RV Reducer vs. Harmonic Drive Gearbox

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Compared with general-purpose reducers, precision reducers for industrial robots require short drive chains, small size, high power, light mass and easy control. A large number of reducers used in articulated robots are mainly RV reducers and harmonic drive gearboxes.

Working principle of RV reducer and harmonic drive gearbox

The RV reducer is used for the three joints of the leg waist and elbow of the robot with large torque, and the industrial robot with a large load uses RV for one, two and three axes. Compared with the harmonic reducer, the key to the RV reducer lies in the processing and assembly process. RV reducer has higher fatigue strength, stiffness and life. It does not have a significant decrease in motion accuracy over time like harmonic drives, and its disadvantages are heavy weight and larger dimensions.

A harmonic drive gearbox is used for industrial robots with small loads or several axes at the end of large robots. A harmonic drive gearbox is a kind of harmonic drive, which includes a harmonic accelerator and harmonic reducer. The harmonic reducer mainly includes a rigid wheel, flexible wheel, bearing and wave generator. Among them, the number of teeth of the rigid wheel is slightly larger than that of the flexible wheel. The features of the harmonic reducer are small size, lightweight, large bearing capacity, high motion precision, and large single-stage transmission ratio.

The two types of reducers mesh with less tooth difference. The difference is that a key gear in the harmonic is flexible. It needs repeated high-speed deformation, so it is relatively fragile, and its bearing capacity and life are limited. RV usually uses cycloidal pinwheels. Harmonic reducers used involute teeth in the past. Now some manufacturers use double arc teeth, which are much more advanced than involutes.

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Different features

Strain wave harmonic drive gearbox is composed of four basic parts: "flexible wheel, wave generator, rigid wheel and bearing". The outer diameter of the flexible wheel is slightly smaller than the inner diameter of the rigid wheel, and the flexible wheel usually has two teeth less than the rigid wheel. The elliptical shape of the wave generator determines that the contact points of the teeth of the flexible wheel and the rigid wheel are distributed on the two opposite sides between the center of the ellipse. During the rotation of the wave generator, the contacting parts of the teeth of the flexible wheel and the rigid wheel start to engage. For every 180° clockwise rotation of the wave generator, the flexible wheel is equivalent to a counterclockwise rotation of the rigid wheel by one tooth difference. At the two 180° symmetrical locations, more than 30% of all teeth are engaged simultaneously, which also creates its high torque transmission.

Compared with strain wave harmonic drive gearbox, RV transmission is a newly emerged transmission, which is developed on the basis of the traditional pin-oscillating planetary gearbox. It not only overcomes the shortcomings of general pin-oscillating transmission but also has a series of advantages such as small size, lightweight, large transmission ratio range, long life, stable accuracy maintenance, high efficiency and smooth transmission. They are widely used in industrial robots, machine tools, medical testing equipment, satellite receiving systems and other fields.

The housing and cycloid wheel of the RV reducer has a high load capacity as the transmission occurs through solid steel. In contrast, the flexible wheel of the harmonic reducer can continuously deform to transmit torque, which determines the harmonic reducer's limited ability to withstand high torque and shock loads, and is therefore generally used in the front end.

Advantages and disadvantages of RV reducer and harmonic reducer

Starin wave harmonic drive gearbox is simple and compact, suitable for miniaturization, low and medium load applications. RV reducer has good rigidity, high impact resistance, smooth transmission, and high precision, suitable for medium and heavy load applications, but RV reducer needs to transmit a large torque, withstand a large overload shock, to ensure the expected working life, and therefore uses a relatively complex over-positioning structure in the design, manufacturing process and cost control The bearing of RV reducer is the weak link, and it is easy to break through the limit of shaft bearing force and cause abnormal bearing wear or rupture when the force is applied. This problem is more prominent at high speed, so the rated torque of the RV reducer decreases very significantly with the input speed.

RV reducer is a high-precision control reducer with a cycloidal pin wheel reduction structure. Due to the large number of gears engaged at the same time, the reducer has the characteristics of small size, lightweight, high rigidity, and overload resistance. In addition, due to the small backlash, rotary vibration, and inertia, it has good acceleration performance and can achieve smooth operation and obtain the accuracy of the correct position.

When the wave generator is installed into the inner circle of the flex spline, it forces the flex spline to produce elastic deformation and to become elliptical, so those flex spline teeth at the long axis are inserted into the gear groove of the circular spline, which becomes a full engagement state; while at the short axis, the gear teeth of both flex spline and circular spline are completely untouched and in a separation state. The process between engagement and separation is in the state of meshing in or meshing out. When the wave generator rotates continuously, it forces the flex spline to deform, which makes the teeth of the two splines change their working state during the process of meshing in, engagement, meshing out, and separation, resulting in the so-called staggering teeth movement, thus realizing the movement transmission between wave generator and flex spline.

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