BS 21:1985 download.Specification for Pipe threads for tubes and fittings where pressure-tight joints are made on the threads (metric dimensions).
1 Scope
BS 21 specifies requirements for the following pipe threads.
a) Jointing threads, which are pipe threads for joints made pressure-tight by the mating of the threads and are taper external, taper internal or parallel internal threads.
NOTE 1 Parallel external pipe threads are not suitable as jointing threads.
b) Longscrew threads, which are parallel pipe threads used for longscrews (connectors) specified in BS 1387 where a pressure-tight joint is achieved by the compression of a soft material on to the surface of the external thread by tightening a backnut against a socket.
Thread sizes from !i to 6 inclusive are covered by this standard and requirements for thread forms, dimensions and tolerances are given, together with the designation of each type of thread.
NOTE 2 Appendix B gives methods of verification of jointing thread dimensions and form using recommended gauging systems described in Appendix A.
NOTE 3 Reference should be made to BS 2779 for requirements for pipe threads where pressure-tight joints are not made on the threads.
NOTE 4 The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purposes of BS 21 the following definitions apply (see Figure 1).
2.1
gauge diameter
the basic major diameter of the thread, whether external or internal, at the gauge plane
2.2
gauge plane
the plane, perpendicular to the axis, at which the major cone has the gauge diameter
NOTE When there is a chamfer at the start of the thread not exceeding one pitch in length the gauge plane is theoretically located for internal threads at the face of the thread, and for external threads at a distance equal to the basic gauge length from the small end of the thread.
2.3
gauge length
on an external thread, the distance parallel to the axis, from the gauge plane to the small end of the thread
2.4
complete thread
that part of the thread which is fully formed at both crest and root
NOTE When there is a chamfer at the start of the thread not exceeding one pitch in length. it is included in the length of complete thread.
2.5
incomplete thread
that part of the thread which is fully formed at the root but truncated at the crest by its intersection with the cylindrical surface of the product
2.6
washout thread
that part of the thread which is not fully formed at the root
NOTE The washout thread is produced by the bevel at the start of the threading tool.
2.7
vanish cone
an imaginary cone the surface of which would pass through the roots of the washout thread
2.8
major cone
an imaginary cone which just touches the crests of a taper external thread or the roots of a taper internal thread
2.9
useful thread
the complete thread and the incomplete thread, excluding the washout thread
2.10
total thread
the complete thread, the incomplete thread and the washout thread
2.11
fitting allowance
the length of useful thread beyond the gauge plane of an external thread required to provide for assembly with an internal thread at the upper limit of the tolerance
2.12
wrenching allowance
the length of useful thread which is provided to accommodate the relative movement between the pipe end and the internally threaded part required for wrenching beyond the position of hand engagement
3 Symbols
For the purposes of BS 21, the following symbols apply.
Symbol Term
Height of the triangle of the thread profile perpendicular to the thread axis Height of the thread profile between rounded crests and roots perpendicular to the thread axis
Radius of rounded crests and roots Pitch
Gauge diameter (basic major diameter) of the thread at the gauge plane
Basic minor diameter of the thread at the gauge plane (d1 = d — 1.280 654p)
Basic pitch diameter of the thread at the gauge plane (d2 d — 0.640 32’7p)
Tolerance on the position of the gauge plane on external threads (see Figure 1) Tolerance on the position of the gauge plane relative to the face of internally tapered threads
NOTE Additional symbols are used in Table 1, Table 3, Table 4
and Table 5; these are not defined because they are for reference
purposes only when used in conjunction with Figure 4, Figure 7,
Figure 8, Figure 9. Figure 10, Figure 11 and Figure 12.
4 Basic forms of pipe threads
4.1 Parallel threads
The basic form of the parallel internal pipe thread and of the parallel external longscrew thread shall be the basic Whitworth form as follows.
The Whitworth thread form (see Figure 2) is that of a symmetrical V-thread in which the angle between the flanks, measured in an axial plane section, is 55°; one-sixth of this sharp V is truncated at the top and the bottom, the threads being rounded equally at crests and roots by circular arcs blending tangentially with the flanks, the theoretical depth of thread being 0.640 327 times the nominal pitch. The basic thread depth, calculated from this, is rounded off to the nearest 0.00 1 mm.
4.2 Taper threads
The basic form of the taper pipe thread shall be as follows (see Figure 3).
It is based on the Whitworth thread form and it too has an angle of 550, the flanks making equal angles with the axis. The crests and roots are rounded off symmetrically in such a manner as to give the same basic differences between major, pitch and minor diameters as in the Whitworth thread of the same nominal pitch.
The taper is I in 16, measured on the diameter.
5 Jointing threads
5.1 General
The design of internally threaded parts (see Figure 4) shall be such that they can receive pipe ends up to the lengths given in column 13 of Table 2 and the minimum lengths of useful thread shall be not less than 80 % of the values given in column 14 of Table 2.
NOTE It is common practice to apply a jointing medium to the threads before assembly to ensure that a pressure.tight joint is made.
5.2 Compliance
NOTE No method is specified for verification of jointing thread dimensions and form but the methods described in Appendix B, using the gauging systems described in Appendix A, are recommended.