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

Benefits of PTFE

From a friction/lubricity standpoint, neat fluoropolymers – such as PTFE (Teflon) - have long been accepted as an ideal non-lubricated bearing material for low speed applications at moderate temperatures. They are maintenance free, offer extremely low coefficients of friction without lubrication, and are immune to virtually all chemicals. Linear bearings and other wear parts are commonly made from PTFE, to take advantage of their low friction properties and chemical resistance. 

Adding fillers to the resin blend improves long-term performance, especially under high temperature and load conditions.  A major reason for the application of plastic materials like PTFE, in sliding systems (like bearings) is their capacity to run – especially against metals – without lubrication. As such they surpass metals under conditions where the use of lubricants is impossible or undesirable. Also, if lubrication is possible, but full film lubrication is not achieved (e.g. at low speeds), PTFE maybe an attractive alternative for metallic running surfaces.

Additives such as Polyphenylene Sulfone (PPS) may be further added which would help the formulations to enhance the PTFE bearings last longer, handle higher loads, at higher temperatures, and resist cold flow under higher temperatures. Moreover, these gains are achieved without compromising the chemical resistance and/or lubricity of the bearing. The same PPS additives also improve bonding of PTFE to metallic reinforcement rings.

Past remedies have included reinforcing fibers and PAR or PI additives. Each has improved life and expanded the operating envelope of PTFE. Reinforcing fibers increase resistance to wear and cold flow, but only at the expense of lubricity. Fibers on the bearing surface can also mar metal parts that move against the fluoropolymer, increasing friction, heat and wear.

Formulated PTFE, on the other hand, provides more durable alternative to fibers in harsh environments, or those involving chemical exposure. Also, as a customized PTFE bearing material, it offers greater resistance to both wear and cold flow while retaining the lubricity and chemical immunity of neat fluoropolymers.

Some of the commonly used fillers of PTFE:

Practically any material that can withstand the sintering temperature of PTFE can be used as filler. Characteristics such as particle shape and size and the chemical composition of the filler greatly affect the properties of the compound. Through a proper combination of base resin and one or more fillers, compounds could be tailor-made for many end-uses. And compounding technology continues to develop and given the wider scope of this innovation, it is likely to go on doing so.

• Improved resistance to cold flow or ‘creep’
• Reduce wear and friction
• Increased stiffness
• Increased thermal conductivity
• Increased thermal dimensional stability
• Increased surface hardness
• Increased electrical conductivity

Glass fiber is a widely used filler. It improves the creep resistance of PTFE both at low and high temperature. It is chemically stable (except to strong alkalis and hydrofluoric acid-HF).

Amorphous carbon is one of the most inert fillers, except in oxidizing environments where glass performs better. Addition of Carbon adds to the creep resistance, increases the hardness and raises the thermal conductivity of PTFE. Filled PTFE compounds with Carbon have excellent wear properties, more so when combined with Graphite. This combination of properties makes carbon/graphite compounds the preferred material for non-lubricated piston rings.

The use of a softer carbon has the additional advantage that it lowers tool wear during machining, thus allowing machining to very close tolerances. Also, Carbon-containing compounds have some electrical conductivity and are therefore antistatic.

The addition of Carbon fiber to PTFE changes its physical properties in the same way that Glass fiber does: Lower deformation under load, higher compressive and flex modulus and increased hardness.  Fillers with carbon fiber in PTFE have the advantage of higher thermal conductivity and lower thermal expansion coefficients than glass-filled ones with the same filler percentages, and they are lighter. They wear less in contact with most metals and are also less abrasive on the mating surface. The wear in water is particularly low. This makes carbon-fiber-filled PTFE an excellent bearing material, especially when lubricated with water. It is widely used in the automotive industry for bearings and seal rings, for example in water pumps and in shock absorbers.

Graphite is a crystalline modification of high purity carbon. Graphite-filled PTFE has one of the lowest coefficients of friction. It has excellent wear properties, particularly against soft metals, displays high load-carrying capabilities in high-speed contact applications and is chemically inert. It is often used in combination with other fillers.

