I'm looking at a hobby micro servo, which has a 5mm splined shaft output. Specifically, this is a Tower Pro model SG90 or clone. Close-up photos of the shaft below.

I've found online diagrams for both a 30 degree Flat Root Spline shaft, and a 30 degree Filet Root Spline. Can someone please explain the difference between these spline designs?

Micrograph of splned output shaft

Closeup of spline teeth

I'm adding a drawing approximating the photos (and a hub) of a 21 tooth spline with a 5mm pitch circle and 45 degree pressure angle. (To simplify, I've not used involute teeth.)

I have filleted the exposed corners of the teeth at 0.06 mm. Is it these that make it a fillet root spline?

I've also added a fillet to the empty spaces joining the faces.

detail drawing of spline

P.S. here's a link to my in-progress CAD model. Feedback welome.


Fillet root splines have a smooth curve between adjacent teeth and the root diameter, flat root splines may not and will have a flat portion of tooth space between teeth. Fillet root teeth provide better bending strength and thus higher load capacity.

The spline in your picture appears to be a fillet root, although the pressure angle looks a lot higher than 30 degrees.

  • $\begingroup$ Thanks. I certainly agree the servo spline looks like a fillet, and I'm guessing its a 45 degree pressure angle. $\endgroup$ – Burt_Harris Feb 25 at 18:24
  • $\begingroup$ I'm not sure exactly what you mean by the "root diameter", I've updated my question with a diagram to help clarify what I'm asking. $\endgroup$ – Burt_Harris Feb 25 at 18:25
  • $\begingroup$ The root diameter in splines and gears is the diameter to the bottom of the teeth. $\endgroup$ – jko Feb 25 at 19:16
  • $\begingroup$ Thanks. I managed to get a look at the ISO spec for exact definitions, but I understand yours better now. $\endgroup$ – Burt_Harris Feb 26 at 1:19

According to ISO 4156-1:2005, the following definitions apply:

3.6 fillet root spline

spline having a tooth or space profile in which the opposing flanks are connected to the root circle (Dei or Die) by a single fillet.

3.7 flat root spline

spline having a tooth or space profile in which each the opposing flanks are connected to the root circle (Dei or Die) by a fillet.

[emphasis added]

The distinction is quite subtle, as even flat root splines have fillets.

From what I now understand, the photos in my question are fillet root splines. However, the diagram is sort of a hybrid. I'll try to draw a second diagram to illustrate the difference and add it here.


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