Three kinds of commonly used porous material cages and the necessity of long-life supplementary oil supply and common structural forms
Abstract: The lubrication characteristics and lubrication principles of long － life bearings used in aerospace are summarized，the three kinds of porous materials cages and the necessity of long － life oil supply and its common structure forms are introduced，the development trends of long － life lubricating technology for bearings used in aerospace are summed up and prospected．
Key words: aerospace bearing; lubricating technology; porous nylon; porous cotton － phenolic; porous polyimide
Bearing and bearing unit determine the precision, life and reliability of spacecraft, and are the key components of spacecraft. Bearing lubrication is the key factor to realize the function, and most bearing failures are caused by lubrication failure. Compared with the conventional bearing lubrication technology, the aerospace bearing lubrication technology has its particularity, which is mainly manifested in that the space lubrication materials, environment and operating conditions are very different from the atmospheric environment, usually involving ultra-high vacuum, high and low temperature alternating, high and low speed and multiple startup and shutdown; And once the spacecraft is launched, it is impossible to replenish lubricating oil. Therefore, the aerospace bearing is required to have low friction, high precision, long service life and high reliability, and must always maintain a good lubrication state. This demand brings many challenges to the material and design of bearings, especially in the lubrication system.
1 Lubrication characteristics of aerospace long-life bearings
1.1 Particularity of aerospace bearing lubrication
The special working conditions, low friction, long service life and high reliability requirements of aerospace bearings determine that they can not use conventional lubrication technology, but use solid lubrication technology or one-time thin oil lubrication technology, and the lubrication method is relatively special. For high-speed, precision and long-life bearings, such as momentum wheel bearings, both at home and abroad adopt one-time thin oil lubrication technology, that is, the cage uses porous materials. In addition to separating the rolling elements when the bearing rotates, the cage also uses porous forms as the source or carrier of lubricating oil, which plays an important role.
1.2 Lubrication principle of aerospace long life bearing
The porous material cage used in aerospace long-life bearing is made by pressing and firing polymer resin or resin containing cotton fiber through a special process, and finally putting lubricating oil pressure into the microporous material. The cage has certain mechanical strength, and there are micro holes inside or on the surface. The micro holes containing lubricating oil become a channel for the mutual circulation of lubricating oil.
When the bearing operates, the cage operates in the same direction. Due to centrifugal force, the lubricating oil in the porous material cage tends to transfer outward; At the same time, the heat generated by friction during the operation of the bearing causes the temperature rise of the cage, and the volume expansion coefficient of the lubricating oil is larger than that of the cage material. The volume expansion of the lubricating oil contained in the micropores in the cage material is larger than the increase of the micropores' volume, generating an internal pressure that forces the lubricating oil to penetrate into the outer surface. Under the action of these two forces, the lubricating oil overflows from the porous material to the surface. With the increase of cage temperature, the viscosity of internal lubricating oil decreases, its fluidity increases, and the transfer rate of lubricating oil to the surface of rolling element accelerates. When the rolling element rotates, the lubricating oil on the cage surface is transferred to the contact raceway surface, gradually forming a layer of oil film to separate the surfaces in contact with each other so as to play a lubricating role.
The unique feature of the porous material cage is that it has the ability to recover lubricating oil. The capillary force generated by the pores in the cage material on the lubricating oil is inversely proportional to the diameter of the pores and is directly proportional to the surface tension between the lubricating oil and the porous material. This force prevents the rapid loss of lubricating oil in the cage. When the bearing stops running, the internal temperature decreases. Similarly, due to the different thermal expansion coefficients of the two materials, a negative pressure is formed in the micropore to suck the contact excess lubricating oil into the cage hole. When the speed, load, temperature, lubricating oil characteristics and other external conditions are certain, a dynamic balance will be established through operation. At this time, the oil output and oil absorption rates are equal, and the oil film thickness is stable. This is an ideal balance state, and the porous material cage enters a stable working area.
The flow process of aerospace long-life bearing lubricating oil with porous material cage is: cage → rolling element → raceway → rolling element → cage.
