This article contains a complete guide to planetary gears and their use.
You will learn:
A planetary gear is an epicyclic gear that consists of a central gear, referred to as the sun gear and serves as the input gear, which has three or more gears that rotate around it that are referred to as planets. The planet gears engage with a ring gear that encircles the sun and planet gears and has the shape of an internal spur gear. The design of planetary gears makes them exceptionally sturdy and easy to convert to different gear ratios.
Planetary gear sets are a method for converting reciprocating motion to rotary motion. They were developed by William Murdoch in 1781 for use with steam engines as a replacement for cranks that were used to produce rotary motion. A common application for planetary gears is the reduction of speed in a limited space.
The main and essential components of a planetary gear are the sun gear or central gear, several other gears known as planets, an outer ring gear, and a planet carrier. The four components of a planetary gear provide stability because of the even distribution of mass and rotational stiffness. Torque is applied radially to the gears and transferred radially without placing pressure on gear teeth.
The sun gear is the central gear in a planetary gear set that receives input for the gear set. As the sun gear is driven by the input shaft, the planet gears are empowered to drive the ring gear. The structure of a planetary gear is configured such that the pinions of the carrier, which is attached to the planet gears, are meshed with the sun gear.
The planet gears are mounted on a carrier and move in accordance with the motion of the sun gear. As they rotate against the sun gear, their teeth mesh with the teeth of the ring gear. As the planet gears move, they rotate around the sun and ring gears. Planetary gears have one or more planet gears and control the speed of the transmission of power.
The outer ring of a planetary gear system is the ring gear or annulus gear. It has teeth on its inner circumference and is smooth on its outer edge. The teeth of a planetary ring gear are pointed inward to mesh with the planet gears. The carrier and planet gears are bound and surrounded by the ring gear that is fixed in its position. All of the teeth of the planet gears mesh with the sun gear and ring gear. As the planet gears rotate around the sun gear, the ring gear is forced to rotate in the same direction as the sun gear.
The planet gears are fixed to a carrier that connects their centers with the sun gear and ensures that they mesh such that their pitch circles roll without slippage. Normally, the carrier is movable and rotates relative to the sun gear. It is connected to the centers of each of the planet gears to provide support. The simplest form of planetary gear has one sun, one planet, a ring gear, and one carrier. Compound planetary gears have more than one of each of the various gears, which become more complex as the number of planet gears increases.
Planetary gears require the defining of multiple inputs to get a specific output. The sun gear is in the center of the assembly and meshes with the teeth of the planet gears, which are small and connected to the carrier framework that is made of aluminum, cast iron, or steel. The carrier has a shaft for each of the planet gears that surround the sun gear and are surrounded by the ring gear.
The construction of a planetary gear set can include spur or helical gears. The difference between the two gears is their helix angle with spur gears having a zero-helix angle while helical gears have a helix angle of 10 to 30 degrees. With either form of gear, bearings play an important part in a planetary gear set's torque transmission. Needle bearings have the right size but are unable to tolerate the axial forces. The ideal type of bearings are tapered roller bearings for axial forces and are larger than needle bearings.
The various types of planetary gears are separated by their performance, efficiency, and versatility. Each type has the ability to change two inputs into a single output, a factor that makes them complicated to design and analyze. The most common form of planetary gear set is a single sun and single planet with a carrier and outer ring gear.
Planetary gear sets behave counterintuitively, a factor that makes their analysis difficult, yet intriguing and interesting. They provide excellent torque with appropriate stiffness and little noise in a compact footprint. Planetary gears come in a wide variety of forms and designs, which is one of the reasons for their wide use.
A short list of planetary gears includes:
These ten planetary gear sets are a very small sampling of the complete list of planetary gears, which includes specialty gears designed for unique and unusual applications. It is important to note that each type of planetary gear set has its advantages and disadvantages. Their use is determined by how they meet the needs of an application.
A single-stage planetary gear set consists of a sun gear, three planet gears, a carrier, and the ring gear. The input shaft rotates the sun gear, which causes the planet gears to rotate the carrier and the output shaft. Different sized gears with different numbers of teeth change the speed and torque of the output. Single-stage planetary gear sets are used with heavy machinery, industrial equipment, and automotive transmissions. As with most planetary gears, single-stage planetary gears are used for their compact design and weight.
Multi-stage planetary gear sets have two or more planetary gears to achieve the required speed and torque. They have the same construction as single-stage planetary gear sets but have several more gears added to the input shaft in the first set, which is connected to the sun gear of the first set. The carrier for the first set is connected to the sun gear of the second set, a pattern that continues to all planetary gears in the multi-stage planetary gear set. Multi-stage planetary gear sets are used in applications that require high torque and several output speeds. They achieve high gear ratios in a compact lightweight package.
