Feature Bearings

Bearing the material in mind

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There are so many bearings for so many applications and situations, each of which has a different set of advantages and disadvantages. Matthew Aldridge at igus UK asks: does the material of the bearing make a difference to its performance?

ne of the earlier ball bearings developed was made out of wood! It was used to support a rotating table and, for the neces-

sary load and wear requirements, it was the designer’s material of choice. As the type of material used does

have an affect on bearing performance, its choice largely depends on what the bearing is going to be used for. For many years, engineers have

trusted in the robust and reliable quali- ties of metals, such as stainless steel and bronze, to fulfil their bearing needs. Moving on, and ceramics have come to the fore due to their ability to withstand exceptionally high speeds and rapid accelerations. For many bearings periodic mainte-

nance to prevent premature failure is paramount. This is especially true for bearings used in high cycle opera- tions, where periodic lubrication and cleaning, as well as the occasional adjustment, are needed to minimise the effects of wear. Over and over again the main

reasons cited as the causes of bearing failure are lubrication issues. Whether it is because of improper maintenance or contamination of the lubrication system, bearing failure often leads to significant periods of machine downtime – resulting in losses in production. Plastic bearings have been around for

some time now and, in many applica- tions, they can be used to eradicate these issues. Not only are they resistant to dirt, dust and chemicals, but they are self-lubricating and can endure high temperatures, heavy loads and high speeds. Some engineers, however, hesitate to use plastic bearings in their designs because of the extensive know- how and experience they have already with the more conventional materials. So let’s take a closer look at some traditional plain bearing types.

Preventing friction

With sintered-bronze bearings, oil is drawn from the bearing as it rotates on the shaft (minimum speed of 61m/s). The oil creates a thin film that then separates the bearing and shaft, pre- venting wear and shaft damage. At high speeds, a low coefficient of friction (COF) is achieved. However, shaft oscil- lation, slow speeds and irregular use or

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iglidur bearings can provide an alternative to expensive bronze, metal-backed and custom

uneven loads can impede film lubrication from being maintained. As a result, the COF and wear

rates can increase dramatically. Metal-backed PTFE-based (polytetra-

fluoroethylene) polymer bearings on the other hand have a steel backing bonded to a porous bronze sinter layer. The layer is impregnated and overlaid with the filled PTFE bearing lining which in ideal conditions provides a very low COF. Unfortunately this thin lining can be scratched off quite easily by contaminates, causing metal-to- metal contact between bearing and shaft, resulting in increased COF and higher wear rates and shaft damage. Self-lubricating polymer bearings,

such as those available from igus UK, contain solid lubricants embedded in millions of tiny chambers of the mostly fibre-reinforced material. During operation, the bearing transfers lubricant onto the shaft to help lower the COF. Unlike a sintered- bronze bushing, polymer bearings release solid lubricants as soon as the bearing or shaft is set in motion. The fibre-reinforced materials inside the bearing withstand any high forces or edge loads. In addition, because of the design flexibility of plastic bearings, these can also be used on many different shaft types. High-performance plastic bearings

must not be confused with plastic bearings from, say, a local injection moulder. With iglidur plastic plain bearings, you can accurately calculate the life of a bearing according to wear rates, actual testing results and specific application parameters. igus iglidur plastic plain bearings constitute

injection-moulded bearings

the step from a simple plastic bushing to a tested, predictable and available machine component. They offer many advantages, but these five would make the top of any engineer's list: • Freedom from maintenance: Plastic

plain bearings can replace bronze, metal-backed and custom injection- moulded bearings in almost any application. Their resistance to dirt, dust and chemicals make plastic bearings a 'fit-and-forget' solution. • Cost savings: Plastic plain bear-

ings can reduce costs by up to 25%. They feature high wear resistance, a low coefficient of friction and can replace more costly alternatives in a variety of applications. • No messy lubricants: Self-

lubricating bearings transfer lubricant onto the shaft to help lower the coeffi- cient of friction and are impervious to dirt, dust or other contaminants. • Consistent COF: Plastic bearings

are designed to maintain a low COF consistently over the lifetime of the bearing. Compared to metal-backed bearings, which can become scratched and increase the COF, plastic bearings often last longer. • Corrosion and chemical resistance:

Plastic bearings can be used in wash- down applications, using salt water and harsh chemicals without compromising performance. Water can even be consid- ered as a lubricant for plastic bearings.

iglidur

self-lubricating plastic bearings deliver long life at lower cost and are totally

maintenance-free

Meeting design requirements

So, if you are looking for a bearing to improve the uptime of your equipment and reduce maintenance or lubrication costs, perhaps metal and bronze may not meet your needs. Over the years, igus has collected

real-world test data from thousands of designs and used it to develop an extensive database. Available online, the ‘Expert System’ allows an engineer to enter the maxi- mum loads, speeds, temperatures, and shaft and housing materials of their design. The system then references this information to the test data to recommend the most appropriate plastic bear- ing, with expected lifetime.

igus T: 01604 677240 Enter 236 www.igus.co.uk

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