Bearings: plastic versus metal
16 October 2017
High-performance plastic bearings have replaced their metal counterparts across a range of applications. The motives for engineers to choose plastic over metal are numerous; Robert Dumayne, dry-tech director, igus highlights some of the key drivers
All over the world, design engineers are installing plastic bearings as long-lasting and cost-effective alternatives their metal counterparts. This is due to their special material properties as well as their excellent performance in motion, where wear is unavoidable.
To this end, igus has its own material development department, which is continually coming up with new iglidur materials to match specific areas of use. These engineering plastics consist of thermoplastic polymers that ensure the bearing's good basic resistance to wear. Fibres and filling materials increase the bearing's load-carrying capacity. In addition, solid lubricants are included in the material. All the components work together in this homogeneous structure, resulting in very good sliding and abrasion properties.
In contrast, rolled metal bearings have a layered structure. The inner layer is very thin and, in the event of high loads, edge pressure or vibrations, can easily be stripped off or damaged by dust particles. Moreover, the sliding layer represents a limit to the degree of possible wear. The homogeneous composition of the plastic compounds is considerably more robust as the wall thickness of the bearing acts as a ‘wear zone’.
Unlike iglidur bearings which are self-lubricating, bearings made of metal alloys have to be lubricated, a disadvantage that is cost-intensive and also time-consuming. What's more, lubrication with grease or oil is becoming increasingly unpopular for hygienic and ecological reasons.
Sintered bearings have oil lubrication incorporated into them and, are therefore not dry-operating. The same applies to roller bearings, which, especially in the case of needle bearings, are often being replaced with iglidur plastic bearings for technical and economic reasons.
The iglidur bearings that are used in agricultural machinery are extremely resistant to wear and can cope with high surface and edge pressures. They are non-corrosive and resistant to impacts and knocks. Fertilizers, slurry or fuels have no effect on their ability to function. The bearings are self-lubricating and maintenance free, therefore the machines can be used reliably even after being out of use for a long time – especially where machines can be exposed to extreme amounts of dirt.
igus has developed special materials in the form of iglidur Q2 and Q290, which are suitable for the slow pivoting applications that involve high loads and are typical in the agricultural sector. Bearings made of the low-cost iglidur G are also insensitive to dust and dirt and are therefore suitable for agricultural machinery.
The food industry
In the food and packaging industry, the absence of lubrication also plays an important role in view of the strict hygienic and safety standards in these sectors. According to the regulations of the Food and Drug Administration (FDA), it must be ensured that products are not contaminated by lubricants or other substances. As a consequence, only plastics that are FDA-compliant and suitable for use with food are used in these sectors.
In contrast to metals, plastics are always non-corrosive due to their organic structure and are also resistant to anorganic media, including acids, lyes and aqueous salt solutions.
For most typical bearing applications, five standard iglidur materials are used. However, demand is growing for materials that have been adapted to particular production environments; in the sterile environment of medical and laboratory equipment, for example, devices must not only be free of oil and grease but anti-bacterial in nature. In turn, other sectors demand materials that are insensitive to air-borne dust particles and fibres; in the textile industry, for example, antistatic plastics that are electrostatically dissipative are used to ensure that statically charged bearings do not attract any lint or dust.
From underwater use to high-load applications, from medical technology to the automotive industry, igus has developed tailor-made materials for all these cases. Which material is the right one for the respective use can be quickly determined with the help of online tools such as the Product Finder, the Configurator or the Service-life Calculator.
In all branches of industry, whether in the agricultural and offshore sectors, facade technology or in hydraulic cylinders and actuators, polymer bearings have been replacing metal bearings for a long time now. New materials, and properties are continually opening up further possible uses for polymer bearings. Given that development in the area of shaft materials and surface treatment does not stand still, continuous tests that react to shaft changes and are aimed at a perfect matching of shafts and bearings are absolutely critical. In addition to the cost benefits associated with plastic bearings, the user can also utilise a non-hardened shaft and thus reduce costs even more.
Material of the future
Consistent continuation of the research and development work is necessary in order to be able to offer bearings that are of the best quality and, at the same time, are the most cost-effective. 3D printing is opening up new potential for igus and its customers as, with additive manufacturing and new kinds of high-performance material, two modern technologies that complement each other perfectly have now been brought together. In combination, they guarantee a high degree of freedom in engineering design and, at the same time, a high level of wear-resistance of the components that are used. In order to achieve this, igus is researching and developing new 3D materials that are specially designed for moving applications.
Since the first iglidur tribofilament was introduced in 2014, the range of products has been expanded continually. Given the six tribofilaments that are now included in the product range, bearings or more complex designs can not only be manufactured but can also be used directly in industrial applications. In addition, igus introduced the first tribo SLS powder for selective laser sintering at the end of last year.
But not all companies have their own 3D printer: For these cases, igus offers a 3D printing service that allows customers to print out their desired parts directly – anywhere in the world.
With 3D printing, igus can also make customer-specific injection moulds made of high-performance plastics in a very short time. With the SLS process, the corresponding tools can be produced in a very short time and used in the standard injection-moulding machine. With regard to injection-moulded parts, the customer is offered a large selection of tribologically enhanced iglidur materials whose parameters can be calculated online, including high-performance plastics that can be used in motion applications and are resistant to chemicals, temperature fluctuations or that can withstand especially high loads, can be used universally.
With the printed mould tools, the engineer gets the possibility of obtaining the special part quickly and cost effectively even in larger numbers whilst maintaining application suitability. This results in significant advantages, especially in the trial phase: in the test phase, the special part is very close to the production product and, at the same time, the costs are reduced considerably.