With an in-line planetary gear set, the input and output shafts are in a straight line and consist of a single sun gear and multiple planet gears contained in a ring gear. The planet gears are mounted on a carrier, which rotates around the sun. The input shaft rotates the central sun gear, which turns the planet gears that mesh with the ring gear. The rotation around the central axis sends power to the output shaft. In-line planetary gear sets are used for their efficiency, torque density, and low backlash.
Offset planetary gear sets are known as parallel shaft planetary gears. They have the input and output shafts placed in an offset or parallel position. As with in-line planetary gear sets, offset planetary gear sets have a sun gear with several planet gears mounted on a carrier that mesh with the outer ring gear. Instead of the output being in a straight line with the input, it is aligned differently in relation to the input shaft. Choosing the correct diameter and number of teeth of the idle gear makes it possible for the input and output shafts to be on a common axis.
With right angle planetary gear sets, the input and output shafts are arranged at right-angles to each other. The sun gear, planet gears, carrier, and outer ring are all mounted in the same configuration as the other gear sets. The rotation of the planet gears transmits power to a right-angle bevel gear set that is connected to the output shaft.
Harmonic drive planetary gear sets are also known as strain wave gearboxes and wave generator gearboxes. They use a flexible metal cup and circular spline to transfer torque. The components of a harmonic drive planetary gear set are the circular spline and the flexible cup that is referred to as a wave generator. Additionally, the gear set has elliptical or circular gears called flexspline.
The input shaft drives the wave generator that causes it to deform and create waves that move along the circumference of the cup. The created waves engage the teeth of the flexspline that rotates around the central axis of the harmonic drive gear set. The flexspline meshes with the outer ring gear that sends power to the output shaft. Harmonic drive planetary gear sets have zero backlash, high precision, and high torque density, which makes them ideal for applications where accuracy and repeatability are important such as robotics and automation applications.
A Simpson planetary gear set has a single sun gear and two or three sets of planetary carriers that are connected in a series. The structure of a Simpson planetary gear set has three forward gears, one reverse gear, a neutral, and is used in a three or four ratio automatic transmission. The gear set is named for Howard Simpson, a famous innovator of designs for the automotive industry.
The two sets of planet gears are interdependent due to having a common sun. The first gear set is closer to the input shaft and is synchronized with the second gear set’s ring.
A Ravigneaux planetary gear set is an improvement of the Simpson planetary gear set and consists of two sun gears, two ring gears, and two sets of planet gears on a single carrier, which makes the Ravigneaux planetary gear set smaller, lighter, and less expensive. The two suns are centrally located along a common rotating axis and are of different sizes.
The process for a Ravigneaux planetary gear set involves having the smaller of the sun gears engaging its set of planets that engage the outer ring gear. The larger outer ring gear is connected to the planets of both planetary gears and engages the larger of the two suns. A carrier, with a different radius, holds the sets of planet gears in place and is connected to the drive shaft. It turns as a unit in relation to the sun and ring gears.
With a differential planetary gear set, every gear rotates, which is a key factor in the gear set for driving the wheels of a car. Initially, cars were set up to have one of the wheels driven while the other wheel was mounted freely. Although it was an effective driving method, it was unsafe, which required finding a method to drive both wheels.
The development of the differential planetary gear set required that each wheel have its own drive shaft. Several bevel gears with a carrier are used to change the direction of the rotation of the drive shaft. The gear set has a large ring gear, a carrier, two planet gears, and a sun gear. The ring gear is connected to the shaft of one of the wheels while the sun gear is attached to the other wheel, which makes it possible to change the rotation of the drive shaft to drive the wheels.
Planetary gearboxes are energized by hydraulic motors, electric motors, or combustion engines. They are the most common form of gearbox and are connected directly to a precision motor or are integrated. Planetary gearboxes are exceptionally efficient in transmitting energy from a motor to an output.
A wheel drive planetary gearbox is the simplest form of gearbox and can be connected directly to a wheel with wheels being combined over the housing of the system. The carrier for a wheel drive planetary gearbox is located in the gearbox. The direct connection of the gearbox to the wheels helps reduce the size of the gearbox.
As with other planetary gearboxes, the sun gear rotates the planets, which are housed in a carrier inside the shaft drive gearbox. The ring gear is fixed while the carrier rotates the drive shaft. The housing of the gearbox is affixed directly to the machine with the output being the rotating shaft.
Spindle output gearboxes are just like shaft output gearboxes with the main difference being the output delivered by a flange.
Planetary gears are one of the most popular forms of gear reduction methods. In their simplest form, they have three sets of gears with their own degrees of freedom. The planet gears rotate and mesh with the sun gear and ring gear. The unity between the sun gear and ring gear means that torque is carried in a straight line.
