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Power Transmission for Motion Control
While technology has advanced and there are many new
ways to accomplish useful motion within particular applications,
electric motors and gearheads are still the preferred
and most popular choices for power transmission throughout
commercial, industrial, and automation applications.
Motors are the components that convert electrical power
to useful mechanical power. Gearheads transform the
mechanical rotary power to the desired combination of
speed and torque if the motor cannot do so directly.
Benefits of this combination include low cost, simplicity,
reliability, and versatility.
There are several styles and types of electrical motors
and power transmission devices on the market today.
Depending on the application, a designer might first
choose between AC and DC motors, then between styles
(e.g., induction, universal, and permanent magnet),
and finally between the many types specific to each
style. The designer is also challenged with the task
of selecting a controlling device and amplifier to provide
the proper input power for the electric motor. Within
the layout for the power transmission, one can select
between options such as direct drive, chain/belt drive,
ball screws, direct gearing, and all of the types respective
to those styles.
Input Power
As designers put the pieces of the puzzle in place to
resolve their applications, they usually hit a hurdle
somewhere in the middle; the motor does not provide
the proper output. For example, the motor may be capable
of supplying the necessary power, but not match the
speed or torque requirements. Most motors are rated
to operate for peak output around a certain continuous
load range of torque and speed. To operate the motor
outside that range might either under utilize the motor
and result in wasting money on an oversized motor, or
over utilize the motor and potentially run the risk
of damaging the motor. Although each type and style
of motor operates differently, smaller motors typically
operate more efficiently at higher speeds and lower
torques. Larger motors operate at higher torques and
lower speeds. The dynamics of this is simple to follow
once the motor is viewed as a large moment arm. The
greater the radius of the rotor the larger the moment
that can be created from the motor, resulting in greater
torque.
High-energy permanent magnet servo motors have become
extremely popular in the motion control industry of
late. For these motors, a general rule of thumb is that
if the length of the motor is doubled, then the output
torque capacity of the motor is also doubled. However,
the output torque capacity of the motor will be quadrupled
if the length is kept the same and the diameter of the
motor is doubled. For maximum efficiency, proper sizing,
and a cost effective design, it is best to design the
motor for operation at or slightly under its designed
load point of speed and torque.
Output Power
Gearing is one of the oldest ways of converting and
transmitting rotary power. Most transmission systems
operate on the basis of changing moment arm length to
alter torque characteristics while inversely changing
speed. Torque can easily be doubled by all kinds of
transmission systems while simultaneously cutting the
speed in half. But, in the end, power in is still equal
to power out (less the efficiency). Proper transmission
systems must be chosen based on performance, size, cost,
and various other constraints. While each type of system
has its own pros and cons, gearing systems hold many
advantages in areas regarding economic power capacity
in small packages. Involute spur gears have developed
to certain standards and offer a highly effective means
of transmitting power.
Twenty-degree pressure angle involute gears typically
are the most popular throughout industry. Simple interaction
between two gears results in a change of speed and torque
by the same amount as the ratio of the diameters (or
tooth count) of the two gears. The power is transmitted
through the gear teeth engagements via load sharing
across multiple teeth (contact ratio). Because of the
20-degree pressure angle, resultant radial loading on
the shaft/journals is introduced into the system. Efficiencies
of spur gearing can exceed 98%.
To enhance the output of such a spur gear design,
a style of involute spur gearing, known as epicyclic
gearing (i.e., planetary gearing), was developed. Planetary
gearing dramatically increased the strength of the gearhead
while eliminating radial loading by counterbalancing
the affect of any one-gear engagement. Planetary gearheads
distribute the input power coming from the sun gear
(input pinion) to two or more planetary gears. Since
the planet gears are positioned symmetrical to the pinion
gear, radial loading on the pinion gear is eliminated.
The planet gears are housed within a ring gear, which
has gear teeth cut into the inside diameter in a reversed
tooth profile. The separating forces (radial force)
produced from the planet gears’ tooth engagements
with the sun and ring gears, cancel each other. The
ring gear is stationary, and the input sun gear pinion
drives the planet gears, which then walk in the same
rotational direction as the sun gear.
