BNL USA Inc.
Foxboro, Mass.
Versatile thermoplastic bearings can be molded to any shape to meet both design and budget targets. Materials can also be chosen to withstand varied load, speed, and environmental conditions and design possibilities are more numerous than with traditional multicomponent steel assemblies. Semicrystalline materials are used instead of amorphous varieties, as they have higher yield stress, can withstand strain, and resist abrasion and chemicals. These characteristics are particularly useful where traditional steel bearings limit performance. Figure 1
Resistivity handles static
Thermoplastics offer other advantages as well. Rotating steel bearings can generate static electricity. This energy then discharges through arcs or sparks, which can cause serious electric shock and damage to electronic components, some of which is sensitive to just 20 V. In contrast, thermoplastics are insulators. Base polymers have high resistivity, and are adaptable to exhibit antistatic properties. Antistatic materials suppress initial charge buildup and minimize the charge generated by movement. This is particularly helpful in ATEX environments of explosive atmospheres. (Here, 4.5-m/sec steel-on-steel friction under force greater than 2 kN is classified as an effective ignition source.)
Another design option is thermoplastic bearings made with static dissipative compounds that pass electrostatic charges to ground. These discharge electricity but prevent electrostatic discharge on human contact suitable in paper-handling equipment that requires dissipation, to allow smooth flow of the paper without attracting polar particulates of paper dust. If more conductivity is required, material blends with carbon powder or other combinations provide more grounding.
Thermoplastics are nonmagnetic, so are more suitable than metal for applications where magnetic resonance is used, such as bankcard readers and MRI scanners. Metal can interfere with the effective operation of these devices, due to stray magnetic fields or false signals. Figure 2
Weight and inertia
Heavier thermoplastics (at 1,420 kg/m
Plastic-bearing rotational inertia is typically less than 10% of that of metal, with inertia variability and torque levels down to 0.041 Nmm, compared with 0.235 Nmm for metal. In contrast, steel bearings rely on greases for protection and smooth operation, though they increase torque draw and inertia — not to mention potential contamination.
Manufacturing
Two methods are traditionally used to manufacture plastic bearings: injection molding and computer numerical controlled (CNC) machining. Fully molded plastic bearings offer superior performance, though the process requires an initial investment and development time for mold tools, making it more suitable for large part batches. Figure 4
During injection molding, as the mold tool fills, the material in contact with the tool surface cools rapidly due to a large differential between material melt and tool-surface temperature. A surface skin of fine-grain crystals forms; lower mold temperature creates a thicker surface layer. The core filling is insulated by this outer skin and cools at a slower rate, growing into larger crystals. The fine-grain surface layer is resistant to fatigue wear while the large crystal filling provides support strength.
Fully machined bearings have a surface of exposed large crystals, which is not as wear resistant as a fine crystalline surface. Even so, where tool budget or volume is restrictive, parts can be produced by CNC machining. Machining can also sometimes be used to add raceways to a molded bearing if part geometry requires it.
Additional manufacturing techniques enhance plastic bearing life. Insert molding — where a part typically made of metal is placed in a tool and overmolded with plastic — can increase load-bearing capacity and wear resistance. Thermoplastic materials can also be blended or filled to improve rigidity or conductivity. (In comparison to virgin polymer, these are less wear resistant.)
True life
If operating conditions are well defined, a designer can accurately predict bearing life and use a product to suit requirements. How is this defined exactly? Bearing life is determined by criteria for failure. Generally this is the ability to resist load, speed, and environmental conditions, all of which limit the number of revolutions.
Sometimes, a subassembly or system is replaced or discarded before total bearing failure. For example, in an application that requires quiet operation, a steel bearing performs successfully until it becomes noisy, at which point it is replaced. It effectively reaches the end of its useful life, even though that is a small percentage of achievable life. Under identical conditions, a plastic bearing creates less noise, running quietly for longer due to the sound-deadening properties of thermoplastics: The plastic bearing has a longer useful life, despite shorter overall achievable life.
Environmental conditions
Even though a plastic bearing can be designed to meet varied load and speed requirements, environmental factors can affect its performance. To illustrate, an insert-molded bearing can improve life expectancy under high loads and speeds, but metal parts are susceptible to corrosion. Conversely, thermoplastic materials filled or reinforced to suit specialized environments are typically less wear resistant under high load or speed. That said, some environments enhance thermoplastic bearing performance: Water can act as a lubricant and expel heat during operation, reducing wear rates.
