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During the past three decades, there has been an evolution in the advancement and use of highly engineered plastics in bearing applications

INDUSTRIAL BEARINGS Plastic bearings have staying power

Nov 26, 2018

During the past three decades, there has been an evolution in the advancement and use of highly engineered plastics in bearing applications. However, they are often not believed to be a feasible option in the engineering sector. The article explores how plastic bearings last longer than their metal equivalents and are more cost-effective.

High-performance plastic bearings are working to shed their negative image and continue 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 to metal. The truth is, composite plastic bearings can outperform their metal counterparts in countless rotary, oscillating, and linear motion applications.

Plastic bearings vs bronze bearings

Maintenance-free plastic bearings regularly deliver a longer service life and a cost savings of up to 40 per cent when compared to oil-impregnated sintered bronze bearings. Plastic bearings not only outlast their metal counterparts, but prove to be more economical, as no grease or oil lubricants are required, eliminating the need for routine maintenance.

With sintered bronze bearings, oil is drawn from the bearing as it rotates on the shaft (minimum speed of 1 m/s). The oil creates a thin film which then separates the bearing and shaft, preventing wear and shaft damage. At high speeds, a low COF is achieved. Shaft oscillation, slow speeds, irregular use or uneven loads can impede film lubrication from being maintained. As a result, the COF and wear rates increase. In addition, if movement stops completely, the oil on the bearing surface dries up and causes higher friction and squeaking. High temperatures can also break down the oil. In addition, oil film on the shaft can act like a magnet for dust, dirt and airborne debris.

Common misconceptions

Despite the performance advantages, several misconceptions may make engineers reluctant to take full advantage of the benefits of thin-walled plastic bearings:

• The wall thickness of either bearing does not directly correlate to its strength. Other factors that are more important and should be taken into consideration include the weight, coefficient of friction and wear capabilities of the bearing.

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

• It is reasonable to assume that since a plastic bearing has less material (a thinner wall), it will not last as long as a thick-walled bronze bearing. This is incorrect because the thin wall of a plastic bearing helps to dissipate any heat buildup, which actually prevents wear. It is important to remember that wear is dependent on the makeup of the bearing material and not on the wall thickness. 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.

Reasons to replace PTFE-lined bushings

Plastic bushings are now designed to handle high speeds, loads, temperatures, caustic chemicals and a wide array of other application factors. Here are the top four reasons for replacing PTFE-lined bushings with plastic bushings, which offer more design flexibility.

Thinner wear surface

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

In comparison, plastic bushings are comprised of advanced compounds, which contain solid lubricants embedded in millions of tiny chambers throughout the material. During operation, lubricant is transferred onto the shaft to help lower the coefficient of friction and wear, and unlike PTFE-lined bushings, plastic bushings 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 mm).

Increased weight

PTFE-lined bushings weigh more than plastic bushings. When using a heavier bushing, no matter what material it is comprised of, more energy is required for the bushing to operate. This can be troublesome, especially in automotive, aerospace, recreational vehicle, and bicycle applications.

In contrast, plastic bushings are lightweight, which helps decrease fuel consumption and carbon dioxide output. The reduced weight can also help reduce carbon dioxide output, lower masses and subsequently, lower energy consumption.

In a weight comparison, an iglide® plastic bearing weighs approximately 80 per cent less than a PTFE-lined bushing.

• plastic bearing = 6.53 grams per piece

• PTFE-lined bearing = 34.02 grams per piece

No corrosion or chemical resistance

The metal shell of a PTFE-lined bushing is not ideal for applications where water or caustic chemicals are present. In these types of applications, PTFE-lined bushings can rust, corrode, contaminate sensitive areas, and ultimately fail. Since plastic bushings are made solely of high-performance polymers, they offer both corrosion and chemical-resistance and operate unaffected in those types of environments.

Reasons to use plastic bearings

All iglide® bearings contain thermoplastics, fibres and/or filaments, and lubricants, which have excellent anti-friction and lowb wear characteristics, producing a self-lubricating effect. This is especially important during the start-up phase of an application—with metal bearings, the lubricant film has not yet formed and the bearing begins operating dry, which can accelerate wear.

In comparison, iglide® plastic bearings are homogeneously impregnated with solid lubricant and run ‘lubricated’ from the start. During operation, plastic bearings transfer lubricant onto the shaft to help lower the COF. This also minimises slip-stick conditions and wear, as well as increases operating life, unlike with plain metal, ball, and needle bearings. Plastic bearings can also be used on many different shaft types.

In extremely dirty conditions, particles simply embed into the wall of a plastic bearing with little effect on performance. Plastic bearings offer other advantages as well, including the ability to withstand chemicals and certain types of corrosives, such as, hydrocarbons, alcohols and alkaline solutions. igus also offers FDA-approved plastic bearings that permit contact with food and pharmaceuticals.

Many engineers are also surprised to learn that plastic bearings can be used in high temperature applications. For example, certain igus bearings can operate continuously at temperatures approaching 250°C as well as withstand peaks upto 315°C and lows of –100 °C.

Plastic bearings also run quietly and absorb mechanical vibrations. The so-called mechanical loss factor, an indicator of vibration-damping capability, is up to 250 times higher than that of plain-metal bearings.

For applications where weight and fuel economy are an issue, for example, in racing bikes, snowmobiles, automobiles, and motorcycles, a thin-walled plastic bearing is ideal. The image below compares the weights of different bearing materials.

Successful applications

Plastic bearings have already replaced plain metallic bearings in thousands of applications, from a wide range of different industries, including agricultural machinery, medical equipment, fitness equipment, packaging machinery and more. Engineers are increasingly turning to plastic bearings in a wide range of challenging applications.

The argument for plastic

igus® designs and develops its high-performance, cost-effective iglidur® plastic bearings for almost any application. The company’s plastic bearings are an off-the-shelf solution, and are available in more than 120 different plastic compounds. Each material is comprised of three parts:

• Base plastics, which are responsible for the resistance to friction & wear

• Reinforcing fibers and filaments, which make the bearings ideal for high forces and edge loads

• Solid lubricants, which are blended into each material and eliminate the need for any external oils or grease

The ability to predict bearing life

igus® offers a service-life calculator called The Expert System for its lines of iglidur® plain bearings and DryLin® linear bearings. These convenient tools are based on an extensive tribological 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. Expert system calculates and delivers results, including life in hours and travel distance for various suitable products.

The expert system delivers trusted results, but igus® also encourages testing a 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.

Image Gallery

  • The edge load is usually a deciding factor for or against the use of bearings. iglide® G300 bearings solve this, also giving high wear resistance, low costs, resistance to corrosion and dirt

  • Reduction of the drive power for bottle washing machines by using plastic bearings under the most difficult conditions in a 2–3% caustic soda and temperature of +176°F

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