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Form Circle is the circle which defines the deepest points of involute form control of the
tooth profile. This circle along with the tooth tip circle (or start of chamfer circle) deter-
mines the limits of tooth profile requiring control. It is located near the major circle on the
internal spline and near the minor circle on the external spline.
Form Clearance (cF) is the radial depth of involute profile beyond the depth of engage-
ment with the mating part. It allows for looseness between mating splines and for eccen-
tricities between the minor circle (internal), the major circle (external), and their respective
pitch circles.
Form Diameter (DFe, DFi
) the diameter of the form circle.
Internal Spline is a spline formed on the inner surface of a cylinder.
Involute Spline is one having teeth with involute profiles.
Lead Variation is the variation of the direction of the spline tooth from its intended direc-
tion parallel to the reference axis, also including parallelism and alignment variations (see
Fig. 1a). Note: Straight (nonhelical) splines have an infinite lead.
Length of Engagement (Lq) is the axial length of contact between mating splines.
Machining Tolerance (m) is the permissible variation in actual space width or actual
tooth thickness.
Major Circle is the circle formed by the outermost surface of the spline. It is the outside
circle (tooth tip circle) of the external spline or the root circle of the internal spline.
Tooth Proportions.—There are 17 pitches: 2.5⁄5, 3⁄6, 4⁄8,5⁄10, 6⁄12, 8⁄16, 10⁄20,
12⁄24, 16⁄32, 20⁄40, 24⁄48, 32⁄64, 40⁄80, 48⁄96, 64⁄128, 80⁄160, and 128⁄256. The
numerator in this fractional designation is known as the diametral pitch and controls the
pitch diameter; the denominator, which is always double the numerator, is known as the
stub pitch and controls the tooth depth. For convenience in calculation, only the numerator
is used in the formulas given and is designated as P. Diametral pitch, as in gears, means the
number of teeth per inch of pitch diameter.
Table 1 shows the symbols and Table 2 the formulas for basic tooth dimensions of invo-
lute spline teeth of various pitches. Basic dimensions are given in Table 3.4019