1、BRITISH STANDARD CONFIRMED APRIL 1998 BS 1657:1950 Buttress threads UDC 621.882.082.4BS1657:1950 This British Standard, having been approved by the MechanicalEngineering IndustryStandards Committee and endorsed by the Chairman ofthe Engineering Divisional Council, was published under theauthority of
2、 the General Council on 17 August 1950 BSI 10-1999 ISBN 0 580 35516 0 Co-operating organizations The Mechanical Engineering Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific a
3、nd industrial organizations: Admiralty Air Ministry Associated Offices Technical Committee Association of Consulting Engineers Incorporated British Chemical Plant Manufacturers Association British Compressed Air Society British Electrical and Allied Manufacturers Association British Engineers Associ
4、ation British Internal Combustion Engine Manufacturers Association British Iron close class tolerances are two-thirds of the medium class tolerances, and free class tolerances are1“ times the medium class tolerances. b) Tolerances on major diameter of screw and minor diameter of nut. Unless otherwis
5、e specified these tolerances should be the same as the effective diameter tolerance for the class used. The necessity of reducing these tolerances may arise in cases where the crest surfaces of the screw and nut form datum surfaces for the measurement of thread angles or pitch, as in large diameter
6、threads, or where the resulting reduction in the depth of engagement has to be limited. In these particular cases the following tolerances are recommended: c) Tolerances on minor diameter of screw. It is important that at the minor diameter of the screw there should be a well formed radius. Unless o
7、therwise specified the tolerance on the minor diameter of the screw shall be twice the effective diameter tolerance. d) Tolerances on major diameter of nut. It is preferable that the root of the nut should be of rounded form. Unless otherwise specified the tolerance on the major diameter of the nut
8、shall be twice the effective diameter tolerance. where D = basic major diameter of screw (assuming no allowance) L = length of engagement p = pitch of thread. Nominal diameter in inches Tolerance on major diameter of screw and minor diameter of nut, in inches From 1 to 1“ Over 1“ to 2“ Over 2“ to 4
9、0.002 0.002 5 0.003 Over 4 to 6 Over 6 to 16 Over 16 to 24 0.004 0.005 0.006 0.002 D 3 0.00278L0.00854p + = 0.002D 3 0.0173p. +BS1657:1950 BSI 10-1999 3 7 Allowances for easy assembly An allowance (clearance) should be provided on all buttress screws in order to secure easy assembly of parts. The am
10、ount of the allowance is deducted from the nominal major, effective, and minor diameters of the screw member in order to determine the maximum metal condition of the screw. The minimum nut is basic. The amount of the allowance is the same for all three classes and is equal to the close class effecti
11、ve diameter tolerance as calculated under Clause6 a) above. The allowances for various combinations of pitch and diameter are given inTable 5. Example: Screw and nut. Standard thread form. Medium class2 in. nominal diameter (D). 8 t.p.i. Screw: Nut: Tolerances for screw: Tolerances for nut: Limits f
12、or medium class: Figure 2 shows the relative tolerances and allowances in diagrammatic form for the three classes, applied to the size of thread2in. diameter 8t.p.i. in the above example. 8 Designation of British Standard buttress threads It is recommended that the following system should be adopted
13、 for designating the threads. If the thread is required to conform to the appropriate tolerances and allowances inTable 2 to Table 5 and to the normal tolerance on major diameter of screw and minor diameter of nut Clause6 b), the designation should be in the general form: 2.0 BS Buttress thread8t.p.