1.3 Space long life bearing porous material cage
At present, the aerospace long-life bearing mainly adopts porous polyamide material cage, porous cotton phenolic material cage and porous polyimide material cage.
1.3.1 Porous polyamide cage
In 1959, the Massachusetts Institute of technology first developed porous nylon material with interconnected micropores, and then the porous polyamide cage was successfully applied to American Titan-2 gyro bearing. When the relative humidity is 50%, the dry porous nylon material can absorb 3% w /w of water, and only 0.6% w /w of water can be absorbed after being filled with oil, indicating that only a small amount of water can enter the material that is already filled with lubricating oil.
Compared with porous cotton phenolic cage material, it has higher oil content and can form a circulating lubrication system from cage to raceway surface and from raceway surface to cage. However, the material can not independently adjust or change the pore diameter or total pore volume according to the needs, has large resistance, poor chemical stability and temperature resistance, poor tribological performance, poor space environment resistance and other shortcomings, which limits its wider application.
1.3.2 Porous cotton - phenolic material cage
Porous cotton cloth phenolic material takes cotton cloth as the base material, phenolic resin as the binder, and thermal reactive foaming agent as the pore forming agent. The tubular material is made by hot rolling and sintering process. It uses the capillary action of cotton fiber to store and transfer lubricating oil to the cage surface, thus playing a long-term lubricating role. Its heat resistance is better than that of porous polyamide cage material, with high dimensional stability. It is a traditional porous cage material.
The porous cotton phenolic cage material is very sensitive to moisture. When the porous cotton phenolic cage material that has not been soaked in oil is exposed to air with a certain humidity, the material will expand due to moisture absorption and prevent the absorption of oil by the pores connected with the cotton fiber in the material. The diameter of the porous cotton phenolic cage increases by 0.2% ~ 0.4%, and the cage becomes larger and clings to the outer ring of the bearing. When the fully oiled porous cotton phenolic material cage is exposed to air with a certain humidity, the moisture absorption forces up to 1 /3 ~ 1 /2 of the lubricating oil in its pores to flow out. Therefore, the porous cotton phenolic material cage shall be protected from the influence of air humidity during manufacturing, oil immersion, bearing installation and storage.
In addition, the porous cotton phenolic cage material has the disadvantages of relatively low porosity, oil content and oil retention, which limits its application.
1.3.3 Porous polyimide material cage
The porous polyimide material cage is made of polyimide molding powder by cold pressing free sintering or constant volume sintering process. There are through holes inside, and the porosity, pore diameter and distribution can be controlled by changing the preparation process. The pore diameter and porosity can be accurately controlled by using molding powder with different particle sizes and adjusting the molding process parameters.
Compared with porous cotton phenolic and porous polyamide cage materials, porous polyimide cage materials have good chemical stability and temperature resistance, excellent friction and wear performance, high mechanical strength, good space environment resistance, high oil content and oil retention, which are controllable and adjustable. It can provide good and continuous lubrication during the use of bearings, and has become the focus of research and application.
2 Long life supplementary oil supply technology
2.1 Necessity of long-life supplementary lubrication
The lubricating oil in the porous material cage of aerospace long-life bearing is lost or deteriorated, which shortens the lubrication cycle. The main reasons are as follows: (1) under the action of centrifugal force, some lubricating oil is lost to the outside of the bearing raceway, and the bearing cannot be lubricated continuously; (2) under vacuum conditions, the evaporation of lubricating oil is intensified, and some lubricating oil is lost; (3) during operation, the lubricating oil will also undergo chemical changes to generate non lubricating substances, which is more obvious in high temperature environment. At the same time, the mission cycle of spacecraft is getting longer and longer. For example, the mission cycle of NASA's space probe is 30 years, which puts forward more stringent requirements for the lubrication life. The oil immersion in the porous material cage alone cannot meet the demand. Therefore, it is necessary to develop a long-life supplementary oil supply technology.
