Epicyclic Gear Train

 

              

   DYNAMICS OF MACHINES

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  Pranjal Sarode – 33

  Rachit Singh– 45

 Prathamesh Sinkar- 46

 Atharva Sonaje– 49

 Sourav Babar – 51

Guide: Prof. Rajkumar Bhagat

❖         INTRODUCTION: -

Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer, or planet, gears or pinions, revolving about a central sun gear or sun wheel. Typically, the planet gears are mounted on a movable arm or carrier, which itself may rotate relative to the sun gear. Epicyclic gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.

 

At around 500 BC, the Greeks invented the idea of epicycles, of circles travelling on circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD was able to predict planetary orbital paths. The Antikythera Mechanism, circa 80 BC, had gearing which was able to approximate the moon's elliptical path through the heavens, and even to correct for the nine-year precession of that path. (The Greeks would have seen it not as elliptical, but rather as epicyclic motion.)

In the 2nd-century AD treatise Almagest, Ptolemy used rotating deferent and epicycles that form epicyclic gear trains to predict the motions of the planets. Accurate predictions of the movement of the Sun, Moon and the five planets, Mercury, Venus, Mars, Jupiter and Saturn, across the sky assumed that each followed a trajectory traced by a point on the planet gear of an epicyclic gear train. This curve is called an epitrochoid.

Epicyclic gearing was used in the Antikythera Mechanism, circa 80 BC, to adjust the displayed position of the moon for the ellipticity of its orbit, and even for the apsidal precession of its orbit. Two facing gears were rotated around slightly different centres, and one drove the other not with meshed teeth but with a pin inserted into a slot on the second. As the slot drove the second gear, the radius of driving would change, thus invoking a speeding up and slowing down of the driven gear in each revolution.

 

It consists of  two gears mounted so that the centre of one gear revolves around the centre of the other. A carrier connects the centres of the two gears and rotates to carry one gear, called the planet gear or planet pinion, around the other, called the sun gear or sun wheel. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gears roll around the sun gear. An epicyclic gear train can be assembled so the planet gear rolls on the inside of the pitch circle of a fixed, outer gear ring, or ring gear, sometimes called an annular gear. In this case, the curve traced by a point on the pitch circle of the planet is a hypocycloid.

The combination of epicyclic gear trains with a planet engaging both a sun gear and a ring gear is called a planetary gear train. In this case, the ring gear is usually fixed and the sun gear is driven.

Planetary gears (or epicyclic gears) are typically classified as simple and compound planetary gears. Simple planetary gears have one sun, one ring, one carrier, and one planet set. Compound planetary gears involve one or more of the following three types of structures:

i.           Meshed-Planet: There are at least two more planets in mesh with each other in each planet train.

ii.           Stepped-Planet: There exists a shaft connection between two planets in each planet

     train.

iii.          Multi-Stage Structures:  The system contains two or more planet sets.

❖         PRINCIPLE OF OPERATION:

An Epicyclic Gear Train (also known as planetary gear) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates to carry one gear, called the planet gear or planet pinion, around the other, called the sun gear or sun wheel.

The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear.

          The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is called a planetary gear train. In this case, the ring gear is usually fixed and the sun gear is driven.

❖         Simple Planetary Gears VS Compound Planetary Gears: -

Compared to simple planetary gears, compound planetary gears have the

 advantages of

i.                larger reduction ratio

ii.             higher torque-to-weight ratio

iii.           more flexible configurations

❖  ADVANTAGES OF PLANETARY GEARS: -

●  Planetary gear trains provide high power density in comparison to standard parallel axis gear trains.

●     They provide a reduction in volume, multiple kinematic combinations, purely torsional reactions and coaxial shafting.

●   The load in a planetary gear train is shared among multiple planets; therefore, torque capability is greatly increased. The more planets in the system, the greater the load ability and the higher the torque density.

●     The planetary gear train also provides stability due to an even distribution of mass and increased rotational stiffness.

 

❖  DISADVANTAGES OF PLANETARY GEARS:

Planetary gearboxes do have some disadvantages. Their complexity in design and manufacturing tends to make them a more expensive solution than other gearbox types. And precision manufacturing is extremely important for these gearboxes.

If one planetary gear is positioned closer to the sun gear than the others, imbalances in the planetary gears can occur, leading to premature wear and failure.

Also, the compact footprint of planetary gears makes heat dissipation more difficult, so applications that run at very high speed or experience continuous operation may require cooling.

So, to summarize these are its disadvantages:

●     High bearing loads.

●     Constant lubrication requirements.

●     Inaccessibility.

●     Design complexity.

❖  PROPERTIES OF PLANETARY GEARS: -

Increased Repeatability: Its greater speed radial and axial load offers reliability and robustness, minimizing the misalignment of the gear. In addition, uniform transmission and low vibrations at different loads provide a perfect repeatability.

Perfect Precision: Most rotating angular stability improves the accuracy and reliability of the movement.

Lower Noise Level: There is more surface contact. Rolling is much softer and jumps are virtually nonexistent.

Greater Durability: Due to its torsional rigidity and better rolling. To improve this feature, your bearings help reduce the losses that would occur by rubbing the shaft on the box directly. Thus, greater efficiency of the gear and much smoother operation is achieved.

Efficiency: Planetary reducers offer greater efficiency and thanks to its design and internal layout losses are minimized during their work. In fact, today, these types of drive mechanisms are those that offer greater efficiency.

Increased Torque Transmission: With more teeth in contact, the mechanism is able to transmit and withstand more torque. In addition, it does it in a more uniform manner.

Maximum Versatility: Its mechanism is contained in a cylindrical gearbox, which can be installed in almost any space.

❖  Where are Planetary Gear Systems Used?

Planetary gears are often used when space and weight are an issue, but a large amount of speed reduction and torque are needed. This requirement applies to a variety of industries, including tractors and construction equipment where a large amount of torque is needed to drive the wheels. Other places you will find planetary gear sets include turbine engines, automatic transmissions, and even electric screwdrivers.

Planetary gear systems are able to produce a lot of torque because the load is shared among multiple planet gears. This arrangement also creates more contact surfaces and a larger contact area between the gears than a traditional parallel axis gear system. Because of this, the load is more evenly distributed and therefore the gears are more resistant to damage.

              

❖  Manufacturing Planetary Gears:

The skills required to manufacture planetary gears are the same as those required for any other type of precision gear manufacturing. Gear Motions is a leading precision gear manufacturer specializing in supplying custom cut and ground gears. We have an extensive portfolio of gear production capabilities which includes the ability to produce all of the individual gears that make up a planetary gear system. For specific manufacturing capabilities such as minimum and maximum diameter, pitch diameter, and face width, please visit our gear production capabilities page.

Gear Motions also has extensive experience when it comes to gear engineering and design. Whether you need to design a gear system from scratch, or need assistance with a redesign, we will work with you through the entire process to ensure your gears are designed and manufactured with precision. Engineering services include reverse engineering, design for manufacturability, prototyping, and redesign.

❖  APPLICATIONS: -

●    3D Printing: 

                                   

Planetary gears have become popular in 3D printing for a few different reasons. Planetary gear boxes can provide a large gear ratio in a small, light-weight package. Some people install such gearboxes to get more accurate 3D prints by gearing-down the movement of their stepper motors.

Planetary gearboxes may help produce very high-quality 3D prints. One popular use of 3D printed planetary gear systems is as toys for children.

Transmissions of helicopters, automobiles, wind turbines, aircraft engines, etc.

     ●     Angle gearbox for rotary tables

    ●     Height adjustment of operating tables

     ●     Height adjustment of cleaning systems