What is the Advantage and Disadvantage of rv reducer

Mechanical transmission system with flexing

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• Outer circle: circular spline (fixed)
• Middle circle: flex spline (attached to output shaft, not shown)
• Inner oval: wave generator (attached to input shaft; inner ball bearing and shaft, not shown)

Harmonic Drive SE strain wave gear set consisting of wave generator bearing (top left), flexspline cup (top right) and circular spline ring (bottom).

Strain wave gearing (also known as harmonic gearing) is a type of mechanical gear system that uses a flexible spline with external teeth, which is deformed by a rotating elliptical plug to engage with the internal gear teeth of an outer spline.

The German company Harmonic Drive SE manufactured the first series-produced gears under the product name or registered trademark Harmonic Drive.

Strain wave gearing has some advantages over traditional gearing systems such as helical or planetary gears, including:

• no backlash,
• compactness and light weight,
• high gear ratios,
• reconfigurable ratios within a standard housing,
• good resolution and excellent repeatability (linear representation) when repositioning inertial loads,

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• high torque capability,
• coaxial input and output shafts.

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High gear reduction ratios are possible in a small volume (a ratio from 30:1 up to 320:1 is possible in the same space in which planetary gears typically only produce a 10:1 ratio).

Disadvantages include a tendency for 'wind-up' (a torsional spring rate) in the low torque region.

Strain wave gearing is commonly used in robotics[3] and aerospace.[4] It can provide gear reduction but may also be used to increase rotational speed[citation needed], or for differential gearing.

History

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The basic concept of strain wave gearing (SWG) was introduced by C.W. Musser in a patent[5][6] while he was an advisor at United Shoe Machinery Corp (USM). It was first used successfully in by USM Co. and later by Hasegawa Gear Works under license of USM.[citation needed] Later, Hasegawa Gear Work became Harmonic Drive Systems located in Japan and USM Co. Harmonic Drive division became Harmonic Drive Technologies.[7][8]

Mechanics

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Cross section of a harmonic gear.

1. input shaft
2. wave generator
3. flexspline
4. circular spline
5. output shaft
6. housing

The strain wave gearing uses the elasticity of metal. The mechanism has three basic components: a wave generator (2 / green), a flex spline (3 / red), and a circular spline (4 / blue). More complex versions have a fourth component normally used to shorten the overall length or to increase the gear reduction within a smaller diameter, but still follow the same basic principles.

The wave generator is made of two separate parts: an elliptical disk called a wave generator plug and an outer ball bearing. The elliptical plug is inserted into the bearing, forcing the bearing to conform to the elliptical shape but still allowing rotation of the plug within the outer bearing.

The flex spline is shaped like a shallow cup. The sides of the spline are very thin, but the bottom is relatively rigid. This results in significant flexibility of the walls at the open end due to the thin wall, and in the closed side being rigid enough to be tightly secured (to a shaft, for example). Teeth are positioned radially around the outside of the flex spline. The flex spline fits tightly over the wave generator, so that when the wave generator plug is rotated, the flex spline deforms to the shape of a rotating ellipse and does not slip over the outer elliptical ring of the ball bearing. The ball bearing lets the flex spline rotate independently to the wave generator's shaft.

The circular spline is a rigid circular ring with teeth on its inside. The flex spline and wave generator are placed inside the circular spline, meshing the teeth of the flex spline and the circular spline. Because the flex spline is deformed into an elliptical shape, its teeth only actually mesh with the teeth of the circular spline in two regions on opposite sides of the flex spline (located on the major axis of the ellipse).

Assume that the wave generator is the input rotation. As the wave generator plug rotates, the flex spline teeth which are meshed with those of the circular spline slowly change position. The major axis of the flex spline's ellipse rotates with wave generator, so the points where the teeth mesh revolve around the center point at the same rate as the wave generator's shaft. The key to the design of the strain wave gear is that there are fewer teeth (often for example two fewer) on the flex spline than there are on the circular spline. This means that for every full rotation of the wave generator, the flex spline would be required to rotate a slight amount (two teeth in this example) backward relative to the circular spline. Thus the rotation action of the wave generator results in a much slower rotation of the flex spline in the opposite direction.

For a strain wave gearing mechanism, the gearing reduction ratio can be calculated from the number of teeth on each gear, in a similar manner to a cycloidal drive:

R = flex spline teeth &#; circular spline teeth flex spline teeth , &#; 1 < R < 0 {\displaystyle R={\frac {{\text{flex spline teeth}}-{\text{circular spline teeth}}}{\text{flex spline teeth}}},\quad -1<R<0}

Note that the reciprocal of the reduction ratio is sometimes referred to with the same phrase and symbol.

For example, if there are 202 teeth on the circular spline and 200 on the flex spline, the reduction ratio is (200 &#; 202)/200 = &#;0.01

Thus the flex spline spins at 1/100 the speed of the wave generator plug and in the opposite direction. Different reduction ratios are set by changing the number of teeth. This can either be achieved by changing the mechanism's diameter or by changing the size of the individual teeth and thereby preserving its size and weight. The range of possible gear ratios is limited by tooth size limits for a given configuration.

This reduction ratio is applicable to the configuration where the circular spline is fixed, the wave generator the input and the flexible spline the output. In case the circular spline also rotates, the following relation holds between the rotational velocities of the three parts:[9]

ω F S = R ω W G + ( 1 &#; R ) ω C S {\displaystyle \omega _{FS}=R\omega _{WG}+(1-R)\omega _{CS}}

Mind that R {\displaystyle R} is negative and small.

