Understand the real quality and service life of the conductive slip ring

Super long life conductive slip ring enters a new era of standardized design

Understand the real quality and service life of the conductive slip ring

Three wrong views on the quality and service life of conductive slip ring——

In recent years, ultra long life and ultra-high speed conductive slip rings have become the focus of publicity by many slip ring manufacturers. The declared long life is generally from 20 million to 200 million revolutions, and the high speed ranges from 1000 to 60000 revolutions per minute (RPM). The implementation methods of each product scheme are different, and the design principles are similar, mainly including three aspects: material, brush pressing and surface treatment process. However, the concept of ultra long life and ultra-high speed of conductive slip ring is described vaguely, and the reliability is not high. It can only provide some theoretical data, or it is still in the sample stage, or it lacks sufficient test data support, and even there are illogical and contradictory data. We might as well take a look at the following wrong views, plausible basis and our analysis of their problems related to the service life, speed and quality of the slip ring.

 Wrong view 1:

The brush design close to zero brush voltage reduces the contact surface and prolongs the service life of the slip ring by 1 / 3.

 Error basis 1:

Because the brush pressure is small, the smaller the contact surface, the higher the surface smoothness of the contact, the less the wear of the contact, so the service life of the slip ring is prolonged.

Problem analysis 1:

Brush voltage is indeed an important factor affecting the service life, but the smaller the brush voltage is, the stability is extremely difficult to control, and the larger the dynamic fluctuation value of current will be, which will increase the instability of electric power or signal transmission. At the same time, the smaller the contact surface is, the greater the resistance will be. When rotating at high speed, it is easier to cause high temperature and even spark, resulting in contact sintering, which shortens the service life of the slip ring and has great potential safety hazards.

 In fact, the resistance becomes smaller and stable when the brush pressure is increased within the reasonable wear-resistant design pressure range; When the pressure is stable, the contact surface is increased and the pressure is reduced, the wear rate of the contact is reduced, so that the service life of the slip ring is longer and the signal transmission is more stable.

 Analysis conclusion 1:

Reasonable and stable brush pressure can ensure the maximization of the service life of the conductive slip ring.

Wrong view 2:

The higher the speed of the conductive slip ring, the better the performance of the product.

 Error basis 2:

The higher the speed of the slip ring, the higher the machining accuracy and the more stable and reliable the performance.

 Problem analysis 2:

The technical principle of conductive slip ring determines that its service life is inversely proportional to the speed. The higher the speed, the shorter the service life. For example, if the slip ring with a speed of 60000 rpm is calculated based on the service life of 200 million revolutions (both are the limit values declared in the current market), the theoretical service time is only 5.5 hours. Therefore, the higher the speed of slip ring, the smaller its practical application field. At present, ultra-high speed slip ring is only used in high-speed detection such as high-speed railway and aircraft engine.

 The faster the speed of the slip ring is, the higher the probability of safety risks such as dust, wear, dislocation, high temperature and short circuit caused by contact friction will be. Therefore, the high-speed slip rings have a design feature of small volume and low current. Generally, the maximum is only 1 ampere. Many of them are of Ma level. Conductive slip rings with more than 10000 RPM must also be equipped with cooling system design. The speed of the high current slip ring is designed in the range of more than ten to tens of revolutions per minute.

 The rotating speed of the conductive slip ring depends on the bearing it uses. There are 8000, 10000, 20000, 40000 and 60000 RPM high-speed slip rings advertised and promoted in the domestic market. The maximum continuous running speed of general bearings in China is about 4000 rpm, and professional fields such as 12000 RPM of rotor support bearing of driving motor of passenger cars; The speed of high-speed motors of well-known foreign brands is only 18000 ~ 24000 rpm, the speed of Brushless DC motors is less than 40000 rpm, and the speed of civil aircraft engines is 30000 rpm; The speed of ordinary desktop electric fan familiar to everyone is only in the range of 500-1500 rpm.

 The world's most famous slip ring manufacturer produces industrial grade high-speed slip rings with a speed of 1000-2500 rpm, while 20000 rpm is the limit, and is equipped with 1.4kg/cm2 cooling air for forced cooling. This is not to say that the speed of domestic slip rings cannot exceed that of foreign countries, but according to these industry data, I believe that everyone should have a judgment on the high-speed slip rings with tens of thousands of revolutions.

 Returning to the essential meaning of conductive slip ring, it is only a conductor, not a motor or bearing. The quality of its transmission is not the stability of its speed.

