About Polymer Bearings

In an already crowded bearing space, polymer bearings have made their mark for a host of different reasons. As a result, an area that was once dominated by steel and phosphor bronze is increasingly giving way to high performance polymers such as PTFE, PEEK, POM, and Nylons, where the sheer breadth of grades and fillers allows for a whole range of properties tailored to match the end-application and offer a solution that far exceeds what metal bearings were able to hitherto provide.

The advantages and disadvantages of polymer bearings against metals can be shown on the chart below:

As shown above, metallic bearings are typically preferred where the loads and possibly the temperatures are much higher. Here too, however, certain polymers such as PEEK and Polyimide (Kapton), can bear enormous loads and remain functional in temperatures of 300°C+. However, such polymers come at a price and are therefore limited in applications such as aerospace and medical, where cost may not be a key criterion.

However, for many applications, polymer bearings find that their advantages are highly sought after. Key among this is the ability to self-lubricate. Self-lubricating polymers such as PTFE, POM, and UHMWPE - to name just a few – offer dry-running capabilities which greatly reduce the need for external lubrication. This is especially valuable in consumer goods, where the structure of the device or appliance is such that the user will not have access to the moving parts. Similarly, in certain industrial applications, self-lubrication ensures minimal down time and greatly reduces the wear and load due to the build-up of friction.

Types of Polymer Bearings

Polymer bearings come in various shapes and sizes and can be either machined from a drawing or reverse-engineered from an existing part. Some of the typical bearings offered by Poly Fluoro Ltd. include:

1. Flange bearings
Flange bearings are designed to handle both axial and radial loads. In some designs the flange is also used as a locating mechanism to hold the sleeve in place.

Flange bearings can be machined either from stock rods or moulded. Polymer grades used would include PTFE (usually with a glass or bronze filling), PEEK (virgin or carbon filled), PPS (usually with a glass filling), and POM.

Flange bearings require a little more machining to the housing but can solve the unique load conditions of a shaft and some type of thrust surface.
 

2. Mounted bearings
Mounted bearings are machined with a double flange in order to sit within a pillow block. These bearings can be fabricated using several different plastic bearing materials to improve wear and reduce or eliminate lubrication.
 

3. Thrust bearings
Put simply, thrust bearings are washers made from any number of materials such as PTFE, PEEK, PPS, POM, Nylons, or Polyimides. They are generally thin, easy to install and prevent metal on metal contact in any thrust load conditions. They are easy to use and do not require lubrication of any kind in most conditions.

Although the design is simple, there is a need to machine the part so that the surfaces are perfectly parallel. This is where Poly Fluoro excels.
 

4. Sleeve bearings
These are the most common bearings, with a simple ID, OD, and length. However, as with the washers, care needs to be taken to ensure the tolerances are tight. Where most manufacturers would only offer a 100 Micron tolerance on linear dimensions, Poly Fluoro is able to go down to as low as 10 Microns in some cases.

The bearings are designed to carry linear, oscillating, or rotating shafts. The key to successfully designing a plastic sleeve bearing is paying attention to temperature, P, V and PV ratings for the material and match it with your application.
 

5. Spherical bearings
Spherical bearings are designed to allow for shaft misalignment, as they can rotate in two directions. Spherical bearings typically support a rotating shaft in the bore that calls for both rotational and angular movement.

Using self-lubricating polymers with very low static coefficients of friction, Poly Fluoro is able to ensure that even minor variations in alignment are immediately accommodated by the bearing to allow for non-stop performance.

While the above bearings are most common, application engineers are constantly finding new areas in which to apply the bearing properties of polymers. Ultimately, any application with repeated motion will benefit from a polymer bearing as it offers an unmatched ability to reduce wear and friction over a very long period of running time.

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The inert nature of PTFE (Teflon) has naturally endeared it to applications involving corrosive chemicals. Not only does PTFE stay non-reactive to almost all chemicals (notable exceptions being sodium and alkalis at elevated temperatures), it also exhibits its properties all the way up to temperatures of 250°C. This seemingly invincible nature allows fabricators and systems designers to incorporate PTFE without worrying which chemicals may or may not present within a system.

Many chemical baths are designed to hold a host of different chemicals. The need to uniformly heat or cool these chemicals is met by the use of universal heat exchangers. In its most basic design, the heat exchanger consists of two adaptors, connected to one another by many lengths of tubes. The adaptors are round blocks with multiple holes through them, each meant to house one end of the tube. The adaptor in turn is connected with a device that pumps out fluids, which then pass through the tube and out the other adaptor. The fluid may be heated or cooled to accordingly heat or cool the chemical bath, respectively. The tubes – which may be many meters in length – are submerged in the chemical bath, allowing for the heat transfer to take place between the chemical and the fluid within the tubes. The size, length and quantities of these tubes will define the volume of fluids that can be passed through the heat exchanger. Furthermore, the wall thickness of the tubes used will add to the efficiency of the heat transfer.

