This article contains a complete guide to planetary gears and their use.
You will learn:
A planetary gear set, also known as an epicyclic gear system, features a central sun gear that acts as the input gear. Surrounding this central gear are three or more planet gears that revolve around it. These planets engage with a surrounding ring gear, which has the form of an internal spur gear. This specific arrangement of gears in planetary gear systems enhances their robustness and adaptability to various gear ratios.
Serving as efficient speed reducers, planetary gears are employed in automotive and off-road transmissions, wheel drives, and industrial conveyors. Their reputation for high ratio capability, space-saving design, and robustness makes them optimal for diverse and specialized uses. The resilience of planetary gears results from an even load distribution among the sun gear and planet gears, allowing them to endure higher torque and offering greater reduction capabilities due to multiple gear mesh points.
Planetary gear sets are used to convert reciprocating motion into rotary motion. Developed by William Murdoch in 1781, these gears were introduced as a more efficient alternative to cranks in steam engines. They are commonly employed to achieve speed reduction within a confined space.
The core components of a planetary gear system include the sun gear (central gear), several planet gears, an outer ring gear, and a planet carrier. This arrangement ensures stability through the even distribution of mass and rotational stiffness. Torque is applied radially to the gears and transmitted without exerting pressure on the gear teeth, enhancing the system’s efficiency.
The sun gear acts as the central gear in a planetary gear system, receiving input from the input shaft. As the sun gear rotates, it drives the planet gears, which in turn drive the ring gear. The configuration of a planetary gear system ensures that the pinions of the planet carrier, which supports the planet gears, are engaged with the sun gear.
The planet gears are mounted on a carrier and follow the motion of the sun gear. As the planet gears rotate, their teeth mesh with those of the ring gear. This movement causes the planet gears to orbit around both the sun and ring gears. A planetary gear system can include one or more planet gears, effectively controlling the speed and transmission of power.
The outer ring of a planetary gear system, known as the ring gear or annulus gear, features teeth on its inner circumference while remaining smooth on the outer edge. The inward-pointing teeth of the ring gear mesh with the planet gears. Fixed in place, the ring gear surrounds and binds the carrier and planet gears. As the planet gears rotate around the sun gear, they engage with both the sun gear and the ring gear, causing the ring gear to rotate in the same direction as the sun gear.
The planet gears are mounted on a carrier that aligns their centers with the sun gear, ensuring proper meshing without slippage as their pitch circles roll. Typically, the carrier is movable and rotates relative to the sun gear, providing support to the planet gears. The simplest planetary gear system includes a single sun gear, one planet gear, a ring gear, and one carrier. In contrast, compound planetary gear systems feature multiple instances of each gear type, increasing in complexity with the number of planet gears used.
Planetary gears operate with multiple inputs to achieve a specific output. The sun gear, located at the center of the assembly, meshes with the teeth of the planet gears. These planet gears are small and mounted on a carrier framework, which is typically made of aluminum, cast iron, or steel. Each planet gear is supported by a shaft within the carrier, which surrounds the sun gear and is encased 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.
Planetary gears differ in performance, efficiency, and versatility, each capable of converting two inputs into a single output. This complexity makes their design and analysis challenging. The most common planetary gear configuration features a single sun gear, a single planet gear, a carrier, and an outer ring gear.
Planetary gear systems often exhibit counterintuitive behaviors, making their analysis both complex and fascinating. They offer high torque with appropriate stiffness and minimal noise within a compact design. The variety of planetary gear forms and designs contributes to their widespread use.
Here is a brief overview of some planetary gear types:
The ten planetary gear sets listed are just a small subset of the full range of planetary gears, which also includes specialty gears for unique and specific applications. Each type of planetary gear set comes with its own set of advantages and disadvantages. The choice of gear set depends on how well it addresses the requirements of a particular application.
A single-stage planetary gear set includes a sun gear, three planet gears, a carrier, and a ring gear. The input shaft drives the sun gear, which in turn rotates the planet gears, causing the carrier and output shaft to rotate. By varying the sizes and tooth counts of the gears, the speed and torque of the output can be adjusted. Single-stage planetary gear sets are commonly used in heavy machinery, industrial equipment, and automotive transmissions. Their compact design and lightweight construction make them ideal for these applications.
Multi-stage planetary gear sets consist of two or more stages of planetary gears to achieve the desired speed and torque. They share a similar construction with single-stage planetary gear sets but include additional gears. In a multi-stage setup, the input shaft drives the sun gear of the first stage, which is connected to the carrier of the first set. This carrier then connects to the sun gear of the second stage, and this pattern continues through all stages. Multi-stage planetary gear sets are ideal for applications requiring high torque and multiple output speeds, offering high gear ratios in a compact and lightweight design.
