This guide aims to give you a basic understanding of servos, the different types and their uses.
Servos are small motorised units that turn the signal from your transmitter into movement of the control surfaces (ailerons, elevator, rudder) on your model. They can also be used to operate retractable undercarriges, otherwise know as retracts.
Servos come in many shapes and sizes, but normally consist of a wire that connects to the model's receiver, a motor to create the movement, gears to handle the output from the motor and circuitry to process the signal passed from the receiver to the motor. Depending on the size and loading on the gears, a bearing may also be used.
Depending on the size of your model different servos
are used. Sizes range from Sub-Micro for the smallest models up to Giant/Monster for the largest models.
Most models have existing servo enclosures built-in for a specific size of servo that is appropriate for use with that model. Always check the instruction manual to see which size servo is required for your model.
Normally a reference servo make and part number is provided.
Whilst servos fit into size categories, e.g. Sub-Micro, Micro, Mini, Midi, Standard, Low-Profile and Giant/Monster, there
are no defined industry standards so one model manufactur's requirement for a Micro servo may actually be a servo
manufactures Mini sized servo. It's always best to find a reference servo noted in the model manual, find its dimensions
and then use these if you are seeking an alternative servo.
Gears are used in conjunction with the motor to create the movement that drives the servo output shaft. Whilst some torque (the turning force) is provided by the servo motor, gears are also used to regulate the torque and speed of the output.
Different materials are used for gears depending on the servo application which in turn can
affect its size, weight, durability and cost. In some servos they will feature one or more types of gears. Common gear materials include:
- Nylon/plastic gears offer the lightest weight, but with lower durability and are normally found in lightweight models where low torque is required.
- Metal gears offer higher durability and can handle higher torque output, but they are heavier than nylon gears.
- Composite gears, such as Karbonite, are now available which are aimed at providing the lightweight characteritics of nylon gears, but with the durability and high torque applications of metal gears. Due to
the complex nature of manufacturing, the material of these are normally high cost.
The torque of a servo is measured in ounces an inch (oz/inch) or kilograms a centimeter (kg/cm) at one or more voltage, normally 4.8, 6.0 or 7.4 Volts. Speed is measured in seconds from the time it takes to move from neutral to a sixty degree angle, e.g. 0.12 (60°/sec, 6V) - this servo takes twelve hundreths of a second to move to sixty degrees when powered at six Volts.
Increasing the voltage going through a servo does not always mean the servo will output more torque or move faster as the motor and gears used are matched to operate at
a set range of voltages and loadings. When you come to choose a servo always check the servo specification has output figures within the voltage range
that you plan to use in your model.
In servos that produce a higher torque and/or speeds, a ball bearing is added to the servo output shaft to provide support and smooth running of the output shaft as it rotates. Bearings normally feature one or more metal balls in the
casing. Other types include solid plastic and even plastic ball bearings.
Sub-Micro servos do not normally feature ball bearings due to light torque loadings and to keep the servo as lightweight as possible. They will normally have a heavily greased solid plastic bearing.
To be able to move a control surface the movement of the servo output shaft is used by adding a servo horn to the servo output shaft. A linkage can then be connected from the horn to the control surface. Horns come in a variety of shapes and sizes.
See the picture to the right for some examples.
Horns are fitted to the servo output shaft which has a number of teeth or splines and is held in place with a screw going through the horn to the servo output shaft. Note that different manufacturers use a certain number of teeth on their servo output shafts so its essential to use the supplied accessories or purchase the correct servo horns to ensure a correct fitting. Using the incorrect servo horn can damage the output shaft by stripping the teeth off, rendering the servo pretty much useless or cause the horn to fail in use.
The range of motor types used in servos is on the increase. The most popular are brushed motors, but with the advances of motor miniaturisation coreless motors and even brushless motors are
becoming more readably available.
Coreless and brushless motors offer lighter weight, higher torque, higher speed and stronger holding capabilities, but require more power and
currently are used in higher-end competition performance models.
The difference between analog and digital servos is purely the way they process and send the received signal from the radio receiver to the servo motor.
Analog servos send pulses of power to the servo motor when it needs to move. The
size of the power pulse is proportional to the amount the servo is being asked
to move. Therefore for small movements a small pulse over time is sent to the servo. Given the small pulse of power the motor will be slow and sluggish to react.
Digital servos act in the same way as analog, but they use a microprocessor to interpret the signal from the radio receiver and provide power to the servo motor. The microprocessor sends pulses of power to the motor at a shorter interval so over time the motor is receiving more power. As the motor is given more power at a shorter interval the motor reacts quicker and also has stronger holding torque, as it has more power to maintain the position.
Given the increase in performance, digital servos are higher cost and do consume
more power than analog servos. Where you require quick, accurate responses with stronger holding, then digital servos are the way to go.
Otherwise analog servo suit most purposes.
Have a browse of our range of servos by clicking here. If you need any help, just call us on 0845 539 2103 (local rate) or email us at Shop@TopProps.co.uk where we'll be glad to advise you.
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