Not all bearing materials are created equal

The choice of plastic material used for a linear bearing can have a significant effect on its performance during service. For example, the plastic can act as a thermal insulator and deform. Another issue is that in the presence of moisture the plastic material can swell, resulting in higher friction.

As an alternative, composite bearings typically comprise a metal backing and either a spray-on or bonded plastic liner. The typical type of plastic material used is PTFE (polytetrafluoroethylene), which is chemically inert, able to withstand the effects of weather, and has a high temperature resistance and low coefficient of friction. In this design there is no metal-to-metal contact and the PTFE is self-lubricating, eliminating the need for maintenance. The metal backing draws the heat from the plastic, eliminating the problems associated with unwanted heat build-up. However, composite bearings can be limited by their speeds or when carrying a cantilevered load.

Recent advancements in both technology and material sciences have increased the capabilities of plastic linear plain bearings. The wide range of tribologically-optimised iglidur polymer materials available from motion plastics specialist, igus provides a plastic bearing option for just about any application. Load, speed and shaft material specifications are all important criteria for selection.

The mechanical design of drylin linear plastic bearings is unique, with grooves on the inner surface running down in parallel with the shaft. These grooves help minimise thermal expansion, resulting in tighter tolerances than a traditional sleeve bearing. When used in dusty or dirty environments, debris can easily pass through the bearing without affecting operation. The use of a slit down the length of the bearing enables easy installation, requiring no tools.

A new dry-tech plastic, the E7, has been added to the igus linear technology portfolio. This was engineered for use with commonly used ferrous shafting; in particular, case-hardened steel, hardened stainless steel and softer stainless alloys, such as 303/304/316. It too offers optimised service life, low friction and maintenance free operation.

To compare and contrast the performance of a composite PTFE-lined bearing against the new E7 bearing, igus tested both for wear on a selection of common shaft materials, on a test stand with single bearings under a centric load. All shafts had a diameter of 20 mm, a 400 N centric load, with a linear speed of 0.1 m/s. Wear was calculated by measuring the mass of material lost after travel, with the different material densities accounted for.

It is worth noting that the E7 liner is noticeably thicker (1.4mm) than the PTFE-lined bearing (0.5mm). When this is factored in along with the 23 - 51% lower wear per kilometre of travel, the potential operating lifetime of the E7 material is considerably greater than the PTFE-based material. The benefit of this would be particularly seen in high cycle applications, where the need for replacement parts would be lowered significantly.

The igus E7 material is able to slip-fit into the aluminium adaptor, allowing for re-use of the aluminium adaptor with the simple replacement of the liner if necessary. Conversely, the PTFE lined bearing is bonded to the adaptor, meaning the entire product must be replaced, as replacement of the liner itself is not possible. The ability to replace the bearing liner, along with the reusability of the aluminium adaptor, mean that the overall cost to purchase and replace the igus E7 bearing liner is significantly less than the bonded PTFE-lined bearing.

The PV (pressure-volume) relationship, also known as measured frictional heat, is critical to the suitability of a linear bearing for a given application – iglidur plastic bearings are typically intended for high load/low speed or low speed/high load applications. For food and drink applications, for example, iglidur A181 material is FDA-compliant and suitable for most washdowns. For higher temperatures or more aggressive chemicals, iglidur A350 and A500 materials may be more appropriate. On the other hand, for cleanroom environments, where outgassing can be an issue, igus recommends its igidur X material.