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Servo Selection

Servo Selection – Understanding the purpose.

Servos are an important part of the Radio Gear. Servos control the operation of the control surfaces on a plane, throttle on a gas/nitro engine, steering on land vehicles and more. They do so by receiving a signal from the receiver.  This signal is translated in to the rotation of the servo arm which then mechanically moves a linkage rod. The servo operates on the nominal 4.5-6.2v from the receiver.

There are many different types of servos where each one serves a different purpose. Servos vary in power, speed, size, and quality. Depending on the desired performance, size restrictions or task, will decide on which would be best to use.

Servo Selection – Servo Types

Standard Servo – typically would have around 45oz in. of torque.  This would be suitable for most applications. Many RTR vehicles will include this type of servo to operate its controls. For better or increased performance it is possible to upgrade the servo.

High Torque

 – Typically would have in excess of 75in oz. of torque in the standard size case. In many applications it is highly recommended but not required for even smaller, lighter RC’s to have a higher torque servo for steering on a car, buggy or boat. This will ensure full control over the vehicle in terms of direction.  On a plane it is important to have enough torque on the elevator channel.  At higher speeds the load of a control surface is much higher. A stalled servo is a scenario where the servo does not have enough torque to overcome a load. A stalled elevator servo ultimately may result in a crash of your plane, boat, or car. Be certain your RC has the proper torque requirements for a servo.

Mini / Micro Servo

 – A mini servo would typically have around 35in oz of torque where a micro servo would have around 20in oz. of torque. Each servo is best suited to smaller applications where size or weight would be an issue.  It may be beneficial to save weight or size if torque requirements are met utilizing this class of servo.

High Speed – 

High speed servos vary in torque depending on which one is selected.  They usually are in the 65in oz range but can vary over a large range.  These servos have a much quicker response time.  They will rotate the servo arm at a quicker speed then a servo of a different class. This results in quicker snappier controls. Use these when reaction  time is most desired.

1/4 Scale

 – These servos received their name from being best suited for 1/4 scale models. They are larger than the standard servo case size.  It is not un common to see torque figures in excess of 300 in oz.  1/4 scale servos can also have high reaction speed depending on servo model chosen.

Digital Servos – 

 Digital servos can be of any of the other classes except they are controlled digitally.  Digital servos have better precision, and accuracy. They tend to have great centering characteristics which make them an excellent choice when a precise servo is required.

BB / Coreless / Brushless

 – Often times, these abbreviations or names will be used. Each one of them offer an advantage usually at higher cost.  BB stands for ball bearing. The output shaft rotates in a ball bearing offering slightly better servo performance. Coreless refers to a brushed motor not using the iron slots (Core) on the armature. This results in higher servo speed and performance. Brushless refers to the motor itself being brushless increasing performance across the board at a higher cost.

Boat Servo Application Chart

Hull Length (in) Hull Length (mm) Torque
in-oz
Torque
kg-cm
20-24 508-610 35 2.16
24-29 610-740 50 3.60
29-33 740-840 80 5.75
33-37 840-940 120 8.64
37-42 940-1070 150 10.8
42-48 1070-1220 180 12.95
48+ 1220+ 200+ 14.4+
Outboard
27-32 685-810 110 7.92
32-37 810-940 160 11.52
37-42 940-1070 275 19.8
42+ 1070+ 350+ 25.2

 

Car/Truck Servo Application Chart

 

Scale/Type Steering Torque
in-oz
Steering Torque
kg-cm
Throttle Torque
in-oz
Throttle Torque
kg-cm
Electric
1/18 25 1.8
1/16 40 2.9
1/12 80 5.8
1/10 100 7.2
1/8 150 10.8
1/8 MT 180 13.0
1/5 275 19.8
Nitro
1/16 45 3.2 25 1.8
1/12 80 5.8 40 2.9
1/10 100 7.2 50 3.6
1/8 150 10.8 80 5.8
Gas
1/5 275 19.8 150 10.8

 

MT = Monster Truck

Throttle Torque is the amount of torque required when the throttle servo operates both the throttle and brake system. This value of torque may be used for a dedicated brake servo as well.

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