4 — Recommended bolt and nut combinations
Grade of bolt 4.6 4.8 5.6 5.8 6.8 8.8 10.9 12.9
Recommended grade 4 4 5 5 6 8 10 12
Material and manufacture of steel nuts
Method of production

Steel nuts may be produced by cold forging, hot forging or by turning from bar. The choice of method shall be left to the manufacturer.

Chemical composition

The chemical compositions of the steels from which nuts are made shall conform to those given in Annex B
Note : The use of free-cutting steels at temperatures above 250 ºC is not recommended. Free-cutting steel may be used where permitted in Annex B, or otherwise by special agreement between the purchaser and the supplier.

Heat treatment

Nuts shall be heat-treated where this is necessary to obtain the mechanical properties given in clause 14.

Mechanical properties of steel nuts (excluding thin nuts)

Steel nuts (excluding thin nuts) shall meet the requirements for mechanical properties given in Table 9. The nuts shall withstand the proof load stress given in Table 9 when tested in accordance with Annex C. Nuts that are proof load tested shall have a hardness not in excess of the maximum given in Table 9.
Nut which are not proof load tested shall have a hardness not less than the minimum agreed between the purchaser and the supplier and not more than the maximum given in Table 9. The tests shall be in accordance with Annex C.

Drilled bolts with split pin holes

Bolts with split pin holes will be supplied only when specially ordered. The purchaser should state, in his enquiry and order, dimension lp, as illustrated in Figure 2. A tolerance of –0 + 0.8 mm shall be permissible on the specified dimension lp.

The split pin holes shall be drilled through the centre of the bolt as specified in BS EN ISO 1234.

Marking and identification(see also Annex E)
General

The marking and identification requirements of this standard are only mandatory for steel bolts, screws and nuts of 6 mm diameter and larger; manufactured to strength grade designations 8.8 (for bolts or screws) and 8 (for nuts) or higher.

Bolts and screws

Marking of bolts and screws shall be as specified in clause9 of BS EN 20898-2:1994.

Inspection and testing

The manufacturer shall take the necessary steps to ensure that the requirements of this standard are fulfilled but if, in addition the purchaser desires the manufacturer to certify or demonstrate that the bolts, screws and nuts conform to this standard, the details and cost of any further inspection shall be the subject of agreement between the purchaser and the manufacturer.

Tests for mechanical properties shall be in accordance with Annex C and BS EN ISO 898-1.

Table 9 — Mechanical properties of steel nuts

Strength grade designation

4

5

6

8

10

12

Proof load stress N/mm2

400

500

600

800

1000

1200

Brinell hardness (HB) max

302

302

302

302

353

375

All nuts

Rockwell hardnessb (HRC) max

30

30

30

30

36

39

All nuts

Vickers hardness (HV) max.

310

310

310

310

370

395

All nuts

Note :

  • The proof load is calculated by multiplying the proof load stress by the tensile stress area of the bolt.
  • The conversion from Brinell hardness into Rockwell hardness has been calculated according to BS EN ISO 6506-1,2 and 3 and BS EN 10109-1.
  • All nuts other than those exempted by agreement between the purchaser and the manufacturer. Nuts with a specified proof load in excess of 500 000 N (see Table 10) may be exempt from proof load testing. Such nuts shall meet the minimum hardness as agree between the purchaser and the manufacturer.
Chemical composition of steel nuts

The chemical composition of materials from which steel nuts shall be made are given in Table B.1

Strength grade designation

Chemical composition limits (check analysis)

Carbon

max. %

Manganese

min. %

Phosphorusmax. %

Sulfur

Max. %

4, 5 and 6 (See notes)

0.50

-

0.110

0.150

8

0.58

0.30

0.060

0.150

10 and 12 (See notes)

0.58

0.45

0.048

0.058

NOTE 1 Free cutting steel may be used only by special agreement between the purchaser and the supplier. In such cases, the following maximum phosphorus, sulfur and lead contents are permissible:

Phosphorus, 0.12 %; Sulfur, 0.34 %; Lead, 0.35 %.

NOTE 2 Alloying elements may be added if necessary to develop the mechanical properties of the nuts stipulated in clause 15.

Testing of mechanical properties of steel nuts (see clauses16and19)
Proof load test

The proof load test consists of applying the relevant proof load given in Table 10 which was obtained from the proof load stress given in Table 9.

Assemble the nut to be tested on a hardened and tempered mandrel as shown in Figure C.1 and apply the specified load in an axial direction.

The nut shall withstand this load without failure by stripping or rupture, and should be removable by the fingers after the load is released. If the threads of the mandrel are damaged during the test, the test shall be discarded.

It may be necessary to use a manual wrench to start the nut in motion. Such wrenching is permissible providing it is restricted to a half turn and the nut is then removable by the fingers after this initial loosening.

Hardened mandrel

The mandrel shall have a hardness of not less than Rockwell C45. The thread shall be tolerance class 5 Hex cept that the tolerance on the major diameter shall be the last quarter of the 6g range on the minimum material side.

Hardened test plate

The test plate shall have a hardness of not less than Rockwell C38.

Hardness test on nuts

Brinell, Rockwell and Vickers hardness may be determined. Apply the impression to the top and bottom face of the nut, otherwise on the side of the nut.

Perform a Brinell hardness test in accordance with the requirements of BS EN ISO 6506-1 to -3. Perform a Rockwell hardness test in accordance with the requirements of BS EN 10109-1. Perform a Vickers hardness test in accordance with the requirement of BS EN ISO 6507-1 to -3.

Marking Of Bolts

Hexagon head bolts according to this document shall be marked with ISO 898-1.

a)  Property class marking in accordance with EN ISO 898-1.

Example 1 8.8

b)  Identification mark of the RFL. It is permissible for the marking to be either embossed or indented on the top surface of the head.

Example 2 bolt marking

Designation

Example - A hexagon head bolt with thread M16, nominal length l = 100 mm and property class 8.8 is designated As follows:
Hexagon head bolt ISO 4014 - M16× 100 - 8.8.