# Servo horn dimensions - What is 25T?

I wonder that horns for servos are marked in some T units. Example: 25T

Please help me getting out, what does it mean.

## 1 Answer

The T indicates the number of teeth or ridges in the servo spline. You need to match the servo spline to any device splines or they won't mate.

:EDIT: Because it's a good link, and because I hate it when links to resources rot, I figured I'd duplicate the information here from the excellent Servo City:

Servo Splines

Servos have an output shaft that typically uses a splined profile in order to transfer the torque from the output shaft into the servo attachment that is fastened to it. This splined profile is typically classified by the number of teeth, however, there is potential for two very different servo splines to have the same tooth count as it doesn't capture the spline diameter or tooth profile. For example, A15T and D15T both have 15 teeth, however the D15T is a much larger spline size than the A15T. The graphics below are loose representations of the common spline sizes that are found on hobby servos. If you have a servo that we do not offer on the site, you can check to see if your servo has a spline that matches any of the below spline profiles by counting the teeth and measuring the distance across the spline.

• Are those teeth have some standard dimensions? Ex, pitch 2mm and pitch 1mm are comletely different cogwheels – filimonic Oct 15 '20 at 16:13
• @filimonic - Kind of. You need to make sure the ratings match; here's a link that shows different splines. So a servo like this is advertised as a "25T" spline, but the earlier link shows that there could be a B25T or H25T spline, so you need to look further at the datasheet. The product page shows, under the SPECS section, 25 tooth (H25T Spline) spline - so it's an H25T standard spline. B25T splines are not compatible. Hope this helps. – Chuck Oct 15 '20 at 17:23
• @Chuck, Your links to servo teeth are very good. I would give 5 stars (10/10). BTW, I didn't know what is a "spline", so I asked Google: Spline: a rectangular key fitting into grooves in the hub and shaft of a wheel, especially one formed integrally with the shaft which allows movement of the wheel on the shaft. Cheers. – tlfong01 Oct 16 '20 at 1:01