Plastic bearings and staying power
17 June 2015
High-performance plastic bearings are continually working to forge a path into almost every industry – from packaging machines and medical devices, to automotive and many more. However, there are still sceptics in the engineering community due to the misconception that plastic is inferior or weaker than metal. Robert Dumayne, director at igus, looks at how plastic bearings compare to sintered-bronze and PTFE-lined bearings
Oil is drawn from a sintered-bronze bearing as it rotates on the shaft, creating a thin film to lubricate the bearing and shaft, preventing wear and shaft damage. At high speeds, a low coefficient of friction (COF) is achieved but at slow speeds, or when there’s oscillation on the shaft, uneven loads, or due to irregular use the lubrication effectiveness can be reduced; as a result, the COF and wear rates can increase.
Issues can also occur if movement stops completely; the oil on the bearing surface can dry up and ultimately cause higher friction and squeaking. High frictions results in higher temperatures, which can break the oil down. In addition, when oil is present on the shaft it can act like a magnet for dust, dirt and airborne debris, which increases potential for seizing up the bearing or contaminating a product/process, especially in food or medical applications.
igus tribopolymer bearings comprise three basic elements: base polymers, responsible for wear resistance; reinforcing fibres, which make the bearings suitable for high forces and edge loads; and solid lubricants, which are blended into each material to reduce friction making the bearing self-lubricating. These plastic bearings regularly deliver a longer service life and cost savings of up to 40% over bronze bearings. They often outlast their metal equivalents, and are more economical because no lubrication is required, eliminating the need for routine maintenance.
A basic difference between plastic bearings and bronze bearings is the wall thickness. A common misconception is that the thinner wall thickness would make it inferior to bronze, however the wall thickness of either bearing type does not directly correlate to its strength. Even with thin walls, plastic bearings perform equally as well, if not better than their thick-walled counterparts. Other factors that should be taken into consideration include the weight, COF and wear capabilities.
A PTFE-lined steel backed bushes are kind of the halfway house for the plastic bearing sceptic. These types of bearings comprise a metal shell with very thin polymer coating (PTFE) applied to the inside. They typically have a maximum wear surface thickness of 0.06 millimetres; over time, this protective coating is stripped off during operation, creating a metal-on-metal contact between the bearing and shaft, which can lead to serious damage.
In comparison, plastic bearings are comprised of advanced compounds, which contain solid lubricants homogeneously within the material thus lowering the COF and wear, and unlike PTFE-lined bearings, plastic bearings eliminate the danger of metal-on metal contact. This is a benefit since the acceptable amount of wear can be determined by the type of application.
In applications where water or caustic chemicals are present, the metal shell of a PTFE-lined bearing can corrode and contaminate sensitive areas and ultimately fail. Since plastic bearings are made solely of high-performance polymers, they offer both corrosion and chemical resistance and thus operate unaffected. In addition, even after limited exposure to moisture, parts of the PTFE-lined bushings’ metal shell can begin to peel off which cements the argument for replacement with plain plastic bearings.
I believe composite plastic bearings can outperform traditional solutions in countless rotary, oscillating, and linear-motion applications. In addition, they are readily available in many different styles, sizes, materials, and colours to meet the demands of almost any industry. While most plastic bearings, such as those from igus, can endure extreme temperatures, heavy loads and high speeds, it is still important to understand all of the options available.