Not all threaded connections serve similar purposes.The load-carrying needs of an aerospace engine support bolt in a tension assembly greatly exceed those of a simple screw that fastens a cover plate to an electrical wall socket. International thread-acceptance documents and standards recognize this basic engineering fact, and incorporate different thread-inspection requirements into their verification standards. While both MIL-S-8879 and AS8879 offer considerable guidance regarding the default level of thread inspection, the most comprehensive document regarding optimal recommendations for selection of a thread inspection system for general and specific applications is ASME B1.3M,Screw Thread Gaging Systems for Dimensional Acceptability—Inch and Metric Screw Threads. According to the document, there are three basic levels of accountable thread verification: System 21, System 22, and System 23.
Typically verified by traditional Go and Not Go thread gages, System 21 is used in applications where the sole acceptance requirement is the ability to assemble. Under this level of verification, neither pitch diameter nor accuracy of thread form is a necessary acceptance requirement. Given the limited dimensional inspection requirements associated with System 21, thread quality can range from Tiffanys’ jewelry to outright junk. In summary, System 21 only verifies maximum material size. It ignores the minimum material condition.
According to FED-STD H28, System 21 should not be used for any application demanding specific strength requirements, such as tensile or shear. Further, System 21 should not be utilized for applications associated with verification of process or capability, as it cannot assess deviation in key manufacturing variables. As tooling wears or process setup degrades, there is a corresponding degradation in accuracy of thread lead or angle. System 21 cannot detect this process shift because the limitations of its defined inspection scope render System 21 incapable of detecting thread-form error.
System 22 contains all of the inspection criteria contained in System 21, and adds the requirement of minimum material pitch diameter or groove-diameter verification. This incorporation of a ceiling or floor to the pitch diameter dimensional tolerance not only creates the obligation to assess essential thread dimensional strength, but also adds an element of form verification. According to basic design theory, if there is no tolerance on form, the tolerance on size will dictate the form. Maximum material—functional size includes the entire effect of thread angle and thread-lead error on the pitch-diameter size, while minimum material pitch or groove diameter is a pure dimensional measurement, and does not include any of the effects of form deviation. Control of both dimensions limits not only the overall dimensional variation, but also imposes specific constrictions on angle, lead, roundness, and taper variation through the control of size.
System 22 is recommended for those applications where thread strength is a factor in thread design, optimal percentage of flank-to-flank engagement is an important consideration, and for situations where loss of design preload through thread loosening or relaxation may compromise the long-term performance of the threaded assembly. Because there are elements of both size and form control in System 22, it’s also recommended for process control and Production Part Approval Process (PPAP) requirements. Lastly, inspection requirements virtually identical to System 22 are the baseline measurement requirement contained in military and aerospace specifications such as MIL-S-8879 and AS8879.
Finally, there’s a third level of thread inspection that is normally reserved for failure analysis, or extremely critical threaded applications where the singular failure of a threaded component may cause overall system failure or severe economic loss, or compromise health and safety. System 23, termed safety-critical thread inspection in some military standards, extends the requirement of dimensional measurement of maximum material—functional size, and minimum material pitch or groove diameter size, as specified in System 22, to include measurement and quantification of thread angle, lead, taper, and roundness.
If simple assembly of a threaded fastener will satisfy the needs of a particular design, System 21 or simple functional gages are often used as the only method of inspection. If there are specific mechanical strength requirements in addition to the need to assemble, or there is a need to verify process or long-term performance, the measurement of actual thread pitch diameter is often added to the functional-gaging requirement. For those rare applications where the singular failure of a threaded component may cause catastrophic consequences, there are additional limitations on deviation in thread angle, lead, roundness and taper, and this inspection protocol is defined by System 23.
This article was first published in the June 2009 edition of Manufacturing Engineering magazine.
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