Differences between harmonic drive vs planetary gear

Differences between harmonic drive vs planetary gear

Differences between harmonic drive vs planetary gear

In the late 1950s, American engineer C.W. Musser presented a groundbreaking idea that astonished experts in the field of gear-driven machinery. He introduced a flexible gear into the gear drive, which deforms in a wave-like manner while in motion.

The planetary reducer is an industrial product used for power transmission. Its structure is tightly integrated with the gear case by means of an internal ring. The center of the ring contains a sun gear that is driven by an external power source. A set of planetary gear sets, which are equally divided into three trays on the pallet, are carried by the output shaft, the inner ring gear, and the sun tooth support.

When the input power drives the sun gear, it drives the planetary gear to rotate and follow the trajectory of the inner ring to revolve around the center. The rotation of the planet drives the power output from the output shaft of the pallet. The speed converter of the gear is used to decrease the revolutions of the motor to the required number and to create a mechanism with a high torque. Planetary reducers, such as the Bapman planetary reducer, are precise reducers used for power and motion transmission.

The harmonic reducer consists of three fundamental components: a wave generator, a flexible gear, and a rigid gear. The harmonic drive reducer is a wave generator that produces a controlled elastic deformation of the flexible gear, which meshes with the rigid gear. This gear transmission is used to transmit motion and power.

The working principles and transmission modes of planetary and harmonic reducers differ, as do their respective application industries.

Differences between harmonic drive vs planetary gear

Planetary geared motor:

A planetary geared motor, also known as a planetary gear reducer, consists of a sun gear, planetary gears, an outer ring gear, a planet carrier, and a transmission shaft. It uses rigid gears for transmission. The planetary gear reducer typically has multiple stages, with a gear with a small number of teeth meshing with a large gear on the output shaft to achieve deceleration. This type of reducer has the characteristics of low torque and high rigidity.

Harmonic geared motor:

A harmonic geared motor uses a fixed internal toothed wheel, a flexible wheel, and a wave generator to achieve transmission. The flexible wheel is an elastic thin-walled cup whose base is connected to the driven shaft. The flexible wheel has a ring gear on its busbar and is radially deformed by the wave generator. Compared with a conventional reducer, the harmonic reducer has higher accuracy and bearing capacity.

Movement characteristics:

The planetary geared motor has different movement characteristics based on the configuration of its components.

  1. If the carrier is the active member, the follower will be overspeeded.
  2. When the carrier is a follower, it must be slower than the speed of the active member.
  3. When the carrier is fixed, the active member and the driven member rotate in opposite directions.
  4. If the sun gear is the active part, the speed of the follower must decrease. If the planet carrier acts as a passive member, its direction of rotation is in the same direction as the active member.
  5. If the planet carrier acts as the active member, the direction of rotation of the passive member is the same as it.
  6. In the simple planetary gear mechanism, the sun gear has the fewest teeth, the ring gear has a medium number of teeth, and the planet carrier has the most equivalent teeth.
  7. If any two components in the planetary gear mechanism rotate in the same direction at the same speed, the rotation speed and direction of the third component must be the same as the former two, and the mechanism is locked and becomes the direct gear.
  8. If there is only one active member and the other two components are not fixed, it is in neutral.


The harmonic actuator is a type of reducer used in mechanical engineering, transmission, and gear transmission. It is composed of four basic components: the wave generator, flexible gear, flexible bearing, and rigid gear. The harmonic drive reducer is an assembly of the wave generator and flexible bearing, which allows for controllable elastic deformation of the flexible gear. The flexible gear then meshes with the rigid gear to transmit motion and power. This type of reducer is a new development based on the principle of planetary gear transmission, and is commonly referred to as a harmonic gear drive reducer.

The wave generator is a rod-shaped member with rolling bearings at both ends, which is pressed against the inner wall of the flexible gear. The flexible gear is a thin-walled gear capable of generating a large elastic deformation, with an inner diameter slightly smaller than the total length of the wave generator. The wave generator causes the flexible gear to produce a controlled elastic deformation, forcing the section of the flexible gear to change from its original circular shape to an elliptical shape.

The teeth near the two ends of the long axis are completely meshed with the teeth of the rigid gear, while the teeth near the ends of the short axis are completely disengaged from the wheel. The teeth of other segments on the circumference are in a transitional state of engagement and disengagement. As the wave generator rotates continuously in a specific direction, the deformation of the flexible gear changes, and the meshing state of the flexible gear and the rigid gear constantly changes, resulting in a slow rotation of the flexible gear relative to the rigid gear in the opposite direction of the wave generator.

During operation, the fixed gear is stationary, and the motor generator drives the wave generator to rotate, with the flexible gear acting as the driven wheel to rotate and drive the load. The number of cycles of deformation at a certain point on the flexible gear during one revolution of the wave generator is referred to as the wave number, which is commonly two or three waves. The double-wave transmission is the most widely used, as it has a relatively simple structure, low stress on the flexible gear, and allows for a large transmission ratio.

Differences between harmonic drive vs planetary gear

Harmonic drive technology offers several advantages for high load-carrying capacity, transmission ratio, small size, light weight, high transmission efficiency, long life, smooth transmission, and high motion accuracy. The surface contact of the tooth and tooth meshing in harmonic drives with a relatively large overlap coefficient allows for smaller loads per unit area, resulting in higher bearing capacity than other transmission forms. Single-stage harmonic gear transmission has a transmission ratio of i=70~500. However, due to the large alternating load on the flexible wheel, the material requires high fatigue strength, processing, and heat treatment, making the process complicated.

Harmonic gear reducers were first developed in China during the 1960s and 1970s and have since been widely used in various industries such as electronics, aerospace, robotics, and even the chemical industry. They are especially useful in high dynamic performance servo systems, where they outperform other transmission forms. Harmonic gear transmission can transmit power from tens of watts to tens of kilowatts, although high-power harmonic gears are often used for short-term work.

Gear reducer hydrostatic transmission systems can convert mechanical energy into liquid pressure energy, increasing wind steering convenience. The gear reducer and the piping system cannot be separated from the kiln transmission thin oil humidification system, the pressure maintaining wheel, the vertical grinding machine, the ball mill, and the supporting hard tooth surface reducer.

To inspect the gear reducer, analyze the osmotic pressure transmission system’s normality using the shaft temperature and resonance diagram of the gear reducer combined with thin oil lubrication system parameters. The reducer lubrication body system includes a lubrication body system and a side pressure, temperature control system. The lubrication main system consists of a fuel tank and a fuel tank, which in turn are the main pump, the coupling, and the variable frequency motor. The gear reducer has a complete nodulation system that meets zero environmental requirements, which should be strictly enforced.

Harmonic gear reducers have been widely used in aviation, aerospace, energy, marine, shipbuilding, bionic machinery, common ordnance, machine tools, instruments, electronic equipment, mining and metallurgy, transportation, lifting machinery, petrochemical machinery, textile machinery, agricultural machinery, and medical instruments.


Greensky Power Co., Ltd. is a professional manufacturer engaged in the research, development, production, sale of harmonic drive and planetary gear .

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