Pardal,
Just treat this like a gear problem
but use a factor of about .700 for the
Y factor in the Lewis equation as a
starter.  Then try some typical module
spline information to maximize the major
diameter of the shaft spline.  The
major diameter of the internal spline will
be the same plus some clearance to allow
these to interface.

Hi diamondjim :
Thank for your tip , could you send me the lewis equation?
Regards.

Pardal

Pardal

Pardal,
The Lewis equation is basically
Wn = Sby/Diametral Pitch   (Pounds)
S = Allowable Max Stress (PSI)
150 x the BHN for a rough approximation of PSI
b = facewidth of the gear
Y factor published many places
y x pi = Y  some publications list small
y for circular pitch gears.  I have not seen
Y factors for anything greater than 25 degree
pressure agles.  If Y values are in the .400 range
and above the values are Y and not y values.
y values will be below .200 in the tables.
Wn = Tangential Tooth Load  Pounds
This would be the max allowable tooth load
allowed or permitted for shock conditions.
The normal tangential tooth load would be
approximately 1/2 this value for good design.
practice.  

If you are having problems with spline failures you could try a polygon profile (DIN32711). In general you can transfer more torque with a polygon than with a spline.

http://www.beasy.com/images/pdf/publications/papers/polygonal-profiles.pdf