Achilles heel: Load capacity
One advantage of metal bearings is that they can carry heavier loads than thermoplastics. High loads render plastic bearings susceptible to operation-generated heat. Thermoplastics are thermal insulators and do not disperse heat as effectively as metal. High temperatures can result in increased fatigue, adhesion, and slip of raceways, which soften and deform — increasing wear rates. To address the problem, higher loads can be managed by spreading load to minimize wear, by either increasing the number of balls, using larger balls, or using double ball rows. In an integrated product design, the ball pitch circle can often be increased as well, enabling the product to better withstand imposed loads and speed.
Full complement bearings also carry heavier load. These have no cage, but more balls — though under high speed, balls can rub together, creating heat and increasing wear. Another option is metal inners and races; they offer a compromise between plastic and metal without sacrificing integration benefits of thermoplastics. In these designs, steel parts disperse the heat at high speeds and loads, while thermoplastic parts can still be customized.
Plastic bearing applications
Thermoplastic bearings are used in many applications where they are more suitable than metal.
- Automotive
Thermoplastics and chemicals
When trying to achieve top speeds and optimum handling in high-performance vehicles, lightweight, plastic-bearing assemblies can be molded to integrate components, decreasing weight and increasing performance. Thermoplastics replace metal in consumer cars to compensate for weight increases brought about by new safety systems, navigational equipment, and in-car entertainment. However, the main reason for reducing weight is better fuel efficiency. (Carbon emissions can be reduced by more than 70mg/km for each kilogram reduction in weight of a car.) Molded plastic bearing assemblies are replacing multi-component metal alternatives in steering column mechanisms and dashboard controls.
- Figure 7
- Business machines
Plastic bearings are used in mail sorting machinery and multi-function printers. The bearings can be molded to suit internal space restrictions; bearing assemblies can integrate toothed pulleys or shafts. In some cases, shields are incorporated to guard against abrasion as well. Thermoplastics can eliminate the static charge generated by fast transport of paper, preventing attraction of debris into mechanisms.
ATMs move cash at high speeds for validating and sorting. Low-friction antistatic bearings protect machinery circuits here, and glass rollers allow magnetic strip and code reads without interference.
- Water applications
Plastic bearings in pool cleaners, dishwashers, spa jets, and shower enclosures require little torque and work efficiently using only the flow of water. An all-plastic bearing with a plastic cage and polypropylene balls is actually buoyant and almost frictionless. Plastic bearings have minimal water absorption, do not corrode, and resist sanitizing agents. Water does increase inertia, but the low bearing mass combats this, so the cleaner needs less power to drive through water.
- Food and poultry processing
Plastic bearings in poultry processing typically include two double race wheels held on an overhead rail by a plastic yoke, integrated with a chain-driven conveyor system. Thermoplastics are resistant to general food ingredients such as water, acids, and alkalis. The self-lubricated units do not corrode and resist high-pressure washdown. An expensive steel bearing is overengineered for this application, as chains (including wheels) are regularly replaced.
Performance in volatile environments
The natural lubricity and resistance properties of thermoplastics along with their abrasively soft nature are useful in precision bearings employed in LCD glass manufacture. BNL has designed a series of plastic bearings for Japanese MET Co., manufacturer of machinery for production of LCD flat panels. Machined from ultra-high molecular weight polyethylene or UHMWPE and polyetheretherketone with polyethylene balls, the bearings support the fragile, wafer-thin LCD panels as they pass through various processes on the machines – etching, exfoliation, and two types of drying. Conductive versions of the bearings used in areas requiring anti-static/static dissipative properties use carbon-fiber reinforcement. The UHMWPE bearing was a challenge, as it tends to deform after machining. Previously, metal bearings were used and had a typical life of only one run before replacement was required due to chemical attack. With the thermoplastic versions, the product is now capable of being used for many runs, while reducing damage to the LCD panels and minimizing the cost for what was a consumable part. Figure 5
Fully molded bearings promise longer life
GAF Materials Corp., North American roofing and ventilation manufacturer, has improved the performance of its Master Flow rotary roof turbines with a suite of custom-designed plastic ball-bearing assemblies from BNL. Molded from a special grade of acetal, the bearings are quiet. Fully molded bearing raceways boost wear rates, giving the GAF turbine a far greater life expectancy than those using bearings with machined raceways. (The fully molded bearings are predicted to outlast similar machined bearings by up to 700%.) A thicker fine-crystalline region boosts wear resistance of the raceway surface. Competitors typically machine a raceway into this surface as a secondary operation, resulting in the removal of the hard crystalline layer. Figure 6
For more information, call (508) 698-8880, email [email protected], or visit www.bnl-bearings.com. Read other application examples and a full white paper on plastic bearings at www.bnl-bearings.com/applications/apps_1.html.