Plastic bearings have more staying power

by Rob Dumayne, director, igus®

During the past three decades, there has been an evolution in the development and application of engineering plastics in bearing applications. Today’s plastic bearings are replacing their metal equivalents and, while costing and weighing less, what many design engineers still do not realise is that they also often last longer in harsh environments.

High-performance plastic bearings are continually working to forge a path into almost every industry; from packaging machines and medical devices, to automotive, farming equipment, textile machinery, and many more. Plastic plain bearings are an economical replacement for needle, ball, and plain metal bearings. However, they are often not considered a viable choice in the engineering community due to the common misconception that plastic is inferior or weaker compared with metal.

The truth is that composite plastic bearings can outperform traditional solutions in countless rotary, oscillating, and linear-motion applications. In addition, plastic bearings are readily available in many different styles, sizes, materials, and colours to meet the demands of almost any industry. While most plastic bearings, especially the examples from igus®, can endure extreme temperatures, heavy loads and high speeds, it is still important to understand all of the options available. 

Plastic bearings vs. bronze bearings

With sintered-bronze bearings, oil is drawn from the bearing as it rotates on the shaft (minimum speed of 1 metre/second). The oil creates 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. Shaft oscillation, slow speeds, irregular use or uneven loads can impede oil lubrication from being maintained. As a result, the COF and wear rates can increase. In addition, if movement stops completely, the oil on the bearing surface can dry up and ultimately cause higher friction and squeaking. High temperatures can also break the oil down. When oil is present on the shaft it can act like a magnet for dust, dirt and airborne debris, which has increased potential for seizing up the bearing or contaminating a product or process, especially in food or medical applications.

igus® has developed more than 120 different plastic compounds, each comprising three parts: base polymers, which are responsible for the resistance to wear; reinforcing fibres, which make the bearings ideal for high forces and edge loads; and solid lubricants, which are blended into each material to reduce friction and eliminate the need for any external lubrication. These plastic bearings regularly deliver a longer service life and cost savings of up to 40 percent over oil-impregnated sintered bronze bearings. Not only do they outlast their metal equivalents, plastic bearings prove to be more economical because no grease or oil is required, thus eliminating the need for routine maintenance.

In addition to the material, a basic difference between plastic bearings and bronze bearings is the wall thickness. A common misconception held by engineers is that the thinner wall thickness of the plastic bearing would make it inferior to bronze – the wall thickness of either bearing type does not directly correlate to its strength. Other factors that are more important and should be taken into consideration include the weight, COF and wear capabilities of the bearing. In addition, due to their thin walls, plastic bearings not only offer a number of benefits, but also perform equally as well, if not better than their thick-walled counterparts.

Another mistake is to assume the thin wall of press-fit plastic bearings will affect the surface pressure; surface pressure of a press-fit bearing is determined by the load divided by the surface area it acts on: P= L/(D × l) where P = surface pressure; L = load; D = inside diameter; and l = bearing length. Whether using a plastic bearing or a bronze bearing, wall thickness has no effect on surface pressure capabilities.

Another common misconception is that a thin-walled plastic bearing has a shorter life than its thick-walled bronze equivalent. It is reasonable to assume that since a plastic bearing has less material, it will not last as long as a bronze bearing; this is incorrect because the thin wall of a plastic bearing helps to disperse any heat build-up, which actually prevents wear. 

In high speed applications, continually re-lubricating the bronze bearing will help prevent wear. However, if a bronze bearing is being used to facilitate other types of motion; excessive wear can lead to increased clearance between the shaft and the bearing. If this happens, a number of problems will arise. It is important to remember that wear is dependent on the makeup of the bearing alone. For this very reason, igus® is constantly developing new plastic materials, which minimise wear and provide a long-lasting, maintenance-free solution for a variety of applications. 

Plastic plain bearings vs. PTFE-lined plain bearings

A PTFE-lined steel backed bush is comprised of a metal shell and a very thin polymer coating (PTFE) applied to the inside. These types of bearings typically have a maximum wear surface thickness of 0.06 millimetres, but as the PTFE coating is stripped off during operation, the metal shell becomes exposed. This creates a metal-on-metal effect between the bearing and the shaft and can cause serious damage. This problem is common when high edge loads or oscillating movements are present.

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 huge benefit since the acceptable amount of wear can be determined by the type of application (unlike the PTFE-lined bushing, which will fail if the wear rate surpasses 0.06 millimetres).

When using a heavier, PTFE-lined bearing, no matter what material it is comprised of, more energy is required for it to operate – this can cause problems, especially in automotive and aerospace applications. In contrast, plastic bearings are seven times lighter, which helps lower energy consumption and lower the carbon footprint. 

The metal shell of a PTFE-lined bearing is not ideal for applications where water or caustic chemicals are present. In these types of conditions, the bushings can rust, corrode, 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. 

Successful Applications

Plastic bearings have already replaced plain metallic bearings in thousands of applications from a wide range of industries, including agricultural machinery, medical equipment, fitness equipment, packaging machinery and more. For example, leading boat lift company, Quality Boat Lifts, is using iglidur® plastic plain bearings on lifts used by residential and commercial marinas, including private owners, to remove boats from the water. The iglidur® bearings are used in the direct-cable winding drive shaft, which raises and lowers boats in and out of the water and were chosen for their low moisture absorption, low wear rate and high load capacity. The lift uses up to 20 iglidur® bearings to handle the axial loads it encounters — which can range from 2 to 30 tonnes. 

In another example, engineers at a specialist manufacturer of standard and custom hydraulic cylinders, Wye Cylinder Engineering, chose to switch to iglidur® plastic bearings due to their improved reliability, even in demanding environments, and their maintenance-free operation. Wye Cylinder Engineering uses iglidur® bearings in a pivot point that provides articulated coupling at the end of a hydraulic cylinder shaft. While the company uses an extensive range of iglidur® bearings depending on the requirements of its customers, it most frequently uses plain sleeve bearings from the cost-effective general-purpose iglidur® G range.

The ability to predict bearing life

igus® offers a service-life calculator called The Expert System for its iglidur® plain bearings and dryLin® linear bearings. These convenient tools are based on an extensive polymer test database and have been verified with thousands of hours of actual testing that make it possible to predict plastic-bearing life under almost any operating condition. 

The Expert System delivers trusted results, but igus® does encourage testing the selected bearing in the proposed application before releasing a machine to the market. igus® supplies free test samples and advice through its expert sales engineers to select the best material and design for a given application.