Counter balanced input force from the sun gear with
reaction force from the ring gear, results in a tangential
force on the planetary gear axis of rotation to produce
this walking motion. The rotary motion and force of
the individual planetary axes is cumulatively captured
with what is known as a carrier plate and can be directly
coupled to either an output shaft, or to another sun
gear for the process to repeat through multiple stages
(depending on the ratio), see Figure 1.
Depending on the number of teeth of the sun gear and
ring gear, each stage typically generates speed reduction
ratios between 3 and 10 times the input speed. A two
stage planetary gearhead, which has two 10 to 1 stages
(i.e., 10 rotations of the input pinion results in one
rotation of the output), yields 100 times the output
torque (less efficiency) and 1/100 the rpm. If the gearhead
were reversed so that the output shaft becomes the input
shaft, the speed would increase by 100 times; the available
output torque would decrease proportionately.
Options
The speed reducing planetary gearhead as described adheres
to: Ratio = (Ns + Nr)/Ns (where Ns = Number of sun gear
teeth and Nr = Number of ring gear teeth). There are
many variations of epicyclic gearing that can be produced
from this style. In some instances, the ring gear can
be the driving gear or the output gear. Complex planetary
gearing is also possible in which gear clusters are
used in the planetary configurations. An automobile
transmission is a good example of how several styles
of planetary gearing can be utilized together.
Materials, heat treating, gear face width, and altering
tooth profiles are additional factors to consider for
customizing specific advantages or characteristics into
a gearhead. Crowning and tip relief each offer altered
performance characteristics. Manufacturing methods also
play a major role in performance, lead times, and cost
ramifications. Gears can be cast, molded, broached,
hobbed, shaped, honed, or ground. Each method has advantages
and disadvantages. Greater gearhead design evaluation
is necessary if an engineer has specific performance
requirements or special considerations for life, precision,
noise, temperature, and vibration.
The cost is typically higher for planetary gearing than
spur gear clustering because part count and complexity
is higher. However, planetary gears present huge improvements
in power density (torque), torsional stiffness, and
backlash reduction. Typical planetary gearheads also
offer reduced size advantages in comparison to belt/chain
drives, harmonic drives, and spur gearing. They also
usually have greater torque carrying capabilities than
the belt/chain drives and harmonic drives of comparable
size. Harmonic drives, have fewer ratio combinations,
but are exceptionally stiff/accurate and rather expensive.
Other types of gearing that have found their way into
many applications are bevel gearing (to accommodate
non-parallel shafting), worm gearing (for higher ratios
in non-parallel shafting and elimination of back-driving
potential), and helical gearing. Helical gearing is
similar to spur gearing except the teeth of the gears
are not straight or parallel to the axis of rotation.
Instead they have a helical twist to them. The result
of the helix gear mesh is that the contact ratio is
increased and the gearhead can be exceptionally strong
and smooth. Helical gears can be utilized almost anywhere
spur gears are employed. In addition to a cost disadvantage,
helical gearing usually produces axial resultant forces
on both the drive gear and the driven gear. For this
reason all planet gears have to be captivated on both
sides of the gear and both the input pinion and output
carrier plate/shaft have to be supported with bearings
to handle the axial loading. (See Figure 2 for a gear
type comparison.)
Software
Recent technology advances in software have also had
a dramatic affect on changing the characteristics of
gearhead performance through design alterations. One
could normally calculate the center distances and tooth
count for the planet gear as Np = (Nr-Ns)/2 once the
sun gear and the ring gear have been determined. However,
this might not optimize the potential gearhead power
transmission capacity. Changing the planet gear design
allows the engineer flexibility to make changes throughout
the other key components of the gearhead.
Software designed and enhanced modified gear tooth profiles
can optimize the operating center distances as well
as operating pitch diameters of the gears, resulting
in a stronger gear set. Software packages allow gear
producers to maximize performance without adding additional
cost.
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