14、i. medium class. Allowance = G = 0.0058 in. Table 5 Maximum major diameter = D G = 1.9942 in. Maximum effective diameter = D B G = 1.9442 in. Table 1 Maximum minor diameter = D 2A G = 1.8678 in. Table 1 Minimum major diameter = D 2B + 2A = 2.0264 in. Table 1 Minimum effective diameter = D B = 1.9500
15、 in. Table 1 Minimum minor diameter = D 2B = 1.9000 in. Table 1 Major diameter 0.0087 in. Table 3 Effective diameter 0.0087 in. Table 3 Effective diameter +0.0087 in. Table 3 Minor diameter +0.0087 in. Table 3 Screw. Major diameter 1.9855 in. and 1.9942 in. Effective diameter 1.9355 in. and 1.9442 i
16、n. Minor diameter not greater than 1.8678 in. Nut. Major diameter not less than 2.0264 in. Effective diameter 1.9500 in. and 1.9587 in. Minor diameter 1.9000 in. and 1.9087 in.BS1657:1950 4 BSI 10-1999 NOTEIf a non-standard allowance and/or tolerance on the major diameter of the screw and minor diam
17、eter of the nut is used, the designation should carry the additional note “(special fit)” and the actual limits of size for the major diameter of the screw or minor diameter of the nut should be indicated on the drawing, or in relevant correspondence, e.g., 2.0 BS Buttress thread (standard form) 8t.
18、p.i. medium class (special fit.) This implies an allowance of 0.002 in., and a tolerance on major diameter of screw, or minor diameter of nut, of0.0025in. Clause6 b). It is obvious that either the allowance, or the tolerance, or both may be changed from standard, but the same method of designation s
19、uffices. If the allowance only is altered, only the screw requires the special method of designation. 9 Recommended gauging practice To ensure that the sizes of the threads are between their allowable limits, the use of the following gauging system is recommended. The design sizes to which the gauge
20、s are made are given inTable 6, and the thread forms of the gauges are illustrated inFigure 4. A. For screws i) A GO screw ring gauge made to the maximum permissible effective diameter of the screw and having the same thread form as a maximum metal nut, with a flat crest. The theoretical length of t
21、hread of this gauge should be not less than the length of engagement of the component threads, but as it is usual to make screw ring gauges from British Standard gauge blanks 1)i is assumed that this practice is allowable, unless otherwise stated on the drawing. ii) A NOT GO effective (pitch) diamet
22、er screw ring or calliper gauge made to the minimum permissible effective diameter of the screw. The thread form of this gauge has a truncated flat crest and a cleared root so that it bears on the middle part of the flanks. The root clearance should be such as to clear the maximum major diameter of
23、the screw. The length of thread of this gauge is limited to two or three pitches. iii) A GO plain calliper gauge made to the maximum permissible major diameter of the screw. iv) A NOT GO plain calliper gauge made to the minimum permissible major diameter of the screw. NOTEIt may be convenient to com
24、bine the calliper gauges specified in sub-sectionsiii) andiv) above as a single “progressive” calliper gauge. B. For nuts i) A GO screw plug gauge made to the minimum permissible effective diameter of the nut and having the same thread form as a maximum metal screw, with a flat crest. The theoretica
25、l length of thread of this gauge should be not less than the length of engagement of the component threads, but as it is usual to make screw plug gauges from British Standard gauge blanks it is assumed that this practice is allowable, unless otherwise stated on the drawing. ii) A NOT GO effective (p
26、itch) diameter screw plug gauge made to the maximum permissible effective diameter of the nut. The thread form of this gauge has a truncated flat crest and a cleared root so that it bears on the middle part of the flanks. The root clearance should be such as to clear the minimum minor diameter of th
27、e nut. The length of thread of this gauge is limited to two or three pitches. iii) A GO plain plug gauge made to the minimum permissible minor diameter of the nut. iv) A NOT GO plain plug gauge made to the maximum permissible minor diameter of the nut. NOTEIt may be convenient to combine the plain plug gauges specified in sub-sectionsiii) andiv) above as a single “progressive” plug gauge. Major diameter (of screw)= 1.998 0 1.995 5 or Minor diameter (of nut) = 1.902 5 1.900 0 1) BS 1044, “Recommended designs for plug, ring and gap gauges.”
