Industrial robots have been widely used in automobile manufacturing and other fields because of their high repeatability, good reliability and strong ability to resist harsh environment. The widespread application of industrial robots is an effective means for automobile enterprises to realize automatic production, improve product quality and production efficiency, save labor and manufacturing costs, and enhance the competitiveness of the industry.

The domestic industrial robot manufacturing industry started from the "Seventh Five-Year Plan" of science and technology in the 1980s. With the support of the national science and technology project "863", after more than ten years of research, production and application, China's robot industry has made a big step from scratch. At present, from the perspective of China's industrial robot market, imported industrial robots account for a large proportion. Compared with foreign brands, the gap between domestic robots in terms of trouble-free running time, positioning accuracy, positioning speed and accuracy retention degree is mainly reflected in the sub-standard core parts technology. As the key component of industrial robot, the bearing of industrial robot has an important influence on the key performance indexes of the robot such as running stability, repeated positioning accuracy, rotary accuracy and working reliability.

Characteristics and classification of special matching bearings for industrial robots

The development of modern industrial robot tends to be light, which requires its matching bearing to have a more lightweight structure. Because bearings need to meet the performance requirements of the main engine of industrial robots on bearing capacity, precision, stiffness and friction torque in limited installation space, it is difficult to select standard general purpose bearings to meet this requirement, and thin-wall bearings pursue the outer limit size, with light weight, small volume, high precision, long life and other excellent performance, can well solve this problem.

Industrial robot bearings, accurately should be called bearings for industrial robots, mainly including: thin-wall ball bearings, cross cylindrical roller bearings, harmonic reducer bearings, etc. Most of them adopt non-standard, multi-roll design principles.

1. Thin wall bearing with equal section

Different from ordinary bearings, the cross section of this kind of bearings in each series is mostly square, and the size is designed to be fixed (see Figure 1) : in the same series, the cross section size is constant, and it does not increase with the increase of the inner diameter size, so it is called equal section thin-wall bearings. Equal-section thin-wall ball bearings include thin-wall four-point contact ball bearings series, thin-wall angular contact ball bearings series and thin-wall deep groove ball bearings series three series, are mostly used in the waist, elbow, wrist of industrial robots. Under the condition of the same inner diameter and size, the number of steel balls installed in the thin-wall equal-section ball bearing is more than that in the standard rolling bearing, so the internal force distribution of the bearing is improved, the elastic deformation of the contact between the ball and the channel is reduced, and the bearing capacity is improved.

2. Thin-wall cross cylindrical roller bearings

The internal structure of thin-wall cross-cylindrical roller bearings adopts rollers arranged vertically across each other at 90°. A single bearing can simultaneously withstand the combined action of radial force, bi-directional axial force and overturning moment. Spacer or isolation block is installed between rollers to prevent the inclination of rollers or mutual grinding between rollers, effectively preventing the increase of friction moment (see Figure 2). In addition, the roller vertical cross arrangement structure can avoid the roller locking phenomenon; At the same time, because the inner and outer rings of the bearing are divided structures, the clearance is adjustable, and higher rotation accuracy can be obtained even if the pressure is applied. Thin-wall cross roller bearing with its light structure and good performance is mainly used in mechanical arms, key robot shoulders, waist, hips and so on.

3.RV reducer bearing

RV reducer, also known as precision bearing reducer, is widely used in industrial robots for its small size, strong impact resistance, large torque, high positioning accuracy and many advantages. For RV reducer, the bearing shape structure and precision positioning are the key factors for its compact structure, good rigidity and precision transmission. RV reducer bearings include a variety of thin-wall bearing series and cylindrical roller cage assembly series (see Figure 3).

4. Harmonic reducer

Harmonic reducer is a kind of wave generator to make flexible wheel produce controllable elastic deformation, the use of flexible bearing controllable elastic deformation to transfer motion and power, its characteristics are compact structure, high motion accuracy, large transmission ratio, mostly used in small and medium torque robot joints. The flexible bearing is the core component of the harmonic reducer, which can achieve the requirement of high reduction ratio through the elastic deformation of the bearing. Moreover, the life of the harmonic reducer mainly depends on the life of the flexible bearing (see Figure 4). The flexible bearing changes constantly with the elastic deformation of the flexible wheel, which not only bears the cyclic stress load, but also the alternating stress load.

Development status of industrial robot bearing

As one of the key supporting basic parts of the industrial robot, the industrial robot bearing is still a relatively weak link in our country. The existing national bearing standards can not meet and guide the design, production and inspection of industrial robot bearings.

Luoyang Baina Company has been committed to the research and development of industrial robot bearings for many years, has formed cross cylindrical roller bearings, harmonic reducer bearings, cross roller turntable bearing three types of products, products from the inner diameter of 20mm to 1000mm outside diameter, precision from P5 to P2 can be manufactured. Fully able to meet the needs of industrial robot oems.

In the development and production practice of industrial robot bearings, in order to standardize and serialize the production of industrial robot bearings, the company has compiled the samples of harmonic reducer bearings, cross-cross cylindrical roller bearings and cross-roller turntable bearings for industrial robots, and formulated the product technical standards, which have been proved by experts and put on record by the Bureau of Quality and Technical Supervision. Fill the domestic blank, and with international standards.

