Planetary Gearheads –
Their Time is Now
A
s industry becomes
characteristics and advantages, so will
more automated and
know, for instance, when to specify
robotised the need for high
a servo motor rather than a stepper
precision gearboxes in
or which type of coupling to use in
machinery and plant engineering grows.
Ian Carr, Managing Director of Drive
Lines in Bedford, explains how planetary
gearheads have precision and power
designed into them.
given situations.
It is fair to say that precision drive
system designers make regular use
of planetary gearheads. In fact they
are used almost exclusively as the
Machinery and systems such as robots,
major joints in both articulated arm
packaging machines, pick and place
and delta robots, are ideal for many
units, tool changers in machine tools,
bespoke machines and are a firm
printing presses and welding systems
favourite for medical scanners. So let
all rely on precision motion. Likewise,
us look first at how they work, then
medical equipment, navigational aids and
at why they are good for precision
astronomical telescopes also need cutting
drives.
edge motion systems.
While at first glance some of these
applications may seem to have little in
common, in fact their power and motion
systems are based on similar fundamental
principles. Looked at as a drive system, the
main requirements are in fact very similar:
high torque must be provided, whilst low
backlash is essential.
Operating principle
Planetary gears are also called
epicyclic gears or sun-and-planet
gears, the latter being useful for
helping visualise - their operating
principle. Dismantle a planetary gear
and at first it looks complicated, but
you can soon identify a central sun
gear, surrounded by, and meshing
To achieve this, torsional stiffness is
with, some planet gears. (Typically
required, and high acceleration and
there are three or four planet gears,
braking are often demanded. More often
but it could be fewer or more.)
than not, there is also the need for a
Holding the sun and planet gears
compact design, or the need to fit within a
together are an outer ring gear with
demanding space envelope.
internal teeth that mesh with the
Such drive systems have a high design
content. Many are truly bespoke, one-off
designs, while others may be customised
variations of a standard design. However,
planets. There is also a planet gear
carrier, but this does not transmit
power rather being a structural
element that keeps the gears in place.
like all drives systems they are typically
Significantly, while the sun gear
made up in large part from standard
rotates relatively quickly, the outer
components, servomotors, gearheads,
ring gear rotates more slowly. In
couplings etc.
fact the ratio of their two speeds is
Thus a precision drive system designer
will select components, optimising one
with the next with the ultimate goal of
achieving each project’s performance
specification. They will also be aware
that each type of component has certain
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PECM Issue 20
derived from the ratio of their two
diameters – the planet gears act as
idlers, their role being to transmit
power between the sun and ring, in
other words their diameter does not
affect the overall speed ratio between
input and output.