What is a spline?
A spline is any shaft that features a series of teeth that match spaces of a corresponding shaft.
By mating an internal spline with an external spline, rotational movement and force is transferred along the same axis with performance dictated by the designed form and fit. Splines can be either external or internal, and they may comprise a variety of tooth shapes to serve various functions.
Tooth profiles of external splines can be cut by shaping, hobbing, form-milling, or power-skiving. The hobbing process typically offers the best balance between productivity and flexibility.
However, due to sweepout clearances and tight part features, shaping methods are also commonly used. Although legacy applications often require parallel key splines, involute splines with specific fit and form dominate standards today.
Internal splines are produced by the same methods as internal gear production, such as shaping, form-milling, or power-skiving. Because splines are straight-toothed, they are also commonly broached for efficient, effective production.
A milling cutter can take the exact conjugate form of the spline tooth space. By using this type of tool, a spline can be made one tooth space at a time, successively indexing from space to space.
The shaping process offers balance between flexibility and productivity. Additionally, external splines must be shaped if hob clearance sweepout is insufficient, common with unique part features such as a shoulder or change in diameter.
Teeth are generated by a hob cutting tool, and as the hob turns in synchronicity with the workpiece, the gashes of the hob successively cut flats along the tooth profile to productively generate several teeth simultaneously.
With major diameter, minor diameter, or side fit this non-involute tooth shape requires unique cutting tools for each spline type.
To achieve desired straightness to the bottom of the cut in minor diameter bearing scenarios, a hob with clearance lugs is required. Clearance lugs promote parallelism by removing fillet interference, ensuring a straight side.
Legacy designs are often full-fillet types where a completely full fillet is generated at the root of the spline. Shoulder-type hobs and elongated-tooth hobs are used where a minimum fillet can be cut (as small as 0.005″ radius).
Spline profiles can take several shapes such as involute and parallel key profiles. Commonly, involute splines are designated by two pitches, such as 24/48 DP, 16/32 DP, 8/16 DP, etc. The two pitches refer to the tooth’s width and height respectively (a spline tooth is wider than it is tall). Common tooth profiles are “twice as wide as they are tall.” The most common pressure angle is 30°.
In North America, splines are often designed to standard diametral pitches, including DP of: 1/2, 2.5/5, 3/6, 4/8, 5/10, 6/12, 8/16, 10/20, 12/24, 16/32, 20/40, 24/48, 48/96
An involute spline can be designed as a major diameter fit or side fit, achieved by varying the depth of the hobbing cut.
In a major diameter fit, bearing is on the major diameter and the fit is controlled by varying the major diameter of the external teeth. In a side fit, bearing is on the sides of the teeth and the fit is controlled by varying the tooth thickness. Side fits are self-centering and help equalize stresses at the bearings of all teeth.
Side-fit involute splines may have a flat-root or fillet-root form.
A flat-root ramp on the hob tooth profile will generate a chamfer on the tooth to provide clearance at the major diameter between the external and internal mating teeth. A fillet-root spline hob has a smooth radius so as not to generate a hard chamfer on the spline teeth tips.
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