The rolling bearing design has gradually changed from the “enhanced” design advocated for a long time to the “antifriction” design.
Abstract: Under the concept of global sustainable development,energy-saving,material-saving and environment-protection have become new technology development trend.The bearing design has been changed from “Enhanced Design” to “Antifriction Design”,which is the remarkable characteristics for new generation of bearings. The practical examples of antifriction design are introduced,and the principle,method and measure of antifriction design are explained based on Tribological theory and mechanism.
Key words: rolling bearing; product design; antifriction
Rolling bearing was directly called “antifriction bearing” in the United States because the friction coefficient of rolling bearing is higher than that of sliding bearing replaced by it μ. It should be 1 ~ 2 orders of magnitude lower (sliding bearing is generally about 0.1 ~ 0.01, rolling bearing is generally about 0.01 ~ 0.001, or even lower). Therefore, the antifriction characteristics of rolling bearing are not only the mission of its birth and development, but also its essence.
1. Antifriction Design Example of Rolling Bearing
Although the friction coefficient of modern rolling bearings has reached a very low level, many famous bearing companies are still committed to further reducing the bearing friction loss and have made remarkable achievements under the new development trend of global sustainable development, saving energy and resources and paying attention to environmental protection. For example:
FAG motors use C series deep groove ball bearings, and the friction torque is reduced by 35%;
NSK's GR series deep groove ball bearings for household air conditioners, vacuum cleaners and electric fans reduce the friction torque by 40% ~ 50%;
NTN automotive transmission uses “self-sealing low friction torque deep groove ball bearing”, and the friction torque is reduced by 80%;
JTEKJ improved “LFT - Ⅲ ultra-low friction moment tapered roller bearing”, the friction torque is reduced by 50%;
SKF adopts polyamide PA46 + GF30 as the standard cage in the new generation of deep groove ball bearing, and the friction torque is reduced by 30%.
The above friction reduction results are not at the expense of excessive sacrifice of other performance and life indicators, but are the results of multi-objective comprehensive optimization, which not only reduces the friction torque of the bearing, but also has the characteristics of noise reduction, weight reduction, life extension and so on.
2. Antifriction Design Method of Rolling Bearing
2.1 Mechanism of bearing friction.
Bearing friction is the impedance to its operation, and its manifestation is friction torque. According to its generation mechanism, it can be divided into four types:
(1) Pure rolling friction;
(2) Differential sliding friction and spin sliding friction on the rolling contact area, as shown in Figure 1;
(3) Sliding friction of sliding contact parts, including sliding between rolling element and cage pocket, sliding between cage and ferrule guide surface, sliding between roller end face and ferrule retaining edge, sliding between seal and ferrule in sealed bearing, etc;
(4) The viscosity of lubricant blocks friction.
The torque generated by the above rolling friction, sliding friction and lubricant friction is called the total friction torque of the bearing, and its calculation formula is
M = ML + MV = ( MD + MS ) + MV ,
ML is the friction moment of load term, N· mm; MV is the friction torque of speed term, N · mm; MD is differential sliding friction torque, N · mm; MS is spin sliding friction torque, N · mm. These friction torques can be calculated by empirical formula, and each bearing company is different.
Fig. 1 distribution of differential and spin sliding velocity and surface friction shear stress on rolling contact ellipse of ball bearing
2.2 Antifriction Design Method of Bearing
To reduce the bearing friction, in addition to the particularly important lubrication aspects (including lubricant, lubrication mode and surface roughness of contact pair), the main task is to reduce the rolling friction and sliding friction belonging to the internal characteristics of the bearing.
The principle of bearing antifriction design is: the antifriction design shall be carried out on the premise of ensuring other performance and service life requirements of the bearing, not just for antifriction, which will seriously affect the realization of other performance, service life and reliability, and make the bearing difficult to meet the specified service requirements.
Antifriction design should be based on Tribological principles and mechanisms. Taking the differential sliding friction moment MD in the load term of deep groove ball shaft as an example, the empirical calculation formula is
MD = Dpw / 2Dw ( Mb + M' b ) + 1 / 2 ( Mb-M' b ) ,
Dpw is the pitch circle diameter of the ball group, mm; Dw is the diameter of the ball, mm; Mb, M' b are the sliding friction torque formed by the contact between the ball and the inner and outer ring channels, N · mm. It can be seen that the bearing friction torque can be reduced by reducing the pitch circle diameter of the ball group or increasing the ball diameter. The same is true for other types of bearings.
In addition, increasing the length of the short axis of the contact ellipse or reducing the length of the long axis of the contact ellipse, reducing the contact area and increasing the contact angle of the tapered roller bearing can reduce the bearing friction torque.
3. Antifriction Design Measures of Rolling Bearing
Taking the most representative deep groove ball bearings and tapered roller bearings as examples, this paper describes the paths or measures usually taken in antifriction design.
