Plastic plain bearing technology in Auto industry
Release date：2008-06-23 Total：1997bout
Bearings can be classified as rolling bearings and sliding bearings on the basis of their operating principle. When operating, rolling bearings give rise to a rolling friction couple, while sliding bearings give rise to a sliding friction couple. Because of this, the coefficient of friction of rolling bearings and sliding bearings differs by at least ten orders of magnitude.
At any certain load and rotation speed, without lubrication, the coefficient of friction of rolling bearings will be largely determined by manufacturing precision of the balls and inner and outer bearing rings, and their mutual gap. Since the amount of friction is not directly correlated with the bearing material, rolling bearings ordinarily require the regular application of lubricants.
At the same load and rotation speed, without lubrication, the coefficient of friction of sliding bearings will be largely determined by the self-lubrication performance of the bearing material. Because sliding bearing materials tend to have excellent self-lubrication performance, sliding bearings can operate for extended periods of time with little or no added lubricant. Because they incorporate self-lubrication technology, sliding bearings can achieve maintenance-free operation, which is why they are increasingly used by design engineers.
Sliding bearings' lubrication mechanism and limiting PV value
When sliding bearings begin working, the sliding friction between the shaft and the bearing contact surface, which is the working surface, will create a lubricating film on the surface, and this will cause the motion pair surfaces to separate. As a result, after a short break-in period, sliding friction will drop rapidly, and the motion pair surfaces will be in a state of indirect contact, which will avoid wear.
Plain bearings offer a large load-bearing capacity and high rotational precision, and the lubricating film enhances impact-resistance. Because of these advantages, plain bearings are widely used in engineering applications.
The formation of a lubricating film is a necessary condition for sliding bearings to function normally. Factors influencing the formation of a lubricating film include the lubrication method, the relative speed of the motion pairs, the physical characteristics of the internal lubricant, and the roughness of the motion pair surfaces.
An important indicator of sliding bearing performance is the maximum limiting PV value, in which the P refers to the bearing's load (expressed as pressure) and V refers to the bearing's linear velocity. In sliding bearings, the greater the limiting PV value, the better the bearing's performance and the longer its service life. Major factors affecting the limiting PV value of sliding bearings include the self-lubrication performance of the bearing material and its bearing capacity and heat resistance. Major external factors affecting the actual PV value of sliding bearings include the load on the bearing, its operating speed, the ambient temperature, and the fitting precision.
In the production design of sliding bearings, a higher limiting PV value is ordinarily better. However, in the use design of sliding bearings, the smaller the PV value, the easier it is to extend the bearings' service life.
In accordance with the bearings' working conditions, the design of sliding bearings generally involves determination of bearing structure, selection of suitable materials, and confirmation of the bearings' geometric parameters.
Sliding bearing technology and R&D directions
Sliding bearings can be classified on the basis of material as metallic and non-metallic bearings. Metallic sliding bearings are usually classified as composite sliding bearings and metal-based sliding bearings. Non-metallic sliding bearings can be classified as Plastic plain bearings and filament-wound bearings.
The structure of composite sliding bearings ordinarily consists of three layers: substrate + copper powder layer + friction reduction layer. When in operation, the lubricating substance will slowly migrate to the surface of the grinding axis, and a friction couple will form between the lubricating films of the bearing friction layer and the surface of the grinding axis. This will reduce the coefficient of friction and the rate of bearing wear.Metal-based sliding bearings are generally classified as solid lubricant embedded sliding bearings (CSB 650 series), bimetallic sliding bearings (CSB 800 series), and solid bearings.Non-metallic sliding bearings represent a current R&D direction in sliding bearings technology research; types of non-metallic sliding bearings include polymer plastic plain bearings (CSB-EPB series) and filament-wound bearings (CSB-CRB series).
The development of polymer plastic sleeve technology has encompassed almost all polymer modification techniques.
From the perspective of manufacturing process, we can distinguish compression sintered Plastic plain bearings and injection molding Plastic plain bearings.
Because compression sintered sliding bearings generally suffer from the problems of low efficiency and poor manufacturing precision, they have little room for development, and are not suitable for large-scale production.Plastic modification technology and injection molding Plastic plain bearing technology are currently making great strides around the world. In China, CSB self-lubrication modification technology is based on more than twenty years of research and development.