2.2 Principle and common structure of long-life supplementary lubrication
In order to effectively prolong the service life of bearings and spacecraft, oil supply systems, such as oil reservoir, oil storage tank and pump auxiliary system, are designed at the installation parts of aerospace bearings. The oil control technology is used to make the internal lubricating oil flow slowly to the bearings to supplement the lost or deteriorated lubricating oil.
According to the working characteristics, the lubrication system in spacecraft can be divided into passive lubrication system and active lubrication system. The passive lubrication system is driven by centrifugal force or surface migration force to continuously provide lubricating oil to the bearing. The common structural forms include centrifugal lubricator, seepage fluid lubricator, oil rope lubrication system, porous oil reservoir, etc. The active lubrication system supplies a fixed amount of lubricating oil to the bearing when there is an external signal command. The common structural forms include active lubrication system, in-situ demand lubricator, static lubrication reservoir, etc.
2.2.1 Centrifugal lubricator
The centrifugal lubricator installed on the rotating part of the bearing unit is the most commonly used lubricator type at present. The circular container is filled with lubricating oil, and the outermost layer of the lubricating oil provides an outflow path of the lubricating oil. When the bearing unit rotates, the lubricator close to the bearing unit also rotates at the same speed. The centrifugal force causes the lubricating oil to flow out through the outflow path to the bearings installed at both ends of the lubricator to supplement the missing lubricating oil.
In the centrifugal lubricator (Fig. 1), the lubricating oil is filled in the metal reservoir. The outer wall of the outer sleeve of the lubricator is provided with an injection hole, and the lubricating oil flows out of the hole under the action of centrifugation. The flow rate is controlled by a limiter installed at the lower end of the injection hole in the oil reservoir. The lubricating oil from the oil reservoir is supplied to the bearing through a specially designed outer spacer fixed in the middle of the bearing. In this design, each set of bearings in the assembly is lubricated by an independent lubricator. The initial minimum flow rate in this lubricator is 4 μ g / h.
Fig. 1 centrifugal lubricator
The biggest advantage of the lubricator with this structure is that it can ensure long-term operation without external force driving or extra attention. Because the flow rate is directly proportional to the oil level height of the oil reservoir, the oil level of the oil reservoir gradually decreases with the extension of time, so it has the disadvantage of reducing the flow rate; The activation system of centrifugal action shall be fully tested in the operating environment before application.
2.2.2 Exudate lubricator
The oozing fluid lubricator is placed on the outer pad of the bearing unit, and the end of the pad forms the outer raceway of the bearing unit. There are precise spiral grooves on the inner and outer surfaces of the lubricator. The grooves bypass axially and transmit lubricating oil to each set of bearings. The flow rate is controlled by the size of the spiral groove and the speed of the bearing transmission shaft. A space barrel bearing system with a fluid oozing lubricator is shown in Figure 2.
Fig. 2 space barrel bearing system with exudate lubricator
2.2.3 Oil rope lubrication system
In this lubrication system, the cotton yarn cloth saturated with oil contacts with the bearing. Friction contact causes a small amount of lubricating oil to accumulate on the contact surface, and the lubricating oil migrates from the contact surface to the bearing. The other end of the gauze strip is in contact with the oil in the oil reservoir, which can absorb oil and maintain saturation.
2.2.4 Active lubrication system
When receiving an external command, the lubrication system transmits a certain amount of lubricating oil to the bearing. When there is a demand for lubricating oil, command to start the lubricator to express this demand by increasing the power or by increasing the bearing temperature due to the increase of bearing friction torque.
An active lubrication system controlled by solenoid valve is developed. In this system, the lubricating oil is stored in the metal bellows and pressurized by the compression spring. The high-pressure oil is driven and released into the bearing by the solenoid valve connected to the oil reservoir. Opening the valve for 125 ms can release 0.2 ~ 5 mg of lubricating oil.
The developed positive pressure feeding system consists of a spring loaded metal bellows. When regulating the release valve, the oil flows to the oil supply line through the metering bellows and metering valve, and the lubricating oil is delivered to the bearing surface. The oil delivery volume is controlled by the metering bellows, and its structure is shown in Figure 3.
