Polymer Beads
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Top Five Polymer Bearing Materials

Polymer bearings can be found in almost any industry and environment, and this includes the clean rooms of electronics to the harsh conditions of the oil and gas industry. And this is no surprise considering the host of benefits that polymer bearings provide, including their resistance to corrosive chemicals, low maintenance, lightweight, and low friction.

So, what exactly are the top five materials used in polymer bearings? The top five polymer bearing materials include Torlon PAI, Bearing Grade PEEK, Bearing Grade PPS, Lubricated PET, and Lubricated Nylon.

1. Torlon PAI

Spring Energized Teflon SealsPAI stands for Polyamide-imide and it is the highest performing polymer that is melt-processable. It offers excellent wear resistance, has an extremely low coefficient of friction,  and can handle operating temperatures up to 500°F. The primary drawback of Torlon PAI lies in its relatively high level of moisture absorption. On the other hand, it has a low coefficient of thermal expansion and a high level of creep resistance, both of which are key characteristics for an effective bearing. Torlon PAI is often used in bushings, bearings, and wear rings.

2. Bearing Grade PEEK

Bearing grade PEEK is known for its excellent wear characteristics, good abrasion resistance, extremely low coefficient of friction, and outstanding chemical resistance. It can handle environmental operating temperatures up to 500°F and performs well even when continuously exposed to hot water and steam. Bearing grade PEEK is also easy to machine, has low moisture absorption, and possesses a high PV rating. 

3. Bearing Grade PPS

PPS (polyphenylene sulfide), like the other bearing grade polymers discussed so far, has excellent wear resistance and a low coefficient of friction. However, it also offers very good wear resistance and dimensional stability even at elevated temperatures. Bearing grade PPS has a rated operating temperature of 425°F and offers outstanding chemical resistance. In addition, bearings can be made to extremely high tolerances when PPS is used.

4. Lubricated PET

Lubricated PET combines the stiffness, wear resistance, and dimensional stability of PET with the low friction demands of bearing applications. It offers extremely low water absorption, good abrasion resistance, and can be machined to very tight tolerances. It is internally lubricated using a dispersed solid and is dry running (needing no additional lubrication). The internal lubrication is released during operation, further reducing the naturally low coefficient of friction that PET possesses.

The primary drawback of PET lies in its limitations with regard to temperature: its continuous service temperature is 210°F, which makes it unsuitable for extreme temperature service conditions. 

5. Lubricated Nylon

Nylon does an excellent job of balancing toughness and strength while combining good abrasion resistance with the ability to be extruded, cast, or machined. Lubricated Nylon, much like lubricated PET, includes a solid dispersal of lubricants that greatly reduces the standard coefficient of friction of virgin Nylon and allows it to be used in dry running applications. One of the more common lubricants used is MDS or Molybdenum Disulfide. 

The primary issue with Nylon is its ability to absorb up to 7% of its weight water, which can affect its dimensions. However, it does have an extremely high limiting PV rating and excellent wear characteristics. 

Conclusion

The top five polymer bearing materials–Torlon PAI, bearing grade PEEK, bearing grade PPS, lubricated PET, and lubricated Nylon–are commonly used to replace metal bearings in a variety of applications. They offer the wear resistance, high PV ratings, low friction, and chemical resistance that are required. If you are in the market for new or replacement bearings, be sure to consider polymer bearings and bushings, also.

Spring Loaded Seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Polymer Bushings: Machine or Injection Mold?

Polymer bushings are found in everything from the food processing industry, where they meet the challenging demands of sanitation and sterilization, to the rugged environments and high shock loads encountered by rear hub bushings on bicycles. 

When it comes to polymer bushing, are machine or injection mold a better choice? Both machine and injection mold polymer bushing provide benefits in different applications. You must consider your unique application to select the polymer bushing that is best suited for your needs.

What’s the Difference Between Machine & Injection Mold Polymer Bushings

Before we dive into this topic, it is important to quickly review why we use polymer bushings. Bushings, which are also referred to as plain bearings, are used to reduce the level of friction between two surfaces that are in rotating or sliding contact with one another. They can also serve secondary functions, such as providing additional support and alignment.

Polymer bushings are replacing more and more metal bushings because polymers are typically lighter-weight, are more corrosion resistant, lower friction, often dry-running, and can be enhanced with fillers to improve properties such as wear-resistance and strength.

Manufacturing Polymer Bushings

When it comes to manufacturing polymer bushings, there are two primary methods to choose from: machining and injection molding. Both of these methods can generate reliable bearings to extremely tight tolerances, but they have significant differences and situations where one is preferred over the other.

Machining Polymer Bushings

When someone refers to machining, most people think of working with metals such as steel and aluminum, but plastics can also be machined.  Machining is a material removal process where the material that is not needed in the final part geometry is removed with a cutting tool and may involve numerous steps.

Depending on the part geometry, this may involve a lathe (for parts with rotational symmetry). Most modern machining facilities use CNC (Computer Numerical Control) and CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) to make parts that meet strict tolerances and are consistent in their dimensions. For polymer bushings, CNC lathes are typically used and multiple process steps are required to achieve the final part geometry and surface finish, including boring, reaming, and facing.

Benefits to machining include a short lead time and cost-effectiveness for low volume production runs of less than 1,000. Machining also avoids residual stresses (which we will discuss in a moment) that can warp the bushing and works extremely well when there are very tight tolerances or thin walls. Machining is also well adapted for situations where there is a need for non-standard shapes or bushing dimensions.

Injection Molding Polymer Bushings

Injection molding is a commonly used polymer manufacturing process that forces molten plastic into steel molds at extremely high pressures. Injection molding machines use a screw system to transport the plastic at high pressure into detailed molds that are typically designed to make multiple parts at a single time. The major expense in injection molding lies in the engineering and fabrication of the molds.

Injection molding of bushings is fast once the tooling is engineered and machined, ideal for production runs over 5,000 parts, and works exceptionally well for bushings that are standard-sized. However, injection molding does have its drawbacks. The most problematic drawback in the context of polymer bushings is residual stresses that develop as the part cools, but it is possible to eliminate these stresses through plastic annealing. 

There can also be issues with shrinkage and dimensional change, which can make it a poor option if the plastic bushings need to meet high tolerances. In addition, injection molding is not a reliable approach if the bushings have thin walls.

Conclusion

The demand for polymer bushings and plain bearings continues to rise. When it comes time to specify the bushings for an application, it is important to choose the best manufacturing method. For situations with small production runs, non-standard dimensions, tight tolerances, or thin walls, polymer bushings should be machined. When a large production run is involved and the bushings have standard dimensions and/or geometries, injection molding is the best option.