In an in-line planetary gear set, the input and output shafts are aligned in a straight line. This setup includes a central sun gear and multiple planet gears encased in a ring gear. The planet gears are mounted on a carrier that rotates around the sun gear. As the input shaft turns the sun gear, it drives the planet gears, which then mesh with the ring gear. This rotational motion around the central axis transfers power to the output shaft. In-line planetary gear sets are valued for their efficiency, high torque density, and minimal backlash.
Offset planetary gear sets, also known as parallel shaft planetary gears, feature input and output shafts arranged in an offset or parallel configuration. Similar to in-line planetary gear sets, offset planetary gear sets include a sun gear, several planet gears mounted on a carrier, and an outer ring gear. However, unlike in-line systems where the input and output shafts are aligned, the output shaft is positioned differently relative to the input shaft. By selecting the appropriate diameter and number of teeth for the idle gear, it is possible to align the input and output shafts on a common axis.
Right angle planetary gear sets have their input and output shafts positioned at a 90-degree angle to each other. They use the same basic components as other gear sets, including the sun gear, planet gears, carrier, and outer ring. The planet gears transmit power to a right-angle bevel gear set, which then transfers the motion to the output shaft.
Harmonic drive planetary gear sets, also known as strain wave gearboxes or wave generator gearboxes, utilize a flexible metal cup and circular spline to transfer torque. The key components of a harmonic drive gear set include the circular spline, a flexible cup known as the wave generator, and elliptical or circular gears called the flexspline.
The input shaft drives the wave generator, causing it to deform and produce waves that travel along the circumference of the cup. These waves engage with the teeth of the flexspline, which rotates around the central axis of the harmonic drive. The flexspline then meshes with the outer ring gear, transferring power to the output shaft. Harmonic drive planetary gear sets are valued for their zero backlash, high precision, and high torque density, making them particularly suitable for applications requiring accuracy and repeatability, such as robotics and automation.
A Simpson planetary gear set features a single sun gear and two or three planetary carriers arranged in series. This gear set typically includes three forward gears, one reverse gear, and a neutral position, and is commonly used in three or four-ratio automatic transmissions. It is named after Howard Simpson, a renowned innovator in automotive design.
The two sets of planet gears in a Simpson gear set are interdependent due to their shared sun gear. The first gear set, positioned closer to the input shaft, is synchronized with the ring gear of the second set.
The Ravigneaux planetary gear set is an enhanced version of the Simpson gear set, featuring two sun gears, two ring gears, and two sets of planet gears on a single carrier. This design makes the Ravigneaux set smaller, lighter, and more cost-effective. The two sun gears are positioned along a common axis and vary in size.
In a Ravigneaux planetary gear set, the smaller sun gear engages with its corresponding set of planet gears, which interact with the outer ring gear. The larger ring gear connects to the planets of both gear sets and meshes with the larger sun gear. A carrier with a different radius holds the planet gears in place and is attached to the drive shaft, rotating as a unit in relation to the sun and ring gears.
A differential planetary gear set is essential for enabling all gears to rotate, a crucial feature for driving the wheels of a car. Early automobiles typically used a setup where only one wheel was driven while the other was allowed to rotate freely. Although this method provided effective driving, it was unsafe, prompting the need for a more reliable solution that could drive both wheels simultaneously.
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 powered by hydraulic motors, electric motors, or combustion engines. As one of the most common types of gearboxes, they are either directly connected to a precision motor or integrated within it. Planetary gearboxes are highly efficient in transmitting energy from the motor to the output.
A wheel drive planetary gearbox is a straightforward design that can be directly connected to a wheel, with the wheels integrated over the gearbox housing. The carrier for this type of gearbox is housed within the gearbox itself. This direct connection to the wheels allows for a more compact gearbox design.
In a wheel drive planetary gearbox, as with other planetary gearboxes, the sun gear rotates the planet gears, which are mounted on a carrier inside the gearbox. The ring gear remains stationary while the carrier rotates the drive shaft. The gearbox housing is directly mounted to the machine, with the rotating shaft serving as the output.
Spindle output gearboxes are similar to shaft output gearboxes, with the primary distinction being that the output is delivered through a flange rather than a shaft.
Planetary gears are a widely used gear reduction method. In their basic configuration, they consist of three sets of gears, each with its own degree of freedom. The planet gears rotate and mesh with both the sun gear and the ring gear. The interaction between the sun gear and the ring gear ensures that torque is transmitted in a straight line.
