美制NPT螺纹标准

1860

AMERICAN PIPE THREADS

PIPE AND HOSE THREADS

The types of threads used on pipe and pipe fittings may be classed according to their intended use: 1) threads that when assembled with a sealer will produce a pressure-tight joint; 2) threads that when assembled without a sealer will produce a pressure-tight joint;3) threads that provide free- and loose-fitting mechanical joints without pressure tight-ness; and 4) threads that produce rigid mechanical joints without pressure tightness.

American National Standard Pipe Threads

American National Standard pipe threads described in the following paragraphs provide taper and straight pipe threads for use in various combinations and with certain modifica-tions to meet these specific needs.

Thread Designation and Notation.—American National Standard Pipe Threads are des-ignated by specifying in sequence the nominal size, number of threads per inch, and the symbols for the thread series and form, as: 3?8—18 NPT. The symbol designations are as follows: NPT—American National Standard Taper Pipe Thread; NPTR—American National Standard Taper Pipe Thread for Railing Joints; NPSC—American National Stan-dard Straight Pipe Thread for Couplings; NPSM—American National Standard Straight Pipe Thread for Free-fitting Mechanical Joints; NPSL—American National Standard Straight Pipe Thread for Loose-fitting Mechanical Joints with Locknuts; and NPSH—American National Standard Straight Pipe Thread for Hose Couplings.

American National Standard Taper Pipe Threads.—The basic dimensions of the ANSI Standard taper pipe thread are given in Table 1a .

Form of Thread: The angle between the sides of the thread is 60 degrees when measured in an axial plane, and the line bisecting this angle is perpendicular to the axis. The depth of the truncated thread is based on factors entering into the manufacture of cutting tools and the making of tight joints and is given by the formulas in Table 1a or the data in Table 2obtained from these formulas. Although the standard shows flat surfaces at the crest and root of the thread, some rounding may occur in commercial practice, and it is intended that the pipe threads of product shall be acceptable when crest and root of the tools or chasers lie within the limits shown in Table 2.

Pitch Diameter Formulas: In the following formulas, which apply to the ANSI Standard taper pipe thread, E 0 = pitch diameter at end of pipe; E 1 = pitch diameter at the large end of the internal thread and at the gaging notch; D = outside diameter of pipe; L 1 = length of hand-tight or normal engagement between external and internal threads; L 2 = basic length of effective external taper thread; and p = pitch = 1 ÷ number of threads per inch.

Thread Length: The formula for L 2 determines the length of the effective thread and includes approximately two usable threads that are slightly imperfect at the crest. The nor-mal length of engagement, L 1, between external and internal taper threads, when assem-bled by hand, is controlled by the use of the gages.

Taper:The taper of the thread is 1 in 16, or 0.75 inch per foot, measured on the diameter and along the axis. The corresponding half-angle of taper or angle with the center line is 1degree, 47 minutes.

E 0D 0.05D 1.1+()p –=E 1E 00.0625L 1

+=L 20.80D 6.8+()p

=

AMERICAN PIPE THREADS1861 Table 1a. Basic Dimensions, American National Standard Taper Pipe Threads, NPT

ANSI/ASME B1.20.1-1983 (R2001)

a Also length of thin ring gage and length from gaging notch to small end of plug gage.

b Also pitch diameter at gaging notch (handtight plane).

c Also length of plug gage.

1862

AMERICAN PIPE THREADS

All dimensions given in inches.

Increase in diameter per thread is equal to 0.0625/n .

The basic dimensions of the ANSI Standard Taper Pipe Thread are given in inches to four or five decimal places. While this implies a greater degree of precision than is ordinarily attained, these dimensions are the basis of gage dimensions and are so expressed for the purpose of eliminating errors in computations.

Engagement Between External and Internal Taper Threads.—The normal length of engagement between external and internal taper threads when screwed together handtight is shown as L 1 in Table 1a . This length is controlled by the construction and use of the pipe thread gages. It is recognized that in special applications, such as flanges for high-pressure work, longer thread engagement is used, in which case the pitch diameter E 1 (Table 1a ) is maintained and the pitch diameter E 0 at the end of the pipe is proportionately smaller.Tolerances on Thread Elements.—The maximum allowable variation in the commer-cial product (manufacturing tolerance) is one turn large or small from the basic dimen-sions.

The permissible variations in thread elements on steel products and all pipe made of steel,wrought iron, or brass, exclusive of butt-weld pipe, are given in Table 3. This table is a

Table 1b. Basic Dimensions, American National Standard Taper Pipe Threads, NPT

ANSI/ASME B1.20.1-1983 (R2001)

Nominal Pipe Size

Wrench Makeup Length for Internal Thread Vanish Thread,(3.47 thds.),

V Overall Length External Thread,L 4

Nominal Perfect External Threads a Height of Thread,h Basic Minor Dia. at Small End of Pipe,b

K 0

Length,c

L 3Dia.,E 3Length,L 5Dia.,E 51?160.11110.264240.12850.38960.18700.282870.029630.24161?80.11110.356560.12850.39240.18980.375370.029630.33391?40.16670.466970.19280.59460.29070.495560.044440.43293?80.16670.601600.19280.60060.29670.630560.044440.56761?20.21430.745040.24780.78150.39090.782860.057140.70133?4

0.21430.954290.24780.79350.40290.992860.057140.910510.2609 1.197330.30170.98450.5089 1.245430.06957 1.144111?40.2609 1.540830.3017 1.00850.5329 1.590430.06957 1.487611?20.2609 1.779780.3017 1.02520.5496 1.830430.06957 1.726520.2609 2.252720.3017 1.05820.5826 2.305430.06957 2.199521?20.2500d 2.703910.4337 1.57120.8875 2.775000.100000 2.619530.2500d 3.325000.4337 1.63370.9500 3.400000.100000 3.240631?20.2500 3.821880.4337 1.6837 1.0000 3.900000.100000 3.737540.2500 4.318750.4337 1.7337 1.0500 4.400000.100000 4.234450.2500 5.375110.4337 1.8400 1.1563 5.463000.100000 5.290760.2500 6.430470.4337 1.9462 1.2625 6.525000.100000 6.346180.25008.417970.4337 2.1462 1.46258.525000.1000008.3336100.250010.529690.4337 2.3587 1.675010.650000.10000010.4453120.250012.517190.4337 2.5587 1.875012.650000.10000012.432814 OD 0.250013.759380.4337 2.6837 2.000013.900000.10000013.675016 OD 0.250015.746880.4337 2.8837 2.200015.900000.10000015.662518 OD 0.250017.734380.4337 3.0837 2.400017.900000.10000017.650020 OD 0.250019.721880.4337 3.2837 2.600019.900000.10000019.637524 OD

0.2500

23.69688

0.4337

3.6837

3.0000

23.90000

0.100000

23.6125

a The length L

5 from the end of the pipe determines the plane beyond which the thread form is imper-

fect at the crest. The next two threads are perfect at the root. At this plane the cone formed by the crests of the thread intersects the cylinder forming the external surface of the pipe. L 5 = L 2? 2p .b Given as information for use in selecting tap drills.

c Three threads for 2-inch size an

d smaller; two threads for larger sizes.

d Military Specification MIL—P—7105 gives th

e wrench makeup as three threads for 3 in. and smaller. The E 3 dimensions are then as follows: Size 21?2 in., 2.69609 and size 3 in., 3.31719.