Planetary Gear
Planetary Gear
A planetary gearhead is a device in which the output and the input shafts are aligned. The primary function of the planetary gearhead is to offer the greatest torque in the most compact manner. They offer a greater torque and a higher reduction ratio per gear train. They are made of good quality composite materials.
Why the name ‘Planetary’ ?
The planetary gearbox gets its name from its resemblance to our planetary system. Just like our solar system, it consists of a ring gear (satellite), a solar gear (sun) and two or more planet gears (planets) set together. The solar gear is usually moving and also puts the planet gears in motion which are bolted in the planet carrier. These make the output shaft. The ring gears are stationary and have a fixed location with respect to the surrounding components. This complete assembly resembles the solar system, also known as the planetary system and hence the name planetary.
Construction of Planetary Gearhead
A basic planetary gearhead consists of four basic components:
Sun gear : The central gear that runs the surrounding planetary gears
Ring gear : The outermost gear
Planet gears : There are multiple planet gears between the sun and the ring gears.
Carrier : It is the assembly that holds the planet gears together. It also keeps the planet gears equally spaced in an orbit.
These three components make one stage of a planetary gearhead. One can devise double or triple stages for greater ratios. Also, the torque density can be increased by using a greater number of planet gears between the sun gear and the ring gear. Planetary gears can be excited with the help of a motor.
Principle of Operation
The planetary gearboxes are used to energise anything, be it a simple plant device or some cutting edge electrical system. These are basically used in applications that require higher torque density, good functional efficiency and higher durability.
In a planetary gearhead, multiple teeth run simultaneously. Due to multiple teeth running simultaneously, the machine offers a high-speed reduction with a comparatively smaller gear. Also, a lower inertia is reflected back to the system.
The collective sharing of the load by numerous teeth allows the planetary parts to give out higher levels of torque.  Â
Consider, the sun gear has s teeth, planetary gears have p teeth and the stationary annulus has a teeth, then
           Reduction ratio per train = (a:s) + 1
Types of Planetary Gears
Depending on their performance, one can classify planetary gears into three types :
Wheel Drive Planetary Gear
The sun gear moves the surrounding planetary gears. In the wheel drive planetary gear, these planetary parts are connected to a carrier. When the sun gear is run, the planetary gears circulate the ring gear which is on the outer side.
The wheels can be put together over the housing of the system. This type has the advantage of optimizing the size of the system by connecting the wheel directly to the gearbox.
Shaft Output Planetary Gear
As in wheel drive planetary gear, the sun gear runs the surrounding planetary gears. In shaft output planetary gear, these planetary parts are housed in a turning carrier. The ring gear is stationary with the turning carrier providing drive to the shaft. The housing part of the system is connected directly to the machine with the output being a turning shaft.
The output shaft consists of gears of different sizes. The gears are in a constant mesh with the countershaft gear. The gears rotate freely from the output shaft but once engaged they are stuck to the output shaft. The output shaft rotates according to the power transferred by the gears.
Spindle Output Planetary Gear
Spindle output planetary gear functions in a similar fashion to the shaft output planetary gears. However, unlike shaft output planetary gearheads, the output is provided as a flange.
Reduction Logic
Ratio Calculation
The gear ratio, also known as the reduction, is obtained by dividing the number of teeth on the larger gear by the number of teeth on the smaller gear.
Torque Calculation
Mo = Mi r μ                                   Â
where,Â
Mo = output torque (Nm)
Mi = input torque (Nm)
r = gear transmission ratio
μ = gear efficiency (%)
Power calculation
Po = Pi μ                        Â
where,
Po = output power (W)
Pi = input power (W)
μ = gear efficiency (%)
Characteristics of Planetary Gearheads
Higher reduction ratio per train.
Higher torques can be obtained
Both input and output of a train have the same direction of rotation
Lesser backlash
Advantages
Requires lesser trains for a specific reduction ratio
Offers higher gear ratio in compact space
For a similar gear ratio, the planetary gearhead will be lighter in weight compared to the traditional gearheads.
Offers greater efficiencies of about 0.92 per stage.
It has a higher torque transmission capability.
It will have lower inertia.
This arrangement offers higher stability.
It has a good service life.
In this arrangement, the driving and driven members are concentric. Hence, the driving and driven equipments can be installed in the same line line. This allows space saving.
Compact design of gearbox.
The load will always rotate in the same direction as the motor, for any number of trains.
Smaller shock on paid reversal of motor rotation.
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Disadvantages
Higher cost compared to the traditional gears.
Complex designing and manufacturing
It is difficult to determine the efficiency of planetary gears.
Gearing needs to be accurate.
Some planetary gear arrangements make additional noise during operation.
The driving and driven members of the planetary gear system must be concentric to avoid any additional gearing.
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Applications
Owing to the numerous advantages, planetary gears have various applications. Following are a few applications of the planetary gears :
Robot, to improve the torque
Printing press, to reduce the speed of rollers
Precise positioning
Packaging machine for reproducible products
Wheel drives
Track drives
Conveyors
Slew drives
Hoist drives
Mixing
Winch drives
Pumps
Coil tubing injectors
Auger and drilling drives
Cutter head drives
Harbour mobile cranes
EOT cranes
Automation assembly
Printing Lathe
Clock
Toys
Turbine engine