1 - release valve; 2 - metering bellows; 3 - adjustable limiter; 4 - metering valve; 5 - feeder; 6 - inner ring (rotation); 7 - cage (inner ring guide); 8 - outer ring (fixed); 9 - oil supply pipe; 10 - oil reservoir; 11 - spring set
Fig. 3 active controllable lubrication system applied to satellite bearings
The developed command lubrication system contains flexible metal bellows in which oil is stored under ambient pressure. As the actuator, the micro stepping motor pressurizes the bellows, and the oil flows into the bearing through a steel pipe with a diameter of 0.5 mm. The system can accurately control the oil delivery volume. When a single operation lasts for 5 s, the oil delivery volume in this mode is shown in Figure 4. The outstanding advantage of this system is that the lubricating oil is stored under ambient pressure, and there is no possibility of omission. The capacity of the bellows is 2.5 g, and the oil delivery volume of each cycle is about 15 mg. If it is operated twice a year, its lubrication cycle will exceed 25 years.
Fig. 4 oil delivery of command lubrication system
2.2.5 In situ demand lubricator
The developed in-situ demand lubricator is composed of a porous material cylinder with an electronic heater. High friction causes the bearing temperature to rise, and the lubricator is driven when lubrication is required. The cylinder is filled with lubricating oil and installed on the fixed collar of the bearing; When heating the cylinder, the lubricating oil flows out of the cylinder because the thermal expansion coefficient of the lubricating oil is higher than that of the porous material; The surface tension of the lubricating oil is lower than that of the bearing material, and the lubricating oil flowing from the barrel migrates to the bearing surface. The feasibility of the system for long-life spacecraft is evaluated and verified by using a spiral orbit friction gauge.
2.2.6 Static lubrication reservoir
The static lubrication reservoir consists of a porous material reservoir mounted on an aluminum sleeve and a heater attached to the sleeve. The volume porosity of the oil reservoir is about 30%, which can carry enough lubricating oil for the whole task. The oil reservoir assembly is installed in the static part of the bearing unit. When the heater is turned on, the oil flows out and lubricates the bearing due to different coefficient of thermal expansion. The oil transfer mechanism of the static oil reservoir is shown in Figure 5.
1 - cage; 2 - ball bearing; 3 - thin oil film; 4 - heater; 5 - heating wire; 6-oil vapor Fig. 5 oil transfer mechanism of static oil reservoir
Fig. 5 oil transfer mechanism of static oil reservoir
3 Conclusion
At present, the porous material cage combined with the long-life supplementary lubrication system basically meets the current needs of spacecraft for long-life lubrication. Among them, the passive lubrication system can supply lubricating oil continuously, which is more suitable for supplementing the lubrication system of spacecraft. With the further extension of the mission period of spacecraft, it is necessary to develop a new lubrication system to supplement lubrication, so as to meet the needs of future spacecraft for long-life lubrication.
More about Marginal Self - Lubricating Bearings:
As a specialized manufacturer of self-lubricating bearings & bushings, Marginal Bearing is devoted to researching and producing new self-lubricating bearing materials.
Self-lubricating bearings, as the name suggests, provide their own lubrication during operation without requiring application of grease or oil lubricants. Due to this, self-lubricating bearings are also referred to as maintenance-free or greaseless bearings as they require no relubrication or grease.
An important distinction to make is that self-lubricating bearings are not bearings that come pre-applied with grease or oil lubricant – these bearings are instead referred to as prelubricated bearings. Prelubricated bearings will require relubrication at some point in their service life.
Self-lubricating bearings work by having lubricant impregnated within the sliding layer of the bearing. This lubricant can either be liquid (oil) or solid (graphite, MoS2, lead) based on the requirements of the application (such as operating temperature). As the bearing operates, the lubricant is released through pores in the sliding layer, lubricating the bearing surface. The lubricant is uniformly dispersed throughout the sliding layer and thus there is no reduction in low friction bearing performance, even if the sliding layer becomes worn. A “running-in” surface is also usually included at the top of the sliding layer to provide low friction bearing performance at start up before the impregnated lubricant reaches the bearing surface.
