PTFE bearings

PTFE bearings, full name Polytetrafluoroethylene bearings, are plain bearings primarily made from polytetrafluoroethylene or composite materials based on this polymer. According to their structural form, PTFE bearings are mainly categorized into several types: solid PTFE bearings, PTFE composite lined bearings (such as self-lubricating spherical plain bearings with PTFE fabric liners), and metal-backed composite bearings (such as steel-backed DU-type bearings). Leveraging the PTFE material's inherent extremely low friction coefficient, excellent self-lubricating properties, outstanding chemical stability, and wide temperature adaptability, PTFE bearings can operate stably for long periods under harsh conditions such as oil-free lubrication, strong corrosion, high or low temperatures, and vacuum. They are hailed as core representatives of "maintenance-free bearings." They are widely used in industries like aerospace, high-end equipment, chemical machinery, food and pharmaceuticals, and automotive engineering, serving as critical fundamental components in modern industry for solving lubrication problems and corrosion resistance issues.

Physical and Chemical Properties

1. Physical Properties

The density of PTFE bearings is approximately 2.10 to 2.30 g/cm³, with a melting point around 327°C. They have a wide continuous operating temperature range, typically from -196°C to +260°C, with some high-performance products even extending this range to -200°C to +280°C. Their thermal conductivity is relatively low, around 0.244 W/(m·K), which is more than 300 times lower than that of metals, meaning frictional heat is not easily dissipated and must be considered in the design. The water absorption rate is extremely low, less than 0.01%, ensuring dimensional stability and no swelling when exposed to water. The friction coefficient is extremely low; the dynamic friction coefficient can be as low as 0.03-0.15 and decreases with increasing load. Below 288°C, the friction coefficient can reach an excellent level of 0.03 to 0.05.

2. Chemical Properties

PTFE bearings possess exceptional chemical stability, resisting corrosion from nearly all chemicals, including strong acids, bases, strong oxidants, and organic solvents, except for a very few substances like molten alkali metals, elemental fluorine, and chlorine trifluoride at elevated temperatures. They exhibit high resistance to most industrial chemicals, as well as solvents like petroleum and alcohol. They also provide excellent electrical insulation, with high volume resistivity and a low dielectric constant. Furthermore, they have good weatherability and are not prone to aging, but their radiation resistance is poor. Some metal-backed composite bearings (such as DAIDYNE) also feature anti-static properties, with contact resistance as low as 1-10Ω/cm².

3. Mechanical Properties

Regarding mechanical properties, the tensile strength of pure PTFE is about 25-27.6 MPa, while the tensile strength of PTFE fiber composite liners can reach approximately 34N/mm², and the tensile strength of their metal backing can be as high as 343N/mm². In terms of compressive strength, static loads can reach a minimum of 482N/mm², and low-speed oscillating loads can reach 220-275N/mm². The coefficient of linear thermal expansion is relatively large, about ten times that of steel, so for thermal expansion clearance must be considered in the design. It is important to note that PTFE exhibits "cold flow" (creep under sustained load) and has relatively low hardness, making the material soft. Therefore, in practical applications, this is often improved by adding fillers or using composite structures, such as adding glass fiber, carbon fiber, graphite, molybdenum disulfide, or bronze powder, to enhance wear resistance, creep resistance, and thermal conductivity.

Application Areas

Depending on the specific type, PTFE bearings are widely used in the following scenarios:

1. Aerospace and High-End Equipment

PTFE fabric-lined self-lubricating spherical plain bearings are core fundamental components in high-end fields such as aerospace, high-end equipment, precision instruments, and humanoid robots. They feature maintenance-free operation, self-lubrication, low friction, high load capacity, high wear resistance, and long life. In high-temperature applications like jet engines, Teflon liners with excellent high-temperature characteristics (such as L-1390 type, usable up to 329°C) can be selected.

2. Chemical and Petroleum Fields

Used in bearing assemblies for valves, pumps, and reactors, leveraging their resistance to strong acids (such as sulfuric acid, hydrochloric acid, hydrofluoric acid), strong bases, and organic solvents. Especially in applications handling corrosive chemicals, PTFE's chemical inertness makes it an ideal choice.

3. General Industrial Machinery

Widely used in hydraulic equipment, textile machinery, packaging machinery, household appliances, and solenoid valves. In compact applications like solenoid valves, thin-walled (minimum wall thickness 0.5mm) metal-backed PTFE bearings contribute to lighter, more compact designs.

4. Food and Pharmaceutical Industries

Utilizing PTFE's non-stick and non-contaminating properties, they are used in food processing lines and pharmaceutical machinery to avoid grease contamination of products and meet hygiene standards.

5. Automotive Industry

Used in various sliding parts of automobile components, such as suspension systems, steering mechanisms, and transmissions, providing maintenance-free, low-noise operation.

6. Special Environment Applications

· Vacuum Environments: Rulon® series PTFE bearings are suitable for dry, wet, or vacuum environments.

· Steam Environments: Can operate effectively in steam environments.