CSB uses advanced polymer modification technology to add special lubricants and functional fibers to high performance polymers. The high performance polymer substrate provides wear resistance, the special lubricant provides lubrication and reduces the coefficient of friction, and the fibers and filler have a strengthening effect.
Because the material from which polymer sliding bearings are made is a modified material with self-lubricating properties, these bearings are superior to metallic composite bearings in terms of friction performance and service life (composite bearings have a layer of friction material only roughly 0.03 mm in thickness).
CSB currently provides CSB-EPB5 high performance polymers plastic plain bearings, which can withstand extended use at a temperature of 250°C and resist almost all chemical substances.Thanks to their extremely low water absorption and high corrosion resistance, CSB-EPB4 bearings can be used underwater for extended periods. The sliding film formed on CSB-LIN linear bearings has an extremely low coefficient of friction and permits a maximum velocity of 10 m/s.
At present, the polymer sliding bearings most commonly used in the auto industry are CSB-EPB general purpose bearings and CSB-EPB3 high-strength Plastic plain bearings.
The new CSB-CRB series of high-load filament-wound bearings are non-metallic sliding bearings. In these bearings, the load-bearing layer consists of heat-resistant epoxy resin reinforced with high-strength glass fibers, and the sliding layer consists of special fibers and PTFE fibers, which gives the bearing superior abrasion resistance and very low coefficient of friction when operating at high load and low speed.
Technological overview of sliding bearing applications in the auto industry
Thanks to sliding bearings' outstanding self-lubrication, they have already starting appearing in widespread auto industry applications.
CSB sliding bearings are currently used by almost every automaker worldwide. In particular, polymer Plastic plain bearings are growing increasingly ubiquitous in motor vehicle applications, and can be used in many types of auto components. These applications range from the simplest car door hinges to elements with complex mechanical structure and stringent requirements, such as the steering gears in auto steering systems and gear shifts in transmission systems.
Apart from the advantages of superb mechanical performance, freedom from lubrication, and ability to operate dry, Plastic plain bearings also feature very low prices.CSB-EPB polymer Plastic plain bearings are less expensive than PTFE-coated metal sliding bearings by as much as 50%. CSB-EPB3 high-strength Plastic plain bearings used in auto door hinge systems can overcome the tendency of metal sliding bearings to rust, which makes the doors noisy or harder to open. They can also prevent the brief jamming that may occur when car doors or trunks are opened and closed.
Sliding bearings used in auto shock absorbing systems must meet extremely strict wear resistance, impact resistance, and fitting precision requirements.
If sliding bearings have poor wear resistance, this will lead to increased seal ring wear, and faulty seal rings will cause hydraulic fluid leakage and loss of shock absorbing function. CSB-40 composite bearings (DP4 material) can withstand this formidable challenge, and have been successfully used with various types of auto shock absorbers.
The use of CSB-RPM5 support sleeves in auto steering systems can ensure that the steering wheel turns smoothly throughout the driving process and the steering system operates normally even when road conditions are very poor.The excellent abrasion resistance and shock absorbing ability of CSB-RPM5 bearings have been verified in auto steering systems.
The use of CSB-EPB4 high performance polymer plastic plain bearings in auto brake systems ensures that drivers enjoy smooth and stable braking, and can brake smoothly and quickly even in emergencies. The bearings can also maintain effective braking no matter whether a car is making a smooth turn or an abrupt emergency turn. The hardness and wear resistance of the EPB4 bearing material ensures an ultra-long service life. These properties are attributable to the high-performance lubricant and functional fibers that are incorporated in this material.
Due to large dimension differences between the sheet metal material or holes punched in auto seat systems and cold rolled steel sheets, the smallest gaps are required within seat systems in order to minimize noise. In order to resolve this problem, CSB-EPB series Plastic plain bearings produced by CSB are increasingly used in car seat systems.
With the continuing improvement of self-lubricating material modification technology, the applications of sliding bearings are evolving at a rapid pace. At present, sliding bearings have been successfully applied in such areas as civil machinery, military equipment, and aviation/space equipment. As sliding bearing applications increase and operating requirements are tightened, we will continue to explore, study, and develop sliding bearing material technologies, processes, and applications.