Not all plastic plain bearing materials are engineered equal
07 March 2017
A key benefit of plain linear bearings is that they are simple to use. In comparison to recirculating ball bearings, which require oil or grease lubricants and periodic maintenance, plastic plain bearings are self-lubricating and therefore run maintenance free.
However not all plastic plain bearing materials are the same. In this article Robert Dumayne, director, igus, discusses how the choice of bearing material can affect the performance and service life of plain linear bearings.
Plastic plain linear bearings have been widely used in industrial applications for many years. They are relatively inexpensive, in comparison with recirculating ball bearings, and even though they are compact, have a high load carrying capability. Requiring no lubrication, they deliver lower cost of ownership due to reduced service levels and are better suited for environments that are dirty or wet. They can also be used without the risk of contaminating food during processing or chemicals in laboratory equipment. Other benefits include that they weigh less, can withstand greater load shocks, both dynamic and static, and they perform well under high accelerations.
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 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. On the outer surface, raised areas help avoid anti-rotation and maintain axial security. When used with the aluminium sleeve, the bearing has the same installation dimensions as a standard recirculating ball bearing.
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. Tests were conducted at the 2750m2 test lab igus operates in Cologne, Germany, the largest laboratory of its kind for testing plastic bearing, cable and energy chain technologies. All shafts had a diameter of 20mm, a 400N centric load, with a linear speed of 0.1m/s. Wear was calculated by measuring the mass of material lost after travel, with the different material densities accounted for.
Case Hardened Shafting: E7 = 23% lower wear and 29% lower friction
440C Stainless Shafting: E7 = 51% lower wear and 18% lower friction
316 Stainless Shafting: E7 = 33% lower wear and 24% lower friction
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 percent 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. A key benefit of iglidur bearings is that there is a material optimised for use with almost every type of shaft material.
With material selection being such an important factor in the performance and lifetime of a plastic linear bearing, igus recommends engineers consult the manufacturer for its recommendation. igus has developed the online iglidur expert selection tool, which uses a wealth of test database and field data to accurately forecast a bearing’s lifetime, based on the specified application requirements. For more complex applications or special conditions, igus technical experts are readily available to assist with material selection.
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