Simple planetary gear sets can achieve speed reductions of up to 10:1, while more complex gear sets with multiple planetary gears can provide even greater reductions. In compound planetary gear sets, the initial gear set serves as the input, and as power flows through the successive planetary stages, the reduction increases progressively. When a right-angle turn is needed, such as in differential planetary gear sets, bevel or hypoid gears are integrated with the inline planetary gear system.
One major advantage of planetary gear sets is their ability to eliminate radial loads caused by tangential forces that could damage the bearings. In a planetary gear set, these forces cancel each other out, preventing radial forces from impacting the shaft bearings. Adding more planet gears enhances load capacity and torsional rigidity, distributing the load more evenly and reducing wear on the gear teeth.
Planetary gear sets can efficiently handle large loads despite their compact size. For applications requiring significant load capacity, helical gears are often used instead of spur gears, as they engage more teeth simultaneously. However, the axial forces generated by helical gears can place additional thrust loads on bearings, potentially causing damage.
In planetary gear sets, wear is typically absorbed by the small bearings that support the planet gears. Although larger bearings could offer advantages, the compact design of planetary gear sets necessitates the use of smaller bearings to fit within the limited space.
To achieve optimal performance from a planetary gear set, the load on the planet gears must be perfectly balanced. Imbalances can occur if one of the planets is misaligned with the sun's axis or if the carrier axis is off-center. As the number of planet gears increases, so does the potential for balancing errors. Depending on the nature of the imbalance, which can be minor, the gear set may gradually wear into it and better distribute the load over time.
Designing planetary gears is a meticulous process that demands attention to even the smallest details. Minor imbalances can lead to significant issues and potentially cause gear set failure. To ensure accuracy and precision, design engineers use high-quality components and materials. During the design phase, potential sources of imbalance are identified and corrected to prevent problems.
Soft mounts can accommodate small radial movements of the sun gear or carrier, allowing for slight adjustments to balance uneven loads. While this method can address balancing issues, most planetary gear systems are designed to be rigid and stiff to maximize efficiency.
Planetary gear sets are generally quieter due to the lower pitch line of the smaller gears. However, noise can still be an issue because multiple planet gear teeth engage at the same frequency, and the input shaft may also contribute to noise. Using high-quality spur gears can help minimize noise, while helical gears, with their smoother and more rapid tooth engagements, can further reduce noise levels.
A key factor in reducing noise from planetary gear sets is proper mounting. When planetary gear systems are mounted vertically to a motor, it ensures that the motor is well-centered on the gear set. Proper mounting allows the gear set to be placed in various orientations, significantly decreasing the noise levels.
Phasing refers to the alignment of multiple gear meshes that operate at the same frequency. Understanding phasing is crucial when discussing planetary gear meshes, as it involves examining how each gear interacts within the gear set.
In-phase phasing occurs when each gear mesh has the same phase angle. Planetary gear sets with in-phase gear meshes benefit from enhanced torque distribution, as thrust forces are effectively canceled out by radial forces.
Sequential phasing involves different phase angles for each subsequent gear mesh. This configuration results in canceled torque and thrust forces, while reinforcing radial forces within the gear set.
Counter phase occurs when opposing planets have opposite phase angles, resulting in the cancellation of all excitation forces.
In mixed phase phasing, various combinations of the different phasing types are present, often occurring with unequally spaced planets. This requires careful calculation of gear mesh phase angles.
Introducing helix angle planetary phasing does not alter the fundamental principles of phasing. Each planetary gear mesh generates a specific meshing frequency. Helix angle gears add axial forces into the gear meshing process. The crucial aspect to consider with planet mesh phasing is its impact on vibrations within the planetary gear set. It is influenced by factors such as the frequency harmonic number, the number of ring gear teeth, and the number of planets. Mesh phasing is not specific to any one planetary gear set but is affected by cyclic symmetry and the response of the mesh frequency.
There are three primary methods for lubricating a planetary gear set: grease, oil splash, and forced oil. The choice of lubrication method depends on factors such as the operating speed of the gear set, the lubricant application method, and the types of gears used.
Grease lubrication is suited for low-speed planetary gear sets as it does not provide cooling and is not suitable for continuous or heavily loaded applications. It is essential to apply the correct amount of grease: too little can lead to inadequate lubrication, while too much can cause drag and reduced efficiency.
Oil splash lubrication is commonly used with helical and bevel planetary gear sets. In this method, gears dip into an oil reservoir, causing the oil to splash onto the other gears and bearings. A potential issue with oil splash lubrication is churning, where gears must push through the lubricant, which can affect performance. This lubrication method is typically designed for tangential speeds of up to 3 m/s.
