Capt Kremen Posted March 12, 2020 Share Posted March 12, 2020 Just taken delivery of some new servos from that wonderful shop with superb service in Frodsham. I have successfully used these little marvels of electronic and mechanical engineering since the 70s, making best efforts to employ the correct linkage geometry for the task i.e. servo arm and control horn positions. One thing which I cannot find definitive reference to, (even on the manufacturers web sites), are the numbers on the servo arms and their meaning. 1 to 4 on the 'cross' arm types and numbers such as '18', '15' and '12' on the HiTec example I'm studying now. Is there a source of information? (Strangely they don't include it in the servo packaging). Are the numbers off-set degrees from 90 and torque for the respective servo arm hole? Obviously from my use down the years, not knowing this info hasn't prevented use but it would be 'nice-to-know'. Quote Link to comment Share on other sites More sharing options...
Brian Cooper Posted March 12, 2020 Share Posted March 12, 2020 Yes, the arms have different offset angles depending on which number is selected. However, I am not sure how many degrees are involved. The differences always seem fairly minimal, but very useful nevertheless. B.C. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted March 12, 2020 Share Posted March 12, 2020 Here is the Hi tec chart Quote Link to comment Share on other sites More sharing options...
John Tee Posted March 12, 2020 Share Posted March 12, 2020 I've known about these different angles for some time. First met them on Futaba servos. Very handy for getting the servo arm as near neutral as possible without resorting to subtrim settings. Only found them by accident when fitting an arm and finding the neutral point changed as I put the arm on in a different position accidentally. John Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 12, 2020 Share Posted March 12, 2020 This is the one thing that Futaba have definitely got right. i.e. Having an odd number of splines so that rotating the servo arm by 90 or 180 degrees, becomes not quite 90 or 180 degrees. This gives a small amount of adjustment of the centre position. I had assumed that the 1, 2, 3 and 4 marks of Futaba cross arms was just arbitrary allowing you to find the best 1 of the 4 possible positions and then remember it by the number. I generally use HiTec servos myself, but always feel that they have missed a trick by having an even number of splines and they also make the arms symmetrical about the 90 and 180 degree axes, giving no adjustment there either. Figure 23 above is the only one they've got right in my opinion - having a 2mm offset on 4 out of the 8 output holes. All the other HiTec arms are symmetrical around the centre hole. (I am aware that the HS8300 and 9300 use the same arrangement as "industry standard" Futaba, so they have learnt something). Quote Link to comment Share on other sites More sharing options...
jrman Posted March 12, 2020 Share Posted March 12, 2020 JR servos have the same ability to adjust the arms around the centre. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 12, 2020 Share Posted March 12, 2020 Posted by jrman on 12/03/2020 17:41:14: JR servos have the same ability to adjust the arms around the centre. Indeed. JR use 23 teeth. As far as I know, HiTec are the only major company that use an even number of teeth (24). Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 12, 2020 Share Posted March 12, 2020 Posted by Brian Cooper on 12/03/2020 14:23:30: Yes, the arms have different offset angles depending on which number is selected. However, I am not sure how many degrees are involved. The differences always seem fairly minimal, but very useful nevertheless. B.C. Well - in the case of Futaba, rotating the arm by 180 degrees will move it by half of one spline. There are 25 teeth, so that means that the adjustment is half of 1/25th of 360 degrees = 7.2 degrees. If it's a 4 way arm, giving 4 different positions, the adjustment is halved again = 3.6 degrees. Quote Link to comment Share on other sites More sharing options...
Tim A Posted March 12, 2020 Share Posted March 12, 2020 Hitec are now using 25T splines on some new servos, I know this because I just brought 4X HS488HB for a new plane. Downside was the arms supplied were too short for the wing servo's and wouldn't clear the wing! So a call to a friend resulted in fitting a couple of longer Futaba arms. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 12, 2020 Share Posted March 12, 2020 Posted by Tim A on 12/03/2020 18:40:14: Hitec are now using 25T splines on some new servos, I know this because I just brought 4X HS488HB for a new plane. Downside was the arms supplied were too short for the wing servo's and wouldn't clear the wing! So a call to a friend resulted in fitting a couple of longer Futaba arms. That's good. Hopefully they'll migrate them all to H25T eventually. Quote Link to comment Share on other sites More sharing options...
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