At present, Luoyang Baina professional engaged in industrial robot bearing research and development, over the years the cumulative production and sales of various robot bearings nearly 100,000 sets, exported to Europe, America, Japan, South Korea, Singapore, China Taiwan and other countries and regions, but also for a number of domestic industrial robots and parts research and development and production of enterprises, scientific research units to provide product support and technical support services.

Key technology of bearing for robot

(1) Precise control technology of thin-wall bearing negative clearance. Industrial robot bearings require smooth operation and appropriate starting friction torque, so there should be an appropriate amount of negative clearance during bearing production and assembly. If the negative clearance of the bearing is too large or too small, it will directly affect the noise, vibration and service life of the bearing. Because the wall thickness of the inner and outer rings of the thin-walled bearing used by the robot is relatively thin, it is easy to lead to the deformation of the rings when the clearance is measured by the loading loading method, and the amount of negative clearance is difficult to control. Special processing and assembly methods and processes and special assembly tools are required.

(2) Precise control of assembly height of thin-wall angular contact ball bearings. The robot has a compact structure, accurate installation space, and the assembly height of bearings is strictly required, and the assembly height of similar bearings abroad is also strictly controlled. Due to the small wall thickness of thin-wall angular contact ball bearing ring, it is easy to produce deformation, the precision of each dimension is difficult to be accurately controlled, and the matching size of inner and outer rings and rolling body is difficult to be strictly controlled, resulting in large assembly height deviation after bearing assembly. Therefore, in order to achieve accurate control of the assembly height of thin-wall angular contact ball bearings, and even achieve the purpose of universal pairing, it is necessary to carry out special groove position design, grinding process formulation, accurate matching of bearings, and at the same time increase the grinding process of bearing protruding amount.

(3) Precise assembly technology of thin-wall bearings. Due to ultra-thin wall thickness and poor stiffness of robot special series precision bearings, it is easy to deform when heating and fitting the sleeve by using ordinary assembly methods and molds. Therefore, in order to realize the precision assembly of thin-wall bearings and achieve the indicators of the finished products, we must adopt the assembly size selection, assembly process formulation and special assembly mold and accessories for thin-wall bearings, carry out strict grinding of the selected bearing rings, develop detailed and strict assembly process, and develop special assembly tools and accessories to ensure the accuracy of bearing assembly.

(4) Non-contact measurement technology of inner and outer diameter of thin-wall bearing rings. The wall thickness of thin-wall bearing rings is very thin, requiring precision vehicles and grinding to achieve the required tolerances. At the same time, the measurement accuracy of thin-wall bearing ring profile parameters is also very high. Traditional detection means, such as D913 instrument used in standard bearing outer diameter measurement and 0.001 torsion spring meter measurement, require a certain force measurement, but the meter is difficult to accurately control. It directly affects the measuring accuracy of the outer diameter of the thin wall ferrule and can not meet the requirements of testing. Therefore, it is necessary to study the measurement method of the inner and outer diameter of thin-wall bearing rings. Based on the non-contact optical precision measurement technology, a set of special measuring instrument for the outer profile of thin-wall bearing rings is developed by integrating motion computer active vision, image processing, precision motion control and computer control, etc.

(5) Bearing comprehensive performance test technology based on robot working conditions. In order to assess and evaluate the performance, life and reliability of robot bearings, simulation tests or sampling tests during mass production must be carried out before installation to ensure stable and reliable bearing performance. Due to the particularity of the robot bearing structure, it is necessary to develop a special bearing test device, and simulate the working condition test, to detect the bearing vibration, temperature, load, speed, friction torque and rotation accuracy and other performance, according to the use and use requirements of the supporting robot, formulate the corresponding test specifications, complete the specified number and time limit of the life test. After the test and the data before the test are compared and analyzed, so as to evaluate whether the service performance, life and reliability of the bearing meet the requirements.

Industrial robot is a new kind of high-tech intelligent machine which can replace part of artificial industrial production. As an industrial robot, three words can be used to describe its requirements: fast, accurate and stable.

For the current robot production technology, fast and accurate are easier to do, but if you do fast, stable and accurate at the same time, it is relatively difficult.

Here a word is defined: repeated positioning accuracy of industrial robots -- industrial robots do a certain action repeatedly (generally 10-20 times), each time the average deviation from the planned position is its repeated positioning accuracy, generally need to be measured by laser interferometer.

For industrial robots, precision is a very important factor, it directly affects the quality of products and production efficiency. And the main factor affecting the accuracy of industrial robots is the bearing, so what kind of bearing can make industrial robots will have higher accuracy?

1. Design of key structural parameters

The bearings of industrial robots have the use requirements of long life, high rigidity and low friction. Therefore, when designing industrial robots, we should not only consider the rated dynamic load as large as possible, 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 bearings.

