ASME B1.20.3-1976 干封管螺纹(英制)

标准号：ASME B1.20.3-1976
中文标准名称：干封管螺纹(英制)
英文标准名称：Dryseal pipe threads (inch)
标准类型：J04
发布日期：1976/11/18 12:00:00
实施日期：1999/12/31 12:00:00
中国标准分类号：J04
国际标准分类号：21.040.20
适用范围：Dryseal pipe threads are based on the USA (American)pipe thread, however, they differ from the USA(American) pipe thread in that they are designed toseal pressuretight joints without the necessity of usingsealing compounds. To accomplish this some modificationof thread form and greater accuracy in manufactureis required. The roots of both the externaland internal threads are truncated slightly more thanthe crests, Le. roots have wider flats than crests, sothat metal-to-metal contact occurs at the crests androots coincident with or prior to flank contact, seeFigure 1. Thus as the threads are assembled bywrenching, the roots of the threads crush the sharpercrests of the mating threads. This sealing action atboth the major and minor diameters tends to preventspiral leakage and makes the joints pressuretight withoutthe necessity of using sealing compounds, providedthat the mating threads are in accordance with standardspecifications and tolerances and are not damagedby galling in assembly. The control of crest androot truncation is simplified by the use of properlydesigned threading tools. Also, it is desirable that bothexternal and internal threads have full thread heightfor the LI length. However, where not functionallyobjectionable, the use of a compatible lubricant orsealant may be used to minimize the possibility ofgalling. This is desirable in assembling Dryseal pipethreads in refrigeration and other systems to effect apressuretight seal.1.1.1 In order to obtain a pressure tight seal usingDryseal pipe threads without a sealer, it is necessaryto hold crest and root truncation of both internal andexternal threads within the limits specified. Unlessthis is done by use of threading tools with the crestand root truncation controlled so as to assure repro-duction on the product threads, it is necessary to usea system of measuring or a system of gaging and measuringto determine conformance.1 .1 .2 Even without truncation gages, the standardpractice of using two separate thread gages for Drysealpipe threads, each with a difference in truncation,length of engagement and point of engagement, providesa more detailed check of angle, lead and taperdeviations than is required for non-dryseal taper pipethreads.1 .I .3 One method of checking crest truncation is byusing 6 step plain gages. It should be recognized thatthis method may give misleading results in that thecrest truncation of the product threads is always less(flat narrower) than that indicated by the position ofthe gages, the degree of inconsistency depending onthe angle, lead and taper deviations present in theproduct thread.1.1.4 Another method employs both crest and roottruncation check gages. The root check gage is madewith a thread form having an included angle of 50degrees and with a smaller gage crest flat than theroot flat to be checked. The major diameter of thisgage is controlled in relation to 6 gaging steps in asimilar manner to the plain 6 step crest check gage.Like the crest check gage, the results may be somewhatmisleading except that in the case of the rootcheck gage, the root truncation of the product threadis always more (flat wider) than is indicated by theposition of the gages.1.1.5 Unless lead, angle and taper of product threadsare veiy well controlled, use of 6 step crest and rootcheck gages will result in product threads with narrowercrest flats and wider root flats than envisionedby the dryseal tolerances. Use of such a gaging systemcould result in rejection of threads which would actuallyconform to the dryseal tolerances specified.The only completely reliable referee method for determiningwhether crest and root truncation has been held within tolerance is direct measurement of the externalthread flats and measurement of these flats ona cast or section of the internal thread.