ProModeler servos use a 25T output shaft. This means our servo arms are compatible with Futaba's (no bad thing in itself). Other manufacturers use 23T, 24T, or 26T. Why did we use 25T? Simple, it's because as even-number of splines offers no advantages in terms of offset when setting up your model while an odd-number of splines does. we refer to 25T as the Goldilocks solution - here's why.
What does this even mean? Ever put a servo-arm on the servo's output shaft and notice it doesn't fit 'square' to the case? Eyeball this second photo (below). Do you see how the servo arm is slightly CW of neutral? The problem with this is you'll get asymmetrical throw. In this instance, a little bit more CCW than CW and this is bad because it may throw off how your model handles.
So the next step a pro does when performing the mechanical setup is to adjust the arm by a spline or two. This, to try for a better neutral position relative to the case. In this second photo, even with a relatively fine 25T spline, moving the arm by just one spline results in the arm going from a tad too far CW to a little bit too far CCW (from neutral).
This is the point where ordinary modelers resort to their transmitter and the sub-trim function. The difference between an ordinary modeler and a pro is the pros know another trick. When using servos with an odd-number of splines (23, 25, 27), simply rotating the arm 180° (flipping it around the other way) often results in the servo arm being much closer to perfectly square (neutral).
And while the photo doesn't really show it, this fourth servo arm photo (below) is nearly perfectly aligned with the case! This is the goal of a good mechanical setup, e.g. the servo control arms is at an almost perfect neutral position!
In this fifth and last photo, notice the numbers molded into the servo arm. Unlike in the second and third photos where the 1 was on the left and the 2 on the right, in this instance the 2 is on the left (look just to the left of the arm mounting screw hole) and similarly, a 1 is molded into the horn just to the right of center - look closely near the base of each horn because the numbers are tiny!
There's one last trick a pro has up his sleeve before using sub-trim. If you have several of the same servo arms, here's what you do; try swapping them out because due to how they're molded, you can actually fine tune things a tiny little bit with different arms. Strange, but true . . . and another trick the pros use! And by the way, do this with the 4-horn and 6-horn servo arms included with your servos because they have numbers molded into them as well.
Anyway, now's the time to use sub-trim and because you're a pro at
performing the mechanical setup, your model will have less sub-trim to
affect subsequent programming (as compared to when an ordinary modeler
does his servo installation).
This brings us to why ProModeler uses 25T splines for our ø6, or 6mm output shaft (nominal diameter). Note; this is roughly 1/4". Was it just to copy Futaba? No, not really. You see, when cutting splines for a 1/4" shaft, you have a choice of 22, 23, 24, 25, 26, or 27-teeth (ditto the broaches for cutting the corresponding inside diameter for the servo arms). Thing is, when you choose an even number, the trick for rotating the servo arm on the shaft doesn't work because the arm centers up the same whether you flip it around 180°, or not. Thus, while the duffer doesn't know or care, an odd-number of splines gives an advantage for the serious modeler to accomplish a better mechanical setup. If you're a pro, this is a big deal.
And this is why we didn't use 24T like some of our competitors, and while we could have used 23T like other competitors, in our experience this results in much too coarse an adjustment when you move it just one spline at a time. Especially for a precise setup! Hence, in our opinion 23T is inferior to 25T for a quality mechanical setup. with respect to using 27T, another problem makes an appearance. This time because the individual teeth being finer (smaller) are a little weaker, also. And this may become an issue with servos outputting over 350oz-in, which some of ours do.
Like Goldilocks and the three bears, 25T is not too coarse, not too fine, but just right!