Choosing Radio Control Servos
We mentioned in our page on Radio Equipment that most outfits now do not include servos. This is because the choice is so great that many modellers prefer choosing radio control servos other than those made by the radio equipment manufacturers.
The choice is actually a bit mind blowing and one could spend weeks, if not months, trawling through the vast array of servos on the market. The fact that most of the radio control equipment manufacturers do not include them does not mean that they do not supply them. It is important to know that the majority of servos on the market feature a standard servo plug that will fit most receiver servo ports.
There are some exceptions but most of the servo manufacturers have adopted the standard applicable to JR, Hitec, Spektrum and Futaba receivers. Here is a diagram showing the four most popular types of servo plug:-
Let me explain the significant differences shown here.
1) Futaba servos have a narrow locating key on the left hand side of the plug as shown here. If you wish to use Futaba servos with any of the other receivers, you will need to trim this key off the plug using a sharp scalpel blade. This will make the plug compatible with most of the other receivers. The purpose of this key is to ensure the plug is not fitted to Futaba receivers the wrong way round.
2) You will notice that the JR and Hitec plugs have a small chamfer on the left side. This matches a similar chamfer on the receiver sockets to ensure correct orientation of the plug. Most independent servo manufacturers have opted to use this type of connector.
3) The Airtronics plug molding is different to the others but will still fit most receivers. The main difference with this plug is that the + (positive) and – (negative) wires are fitted in reverse to every other servo and receiver. It is possible to change these leads over using a sharp pin to release the terminals and swap them over. If you decide to buy servos with this type of plug, I suggest you get someone experienced to show you how to do this.
What They Do
So, what do servos do? Basically they receive pulsed electronic signals from the radio receiver and convert them into a mechanical output using a rotating arm attached to the central output spline of the servo. The position of the arm is controlled by the varying duration of the train of pulses. Hey, don’t worry about it, most people don’t understand exactly how this works. Just accept that they work and are amazingly reliable.
Whose servos you decide to buy will be influenced by your pocket, the mating receiver and probably advice from other club members ( if you’ve joined one!). I have been a big fan of the standard sized servo from Far Eastern manufacturer TowerPro. This is designated the SG5010 – UK (linked to UK supplier, for a USA supplier click this link: SG5010 – US) . It is strong and has a twin ball raced output shaft, and will operate from a voltage supply between 3.5 volts and 8.4 volts. The nylon gear train is ideal for basic sport model flying as it wears better than metal gears and retains its accuracy for longer. Metal gears are obviously stronger but for the loads we will be applying in our trainer, nylon gears are totally adequate.
The output power or “Stall Torque” is very good measuring 8Kg/cm. This means that at 1 cm from the centre of the output spline the servo is capable of holding a weight of 8Kgs against gravity. This is more than sufficient power for our trainer and will not put these radio control servos under any undue mechanical stress. They are also very economically priced!
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