Chemical Engineering December 2013 - 62

Tognum
Engineering Practice
gear units. However, planetary gear
units can often be employed in highspeed
CPI machinery (typically in
the 0.2-30 MW range). These are
popular both for rotating machines
and material-handling units, particularly
where large speed ratios
are necessary.
While double-helical gear units
are simple and popular, planetary
gear units offer smaller size and
higher efficiency compared to other
types of gear units. Because the
transmitted power is divided over
several tooth meshes, planetary
gear units are more compact and
less expensive than other options.
The smaller gear diameters in a
planetary gear unit also produce
smaller mass moments of inertia,
and this substantially reduces
the acceleration and deceleration
torque during acceleration and
braking. Generally, high speed ratios
can be achieved (even 80 or
more). The coaxial design permits
a superior arrangement for many
CPI machineries. However, due to
operational issues and reliability
risks, planetary gear units will require
extra consideration.
Figure 3 shows an example of
a planetary gear unit. They can
achieve a large speed ratio (even 40
to 80) in a very compact unit.
A planetary gear unit typically
consists of three coaxially rotating
components: a sun gear, a carrier
with several planetary gears, and
a ring gear. Among the variants of
planetary gear units, the options
preferred for CPI applications are:
* The star-gear type, with speed
ratios of around 2-12
* The planetary-gear type, with
speed ratios of roughly 3-13
* The star-gear type with the double
planet, with speed ratios of
around 12-40
* The compound-epicyclic gear type,
with speed ratios of about 8-80
The star-gear type and the planetary-gear
type can usually offer a
similar ratio range (both can cover
a speed ratio in the range of 3-12).
The star-gear type is used when the
rotating carrier would cause unacceptably
high pressure on the bearing
journal. The compound-epicyclic
gear type offers one of the
most compact options.
An important loss in a
high-speed gear unit is
windage loss, which represents
the power lost because
of the compression
of the air-lubricant mixture
around teeth roots
during meshing and the
aerodynamic trail of the
teeth in the air-lubricant
mixture. With any planetary-gear
type, it is necessary to take into account
a relatively high windage.
The best operation and perforFIGURE
2. Gas turbines typically have a defined operating
speed range. When used in a machinery train
(with a generator, compressor or pump train), a welldesigned
gear unit is used to match the speed of the
turbine shaft to the driven equipment
mance occurs when the load is
distributed as evenly as possible
across the individual planet gears.
An uneven load balance can create
damaging effects.
The ability to reduce friction can,
theoretically, result in a better-centered
gear-tooth loading. However,
in real-life scenarios, some friction
(and thus some unbalances in the
loading) should be expected. The
mass inertia of the gear-unit elements
can also offer some dynamic
loads. It is desirable to reduce the
masses, but there are always practical
limits in the mass-reduction exercises
(some limits to achieve very
lightweight designs)
The partial-load operations and
particularly the minimum-load
case, can offer some challenges. The
no-load case occurs when the gear
unit and the associated machinery
train are operating with no load -
for example, when a pump unit operates
in the pre-commissioning stage
without actually pumping a liquid,
and thus, the driven gear unit just
rotates with no load transmission.
Some machinery trains should only
operate a very short time in a noload
condition, otherwise damage or
reliability issues may arise.
Usually, most gear units have
several backlash-prone joints and
relatively large masses that are unable
to center themselves when the
load changes, and this may cause a
gear unit to behave unstably. This
issue needs considerable attention,
especially for planetary gear units.
For instance, sophisticated elastic
mountings are usually used for the
62 ChemiCal engineering www.Che.Com DeCember 2013
planetary gears (for example, in the
form of flex pins). These will bend
under a load, with the result that
the planet gears do not skew but are
simply minimally displaced parallel
to the pinion or ring-gear axis. This
solution can ensure an optimum
load balance between the meshed
teeth and the even load distribution
across the entire width of the tooth
face at both full and partial load.
Selection criteria
Key factors influencing the choice
of gear units are load capacity, efficiency,
and successful references (for
comparable applications). In general,
several types of gear types are
available for planetary gear-units,
including spur gears, single-helical
gears, double-helical gears and others.
If single helical gears are used,
the two opposing axial forces acting
on the planetary gear may generate
a large tilting moment (axial bearings
could be required if the helix
angle is not selected properly).
The double-helical gear set usually
requires free axial adjustability
to allow users to achieve an even
load distribution across the two
tooth halves. However, the two gear
meshes can prevent the necessary
movement (see Figure 1). In this
way, two side-by-side gear meshes
do not allow for the tiny movements
required for adjustments
and smooth operation. In addition,
external axial forces may interfere
with the load distribution.
Spur gears (or straight-cut gears)
are the simplest type of gear. Their
slightly better efficiency compared
to the spur gearing is one advantage
for the helical gearing. For
planetary gear units, a single-helical
gearing with an optimum helix
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