by Jackie Johnson Jackie Johnson No Comments

Using PEEK for Bearings

NTRODUCTION:

The first bearings recorded in history were likely made of wood or bronze, but modern engineers can choose from a variety of materials ranging from classical choices such as stainless steel to various types of polymers.  One modern (and fascinating) bearing material is PEEK.  In this article, we will look at five specific characteristics of PEEK that have allowed it to evolve as a popular material for bearings.

What is PEEK?

PEEK (polyetheretherketone) is semicrystalline thermoplastic known for retaining its functionality at high temperatures and in challenging environments. This includes aspects such as strength, wear resistance, dimensional and structural stability, and chemical inertness.  These factors alone have convinced many engineers to switch from traditional bearing materials to PEEK.

PEEK was invented in November of 1978 by UK based company Victrex PLC, and received a US patent in 1982 after being introduced to the US by then Imperial Chemical Industries (ICI). The processing conditions implemented to mold the material influence its crystallinity, or structure, and this results in the superb mechanical and chemical resistance properties of PEEK.

PEEK Efficiency

PEEK is lightweight and strong, giving it a high strength-to-weight ratio.  This means that a metal bearing would weigh significantly more than its equivalent PEEK counterpart.  This not only reduces overall weight, but also the inertia of the system, which in turn can increase efficiency.

A Durable Material

By reinforcing PEEK with carbon fibers, its load bearing capacity and dimensional stability are increased.  The carbon fibers will increase PEEK’s ability to dissipate heat, reducing the chances of thermal expansion.  Such characteristics are vital for bearings in high performance applications, where a slight change in bearing geometry could result in a system failure.

By using PEEK, bearings are better able to withstand high temperatures and corrosion, making it well suited for applications in extreme environments. In fact, PEEK can withstand temperatures up to 662°F!

Plain PEEK has a low coefficient of friction. By adding graphite and PTFE filler, friction can be further reduced.  The ability to add such fillers to PEEK can result in self-lubricating bearings with increased wear resistance.

Manufacture

PEEK is compatible with various manufacturing processes, with bearings typically produced using machining and injection molding.  Both modern CNC machining and injection molding support the efficient manufacture of high precision, complex parts such as bearings.

Evolution of Bearing Materials

Bearing materials have evolved far beyond wood and bronze.  Characteristics such as the potential strength, reduced weight, reinforcement, self-lubrication, and easy manufacture of materials like PEEK continue make them a competitive choice for bearing material design.

Take the next step in bearing design, specific material compatibility should be checked on each application.  Advanced EMC provides a free white paper on Advanced Polymer Bearing Design.

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.