RC Servo Guide

Each and every radio control model out there uses at least one rc servo motor. Just one servo motor is usually found in electric rc cars. When it comes to nitro or gas powered rc cars then there are at least two servos – one for throttle and the other one for steering.

The job of an RC Servo is to move the servo arm as much as you input it on the radio control and maintain that position, also it has to do it as quickly and accurately as possible.

RC Servo Sizes

RC Servo Sizes

There are 3 types of rc servo sizes. These are: micro, standard and 1/4 scale size. These sizes cover most of the servos out there.

There are even more precise sizes for the micro rc servo and you can differentiate them by size.

According to HobbyKing these sizes are:

  • Sub-Micro Servo (1-5g)
  • Micro Servo (5-10g)
  • Mini Servo (11-20g)
  • Park Servo (21-30g)
  • Standard Servi (31-49g)
  • X-Large Servo (50+ g)

 

RC Servo Horns

RC Servo Horns

When you buy a servo it comes with a set of servo horns or servo arms. But keep in mind that there are different servo output shaft types.

23 tooth spline – Sanwa / Airtronics / KO / JR
24 tooth spline – Hitec
25 tooth spline – Futaba / TT / ACE / HPI

RC Servo Torque and Speed

When you read the servos specification you probably spotted operating speed and stall torque.

Operating Speed

This means how fast the servo arm moves to 60°. The less seconds the faster it is. Why 60 degrees? This is a common standard all the manufacturers use. But you always have to carefully check out the specification and their units.

There are no standard speed rates for servos. Speed rates greatly vary for various brands and you just have to pick the one you like the most.

An optimal speed rate is around 0.20s/60° range and nothing faster is required for a beginner that learns to fly rc airplanes.

If were talking about a specific rc helicopter tail rotor servo then the speeds can go as fast as 0.04s/60°.

Torque

In simple words torque is how much weight a servo can pull or push on a given arms length.

rc servo torque

For fellow Americans the values are:

2cm – 1.25 kg = 0.98 in – 44 oz

1cm – 2.5 kg = 0.39 in – 88 oz

0.5 cm – 5.0 kg = 0.19 in – 176 oz

As you can see in the picture. When your pushrod is more away from the center of the servo the less torque you can get, but when placing the pushrod towards the center of the servo the bigger your torque gets.

These specification ratings are given for two voltages. For 4.8 volts and 6.0 volts. This means that at a 6V battery pack the servo will operate a little bit faster than at 4.8V. You don’t want to exceed 6V for the servo or else it will burn. (not actually catch on fire)

Analog Vs Digital Servos

Analog Servo

The analog servo does it’s job very well and if you’re a beginner I would suggest to go this way for now. If we analyze how the analog servo works you’ll find out that it’s much slower than a digital servo in terms of gaining speed and thus torque to rotate the servo horn. The analog servo doesn’t hold the neutral position so well and if you’ll try to move the servo horn you’ll feel it move a little bit.

Digital Servo

Now digital servos are much more expensive, but not without a reason. Digital servos have much faster response times, faster acceleration and it holds neutral position much better. When I say faster response times I don’t mean it will travel full 60° faster than an analog servo. No, what I mean is, if we compare this to a drag race and identical specification servos, but one analog and the other digital, then digital servo will take off much quicker in the start and have much more torque in smaller traveled distance than an analog one.

The digital servo will hold the neutral and any other position very well and if we’d try to move it by the horn you’ll feel it completely stuck in that one position.

There is just one minor minus for the digital servo and that is Power Consumption.  The faster response times eat a bit more power.

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