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Industrial robot is a new type of high-tech intelligent machine equipment that can replace some manual production. As an industrial robot, it can be described in three words: fast, accurate and stable. For the current robot production technology, it is easier to be fast and accurate, but it is relatively difficult to be accurate while being fast and stable. A word is defined here: repeated positioning accuracy of industrial robot – the industrial robot repeats a certain action (generally 10-20 times). The mean value of each deviation from the planned position is its repeated positioning accuracy, which is generally measured by laser interferometer. For industrial robots, precision is a very important factor, which directly affects the product quality and production efficiency. The main factor affecting the accuracy of industrial robot is bearing.
The bearing of industrial robot has the requirements of long life, high rigidity and low friction. Therefore, when designing industrial robot, we should not only consider the maximum rated dynamic load, but also carefully select the value of each structural parameter, so as to improve the contact stress distribution of bearing parts, achieve the contact state conducive to the formation of lubricating oil film and improve the service life of the bearing.
The inner and outer rings of the contact ball bearing of the industrial robot adopt an integral structure, and the general cross-sectional area is only 20% of the standard bearing with the same inner diameter, so the value of the radial wall thickness of the cage is very limited. If the channel position is designed in the center of the bearing width, the bearing will become a symmetrical structure, At the same time, the strength at the bottom of the cage pocket and the installation position of the sealing ring will be limited. Therefore, in order to ensure that the bearing cage has sufficient strength and the sealing ring has sufficient installation space, the groove position adopts the asymmetric design of the end faces on both sides.
When the contact ball bearing of industrial robot is in working state, it must bear a certain radial load and an axial load at the same time. If the axial load is too large, it may cause the contact between the steel ball and the edge of the inner and outer ring retaining edge or the contact ellipse between the retaining edge and the raceway to be truncated. At this time, stress concentration will occur and the bearing ring will be worn, thus affecting the accuracy. Therefore, when designing the retaining edge height, it is necessary to calculate the small retaining edge height J of the bearing ring by using Hertz contact stress theory according to the axial load borne by the bearing, so as to calculate the retaining edge diameter of the bearing ring. In the design, considering the small cross-sectional area of contact ball bearing, on the premise of meeting the use requirements, the value of retaining edge height coefficient can be smaller than that of deep groove ball bearing.
The early failure of industrial robot bearings is generally due to poor sealing. Because of poor sealing, sundries or other pollutants in the air are easy to enter the bearings, resulting in gradual failure of lubricating grease and bearing damage. Therefore, the tightness of the bearing is very important. Therefore, when designing the sealed bearing, we should not only consider the seal, but also consider the specific working requirements of the bearing and the specific structural form of the bearing. Industrial robots are developing towards miniaturization, lightweight and precision. Therefore, the bearing for industrial robot is lubricated by grease with sealing structure.
There are generally two ways to fit the bearing inner ring with the sealing lip:
(1) Contact seal with inner ring sealing lip in contact with inner ring retaining edge
(2) Non contact seal with inner ring sealing lip in contact with inner ring retaining edge. Compared with the non-contact seal, the contact seal has better dust and grease leakage effect, but its friction torque is large and the temperature rise is high. Considering the large friction torque has an adverse impact on the robot host, the non-contact seal structure is generally adopted in the design. In addition, because the bearing ring wall is very thin and the inner and outer rings of the bearing are easy to deform, the inner ring retaining edge can adopt the grooveless structure, but the sealing clearance should be strictly controlled.