By Matt Mowry, Product Manager, drylin linear systems, igus Inc.
Linear plain bearings are becoming commonplace in today’s manufacturing industry. As technologies and material sciences have advanced, so have the capabilities of linear plain bearings, particularly those made of plastic, such as the drylin series of self-lubricating linear bearings from igus.
Originally, linear plastic plain bearings were developed to replace recirculating ball bearings. Unlike ball bearings, plastic plain bearings are designed to run without maintenance or external lubricants, like oil or grease. Not only is the upfront cost per bearing lower, so is the overall cost of ownership. There are also technical benefits to plastic bearings, the main advantage being that they are self-lubricating. Since they do not require any external lubrication they are well suited for dirty or wet environments, and are also ideal for sensitive food or laboratory applications, as they eliminate the risk of contamination. They are lighter weight, able to withstand higher static and shock
loads, are corrosion-resistant, and perform better than ball bearings in applications with high accelerations.
drylin Plastic Linear Bearing Design
drylin linear bearings are designed using dry-tech plastic composites. These consist of a base polymer, reinforcing fibers, and particles of solid lubricants. The bearings are injection molded, homogenously blending the plastic material, and exhibit constant coefficients of friction and self-lubricating properties throughout their lifetime (see images below). As the bearing system begins to cycle, tiny deposits of solid lubricant are transferred into the microfinish of the shaft, lowering friction, increasing lifetime, and eliminating the need for additional external lubrication.
The geometry of drylin linear plastic bearings also makes them unique. Grooves along the bearing’s inner diameter serve two purposes. First, they minimize thermal expansion and result in a more tightly toleranced running clearance than a typical sleeve bearing. Additionally, they act as “dirt channels” in aggressive environments by allowing debris to pass easily through the system.
The bearing liner is also molded with a lengthwise slit, enabling an easy, tool-free, “slip fit” installation into the housing bore. drylin liners are molded with features on the outer diameter to allow for anti-rotation and to maintain axial security. The housing, or adaptor, is an aluminum sleeve, giving a bearing the same installation dimensions as standard recirculating ball bearings.
Polytetrafluoroethylene (PTFE)-lined metal bearings are another option for linear bearing applications. A typical bearing of this type is made up of a thin, self-lubricating layer, sometimes gold in color, embedded with PTFE, which is permanently bonded to an aluminum housing. It should be noted that the liner is not replaceable. Just like drylin, their outer shells are designed for dimensional interchangeability with linear ball bearings.
A new dry-tech material has been added to igus® linear technology, the E7 material. E7 was engineered for optimized service life and low friction on ferrous shafting; in particular, case-hardened steel or hardened stainless steel, which are common materials in the industrial marketplace. The E7 material is also effective on softer stainless alloys, such as 303/304/316, which offer superior corrosion resistance.
Laboratory Test Comparing E7 to PTFE-Lined Bearings
Testing was conducted to compare the friction and wear properties between the E7 material and PTFE-lined bearing technologies at the igus test lab, located in Cologne, Germany.