 Analysis conclusion 2:

The quality and performance of the conductive slip ring depends on the stability of signal transmission. Zero loss and zero fluctuation are the best quality, which has nothing to do with the speed of the slip ring (if any, it is only negatively correlated).

Wrong view 3:

The service life of the conductive slip ring is equal to the number of revolutions used. The greater the number of revolutions marked, the longer the service life and the better the quality.

 Error basis 3:

When many manufacturers mark the service life of the conductive slip ring, it will be expressed by the number of revolutions or "revolution @ speed". The greater the rotation value of the service life of the conductive slip ring, the longer the service life. Therefore, 20 million revolutions must have a longer service life and better quality than 10 million revolutions.

 Problem analysis 3:

Due to the wide application of conductive slip ring, the quality requirements of different industries vary greatly, even completely. For example, the construction machinery industry may have no requirements for the flash off of power signal, and the flash off for 1 or 2 seconds has little impact; However, some high-precision automation equipment industries require no more than 30 flashes per second; Medical equipment and military equipment industries are required to be controlled within microseconds; The data communication industry will also have many requirements, such as data packet loss rate, bit error, transmission frequency and noise. It is not difficult to understand why the same new slip ring used in industrial automation equipment may not meet the application requirements of medical equipment.

 Most domestic conductive slip ring manufacturers mainly focus on the structural design of slip ring, lacking professional technical support from different application industries. The general practice is to take the dynamic fluctuation value of resistance as the measurement standard of power signal transmission quality. Even so, there are still great differences in quality measurement standards. Due to the low requirements of early market application, most slip ring manufacturers use the resistance tester on the market to test the slip ring, but the sampling rate of this instrument is low, less than 20 times / s, resulting in those very short-time flashes will not be captured. Therefore, using more than 20 years of experience and resources in the communication industry, 3KMLink has independently developed and designed a resistance fluctuation detection equipment with high sampling rate, which can test the fluctuation and flashover of the slip ring at the speed of 1000000 / s, greatly improving the accuracy and reliability of the test. At present, it is the flashover detection equipment with the highest accuracy in the industry.

 Secondly, for example, for the conductive slip ring with the same service life of 10 million revolutions, the test speeds used by different manufacturers are likely to be different, and some are tested only for 8 hours a day, some are tested 24 hours a day, some are tested at room temperature, some are tested in different or alternating simulation environments of high and low temperatures, etc. Strictly speaking, it is one-sided and nonstandard to express the service life without several main dimensions such as test methods, continuous duration, speed, environment and dynamic fluctuation value. There is no comparability between them, and they can not be used as the judgment basis of normal service life.

 Therefore, strict manufacturers will first define the service life of the conductive slip ring according to the demand characteristics of the actual application of the slip ring, and then use the application demand industry standards and specifications, and generally mark "please note that the service life of the slip ring depends on the speed, environment and temperature" in the service life. Only the number of revolutions defined in this way is the real service life of the conductive slip ring.

 Analysis conclusion 3:

The service rotation value of the conductive slip ring is not equal to the service life, nor can it represent the quality of the conductive slip ring.

Three factors affecting the quality and service life of conductive slip ring——

Through the analysis of the above problems from three different angles: brush pressure, speed and number of revolutions, we believe that we will have a new and more accurate understanding of the service life and quality of the conductive slip ring, and understand that the service life that meets the technical quality requirements of the market application field is the real service life of the conductive slip ring. Only in this way can we further understand the technical performance of the conductive slip ring and correctly understand the relationship among the three influencing factors of the material, design and process of the conductive slip ring.

 The materials that determine the quality of the conductive slip ring mainly guide the contact materials of the electric slip ring, including gold, silver, copper, rare and precious metals and their synthetic metals with different proportions and manufacturing processes. In different applications of power supply and signal rotary transmission, each material has different characteristics and properties, including the conductivity, hardness, strength, elasticity, brittleness, ductility, wear resistance, corrosion resistance, high and low temperature resistance and heat dissipation of contact materials. In order to ensure the contact reliability and service life of conductive slip ring and brush wire, the hardness of the two materials should be matched as much as possible to minimize the wear rate between contacts.

