This difference is called the backlash or clearance. The fit for a side-fit spline is the difference between the circular-tooth thickness of the external splined part and the circular-space width of the internal part. Also, there is clearance between the outside diameter of the external part and the major diameter of the internal part. A side-fit spline has clearance between the root diameter of the external part and the inside diameter of the internal part. Splines are specified as either side fit or major-diameter fit. Standard cutting tools are available for both types in a range of standard DPs. The root diameter of a flat root external spline is typically larger than a fillet root spline, but the stresses are close to being the same because the fillet root with the smaller root diameter offsets the sharper corner but larger root diameter of the flat-root spline. Splines are made with either a fillet root or a flat root at the interface of the tooth flank and the root diameter. The 30-degree splines are by far the most common, so that is what will be considered here. Involute splines are available in several different pressure angles: 30 degrees, 37.5 degrees, and 45 degrees.

Thus an 8/16 spline has the tooth thickness of 8 DP and a tooth height (whole depth) of 16 DP. Typical involute SAE/ANSI spline teeth are specifies as two numbers: a numerator that specifies the tooth thickness in diametral pitch (DP) and the second number that specified the tooth height in diametral pitch (DP). They have the same number of teeth and fit together as one.

Involute splines have teeth similar to gear teeth except spline teeth are much shorter, and they do not roll. We will only be considering involute splines here because they are much more common. Splines may be straight sided, tapered, or have an involute form. Splines provide a connection between two shafts or other components that transmit torque and rotation.

Sae 6b spline