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Sunday, 3 January 2016


We were discussing in our previous post about the basic of helical gears and also basics of spur gears and we have also seen the various advantages, disadvantages and applications ofspur and helical gear.

Today we will see the concept of planetary gear system, where we will look out the following important points as mentioned below
  1. Mechanism of planetary gears
  2. Various components of planetary gears
  3. Advantages and disadvantages of planetary gears
  4. Applications of planetary gear unit

Planetary gear system: Components 

Planetary gear system will have following components as mentioned below.
  1. Sun gear: Central gear which is meshing with planet gears
  2. Ring gear: Internal gear is the outermost gear, which is meshing with planet gears
  3. Planet gears: There will be two or more than two planet gears between sun gear and ring gear and these planet gears will mesh with internal gear i.e. ring gear and will also rotate around the central gear i.e. the sun gear
  4. Career: Career assembly hold planet gears together as well as equally spaced also in an orbit.
Above figure shows the various components of planetary gearing arrangement.
Planetary gear is one type of epicyclic gear which could be used where we need precise motion control. Generally in industries, there are lots of applications where we need to transmit higher torque in limited space as well as we require light weight unit for transmitting the power, motion and torque from driving equipment to driven equipment.

In case of planetary gear system, we can increase the torque density by using more planet gears between the sun gear and ring gear.

Let us consider one spur gear and one planetary gear unit of similar size. Let in planetary gear unit there are three planet gears, now when we consider the torque transmission capability, we will note it that planetary gear unit torque transmission capacity is 3 times as compared to torque transmission capacity of spur gear unit. If there are four planet gears in planetary gear unit then in that situation, we will secure four times torque transmission capacity as compared to that of spur gear unit.

As we are well aware now that in case of one planetary gear unit, there will be two or more than two planet gears, therefore the torsional stiffness of the planetary gear unit will be higher by a factor of N as compared to a traditional gear train. N is used here to mention the number of planet gears in planetary gear unit.

If there are three planet gears, then planetary gear unit will have three times torsional stiffness as compared to similar size of traditional gear train. Higher torsional stiffness is very important in various applications of repeatability requirement and precise positioning especially under varying loading situations.

Let us see why we need to say such gearing arrangement as planetary gear system? As we are aware that planets move around the sun in universe and in similar manner, two or more than two gears rotates around the central gear in such gearing arrangement. Therefore central gear is termed as sun gear and gears those are rotating around the central gear are termed as planet gears.

In simple, we may say that a planetary gear unit will be compact, higher torque density, lower inertia and generally grease lubricated gearing system.

Advantages of planetary gear system

Planetary gear system has lots of advantage as compared to traditional gearboxes
  1. Planetary gear system could be used to secure higher gear ratio in compact space. Therefore we prefer to use planetary gearing arrangement where we need to reduce the speed in compact space.
  2. For similar gear ratio, planetary gear system will have light weight as compared to traditional gearbox.
  3. Power transmission efficiency will be quite good as compared to traditional gearboxes i.e. higher proportion of input energy will be delivered with the help of such gearing arrangement.
  4. Planetary gear system will have higher torque transmission capability and will have lower inertia.
  5. As we are looking here that load which is being transmitted will be shared between multiple planet gears, hence load distribution will be quite good and torque transmission will also be increased by using planetary gearing arrangement.
  6. In case of planetary gear system, driving member and driven member are concentric and therefore driving and driven equipments could be installed in same line which will result space saving.
  7. Planetary gearing arrangement will provide the higher stability
  8. Service life will also be quite good if we compare it with traditional gearboxes service life for similar load.

Disadvantages of planetary gear system

Planetary gear system has lots of advantages that we have discussed above. There are several disadvantages of planetary gear system also as mentioned below.
  1. Cost of planetary gear system will be high as compared to traditional gearbox
  2. Planetary gear system designing and manufacturing are quite complex
  3. Determination of efficiency of planetary gear system will be difficult
  4. Gearing should be accurate
  5. Some planetary gearing arrangement makes additional sound during operation
  6. To avoid any additional gearing, driving member and driven member must be concentric
Applications of planetary gear system
With lots of advantages of using planetary gear system, there are various applications of planetary gear system and few of them are as mentioned below.
  1. EOT cranes, Harbour mobile cranes and other mobile cranes
  2. Automation Assembly and automation system
  3. Printing Lathe
  4. Planetary gearing arrangement also used in clock, toys and turbine engine
Do you have suggestions? Please provide your feedback and suggestions in comment box, provided at the end of this post.

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