Simple planetary gear sets provide speed reduction as high as 10:1 while more complex and complicated planetary gear sets with multiple planetary gears are capable of providing greater reductions. With compound planetary gear sets, the first gear set is the input. As the power from the input moves along the different planetary gears, power reduction gets greater from stage to stage. When a right-angle turn is required, as in a differential planetary gear set, bevel or hypoid gears are attached to an inline planetary gear system.
A major benefit of planetary gear sets is the avoidance of radial loads that are created by tangential gear forces that can damage the bearings of the gear set. The forces in a planetary gear set cancel out each other making it impossible for radial forces to act on shaft bearings. As more planet gears are added, load capacity and torsional rigidity increase, and the load is distributed. This reduces the wear on the teeth of planet gears.
Large loads are easily driven by small streamlined planetary gear sets. Additionally, helical gears can be used for large load capacities that spur gears are unable to handle. This is due to helical gears having more teeth mesh at once. Helical gears axial reactions cause bearings to handle the thrust load, which can damage them.
Wear in planetary gear sets is endured by the small bearings that support the planet gears. Although larger bearings would be beneficial, small bearings are necessary because of the limited space in a planetary gear set.
The load on planet gears must be perfectly balanced to ensure the best performance of the gear set. Imbalances occur when one of the planets is closer or further from the axis of the sun, or the carrier axis is off. Additionally, as the number of planets increases, the potential for balancing errors increases. Depending on the type of imbalance, which can be small, the gear set may wear into it and gradually distribute the load.
Planetary gear design is a precision process that requires close attention to the slightest detail. The smallest imbalance can lead to an imbalance in a planetary gear set and cause it to fail. Design engineers select high quality components, assemblies, and materials to ensure the accuracy and precision of planetary gears. During the designing phase, areas of potential imbalance are pinpointed such that they can be avoided and corrected.
Soft mounts allow for small radial movement of the sun gear or carrier. It allows for a slight shift in the gear system to equalize uneven loads. Although the method can be used to deal with balancing issues, the majority of planetary gear systems are rigid and stiff for the greatest efficiency.
Planetary gear sets are normally quieter due to the lower pitch line of the smaller gears. The caveat is the many planet gear teeth engaging at the same frequency and the noise of the input shaft. High quality spur gears bring down the amount of noise produced while helical gears, with their rapid tooth engagements, further reduce potential noise levels.
The basic factor that assists in limiting planetary gear set noise is proper mounting. Planetary gear systems are mounted vertically to a motor, which allows the motor to center on the gear set. Proper mounting makes it possible to place the gear set in any orientation and drastically reduces the noise level.
Phasing refers to the multiple gear meshes that have the same frequency. An understanding of phasing has to be considered when discussing planetary gear meshes. Planetary gear set phasing examines how each gear meshes in the gear set.
In-phase is where the same phase angle is found in each gear mesh. Planetary gear sets with in-phase gear mesh have reinforced torque where thrusts are canceled by radial forces.
Sequential phase refers to separated phase angles for subsequent gear meshes. This phase has canceled torque, thrusts, and reinforced radial forces.
Counter phase has an opposite phase angle for opposing planets with all excitation canceled.
In a mixed phase, there are combinations of each of the phasing types and occur with unequally spaced planets and requires that gear mesh phase angles be carefully calculated.
The introduction of helix angle planetary phasing does not change the fundamentals of phasing. Each planetary gear mesh produces a meshing frequency. Helix angle gears introduce axial forces into gear meshing. The key factor to remember regarding planet mesh phasing is how it impacts vibrations in a planetary gear set and is defined by the frequency harmonic number, ring gear tooth number, and the number of planets. Mesh phasing is not related to a particular planetary gear set but is influenced by cyclic symmetry and the response of the mesh frequency.
The three methods used to lubricate a planetary gear set include grease, oil splash, and forced oil. The differentiation between the various methods is dependent on the speed at which the gear set operates, the method for applying the lubricant, and the types of gears.
Grease lubricant is used with low-speed planetary gear sets since it does not provide cooling and cannot be used in continuous duty or loaded applications. A measured correct amount of grease lubricant is a necessity since too little prevents proper lubrication while too much can cause drag.
Oil splash lubrication is used with helical and bevel planetary gear sets and is referred to as oil splash due to the use of an oil reservoir that the gears dip into, which causes the oil to spill onto the other gears and bearings. A problem with oil splash lubrication is churning where the gears have to push through the lubricant. The process is designed for tangential speeds of 3 m/sec.
Forced oil lubrication is used with high-speed planetary gear sets and involves an oil mist, spray, or drop. With the oil mist technique, an atomized oil mist saturates all of the gears. With oil spray, oil lubricant is sprayed on the gears. The oil drop method drops oil directly onto the areas to be lubricated and is used in conjunction with the oil splash method. In all cases, oil is applied from an oil reservoir.
