Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymer Bearings - Self-lubrication Solutions for Critical Applications

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 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:

Polymer Bearing

Advantages

Disadvantages

Lightweight

Limited load capability

Fully customisable

Can be expensive

Self-lubricating

Limiter temperature range

Easy to replace

 

 

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. 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.


Related Posts

1. Solar Tracker Bearings - Considerations for Design and Manufacture

2. Cantilever Load Considerations for PTFE Sliding Bearings

3. Oil Free Polymer Bearings - Fluoropolymer Formulations for Applications Needing Self-lubrication

PCTFE vs PTFE - A Comparison of Two Very Similar Polymers

Even though PTFE remains a niche polymer among more generic materials such as PP (Polypropylene), PVC, PE (Polyethylenes, such as HDPE and LDPE), and even Nylons, within the engineering space it is now quite common. Most applications involving high temperature, corrosive chemicals, high voltages, or high wear/friction now look to PTFE automatically as a solution. 

Despite this, there do exist applications where PTFE does not fit the bill and a compromise must be made. For example, applications where high dimensional stability is needed across a wide temperature range, PTFE tends to fall short. The high linear thermal expansion coefficient of PTFE means that it cannot hold its dimensions as temperatures vary. In such a situation, we have seen PEEK being adopted. While PEEK does do the trick, it is also 10X the cost of PTFE.

Similarly, certain applications where cost is a constraint need to make do with POM (Delrin), or even PVC, where PTFE cannot be used. In such a scenario, we possibly forego some of PTFE’s key properties.

Over the years a variety of new polymers that have been developed to fill the performance and commercial gaps between PEEK and PTFE. These include PFA, FEP, PEK, PPS (Ryton), and PCTFE.

Although not well known, PCTFE forms an ideal substitute for PTFE in certain applications where PTFE is unable to perform adequately. The table below is meant to offer a snapshot comparison of the two, such that any application engineer can evaluate the key differences.

 

 

Unit

PTFE

PCTFE

Remarks

Properties

Tensile Strength

Mpa

20-30

30-35

With a marginally higher tensile strength, PCTFE rates higher than PTFE on this metric

Elongation

%

200-350

100-250

PCTFE is stiffer than PTFE, which means it lacks some of the softness of PTFE when it comes to sealing, but that it also holds its dimensions more easily

Melting Point

Deg. C

350-380

200-220

PTFE is still preferred on outright high-temperature applications

Dielectric Breakdown Voltage

KV/mm

50-100

20-40

PTFE rates higher on outright dielectric strength

Coefficient of Friction

 

 0.03-0.05

0.25-0.35 

PTFE rates higher as a non-stick material 

 

 

 

 

 

 

Processing

Injection Moulding

 

No

Yes

PCTFE has more versatility when processing, allowing for more complex parts

Compression Moulding

 

Yes

Yes

 

 

 

 

 

 

Characteristics

Chemical Resistance

 

Extreme

Very Good

PTFE is still unmatched in chemical resistance

Thermal Stability

 

OK

Very Good

PCTFE rates higher than PTFE when it is a question of stability over a wide range of temperatures

Price

 

Med

High

PCTFE is more expensive than PTFE, and is therefore used in specific applications only

As you can see from the above chart, PCTFE and PTFE each have unique advantages and disadvantages when compared with one another. Like all polymers, the application needs to be properly understood and the commercials need to be weighed in before any decision can be made.

However, it is fair to say that when dimensional stability across a temperature range is a must, PCTFE is growing to become the most effective substitute for PTFE.

Datasheet for PCTFE:

Property

Value

Units

Method

MECHANICAL PROPERTIES

Tensile Strength

4860 - 5710
34 - 39

psi
MPa

D 638

Elongation

100 - 250

%

D 638

Flexural Strength, 73°F

9570 - 10300
66 - 71

psi
MPa

 

Flex Modulus

200 – 243 x 103
1.4 – 1.7

psi
MPa

 

Impact Strength, Izod, 23 deg C

2.5 – 3.5

ft-lb/in

D 256

Compressive Stress at 1% deformation,

1570 – 1860
11 - 13

psi
MPa

D 695

Density

2.10 to 2.17

gm/cu.cm

 

THERMAL PROPERTIES

Coefficient of Linear Expansion

7 x 10-5

K-1

 

Melting Point

410 -414
210 - 212

deg F
deg C

 

Thermal Conductivity

1.45
0.84

Btu·in/h·ft2·°F
W/m·K

ASTM C 177

Specific Heat

0.22
0.92

Btu/lb/deg F
kJ/Kg/deg K

 

Heat Distortion Temperature, 66 lb/sq.in (0.455 MPa)

259
126

deg F
deg C

D 648

Processing Temperature

620
327

deg F
deg C

 

ELECTRICAL PROPERTIES

Dielectric Strength, short time, 0.004”

3000

Volt/mil

D 149

Arc-Resistance

360

sec

D 495

Volume Resistivity, @ 50% RH

2 x 1017

ohm-cm

D 257

Surface Resistivity, @ 100% RH

1 x 1015

Ohm sq-1

D 257

Dielectric Constant, 1 kHz

2.6

ε

D150-81

Dissipation Factor, @ 1 kHz

0.02

 

D150-81

OTHER PROPERTIES

Water Absorption

0.00

% increase in weight

D570-81

Flame Rating+

Non-flammable

 

D 635

Coefficient of friction (Dynamic)

 

0.25-0.35

D 1894

Specific Gravity

2.10 to 2.17

 

D792

Moisture Permeability Constant

0.2

g/m, 24 hours

 

O2 Permeability

1.5 x 10-10

Cc, cm/sq.cm, sec, atm

 

N2 Permeability

0.18 x 10-10

Cc, cm/sq.cm, sec, atm

 

CO2 Permeability

2.9 x 10-10

Cc, cm/sq.cm, sec, atm

 

H2 Permeability

56.4 x 10-10

Cc, cm/sq.cm, sec, atm

 



 

PTFE Heat Exchangers: A Universal Solution for Corrosive Environments

The inert nature of PTFE (Teflon) has naturally endured 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 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 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 than can be passed through the heat exchanger. Furthermore, the wall thickness of the tubes use will add to the efficiency of the heat transfer.

PTFE 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.