Timing signals for R/C
servos are often stated in
milliseconds, but a more
accurate unit of measure is the
microsecond — or millionth of a
second. To convert milliseconds
to microseconds, just move the
decimal point to the right three
digits. For example, if a pulse is
0.840 milliseconds, move the
decimal point over three digits
and you have 0840, or 840
microseconds (lop off the
leading zero; it’s not needed).
A Deeper Look at
Guts of a Servo
The motor inside an R/C
servo turns at several thousand
RPMs. This is way too fast to be
used directly, so all servos
employ a gear train that
reduces the output of the
motor to the equivalent of about 50–100 RPM. Servo gears
can be made of nylon, metal, or a proprietary material.
Besides the drive gears, the output shaft of the servo
receives the most wear and tear. On the least expensive
servos this shaft is supported by a plastic or resin bushing
which obviously can wear out very quickly if the servo is
used heavily. A bushing is a one piece sleeve or collar that
supports the shaft against the casing of the servo.
Metal bushings — typically made from lubricant-impregnated brass (often referred to as Oilite, a trade
name) — last longer but add to the cost of the servo.
The best — and more expensive — servos come
equipped with ball bearings, which provide the longest life.
Ball bearing “upgrades” are available for some servo
models, but it’s usually cheaper (and certainly easier) just to
buy a servo already equipped with them.
When looking at servos, you’ll often see a notion
regarding the bearing type — either bushing or bearing —
and whether it’s metal or plastic. You also may see a
notation for “top” or “bottom;” this refers to the servo
having a bushing or bearing on the top and/or bottom of
the output gear like that shown in Figure 5.
Better servos have two bushings or bearings: one each
on the top and bottom of the gear. That provides the best
support, and the servo will last longer.
Common Servo Specifications
Radio control servo motors enjoy some standardization.
This sameness applies primarily to standardized servos,
which measure approximately 1.6” x 0.8” x 1.4”. For other
servo types, the size varies somewhat between makers, as
these are designed for specialized tasks. Table 1 outlines
typical specifications for several types of servos including
dimensions, weight, torque, and transit time. Of course,
except for the size of standard servos, these specifications
can vary between brand and model. A couple of the terms
used in the specs require extra discussion:
• The torque of the motor is the amount of force it
exerts against a load. One common torque unit of
measure for R/C servos is often expressed in oz-in
(ounce-inches) — or the number of ounces the servo
can lift when the weight is extended one inch from
the shaft of the motor. (Other common
measurements include Newton-meters but the
general concept is all the same.) Servos exhibit high
torque thanks to their internal gearing.
• The transit time is the approximate time it takes for
the servo to rotate the shaft a certain number of
Table 1. Typical Servo Specifications.
Type Length Width Height Weight Torque Transit Time
Standard 1.6” 0.8” 1.4” 1.3 oz 42 oz-in 0.23 sec/60°
1/4-scale 2. 3” 1.1” 2.0” 3. 4 oz 130 oz-in 0.21 sec/60°
Mini 0.85” 0.4” 0.8” 0.3 oz 15 oz-in 0.11 sec/60°
SERVO 07.2014 59
Figure 5. Ball bearings or
bushings may be placed at
the bottom and/or top of
the servo output gear in
order to prolong the life of
the servo motor.