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Examples of use

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The electrically driven wheels of the Apollo Lunar Rover included strain wave gears.[10] Also, the winches used on Skylab to deploy the solar panels were powered using strain wave gears.[citation needed]

See also

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References

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General

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As a precision reducer, the core component of an industrial robot, compared with a general-purpose reducer, the reducer for a robot is required to have the characteristics of short transmission chain, small size, high power, light weight and easy control.

1. RV reducer

RV reducer is a new transmission mechanism developed from traditional cycloid gearbox and planetary gearbox. It has the advantages of compact structure, large transmission, long service life, stable precision and high efficiency. It has been widely used in articulated industrial robots. The power of the industrial robot servo motor is transmitted through the gearbox of the input shaft of the reducer, so that the large gear of the output shaft is linked to achieve the purpose of deceleration. The transmission ratio of the large and small gears is also the transmission ratio.

The working principle of the RV reducer: install a cylindrical pin on the inner ring of the RV reducer housing. The eccentric movement of the RV transmission can cause the pin and the RV transmission to assemble and connect. At the same time, multiple RV gearboxes grab, increasing the load capacity. Since the number of RV teeth is one less than that of the pin, when the eccentric shaft rotates one revolution, when the housing is fixed, the RV gear and the input shaft rotate one tooth in the same direction. The outlet can be the housing or the drive shaft. When the housing is fixed, the transmission shaft is the output, and the output direction is the same. When the transmission shaft is fixed, the housing is the output, and the output direction is just opposite.

The RV reducer is used for the three joints of the leg, waist and elbow of the robot with a large torque, and the industrial robot with a large load uses RV for the first, second and third axes. Compared with the harmonic reducer, the key of the RV reducer lies in the processing technology and assembly process. The RV reducer has higher fatigue strength, stiffness and life, unlike the harmonic drive, the movement accuracy will be significantly reduced as the use time increases, and its disadvantages are heavy weight and large dimensions.

2. Harmonic reducer

Harmonic reducer is a new type of gear reducer transmission mechanism commonly used in low-load industrial robots. It consists of a fixed rigid wheel, a flexible wheel and a wave generator, wherein the number of teeth of the rigid wheel is slightly higher than that of the flexible wheel. The harmonic reducer has the advantages of large transmission, small size, partial parts, and high transmission efficiency. The transmission ratio of this single unit can reach 50-, and the transmission efficiency can reach 92%-96%. A harmonic reducer consists of three main components: a wave generator, a flexible transmission, and a rigid transmission. Wave generators transform flexible transmission into adjustable elastically deformable and rigid transmission grids for motion and power transmission. The transmission can achieve a high reduction ratio, but its load, stiffness and life are not as good as RV, so it is mostly used for light-loaded robots.

The working principle of the harmonic reducer: the harmonic reducer uses a flexible gearbox to generate adjustable elastic deformation waves, so that the relative stepped teeth between the internal rigid gearbox and the flexible gearbox can transmit power to achieve the deceleration goal. This type of transmission is fundamentally different from ordinary transmissions, and has its own characteristics in grid theory, ensemble calculation, and structural design.

Harmonic reducers are used for industrial robots with small loads or several axes at the end of large robots. Harmonic reducers are a type of harmonic transmission, and harmonic transmissions include harmonic accelerators and harmonic reducers. The harmonic reducer mainly includes: rigid wheel, flexible wheel, bearing and wave generator, and the four are indispensable. Among them, the number of teeth of the rigid spline is slightly larger than that of the flexible spline. Harmonic reducers used in small robots are characterized by small size, light weight, large carrying capacity, high motion precision, and large single-stage transmission ratio.

3. Planetary reducer

The planetary reducer is small in size, light in weight, high in carrying capacity, long in service life, stable in operation and low in noise. It has the characteristics of power splitting and multi-tooth meshing. It is suitable for lifting transportation, construction machinery, metallurgy, mining, petrochemical, construction machinery, light industry textiles, medical equipment, instrumentation, automobiles, ships, weapons, aerospace and other industrial sectors.

The planetary reducer is a new type of planetary reducer with wide versatility. The internal gear adopts low carbon alloy steel carburizing and quenching plus grinding or nitriding process. The whole machine has the characteristics of small structure size, large output torque, large speed ratio, high efficiency, safe and reliable performance, etc.

How Planetary Gearboxes Work: Connecting a planetary gearbox to a planetary gearbox causes rotation when the solar transmission rotates under the power of the engine. At the same time, since the other side of the planetary gearbox connects the main body to the hollow inner gearbox on the inner wall, the planetary gearbox rolls in the direction of rotation through the rotational force, forming a "rotating" motion around the sun transmission.

Adantages and disadvantages of RV reducer and harmonic reducer

The harmonic reducer is simple and compact in structure, suitable for miniaturized, low and medium load applications. The RV reducer has good rigidity, strong impact resistance, stable transmission, and high precision, and is suitable for medium and heavy load applications. However, the RV reducer needs to transmit a large torque and withstand a large overload impact to ensure the expected working life. A relatively complex over-positioning structure is used in the design, and the manufacturing process and cost control are difficult. There are no elastically deformed stress elements inside the RV reducer, so it can withstand a certain torque. The bearing of the RV reducer is its weak link. When the force is applied, it is easy to break through the force limit of the bearing and cause abnormal wear or rupture of the bearing. This problem is more prominent during high-speed operation, so the rated torque of the RV reducer decreases significantly with the input speed&#;

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