3.1 Antifriction design of deep groove ball bearing
(1) Reduce the pitch circle diameter of the ball group; (2) Increase the ball diameter; (3) Reduce the tightness between the ball and the channel; (4) Reduce the number of balls; (5) Improve the lubrication performance between the ball and the cage pocket.
The above measures can be adopted comprehensively, or focus on improvement in one aspect. For example, NSK’s GR series deep groove ball bearings can reduce the pitch circle diameter of the ball group to the extent shown in Figure 2; NTN Company has designed an oil storage tank on the punching cage pocket, as shown in Figure 3.
Fig. 2 Comparison of GR deep groove ball bearing of NSK with the original structure
Fig. 3 stamping cage with oil storage tank of NTN
3.2 Antifriction design of tapered roller bearing
(1) Reduce the pitch circle diameter of roller group; (2) Increase the roller diameter; (3) Reduce the roller length; (4) Increase the bearing contact angle; (5) Optimize the contact surface between roller and raceway; (6) Improve the contact between the roller ball base surface and the large retaining edge of the inner ring; (7) Reduce the number of rollers.
The above measures can also be considered comprehensively and highlight the key points. For example, the main antifriction measures adopted by JTEKT “LFT-iii ultra-low friction moment tapered roller bearing” are shown in Figure 4.
Fig. 4 antifriction measures for tapered roller bearing with ultra-low friction torque
Among them, reducing the pitch circle diameter of the roller group and increasing the roller diameter are to reduce the rolling friction; Reducing the length of rollers, the number of rollers and the special convexity shape of rollers and raceways are all to reduce the contact area and reduce the differential sliding friction; Increasing the bearing contact angle is to make the raceway share more axial load and reduce the sliding friction between the roller end face and the inner ring retaining edge; Reducing the inner diameter of the cage and designing the small retaining edge of the inner ring into a special shape to make it a labyrinth structure is to control the lubricating oil flow; Larger contact angle, roller diameter and fewer rollers can improve the pumping and discharging performance, prevent the lubricating oil from stagnating in the bearing and discharge it as soon as possible.
In addition, by increasing the rolling element diameter and bearing contact angle, the loss of fatigue life and rigidity caused by reducing the contact area can be compensated. The special convexity shape of roller and raceway, coupled with special heat treatment, can effectively improve the service life of the bearing.
4. Conclusion
With the continuous and significant progress of steel quality and manufacturing technology, the fatigue life of most purpose bearings has no longer become the main problem, but the performance has become the goal of continuous pursuit. Under the concept of sustainable development, energy saving, material saving and environmental protection have become the new technological development trend. The bearing design has gradually changed from the “enhanced” design advocated for a long time to the “antifriction” design, which is the remarkable technical feature of the new generation of bearings.
The antifriction of rolling bearing is further improved on the basis of already low friction coefficient or friction torque. Therefore, it is difficult to solve the problem only by conventional thinking. For example, reducing surface roughness is the simplest and direct method, but this measure may be effective, or get half the result with half the effort, or it may be ineffective. The most important thing is to thoroughly understand and master the Tribological principle and mechanism of the bearing, and then seek solutions through the design of internal structural parameters, which is the most basic and effective technical path to achieve the purpose of friction reduction in essence.
More about SUNSPEED Rolling Bearings:
Rolling bearings support and guide rotating or oscillating machine elements such as shafts, axles or wheels and transfer loads between machine components. They provide high precision and low friction and therefore enable high rotational speeds while reducing noise, heat, energy consumption and wear. Rolling bearings are cost-effective and exchangeable machine elements that typically follow national or international dimension standards.
Roller bearings also known as rolling-element bearings are similar to ball bearings in that they are designed to carry a load while minimizing friction. However, roller bearings transmit loads using cylinder rolling elements, rather than balls, to maintain the separation between moving parts of the bearing. These versatile bearings can contain single or multiple rows of rolling elements; multiple rows can significantly improve radial load capacity. Also, the use of different roller shapes can further reduce friction and support both radial and axial loads.
SUNSPEED formerly known as Seven Color Bearing co., Ltd. set up in 1996, is a large bearing enterprise in China. The company focuses on 10 categories of bearing products with a total of more than 5000 types. The products are as follows: deep groove ball bearing, tapered roller bearing, self-aligning ball bearing, spherical roller bearing, cylindrical roller bearing, angular contact bearing, thrust ball bearing, thrust roller bearing, needle roller bearing, joint bearing, etc. Bearings are widely used in motor, machine tool, automobile, wind power, mining and other fields.
The company’s private brand “LSCB” is China’s most promising brand, and we always adhere to the principle of “quality first, service first”. The company has established a set of complete sales network and after-service support system, and products are sold both in China and abroad. In order to realize the long-term sustainable development of the company, all SUNSPEED people will keep adhering to the people-oriented concept as well as the principle of “quality first, service first”, sparing no effort in achieving the long-term development of the company.
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