Inspecting Threads With Thread Gages Is NOT Enough

1 About the Author/JOE GREENSLADEJoe Greenslade is President of Greenslade & Company, Inc. in Fort Worth, Texas, a supplierof fastener inspection products, inspection software, and ISO 17025 (A2LA) accredited calibra-tion services. He also provides a variety of consulting services including serving as ExpertWitness in fastener related has been in the fastener industry in a variety of capacities since 1970. He has written over220 fastener technology related articles and has spoken to many fastener industry organizationsthroughout the United States. Joe has been awarded 12 Patents for fastener is an active member of the American Society of Mechanical Engineers (ASME), theAmerican Society for Testing and Materials (ASTM), the National Fastener Distributor s Associa-tion (NFDA), and a member of the Board of Directors of the American Association for LaboratoryAccreditation (A2LA).

2 He is a former member of the Industrial Fasteners Institute (IFI). He was amember of the Fastener Quality Act Task Force representing the fastener industry. Joe alsoserves on the Screw Thread Improvement Task Force working with government and industry toimprove aerospace related Thread specifications, and Thread gaging and calibration Threads with Thread GagesIs NOT EnoughThere is a wide-spread, potentially dangerous mis-understanding by fastener suppliers that if athreaded fastener is acceptable using the appropriatesize GO and NOGO Thread Gages that the Thread isacceptable for ALL characteristics. The use of GO andNOGO Thread Gages to inspect Threads is NOT ad-equate and they do not even inspect the most criticalcharacteristics on the GO and NOGO threadgages only determine if the pitch diameter of the in-spected part is within accept-able size.

3 When a part passesthe GO and NOGO threadgage inspection that only in-dicates that the evaluatedpart will go together with acounterpart that has alsopassed its GO and NOGO Thread gage inspection. Theseinspections do NOT indicatewhether or not these threadswill strip out of a mating partwhen they are placed into ser-vice. The pitch diameter size alone does NOT pre-dict Thread inspection DOES evaluate potentialthread strength?The most critical Thread characteristic on an exter-nal Thread (screws and bolts) is the MAJOR DIAM-ETER. The most critical characteristic on an internalthread (nuts) is the MINOR DIAMETER. It is the amount of overlap between the externalthread s MAJOR DIAMETER and the internal Thread sMINOR DIAMETER that determines whether or not thethreads will strip out of one another when the applica-tions stresses are applied to the product joint.

4 Stan-dard bolt and nut products are designed so that whenthe major diameter of a screw or bolt is within speci-fied size and the minor diameter of the mating nut iswithin specified size the failure mode will always bethe bolt StrippingThread stripping is by far a more dangerous type offastener failure than is bolt breaking. Applicationstresses are determined based on the tensile strengthof the screws or bolts used. When Threads strip, thestrength of the screw or bolt has not been a screw or bolt major diameter is undersizedor when the nut minor diameter is oversized, the amountof Thread overlap is decreased. This means there isless material mass between the external and internalthreads to resist a shearing action between them whenthe stresses of tightening or the stresses of the appli-cation are exerted on the joint.

5 The shearing away ofeither the internal Thread or the external Thread is whatis referred to as Thread stripping. GO/NOGO Thread GagesGO and NOGO Thread ring Gages doNOT inspect the ma-jor diameter of a screwor bolt. The major di-ameter in a ring gageis larger than the maxi-mum product threadmajor ring gagescannot and will notdetect an undersizedproduct Thread s and NOGO threaded plug gagesdo NOT inspect theminor diameter of minor diameterof GO and NOGOBolt entering GO ring not entering GO ring plug Gages are smaller than the smallest al-lowable minor diameter of the nut Thread . The GO andNOGO Thread plug gage will never detect an oversizednut minor should fastener suppliers do?

6 In addition to Inspecting screw and bolt Threads withGO and NOGO ring Gages or Tri-roll Thread Gages , theyshould also inspect the major diameters of screw andbolt Threads using a micrometer, caliper, or a NOGO cylindrical ring gage made at the applicable limits ofthe major addition to Inspecting nut Threads with GO/NOGO threaded plug Gages internal Threads should be inspectedusing GO/NOGO cylindrical plug Gages manufacturedat the applicable internal Thread minor constitutes the minimum acceptablethread inspection process?The simplest acceptable Thread inspection system,as defined by the American Society of Mechanical En-gineers (ASME) in the Thread acceptability standardASME , is System 21.

7 The requirements of a Sys-tem 21 Inspection are the evaluation of the limits orsize of the following Thread characteristics: external Threads GO Maximum Material GO Threaded Ring gage NOGO Minimum Material NOGO Threaded Ring gage Major Diameter MicrometerInternal Threads GO Maximum Material GO Threaded Plug gage NOGO Minimum Material NOGO Threaded Plug gage Minor Diameter GO Cylindrical Plug gage NOGO Cylindrical Plug GageWhen Inspecting screws, bolt, and nut Threads , do notstop with the GO/NOGO Thread gage evaluation. Pass-ing a GO/NOGO threaded gage evaluation providesInspecting bolt s major no assurance of Thread strength. Alwaysinspect the major diameter of external Threads and theminor diameter of internal Threads assurance of poten-tial Thread more information on this or other quality assur-ance matters related to specifications, procedures, and/or equipment, contact the author at threaded plug gage enters threaded plug gage does not enter cylindrical plug gage enters nut minor cylindrical plug Gages does not enter nutminor from Distributor s Link Magazine, Winter 2006