Twin Tweaks ...
DISASSEMBLING THE SAVÖX GEARBOX.
REMOVING THE PIN.
MESSING WITH THE POTENTIOMETER.
62 SERVO 07.2011
rely on servos to power entire drive trains. Even back when
our fledgling FIRST team was experimenting with the EDU-bot, we were working with servos modified for full rotation.
For the Savöx servos to be a truly exciting option for
roboticists, we had to see if they would be amenable to this
Your Rotation has
Most servos are used for maintaining
positions or other limited adjustments, so
the default is to have a limited range of
motion like 90° or 180°. Servos accomplish
this via feedback mechanisms, and in the
case of the Savöx and many other servos,
this includes a potentiometer and a
mechanical stop. The potentiometer is linked
to one of the gears in the gearbox, and the
changing resistance of the potentiometer
allows the servo to know what the angle of
rotation is. A physical stop also keeps the
servo from making a full rotation. Both of
these mechanisms need to be adjusted to
allow the servo to experience the joy and
freedom of 360° rotation.
The full rotation hack is indeed a classic,
and there are numerous resources on the
Web that describe the process step by step
for a variety of servos. One of our favorite
guides is from Acroname Robotics which
offers detailed illustrated instructions for
modifying the Hitech HS-300. There are
many other guides online directed at many
other servos, but none for the Savöx servos.
ServoCity.com even cataloged
a rotation modification difficulty list for
prospective hackers to let them know what
they might be getting themselves into. Some
servos have even been deemed
unmodifiable. One of the main reasons a
servo might not be able to be hacked into
full rotation is because some servos only use
a half gear in the gearbox. This saves space
but also means the gearbox can’t support
Upon initially opening the Savöx servo,
we were afraid that a full rotation hack
would be difficult. The titanium and
aluminum gears looked positively cozy, and a
bearing at the top of the shaft seemed to
decree to the gears below that none shall
pass. As we fretted over visions of cutting
off the bearing with a Dremel tool, we
stumbled upon a way to disassemble the
gearbox that was far less destructive. Using
two screwdrivers and a careful application of
force, the gearbox could be pulled apart
enough so that the large gear — bearing intact — could be
freed from the gearbox. With this keystone gear of sorts
removed, the rest of the gearbox fell away like leaves from
the autumn trees.
The easier of the two major modifications that needed
to be made was dealing with the mechanical stop. In some