Molybdenum disulfide adds to the hardness and stiffness of PTFE and reduces friction. It is normally used in low percentages and together with other fillers.

Calcium fluoride is suitable filler for PTFE in uses where it encounters chemicals that attack glass, such as hydrofluoric acid and strong alkalis.

Alumina or aluminum oxide is an excellent electrical insulator and is used to improve mechanical properties of compounds. As it is very hard, machining of the sintered part should be avoided whenever possible

In recent years, Polymeric fillers with sufficient heat stability to be used in PTFE have become available. Some remarkable properties have been obtained with polymer-filled compounds, particularly with respect to friction against soft metals.

Factors influencing tribological properties:

• Load
• Velocity
• Sliding Movement-rotating/reciprocating
• Degree of coverage
• Ambient temperature
• Filler-Percentage nature morphology
• Preparation method of the finished part
• Running-in conditions
• Mating Surface
• Material-surface roughness
• Lubrication - Environment
• Entrapped wear debris

The “pv” factor

The performance of PTFE bearings depends on the load and speed conditions to which they are submitted in the final application

The “pv” value is the product of the load “p” and the speed “v” and represents the reference factor for dry operating conditions.

By working within the recommended limits for the pv value, the temperature generated by friction, between the surfaces reciprocally sliding against each other, is assured to be well within the desired operating range.

In case the recommended pv limits are exceeded in dry operating, higher wear rates can be expected. In order to avoid that it is recommended to provide lubrication or a proper cooling of the bearing.

The limit ‘pv” values for dry operating of Fluoropolymer, PTFE bearings are:

• For continuous use without any noticeable wear traces 0.35 N/mm2.m/s
• For short time 1.7 N/mm2.m/s.

The specific load

The specific load limit for these bearings should not exceed 7 N/mm2. Load induces a deformation during the time, which is also function of the bearing wall thickness and that can negatively influence the bearing performance.

Higher values, up to 7 N/mm2, can be adopted by reducing the wall thickness using, for example, solutions obtained from tapes of thin thickness made through peeling of the desired formulated fluoropolymer cylinders.

The sliding speeds

The maximum sliding speed in dry conditions is 2 m/s. Higher speeds can be adopted provided lubrication or a proper cooling of the bearing is adopted.

Absence of lubrication

In many applications, lubricating two moving parts that are sliding against each other can pose a tough problem, because of the nature of the application itself, or for economical cost reasons. It is therefore essentially important, in such cases, to have the possibility to select bearings able that operate completely dry, offering in the meantime a proper performance level.

PTFE compounds do not need to be lubricated; that is one of their main attractions in use for bearings. Yet there can be circumstances where a lubricant or a process liquid is in contact with the surface. Almost without exception, the coefficient of friction of compounds is decreased by the presence of a lubricant, whether it is water, a lubricating oil or a solvent, but – perhaps surprisingly – the wear rate goes up in the presence of a lubricant. The reason, however, is simple. Under non-lubricated, dry conditions a very thin layer of PTFE is transferred to the mating surface and acts as an effective by lubricant. But the presence of liquid prevents or hinders this PTFE transfer. The degree to which it nevertheless happens depends on the nature of the liquid.

Water greatly increases the wear rate of PTFE compounds especially that of glass-filled types. Carbon/graphite compounds show the best performance. When surfactants are added to the water, and the surface tension is decreased to less than 27/dyne/cm1 the wear rate decreases sharply.

Generally PTFE formulated bearings, can operate:

• For continuous use in dry conditions, up to 0.35 N/mm2 m/s, without any noticeable wear traces
• For continuous use in dry conditions, up to 0.70 N/mm2 m/s, with extremely reduced wear rates
• For intermittent use in dry conditions, up to 0.70 N/mm2 m/s.

The coefficient of friction

The dry coefficient of friction of formulated oil-free PTFE bearings depends on the load and speed conditions, as well as on the grade of finish and on the nature of the counter surfaces. It is generally comprised in the range between 0.04 to 0.28. If lubrication is available, the coefficient of friction is further reduced to a value around 0.02 to 0.04.