Forced oil lubrication is employed for high-speed planetary gear sets and can involve methods such as oil mist, spray, or drops. The oil mist technique uses atomized oil to saturate all gears. The oil spray method involves spraying lubricant directly onto the gears. The oil drop method delivers oil drops directly to specific areas needing lubrication and is often used alongside the oil splash method. In all these approaches, oil is supplied from an oil reservoir.
Planetary gear sets are widely used due to their compact size, low weight, and efficient design. They function as speed reducers, slowing down motors while increasing torque. These gears are popular across various industries for their ability to generate significant torque and distribute loads among multiple gears, thanks to their increased contact surfaces. This load distribution enhances their durability and resistance to damage.
Planetary gear sets are well-suited for rugged applications due to their robust design, which can handle high torque and reductions. Their even load distribution and self-aligning properties enable them to endure high shock and overhung loads effectively.
Planetary gear sets are commonly used in wheel transmissions due to their lower induction losses and fewer gear changes. They offer advantages in terms of weight, efficiency, and noise reduction compared to larger transmissions, with a torque capacity of up to 332,000 Nm. Additionally, planetary gear sets are employed in various applications including 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.
In contrast to wheel conveying systems, planetary gearboxes offer enhanced point-to-point reliability at higher speeds. Their compact design makes them well-suited for high-speed product movement. With their higher torque capacity, planetary gearboxes help maintain acceptable vibration levels, reduce drive failures, and minimize operator fatigue.
Planetary gearboxes are employed in pumping systems because of their efficiency, quiet operation, and durability. They deliver the necessary performance and torque for these systems. Additionally, planetary gearboxes excel in harsh conditions and offer a cost-effective installation solution.
Wind turbines need gearboxes that can amplify rotor speed, making planetary gearboxes a preferred choice due to their high power density and concentric input and output. The substantial load from the blades requires robust gear systems. Planetary gearboxes excel in load sharing, which enhances power density and allows for increased gear ratios and power capacity within a compact 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.
With a 97% energy input efficiency and the capability to reduce speeds up to 10:1, planetary gearboxes transmit energy with minimal loss. These attributes make them the preferred choice for modern equipment and industrial machinery.
Planetary gearboxes play a crucial role in the joints of robot limbs, determining their speed and direction of movement. A common type used in robotics is the harmonic drive, known for its higher transmission ratio and larger meshing teeth compared to other gearboxes. Harmonic planetary gearboxes effectively handle variations in kilowatt power used by robots, operating with minimal impact and noise. They are capable of functioning in vacuum conditions, corrosive environments, and harsh media, while enabling high-speed motion.
Planetary gears are available in various forms, each designed for specific functions. Manufacturers produce high-quality planetary gears tailored to meet the unique requirements of any application. The key factors in their production are precision, accuracy, and strength—concepts essential to their performance.
The AD-Series features a one-piece planetary cage design with a rigid and precise rotating flange. This high-torque, low-backlash helical planetary gear set is compact, allowing it to fit into the smallest available spaces while delivering top-quality speed reduction without failure.
The P2KA18 planetary gearbox is designed for high-power unit solutions, available in 27 sizes and seven basic types. It supports torque ranges up to 2,600,000 Nm and transmission ratios of 4000:1. Its modular design makes it versatile for various industrial applications. The P2KA18 offers high performance at a reasonable cost, featuring compact size, exceptional reliability, ease of installation, and low maintenance.
The GBPH-060x is engineered with the highest torque density, making it ideal for motion control, automation, and robotic applications. It fits the NEMA 23 frame size and is compatible with servo, stepper, brushless, and AC or DC motors. The GBPH-060x aims to deliver a cost-effective, high-quality planetary gearbox, offering exceptional value for a wide range of applications and processes.
The Rexnord Atlas gear drive offers a gear ratio range from 4.39 to 1255 and a torque range of 43,000 to 26,000 inch-pounds. It features a foot-mounted drive that can be positioned either horizontally or vertically, and comes with accessories such as flanged motor mounting, slide base, top motor mounting, internal backstops, and shaft fans. The Rexnord Atlas gear drive is backed by a heavy-duty three-year warranty, ensuring its reliability.
The GPT gearhead is designed for high-torque applications, featuring up to six planets per stage. Its compact design delivers exceptional performance while maintaining a low volume. The GPT series utilizes hardened stainless steel gears for reliable force transmission. All components are welded and lubricated to ensure outstanding performance and durability.
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