The wide use of planetary gear sets is due their size, weight, and compact design. They act as speed reducers that slow down motors and increase torque. Planetary gears are used in several industries due to their ability to produce torque and share the load with multiple gears because of more contact surfaces and area between gears. The load is evenly distributed, which makes gears more resistant to damage.
Planetary gear sets are used in rugged applications due to the even distribution of the load and their robust and power dense design that is capable of handling high torque and reductions. They can withstand high shock loads and overhung loads because of their self-aligning properties.
Planetary gear sets are used in wheel transmissions because they have lower induction losses and fewer gear changes. They are lighter, more efficient, and quieter than larger transmissions with a torque capacity of 332,000 Nm. Planetary gear sets are used with feed machine tools, presses, and conveyors.
Track drives and trolley systems are used to power the main drives of railway systems such as locomotives, stationary electric motors, self-propelled lawn mowers, and military tanks. Switching gears in planetary gearboxes increases the torque necessary to accelerate and brake trains. The wide range of output speeds and torque provide the power necessary for the traction to move a train and improve safety.
Unlike wheel conveying systems, planetary gearboxes have greater point to point reliability at higher speeds. Since they take up less space, they are ideally suited for high-speed product movement. Planetary gearboxes have higher torque, which is necessary to maintain acceptable vibrations levels, a reduction in drive failures, and the elimination of operator fatigue.
Planetary gearboxes are used with pumping systems due to their efficiency, low noise level, and longevity. They provide the performance and torque that is required by a pumping system. An extra advantage for using planetary gearboxes is their ability to withstand harsh and hustle conditions as well as their low cost of installation.
Wind turbines require gearboxes capable of increasing rotor speed. Planetary gearboxes are preferred due to their high-power density and concentric input and output. The input from the blades provides the largest load that a planetary gear system has to endure. The load sharing capacity of a planetary gear system ensures a higher power density, which allows designers to increase the gear ratio and power capacity in a very limited space.
Planetary gearboxes are an important part of automatic transmissions and are widely used in DC motors and servo motors. This is due to the rigidity, compactness, and torque of planetary gearboxes. They work as coaxial speed reducers or increasers with speed reduction being their most important function. Planetary gearboxes are capable of increasing torque by slowing down brushed, brushless, or servo motors due to their many contact points.
At a 97% energy input efficiency and the ability to reduce speeds up to 10:1, energy is transmitted through a planetary gearbox with very little loss. These factors are the reason that they are used in most modern equipment and industrial machinery.
Planetary gearboxes are part of the joints of robot limbs and determine the speed and direction of the movement of the robot. The common form of planetary gearbox used in robotics is a harmonic drive, which has a higher transmission ratio than other gearboxes with larger meshing teeth. Harmonic planetary gearboxes are able to handle the variations in the kilowatts used to power robots and has no impact or noise. They are able to operate in a vacuum, corrosive environment, and harmful media with the capability to implement high speed motion.
Planetary gears come in several forms with each style designed to perform a specific function. Manufacturers produce high quality planetary gears capable of meeting the specific needs of any application. The key factors in their production are precision, accuracy, and strength, concepts that are critical to their performance.
The AD-Series has a one-piece planetary cage design with a stiff and accurate rotating flange. It is a high torque, low backlash compact helical planetary gear set. The compact design of the AD-Series makes it possible to fit into any the smallest available space but provide the highest quality speed reduction without failing.
The P2KA18 planetary gearbox is rated for high power unit solutions. It comes in 27 sizes and seven basic types with torque ranges of up to 2,600,000 Nm and transmission ratios of 4000:1. The modular design of the P2KA18 makes it applicable to a wide range of industrial applications. The high performance of the P2KA18 is available at a reasonable cost in a compact size and includes exceptional reliability, simple installation, and low maintenance.
The GBPH-060x is designed with the highest torque density for motion control, automation, and robotic applications. It is the ideal solution for NEMA 23 frame size and can be used with servo, stepper, brushless, and AC or DC motors. The goal of the GBPH-060x is to provide a cost effective, high quality planetary gearbox. The extraordinary value of the GBPH-060x makes it the perfect solution for a wide variety of applications and processes.
The Rexnord atlas gear drive has a gear ratio range of 4.39 up to 1255 with a torque range of 43,000 to 26,000 inch pounds. It has a foot mounted drive that can be positioned horizontally or vertically with accessories that include flanged motor mounting, slide base, top motor mounting, internal backstops, and shaft fans. The dependability of the Rexnord atlas gear drive is guaranteed with a heavy duty three year warranty.
The GPT gearhead has been developed for high torque applications and has up to six planets per stage. The high torque is enclosed in a short length that offers exceptional performance with low volume. The GPT series contains gears made of hardened stainless steel to ensure reliable transmission of forces. Connections are welded with all components of the gearhead being lubricated to ensure outstanding performance.
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