Machinery's Handbook, 31st Edition
1946 Screw Threads Screw Thread: A screw thread is a continuous and projecting helical ridge usually of uniform section on a cylindrical or conical surface. Sharp Crest (Crest Apex): The apex formed by the intersection of the flanks of a thread when extended, if necessary, beyond the crest. Sharp Root (Root Apex): The apex formed by the intersection of the adjacent flanks of adjacent threads when extended, if necessary, beyond the root. Standoff: The axial distance between specified reference points on external and internal taper thread members or gages, when assembled with a specified torque or under other specified conditions. Straight Thread: A straight thread is a screw thread projecting from a cylindrical surface. Taper Thread: A taper thread is a screw thread projecting from a conical surface. Tensile Stress Area: The tensile stress area is an arbitrarily selected area for computing the tensile strength of an externally threaded fastener so that the fastener strength is consistent with the basic material strength of the fastener. It is typically defined as a function of pitch diameter and/or minor diameter to calculate a circular cross section of the fastener correcting for the notch and helix effects of the threads. Thread: A thread is a portion of a screw thread encompassed by one pitch. On a single- start thread it is equal to one turn. (See also Threads per Inch and Turns per Inch .) Thread Angle: See Included Angle . Thread Runout: See Vanish Thread. Thread Series: Thread Series are groups of diameter/pitch combinations distinguished from each other by the number of threads per inch applied to specific diameters. Thread Shear Area: The thread shear area is the total ridge cross-sectional area inter sected by a specified cylinder with diameter and length equal to the mating thread engage ment. Usually the cylinder diameter for external thread shearing is the minor diameter of the internal thread and for internal thread shearing it is the major diameter of the external thread. Threads per Inch: The number of threads per inch is the reciprocal of the axial pitch in inches. Tolerance: The total amount by which a specific dimension is permitted to vary. The tol erance is the difference between the maximum and minimum limits. Tolerance Class: ( metric ): The tolerance class (metric) is the combination of a tolerance position with a tolerance grade. It specifies the allowance (fundamental deviation), pitch diameter tolerance (flank diametral displacement), and the crest diameter tolerance. Tolerance Grade: ( metric ): The tolerance grade (metric) is a numerical symbol that des ignates the tolerances of crest diameters and pitch diameters applied to the design profiles. Tolerance Limit: The variation, positive or negative, by which a size is permitted to depart from the design size. Tolerance Position: ( metric ): The tolerance position (metric) is a letter symbol that des ignates the position of the tolerance zone in relation to the basic size. This position pro vides the allowance (fundamental deviation). Total Thread: Includes the complete and all the incomplete thread, thus including the vanish thread and the lead thread. Transition Fit: A fit having limits of size so prescribed that either a clearance or an inter ference may result when mating parts are assembled. Turns per Inch: The number of turns per inch is the reciprocal of the lead in inches. Unilateral Tolerance: A tolerance in which variation is permitted in one direction from the specified dimension. Vanish Thread: ( Partial Thread, Washout Thread, or Thread Runout ): That portion of the incomplete thread which is not fully formed at the root or at crest and root. It is pro duced by the chamfer at the starting end of the thread forming tool. Virtual Diameter: See Pitch Diameter, Functional Diameter. Washout Thread: See Vanish Thread.
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