The (mating) counter-surfaces

Oil free bearings give the best performances in contact with hard counter surfaces. The bearings, if formulated correctly, adapt equally well against softer surfaces, such as the stainless steel, copper and aluminum alloys or plastic materials.

With regards to the finish of the counter surfaces, values between an average roughness Ra=0.2-0.6µm is recommended. Further, it is not advisable that the maximum roughness value should exceed Ra=0.8 µm.

Material and surface roughness of the mating surface are important variables in predicting wear. Against hardened steel, the general wear pattern remains the same, although the wear rate is usually lower. Soft metals should be used in combination with less abrasive fillers. Bronze or copper alloys should not be used in contact with bronze-filled compounds.

Both friction and wear increase as surface roughness increases. Where the mating surface is rough, compounds with abrasive components like glass or ceramic perform better as they tend to polish the opposing face. This also explains why initial wear on a clean surface is usually different from wear after an initial running-in period. The wear rates indicated refer to “steady state” conditions after such a running-in period.

High temperature applications

PTFE bearings can be used both at very low temperatures (-2600C) and in high temperatures (+2600C) – an exceptionally extreme range where normal lubricants would fail.

For high temperature applications, the coefficient of thermal expansion of the formulated PTFE bearings should be adequately considered.

Adopting proper formulation is therefore important, so that the customized polymeric matrix responds to thermal variations. These bearings modify their original tolerances and dimensions much more than metals.

Also, if appropriate precautions are not taken in the design phase, it can cause high mechanical stresses, for example in case of press fit installations in metallic housings, with consequent damages to the bearing assembly.

In applications where a wide temperature range is predictable, the use of tape bearings made from PTFE formulations provides the ideal solution.

A peculiar characteristic of these bearings is that, in case of anomalies in the application that could generate extreme conditions of load or temperature, they do not break, but simply deform, thus avoiding seizure or unwanted debris with consequent damages to other parts of the machine.

Environment

The environment also affects the wear properties of PTFE compounds. At very low moisture levels in air (40ppm or lower) wear rates increase. This is particularly the cause for compounds containing graphite, as this material tends to disintegrate when no moisture is present. The nature of the ambient gas also has an influence. Wear rates in nitrogen and helium is reported to be lower than in air.

Entrapped wear debris

During the process of wear, debris is formed, consisting of particles of the filler, of PTFE, and of the mating surface. Thus, wear rates can be lowered if the geometry of a bearing allows debris to expelled from the bearing. This can be done by machining radial grooves in a bearing surface. This is particularly effective with bronze-filled compounds

Space and weight saving

With respect to the conventional metallic bush, or ball bearings, oil-free PTFE bearings offer substantial saving of installed weight. In comparison to ball or roller bearings, PTFE bearing offer a consistent reduction of the radial space required.

Basic considerations on the operating and the design of oil-free PTFE bearings.

Summarizing, the adoption of oil free PTFE bearings offer the following advantages:

• Dry running capability/self-lubricating properties
• Low coefficient of friction
• Low relative abrasiveness
• Resistance to high loads
• Operating temperatures from -2600 C up to 2600 C
• Thermal dissipation and antistatic properties
• Dielectric properties
• Chemical inertness and non-toxicity
• Dimensional stability
• Flexibility and fatigue stress resistance
• Vibrations dampening/low noise running
• Low drying characteristics