2. Position of ferrule groove

The inner and outer rings of contact ball bearings of industrial robots adopt an integral structure, and the general cross-sectional area is only 20% of standard bearings with the same inner diameter, so the radial wall thickness of the cage is limited. If the groove position is designed in the center of the bearing width, the bearing becomes a symmetrical structure, and the strength of the bottom hole of the cage and the installation position of the sealing ring will be limited. Therefore, in order to ensure that the bearing cage has enough strength and the sealing ring has enough space to install, the groove position is designed to be asymmetric to the end faces on both sides.

3. Determine the height of the retaining edge

When the contact ball bearing of industrial robot is in the working state, it should bear a certain radial load, but also bear a certain axial load. If the axial load is too large, it may cause the contact between the steel ball and 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, in the design of the flange height, the minimum flange height J of the bearing ring should be calculated by Hertz contact stress theory according to the axial load bearing, so as to calculate the diameter of the flange of the bearing ring. In the design, considering the small cross-sectional area of contact ball bearings, under the premise of meeting the requirements of use, the high coefficient of retaining edge can be smaller than that of deep groove ball bearings.

4, sealing structure design

The early destruction of the bearings of industrial robots is generally because of poor sealing. Because of poor sealing, debris or other pollutants in the air are easy to enter the bearings, so that the grease gradually fails, leading to the damage of the bearings. So the sealing of the bearing is very key. Therefore, when designing sealed bearings, we should not only consider sealing, but also consider the specific working requirements of bearings and the specific structural form of bearings.

Industrial robots are developing towards miniaturization, lightweight and precision. Therefore, the lubrication of bearings of industrial robots adopts the lubrication method of sealing structure for grease lubrication.

5, according to the characteristics of the industrial robot bearing wall thin

There are generally two ways to match the bearing inner ring with the sealing lip:

(1) Contact seal with inner ring sealing lip and inner ring retaining edge

(2) Non-contact seal with inner ring sealing lip and inner ring retaining edge

The contact seal has better dustproof and grease leakage effect than the non-contact seal, but its friction torque is larger and the temperature rise is higher. Considering that the friction torque is larger, it has an adverse effect on the robot main engine, so the non-contact seal structure is generally adopted in the design.

In addition, because the bearing ring wall is very thin, the inner and outer rings of the bearing are easy to deform, so the inner ring retaining edge can be made of groove-free structure, but the sealing clearance should be strictly controlled.

Research status of generic technology of robot bearing

Domestic industrial robot bearings mostly rely on imports, a few manufacturers although the production and manufacturing of industrial robot bearings, but the batch is small, fewer varieties and specifications, parts of the low degree of universality, long supply cycle, high cost, and the quality is unstable, these factors seriously restrict the normal development of the domestic industrial robot industry.

At present, our country engaged in industrial robot bearing research and development units mostly concentrated in Henan Luoyang. Luoyang Shaft Research and Technology Co., LTD. (Shaft Research Institute) first launched robot technology research during the Seventh Five-Year Plan Period, and undertook the sub-project "Development and Research of Special Bearings for Robots". It carried out the design, trial production, performance testing and life test of thin-wall four-point contact ball bearings and thin-wall cross cylindrical roller bearings for welding, spraying and assembling robots. Compared with YAO bearings in Germany and THK bearings in Japan, the results have reached the international technical level of similar products. The developed products are applied in Dalian Institute of Modular Machine Tool 12kg welding robot, Guangzhou Institute of Machine Tool 50kg handling robot and Institute of Machine Automation 75kg welding machine to meet the main machine requirements. In recent years, Shaft Research Technology Co., Ltd. has studied the structural characteristics, stiffness, precision, friction torque, preload and application of thin-wall bearings, and qualitously analyzed the problems to be considered in thin-wall bearing design and the influence of shafting support stiffness on bearing deformation, which provides a strong technical support and research basis for the development of a series of special matching bearings for domestic industrial robots.

Luoyang LYC Bearing Company has carried out in-depth research on the types, structural characteristics, bearing materials and heat treatment process, dimensional tolerances and rotation accuracy, rolling body accuracy, clearance and inspection methods of flexible bearings for harmonic drive. Henan University of Science and Technology put forward the structural design method of lightweight green long-life thin-wall four-point contact ball bearings, studied the relationship between bearing effective wall thickness and bearing capacity, starting friction torque and other key performance indicators, found the minimum wall thickness under the condition of guaranteed life and reliability, so as to improve the utilization rate of bearing materials, reduce bearing weight, save energy, The performance index exceeds that of similar foreign products.

On the premise of considering the working characteristics of the flexible bearing, the centrifugal force of the rolling body and the influence of EHL oil film, a method for analyzing and calculating the load distribution and contact stress of the flexible bearing was proposed by Beijing Harmonic Technology Institute, which has a certain significance for calculating the allowable load of the flexible bearing according to the dynamic load capacity.

Luoyang Weisger bearing, Luoyang Hongyuan bearing, Shanghai Lieli bearing and other units are also by virtue of their own research and development and production of industrial robot special supporting bearings or application engineering projects active in today's domestic and foreign industrial robot special bearing market. In addition, some research institutes and universities are also engaged in the research and development of high-end industrial robot bearing manufacturing technology and application projects.