· Underwater Applications: Excellent water resistance and low water absorption make them suitable for submersible pumps, marine equipment, etc.

· High-Temperature Applications: Graphite-filled PTFE bearings can operate at extreme high temperatures of 400-500°C.

Advantages Compared to Other Products

1. Fully Self-Lubricating, Maintenance-Free

The most significant advantage of PTFE bearings lies in their self-lubricating property. During the initial break-in period, of PTFE transfers to the mating surface, forming a transfer film with an extremely low friction coefficient, causing the bearing to essentially slide against itself. This enables maintenance-free operation without the need for regular greasing, significantly reducing maintenance costs.

2. Extremely Low Friction Coefficient

The friction coefficient of PTFE is approximately 0.03 to 0.15 and decreases with increasing load. Some products (such as Rulon®) exhibit no stick-slip characteristics, eliminating "crawling" during low-speed motion, making them particularly suitable for precision positioning and high-speed reciprocating applications.

3. Extremely Strong Corrosion Resistance

They can function effectively in harsh chemical media such as strong acids, bases, and organic solvents – a capability unmatched by metal bearings or ordinary rubber bearings.

4. Wide Temperature Adaptability

They maintain stable performance across a broad temperature range from -200°C to +280°C, suitable for extreme conditions like cryogenic treatment and high-temperature baking.

5. High Load Capacity

PTFE composite lined bearings possess excellent load-bearing performance, with static loads reaching a minimum of 482N/mm² and low-speed oscillating loads reaching 220-275N/mm². Metal-backed composite bearings (such as DAIDYNE) have a maximum specific pressure up to 137MPa.

6. Mating Surface Friendly

The PTFE transfer film protects the shaft or cylinder bore surface, reducing wear. They do not damage the shaft and significantly extend the service life of mating components.

7. Clean and Contamination-Free

Being free of oil or grease, they do not suffer from aging or deterioration, making them particularly suitable for industries requiring cleanliness or avoiding contamination, such as food, pharmaceutical, and electronics.

8. Extremely Low Operating Noise

PTFE bearings operate with very low noise, making them suitable for applications with strict noise requirements.

Usage Precautions

1. Never Use External Lubricants

PTFE lined bearings must absolutely not be lubricated with traditional lubricants such as grease or lubricating oil. External additives will interfere with the formation of the transfer film, attract contaminants, and ultimately lead to premature failure. Do not apply grease or lubricating oil; it is not only ineffective but will attract dirt and cause abnormal wear.

2. Poor Thermal Conductivity

PTFE material has a low thermal conductivity coefficient, so frictional heat is not easily dissipated. In high-speed or heavy-load conditions, if the manufacturer's specified PV limit is exceeded, the self-lubricating film may become overloaded, leading to a rapid increase in friction and temperature, and ultimately failure. Therefore, operating limits must be strictly observed, and modified PTFE filled with carbon fiber or bronze powder should be considered when necessary to improve thermal conductivity.

3. High Coefficient of Thermal Expansion

PTFE's linear expansion is about ten times that of steel. When designing and installing, sufficient clearance must be to prevent seizure or sticking due to expansion at high temperatures. Dimensional changes due to temperature variations can be significantly larger than those of the connected steel structure; the design must account for this expansion and contraction.

4. Poor Creep Resistance (Cold Flow)

Pure PTFE is prone to permanent deformation under sustained compressive load, a process known as creep. For heavy-load applications, it is essential to choose reinforced PTFE (e.g., filled with glass fiber, carbon fiber, or bronze). Design loads must always be kept well below the material's specified limits to ensure long-term stability.

5. Chemical Compatibility

While PTFE is resistant to most organic compounds and solvents, contact with strong acids (such as nitric acid and sulfuric acid) and strong alkaline liquids must be avoided. Contact with these chemicals can degrade the polymer liner and lead to bearing failure. When used in acidic or alkaline environments, that phenolic resin backings may corrode or peel off.

6. Requirements for Mating Shaft

· Surface Hardness: Recommended to be no less than HRC 50. Chrome plating is particularly effective under severe usage conditions.

· Surface Roughness: Should be below 0.2μm Ra. If the roughness exceeds this, the service life of the plain bearing will be significantly shortened.

· Corrosion Protection: Teflon-lined plain bearings do not use grease, so the corrosion protection of the shaft must be considered.

· Shaft End Chamfering: The R chamfer at the front end of the shaft should be 15° to avoid sharp edges damaging the liner surface.

7. Installation Precautions

· Avoid Impact: Please avoid inserting the bearing into the housing using methods that cause impact, such as hammering.

· Protect Liner Surface: Burrs or sharp edges on the shaft front end will damage the liner surface; when inserting bolts, ensure the threaded does not damage the liner surface.

· Housing Preparation: The housing opening must be chamfered to prevent burrs from damaging the bearing's outer circumference.

· Contamination Protection: The bearing surface must be protected from weld spatter, paint overspray, and metal chips from grinding or cutting.

8. Clearance

· Oscillating Motion: Standard fit is 0 to 0.038 mm clearance.

