Introduction of Involute Spline
Pressure Angle in Gears
There are five most common types of involute splines.  

Pressure Angle of 30° 
It is for ASME, SAE, AGMA and ASA standards and belongs to a short cut tooth with a depth of 50% and 6 teeth at least.

Pressure Angle of 45° 
The depth of tooth is also 50% and used for an involute sawtooth spline of a fine pitch. The thickest pitch is Dp = 10 with 6 teeth. It is usually used for angle control of multiple teeth with a larger axis diameter for higher capacity.

Pressure Angle of 20° 
The depth of tooth is 75% and similar to a short cut tooth. If it is used for cars, the depth of tooth is 50%. This type has better contact than the previous two by 50%. As it is deeper, gear scraping and manufacturing is easy.

Pressure Angle of 25° 
The depth of tooth is 70% with an enough involute for scraping. It has better contact than the first two types by 40% and better automatic alignment than the third type.

Pressure Angle of 14.5° 
The depth of tooth is 30%. This type can’t be scraped and unable for automatic alignment; therefore, it is rarely used in sliding splines. As it has a very short depth of tooth and a small pressure angle, it is usually used for greater axis diameter. Its breaking stress is small and the wall inside the spline is thinner, which results to a bigger addendum width. It is particularly suitable for a fixed spline mechanism whose major diameter can be fit.

Glossary for involute spline
1.  active spline length (L): It refers to the contact length of a pair of external and internal splines when they are engaged slightly. The active spline length exceeds the length of engagement for sliding splines. 

2.  actual space width (S): the arc width of any space on the pitch circle 
3.  actual tooth thickness (t): the arc thickness of tooth on the pitch circle of any tooth 
4.  alignment variation: the variation of the actual splined shaft corresponding to the reference axis 
5.  base circle: the circle formed by involute splined teeth 
6.  base circle diameter (Db ): diameter of the base circle 
7.  basic space width : For a pressure angle at 30°, the basic space width equals to half of the circular pitch. For a pressure angle at 37.5° or 45°, the basic space width is greater than half of the circular pitch, which is because the size of a tooth increases in proportion. To make the thickness of tooth of the external spline at base the same as that of the internal spline at the form diameter, minor diameter of the internal spline is increased. 
8.  circular pitch (P): the arc distance between two adjacent teeth on the pitch circle 
9.  depth of engagement : the distance between the minor circle of the internal spline and the major circle of the external spline subtracting semitopping or tooth backlash 
l0.  diameter pitch (P): It refers to the number of teeth per inch pitch diameter. The diameter pitch may decide the circular pitch, the basic space width or the basic tooth thickness. If working with the thickness of tooth, the pitch diameter can be obtained. 
11.  effective clearance (Cv ): It refers to the effective space width of the internal spline subtracting the effective tooth thickness of the external spline. 
12.  effective space width (Sv): Idealistically, the effective space width of an internal spline should be equivalent to the arc tooth thickness of the pitch circle of an external spline. In this way, no interference or looseness exists (for axial touch). As the minimum effective space width is a design standard, requirements for common difference will be met by increasing or decreasing the tooth thickness of the external spline. 
13.  effective tooth thickness (tv): Idealistically, the effective tooth thickness of an external spline should be equivalent to the effective space width of the pitch circle of an internal spline. In this way, no interference or looseness exists (for axial touch). 
14.  effective variation: the cumulative total variation of various variations while internal and external splines are engaged 
15.  form circle: It refers to the circle at the limit of involute design, which is close to the major circle of the internal spline and the minor circle of the external spline. 
16.  form clearance (CF): It refers to the radial depth of the involute profile subtracting the engaged part. The purpose of this value is to ensure there is a sufficient clearance between the minor circle of the internal spline and the major circle of the external spline when these two concentric circles are engaged. 
17.  form diameter (Df): the diameter of the form circle 
18.  lead variation: It refers to the variation between the tooth direction of the spline and the direction of the reference axis, including parallelism and alignment variation. 
19.  length of engagement (Lg): the total axial touch length of the spline tooth 
20.  machining tolerance (m): the permitted tolerance of actual space width or tooth thickness 
21.  major circle: It refers to the utmost circle on the spline surface. It means the outer diameter circle for an external spline and the dedendum circle for an internal spline. 
22.  major diameter (D0): diameter of the major circle 
23.  minor circle: It refers to the innermost circle on the spline surface. It means the dedendum circle for an external spline and the addendum circle for an internal spline. 
24.  minor diameter (D): diameter of the minor circle 
25.  nominal clearance: It refers to the actual space width of the internal spline subtracting the actual tooth thickness of the external spline. 
26.  Pitch: The ratio of 1:2 represents parameter proportions of the spline. The denominator refers to the normal diametral pitch and the numerator stands for the stub pitch. 
27.  Profile variation: It means the vertical variation from the standard profile to the actual flank of tooth. 
28.  stub pitch: It refers to the radial distance from the pitch circle of the external spline to the major circle and that from the pitch circle of the internal spline to the minor circle. In ANSIB92.1, the stub pitch is two times more than the diametral pitch. 