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254 USING SERVO MOTORS

• Sail winch servos are designed with maximum strength in mind and are primarily intended
to move the jib and mainsail sheets on a model sailboat.
See the “Typical Servo Specifications” table, later in this chapter, for a size comparison of
these various types.

Gear Trains and Power Drives

The motor inside an R/C servo turns at several thousand RPMs. This is way too fast to be
used directly; all servos employ a gear train that reduces the output of the motor to the
equivalent of about 50 to 100 RPM. Servo gears can be made of nylon, metal, or a proprie-
tary material.

• Nylon gears are the lightest and least expensive to make. They’re fine for general- purpose
servos.
• Metal gears are much stronger than nylon and are used where brute- force power is needed,
but they significantly raise the cost of the servo. They’re recommended for heavier walking
robots or large robotic arms. On many servos only some of the gears are metal; the rest
are a heavy- duty nylon or other plastic material.
• Gears made of proprietary materials include Karbonite, found on Hitec servos. These
materials are offered as stronger alternatives to plastic.

G Replacement gear sets are available for many servos, particularly the medium- to high- priced
ones ($20+). Should one or more gears fail, the servo can be disassembled and the gears
replaced.

Output Shaft Bushings and Bearings

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

Output shaft

Upper bushing or bearing

Output gear

Lower bushing or bearing
Figure 23- 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.

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