Typical Applications segments of Fluoropolymer PTFE Bearings

Applications	Design Considerations	Product typology	Benefits
Cars and their components	Low coefficient of friction High wear resistance Vibration and noise absorption Reduction of weight	Bearings and bushings also of special conformation	Weight and cost saving versus metallic solution. Long life/reliability
Textile machines and components of the textile industry	Dry operation Low coefficient of friction and high wear resistance in high sliding speeds conditions Operating in wet environment Chemical resistance Resistance to abrasion caused by dusty environments Low dirtying	Standard and ad hoc bearings and bushings Lining of flexible rapiers Sealing elements Parts of special conformation	Plain PTFE and formulations offer the most suitable solution to customize the specific application requirements with standard or tailor-made products
Packaging and packing machines	Dry operating Low coefficient of friction Food compatibility Reduction of installed weight	Standard and d hoc bearings and bushings Parts of special conformation	Dry operating capability Non-toxic No risk of contamination the handled products and of the plastic films used for packaging
Machines of the food industry	Dry operating Food compatibility / chemical inertness Abrasion resistance Mild counter surfaces	Standard and ad hoc bearings and bushings Parts of special conformation Special seals	Dry operating capability Non-toxic No risk of contamination the handled products Low relative abrasiveness
Machine tools	Low coefficient of friction/ non-stick-slip	Tapes 1.5mm thick, etched for bonding, for slide ways lining	Very low coefficient of friction in the specific operating conditions
Dry reciprocating compressors	Non-lubricated or wet operating conditions High wear resistance Compression resistance	Piston rings Cup seals Bearer rings	Long lasting Self-energizing qualities Fatigue resistance
Valves and solenoid valves	Low deformation under load Chemical resistance Dimensional stability	Valve seats Solenoid valve shutters	Resistance to deformation under load also at high temperatures Chemical inertness
Bearing and sliding pads	Low deformation under load Absence of stick-slip Weathering resistance Long lasting life	Sheets and tapes cut to size 2.5 mm thick, etched for bonding	Resistance to deformation under load Stick-slip free operating Long life without appreciable wear

 

Wear testing

For the proper selection of dry running materials, tribological data on the material combinations in question are required. The tribological behavior of a material in given situations may strongly depend on their actual composition and structure. Since a general theory to predict this behavior from the first principles is not available, tribological characterization of the material must be based on experimental results.

The standard pin-on-disc friction / wear testing apparatus performed all the tests under identical conditions. A sample disc is spun under controlled conditions of temperature and velocity while a hard pin bears down on it under a known load. Loss of material due to the load is measured periodically to quantify a wear rate.

Pure PTFE, eight PTFE compounds and a Polyimide compound were tested according to the pin-on-disc test principle against normalized plain carbon steel discs. All specimens were manufactured under processing parameters corresponding to those used to make a commercial quantity of Fluoropolymer PTFE bearings material with 10-30 percent additive levels.  Specific loads were in range of 1 to 4 N/mm2, sliding velocity (v) from 0.1 to 2.5m/s; sliding distance amounted to 180km.

The wear factors (k) of most compounds were found to be in the range from 0.2 to 0.7. The dependence of k and f on the operational conditions is an important consideration in selection of the various compounds.

To measure the effect of surface roughness on service life test materials were tested against two different levels of surface roughness: 1 and 2 µm Rz

Usually, no damage occurred to the steel-mating surface. Glass fiber and bronze fillers however tend to scratch finely the disc at high load and speed. Carbon fiber and carbon graphite containing PTFE compounds may cause slight changes in the surface condition of the normalized steel. The presented data allow for the tribological comparison of the compounds under consideration and may contribute to the tribological proper selection of dry running materials. From these tests a few first comments can be made: the tested glass filled compounds and the bronze filled compound show high friction and wear at elevated speeds while showing relatively low friction and wear at low speeds. These compounds would therefore perform better as slowly running slide bearings than as high-speed compressor seals. The opposite is true for the carbon fiber filled compounds; they show relatively low friction and wear at elevated speeds.

Outlook

Linear drives, transmissions and actuator mechanisms have broad requirements for bearings capable of handling low speeds and high loads with zero maintenance. Examples are found in control linkages, clutch and shift mechanisms, shaft containment and housings. Self-lubricated PTFE bearings have proven to successful in thousands of such applications. With PPS improving wear resistance, dimensional stability and cold flow resistance, oil-free PTFE bearings are now ready to take on even more severe-service applications.

Recent achievements have important implications for designers of linear actuators, low-speed transmission, or gearboxes that incorporate linear, thrust, or plain rotary bearings. This implies that drive and bearing designers can take full advantage of the exceptional lubricity and chemical resistance of PTFE and other Fluoropolymer in higher-load and preload situations.