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Plastic Bearings — The Sensible Alternative

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Bearing

For some applications, there has been a sensible alternative to bearings made of steel and stainless steel for some years now: plastic bearings or polymer bearings.

Bearings are components that decouple moving machine parts from each other. Their task is to enable movements and protect the machine parts from wear. If you compare the different types of bearings, it becomes clear that the variants have advantages and disadvantages.

According to the type of friction, bearings are divided into the roller and plain bearings. roller bearings transmit the forces that occur between the parts that move in relation to each other with the help of roller elements. They are mainly made of hardened chrome steel, more rarely of case-hardened steel, ceramic or plastic.

Lubrication behaviour

In order to minimise friction and wear, needle bearings require continuous lubrication with grease or oils due to the roller elements being made of metal. As soon as the lubricant fails, material damage occurs, up to and including so-called seizure. 38.5 per cent of all roller bearing damage occurs due to incorrect or insufficient lubrication, for example when a machine is at a standstill for a longer period of time. When the machine is started up, the lubricant first has to be distributed again. 

The bearing runs hot, noise increases and the lubrication fails. Another reason for the damage is insufficient sealing: if the lubricant is lost, the system runs dry. Furthermore, penetrating dirt and dust shortens the service life of needle bearings, as the contaminants mix with the lubricant and thus get into the raceway. Bearings made of high-performance polymers, on the other hand, are completely lubricant-free. They consist of a thermoplastic base material that provides good wear resistance. 

To increase the mechanical load capacity, fibres and fillers are added to the material matrix. In addition, there are embedded solid lubricants that are released as soon as the mating partner moves along the sliding surface of the plain bearing.

Vibration

In addition, Iglidur bearings are resistant to shock loads and can absorb vibrations. With needle bearings, on the other hand, high edge pressures occur between rollers and raceways during tilting due to their rigidity, which leads to wear. Although this can be mitigated by crowned profile ends, it cannot be avoided. 

Impacts also create a so-called brinelling effect - the bearing race deforms and rough running surfaces are created in the bearing, leading to increased noise and wear and even failure.

Corrosion resistance

In addition, the corrosion process can severely impair the function of needle bearings: If water, condensation or acids penetrate the bearing, this often leads to stoppages. In comparison, polymer bearings, depending on the choice of material, are characterised by low moisture absorption and are completely corrosion-free. Due to their organic nature, they are resistant to inorganic media, acids, alkalis and salt solutions.

Precision

The advantages of roller bearings are their high precision and rigidity. The more needle rollers are used, the more load-bearing capacity and rigidity the bearing has. In addition, the precision and rigidity can be adjusted by the bearing clearance depending on the operating ratio. 

Needle roller bearings show their strength in precision in machine tools or gearboxes. In many other applications, however, this high precision is not required at all. In addition, the high material input and the complex manufacturing process of needle roller bearings drive up the costs.

Weight

The low specific weight of polymers combined with good strength is an advantage of plastic bearings. In contrast to identical metal solutions, polymer bearings reduce the weight at each bearing point by a factor of seven. In the automotive industry, for example, where every gram counts, expensive needle bearings are therefore replaced by lighter and cheaper Iglidur plain bearings, for example in throttle valves.

Conclusion

When, instead of precision and rigidity, factors such as low weight, media and corrosion resistance as well as high robustness and service life, especially at small angles of motion, are particularly important, plain bearings made of tribo-polymers can show their strengths. Then they represent a real alternative to metallic needle bearings, both technically and above all economically.

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