PTFE (Teflon) heat exchangers follow this design, with both the adaptors and the tubes being made from pure virgin PTFE. However, the fabrication process can be tricky. For one, as PTFE does not easily join even itself, the fusing of the tubes with the adaptor needs to be done in one of two ways:

1. Bonding – the outer diameter of the tube is chemically treated as are the inner holes of the adaptor. Bonding can be done using an industrial grade adhesive, which would also need to show resistance to the chemicals in the bath. The advantage of bonding is that it is much easier to fabricate such an assembly. The disadvantage is that the bond may not hold in the face of high temperatures and in the event that an unexpected chemical enters the mix

2. Welding – welding PTFE is very tricky and is as much an art as it is a science. PTFE welding can only be done if specially modified grades of PTFE are used, which allow themselves to be welding. Grades such as Chemour’s NXT and Inoflon’s M490 are examples of modified grades that can be used to make the adaptors. The tubes too need to be made accordingly. Modified grades of tubing are needed to ensure that both the tube and the adaptors are able to fuse with one another under high-temperature conditions

Aside from temperature and chemical resistance, it is also vital that the heat exchanger assembly is able to withstand the pressure build-up due to the passage of fluid. In the event of higher pressures, both the fused/bonded assembly as well as the tube itself would need to be capable of holding the same. Because of this, the wall thickness of the PTFE plays a dual role. On the one hand, because PTFE is a poor conductor, a lower wall thickness ensures that the heat transfer is more efficient. However, a thinner tube means more complications in bonding and/or welding, while also a lower capacity to withstand higher pressures.

Apart from the use of PTFE in the heat exchanger assembly, expanded PTFE (ePTFE) also finds significant use in this application. Like virgin PTFE, ePTFE also exhibits superior chemical and heat resistant properties. At the same time, the sealing properties of ePTFE ensure that it forms an effective gasket material in any portion of the assembly that may require clamping and effective sealing for the fluids.

Overall, it seems unavoidable that for certain chemical baths, PTFE is the only viable option for a heat exchanger assembly. With the proper fabrication and right materials, it offers a very effective, durable, and versatile solution for all industries where heat exchangers are needed.

Of all the properties of PTFE/Teflon, the one that people tend to know best is that it is non-stick. While this characteristic is usually attributed to the now increasingly discontinued application of PTFE in non-stick cookware, it does have wider applications in surface protection, sliding elements, and self-lubricating bearing materials.

However, there exist many applications where PTFE is required to be bonded to other surfaces. Most notable among these would be in structural, sliding bearings, where a PTFE sheet must be bonded on one side to a metal surface, while the other side is exposed as a sliding element. In such an arrangement, the PTFE sheet is bonded to a metal plate and a stainless-steel plate is placed on top of the PTFE sheet. The low coefficient of friction between PTFE and polished stainless-steel means that the stainless-steel sheet can slide freely along the PTFE sheet’s surface. The stainless-steel plate is itself welded to another metal surface and a vertical load is applied to it. These PTFE sliding bearings are used in infrastructure to accommodate both loads and movement. However, the high vertical loads also mean that shear loads exist, which act directly on the bond between the PTFE and the metal, making it essential that the bonding process is done with the utmost care and technical understanding.

Here we look at some of the factors that affect the bonding and throw light on the precautions and preparations needed to ensure a strong, reliable bond.

Appearance: Virgin PTFE is white in color and does not bond to surfaces unless it is chemically treated (etched) using a special process. The formulation of the chemical etchant is proprietary, with each processor using a method that suits them best. Once etched, the surface of the PTFE changes color to brown. This brown surface can be bonded easily using standard industrial grade adhesives.

Surface preparation: The metal surface to be mounted with PTFE can be prepared by the normal machining methods such as grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with trichloroethylene, perchloroethylene or acetone. As with any bonding process, we would need to ensure the surface of the metal is free from grit and debris. 

Bonding PTFE: For bonding of PTFE the following resin adhesive can be used: Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and PTFE sheet and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Other bonding agents can also be looked into, but usually a good industrial grade agent would be recommended.

Hardening: After mounting the PTFE a clamping pressure of between 10-15 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening times for various temperatures are 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing: After curing of the adhesive, the PTFE can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of PTFE use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open stone’. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively, aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.  

Oil Grooves: PTFE pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the PTFE. Oil grooves should be away from the edges by 6mm.

Maintenance: Bonded PTFE must be maintained, as the strength of the bond can be impacted by adverse environmental factors such as excessive sunlight, corrosion, and heat. Temperatures around the bonded areas should not exceed 120°C, while any presence of corrosive elements – such as sea-air and/or chemical fumes can – affect the metal surface and eat into the bond. Usually, when exposed to adverse elements, the bond strength can get affected along the edges and any corrosion along the metal can slowly eat its way into the middle.

In such a case, the bond can be reapplied along the edge of the sheet after cleaning any debris/rust from the affected area. However, care must be taken to apply a protective coating around the bonded area to ensure long-term functionality.

It should be noted that even with etching, PTFE remains a material resistant to bonding. While the etching process allows for a reasonably strong bond to metals, there is a limit to the strength of this bond. Even well bonded surfaces offer a bond strength of only 4-5 Mpa (40-50Kg/cm2). As such, in areas where the shear load is expected to be higher, the PTFE sheet may need to be supported by clamping or bolting.