Machinery's Handbook, 31st Edition
2332 Involute Splines The tolerance class used in ASA B5.15-1960 is the basis and is now designated as toler ance Class 5. The newer tolerance classes are based on the following formulas:
Tolerance Class 4 Tolerance Class 5 0.71 Tolerance Class 6 Tolerance Class 5 1.40 Tolerance Class 7 Tolerance Class 5 2.00 # # # = = =
All dimensions listed in this standard are for the finished part. Therefore, any compensa tion that must be made for operations that take place during processing, such as heat treat ment, must be taken into account when selecting the tolerance level for manufacturing. The standard has the same internal minimum effective space width and external maxi mum effective tooth thickness for all tolerance classes and has two types of fit. For tooth side fits, the minimum effective space width and the maximum effective tooth thickness are of equal value. This basic concept makes it possible to have interchangeable assembly between mating splines where they are made to this standard regardless of the tolerance class of the individual members. A tolerance class “mix” of mating members is thus allowed, which often is an advantage where one member is considerably less difficult to produce than its mate, and the “average” tolerance applied to the two units is such that it satisfies the design need. For instance, assigning a Class 5 tolerance to one member and Class 7 to its mate will provide an assembly tolerance in the Class 6 range. The maximum effective tooth thickness is less than the minimum effective space width for major diameter fits to allow for eccentricity variations. In the event the fit as provided in this standard does not satisfy a particular design need and a specific amount of effective clearance or press fit is desired, the change should be made only to the external spline by a reduction or an increase in effective tooth thickness and a like change in actual tooth thickness. The minimum effective space width, in this standard, is always basic. The basic minimum effective space width should always be retained when special designs are derived from the concept of this standard. Terms Applied to Involute Splines.— The following definitions of involute spline terms, here listed in alphabetical order, are given in the American National Standard. Some of these terms are illustrated in the diagram in Table 6. Active Spline Length ( L a ) is the length of spline that contacts the mating spline. On slid ing splines, it exceeds the length of engagement. Actual Space Width ( s ) is the circular width on the pitch circle of any single space con sidering an infinitely thin increment of axial spline length. Actual Tooth Thickness ( t ) is the circular thickness on the pitch circle of any single tooth considering an infinitely thin increment of axial spline length. Alignment Variation is the variation of the effective spline axis with respect to the refer ence axis (see Fig. 1c). Base Circle is the circle from which involute spline tooth profiles are constructed. Base Diameter ( D b ) is the diameter of the base circle. Basic Space Width is the basic space width for 30-degree pressure angle splines: half the circular pitch. The basic space width for 37.5- and 45-degree pressure angle splines, how ever, is greater than half the circular pitch. The teeth are proportioned so that the external tooth, at its base, has about the same thickness as the internal tooth at the form diameter. This proportioning results in greater minor diameters than those of comparable involute splines of 30-degree pressure angle. Circular Pitch ( p ) is the distance along the pitch circle between corresponding points of adjacent spline teeth. Depth of Engagement is the radial distance from the minor circle of the internal spline to the major circle of the external spline, minus corner clearance and/or chamfer depth.
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