· Rotary Motion: Standard fit is 0.051 to 0.102 mm clearance.

· Fit with Housing: Standard fit is 0.013 to 0.051 mm interference.

Selection Guide

To ensure a PTFE bearing suits specific operating conditions well, consider the following aspects during selection:

1. Operating Condition Assessment

· Load Conditions: Accurately calculate static and dynamic loads, determining load magnitude (light/heavy) and nature (constant/impact/oscillating/rotary).

· Operating Speed: Determine sliding speed v (m/s) or PV value (Pressure x Velocity).

· Ambient Temperature: Continuous operating temperature range? Are extreme temperature fluctuations involved? Is the environment vacuum, steam, or underwater?

· Contact Media: Chemical composition and concentration of the media? Does it contain particles?

2. Material Selection

· Virgin (Unfilled) PTFE: Offers the lowest coefficient of friction, suitable for applications requiring smooth motion.

· Reinforced PTFE: Filled with glass fiber, carbon fiber, graphite, molybdenum disulfide, or bronze powder, providing higher compressive strength and wear resistance, suitable for higher load applications.

· Glass Fiber Filled: Improves wear resistance and dimensional stability.

· Carbon Fiber Filled: Enhances thermal conductivity, creep resistance, and improves electrical conductivity.

· Bronze Powder Filled: Improves thermal conductivity and hardness, suitable for heavy-load hydraulic wear bands.

· PTFE Fabric Liner: Used in self-lubricating spherical plain bearings, featuring high load capacity and high wear resistance, suitable for high-end fields like aerospace.

· Metal-Backed Composite PTFE: Combines the strength of metal with the self-lubrication of PTFE, suitable for heavy-load, compact designs.

3. Structural Form Selection

· Solid PTFE Bearings: Suitable for light loads, corrosive environments.

· PTFE Composite Lined Bearings: Suitable for oscillating motion, spherical plain bearings.

· Metal-Backed Composite Bearings: Suitable for high loads, shock loads, intermittent operation, and reciprocating motion.

· Flanged Bushings: For applications requiring simultaneous radial and axial load support, the suitability of both the bushing and the flange must be checked separately.

4. Dimensional Design and Life Calculation

· Load Ratio C/P: As a first approximation, a guide value of load ratio C/P = 2 can be used to obtain the required basic dynamic load rating C.

· PV Diagram Verification: Use the pv diagram to verify whether the selected bearing can be used under the actual load p and sliding speed v conditions.

· Basic Rating Life Calculation:

Gh = c1 c2 c3 c4 c5 × KM / (p v)n

Where c1-c5 are coefficients for load, speed, temperature, surface roughness, and load type, KM is the material coefficient (480 for PTFE composite bushings), and n is the exponent (1 for PTFE).

· Note: If p v < 0.025, the value 0.025 should be used for calculation.

5. Temperature Adaptation

· Standard Temperature (≤130°C): Can choose a PTFE layer bonded to a steel backing plate.

· Medium Temperature (≤200°C): Choose a PTFE layer embedded in the backing plate, offering excellent stability.

· High Temperature (400-500°C): Choose graphite-filled bearings.

6. Installation Method Selection

· Full Welding: Creates a permanent, integral bond with the structure.

· Spot Welding: Uses partial welding where full bonding is not required.

· Bolted Connection: Allows precise adjustment, alignment, and easier future replacement.

· Mortar Embedment: Preferred method for integrating bearings into concrete structures.

7. Adaptation to Special Conditions

· High-Speed Conditions (>5 m/s): Prioritize PTFE filled with bronze or carbon fiber; pay attention to PV limits.

· High-Pressure Conditions (>40 MPa): Consider metal-backed composite bearings or add back-up rings to prevent extrusion.

· Water-Based Media: Avoid materials that absorb water and swell; choose carbon fiber-filled PTFE.

· Food/Pharmaceutical Industries: Select pure PTFE or specialized food-grade materials that meet hygiene standards.

· Vacuum Environments: Choose specialized grades suitable for vacuum, such as Rulon®.

Summary

PTFE bearings are high-performance plain bearings based on polytetrafluoroethylene, categorized by structure into various types including solid PTFE bearings, PTFE composite lined bearings, and metal-backed composite bearings. Their core advantages lie in their extremely low friction coefficient enabling self-lubricating, maintenance-free operation; exceptional chemical stability resisting corrosion from various media; wide temperature range adapting to harsh environments; high load capacity meeting heavy-duty requirements; and being friendly to mating surfaces. However, the drawbacks of PTFE – poor thermal conductivity, high thermal expansion, and weak creep resistance – require engineers to comprehensively consider the operating conditions during selection and design. This involves rationally choosing filler materials and structural forms, precisely calculating clearances, strictly adhering to the principle of "never use external lubrication," and following installation guidelines for protection. Correctly selecting and applying PTFE bearings not only effectively enhances equipment reliability and service life but also enables maintenance-free operation and reduces long-term maintenance costs, playing an irreplaceable role as core fundamental components in aerospace, chemical, machinery, food, pharmaceutical, and other industries.