 The design affecting the service life of the conductive slip ring is mainly in two aspects: brush pressure design and structural design (of course, there is also the design of the electronic part, but it is not the focus of this paper, so I won't talk about it). For the same material, such as 0.3, 0.25, 0.2, 0.17, 0.1, 0.08, 0.05mm, the brush wire with different diameters has different stress and can bear different brush pressure; The brush wire with the same material and diameter adopts different forms such as single brush, multi brush and bundle brush, or the ring is designed into flat shape, arc shape, V shape and W shape, and the brush pressure, conductivity and wear rate of the contact are also different; The different shape and length of the brush wire, the contact angle with the loop and the temperature change during operation will affect the change of the brush voltage, resulting in the change of the dynamic fluctuation value of the resistance, and then affect the transmission quality of the power supply and signal. The effects and differences can not be distinguished, realized and controlled only by the designer's many years of design experience, but must rely on a large number of experiments, test databases and long-term analysis and verification accumulation, so as to obtain the optimal solution closest to the objective facts.

 The processes affecting the quality of conductive slip ring mainly include technical treatment process and manual manufacturing process. The technical treatment process focuses on the surface smoothness, wear resistance, corrosion resistance, conductivity and friction dust of the contact. With the continuous development of professional technologies such as conductive materials, electroplating process and surface treatment, the technical treatment process level of major slip ring manufacturers has become closer and closer. The influence on the quality of conductive slip ring is mainly reflected in the gap in manual manufacturing technology. Because the conductive slip ring is widely used, most of the business of the slip ring industry is non calibrated and can only be operated and assembled manually. At present, human factor is still the most important guarantee for the consistency and reliability of conductive slip ring, and it is also the most uncontrollable key node.

Good materials, design and workmanship do not mean good quality——

Many people believe that "good materials + good design + good technology", as well as the detection of finished product technical parameters, as the final quality guarantee, will be able to make good products. In fact, in the highly competitive high-tech era, good products can no longer be superimposed by such a simple way, and the conductive slip ring industry is no exception.

 "The difference is a hundred miles away". We have selected 10 conductive slip rings with the same initial values of various technical parameters to conduct the limit life test of uninterrupted operation at the same time. As a result, the service life of the 10 slip rings are different, the degree of wear is also different, and the variation of dynamic resistance fluctuation value is even different. The biggest difference is close to 60%. The reason is the lack of systematic data support. Even if there are strict standard operation specifications, it is difficult for a technical engineer with more than ten years of manufacturing experience to ensure the consistency of brush wire and brush pressure of slip ring in the manufacturing process from materials to finished slip ring products:

 1. Large brush pressure and fast wear; If the brush voltage is small, the dynamic resistance fluctuation value is too large (the dynamic resistance fluctuation value is the most important index to test the stability of the slip ring, and its stability directly affects the signal transmission quality. When the dynamic resistance fluctuation value changes in a large range, it is easy to spark if it is a power ring; if it is a signal ring, it will affect the effect of signal transmission); If the brush pressure is too large or too small, the temperature of the contact will be high, and the elastic stress of the brush wire will be reduced, which is easy to cause the brush pressure to change with the change of temperature. What's more, it is impossible to determine the corresponding relationship and change law between different brush pressure range and wear rate, dynamic resistance fluctuation value and temperature change.

 2. When assembling and adjusting the brush, the brush wire was bent and straightened again. Although the appearance of the brush wire is identical, there is no difference. But in fact, due to the change of internal stress, its brush pressure is different from the original.

 3. The different processing time and ambient temperature of each process node of the contact, such as quenching, stress release and plating, will lead to great differences in the strength, hardness and surface wear resistance of the finished product.

 Therefore, in the second round of test, we took every 3 conductive slip rings with the same brush pressure as a group and divided them into 5 groups according to different brush pressure levels for test. As expected, five groups of test parameter data are very similar or even the same.

The quality and service life of slip rings have entered a new era of standardized design——

In recent years, the structural design and technical process of conductive slip ring are more and more similar and less differentiated. How to improve the market competitiveness of products? The slip ring manufacturers with foresight have gradually realized that the competitiveness of the conductive slip ring industry is no longer the past design ability, experience and special treatment process, but the enterprise's own standardized production design ability and data production control ability.

 Because only "standardization, digitization and standardization" can run through the design, production and management of materials, processes and finished products, and form a benign optimization cycle system. In order to achieve the best balance between the wear rate, stability and consistency of the conductive slip ring contact; In order to carry out professional grading and batch management of products and ensure the best consistency and reliability of batch products; In order to accurately control the real quality and service life of the slip ring, and specifically meet the various market needs from different industries, so that customers can use it clearly and clearly; In order to make the company stand out in the increasingly fierce market competition, and even lead the slip ring industry into a new era of development.

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