A Design Analysis on an Ultra-Low-Temperature Composite Sliding Bearing
Abstract: Sliding bearings feature smooth, reliable, and noiseless operation and are extensively used in valves. Under liquid lubrication conditions, the lubricating oil separates the sliding surfaces, preventing direct contact. This minimizes friction loss while fully utilizing the vibration absorption capacity of the oil film. To improve the friction characteristics of the bushing surface, an anti-friction material is cast onto its inner surface, which is known as a bearing liner. Under ultra-low temperature (cryogenic) working conditions, conventional bearing liner materials can no longer meet the application requirements. However, polychlorotrifluoroethylene (PCTFE) exhibits excellent low-temperature resistance and lubricating properties. This paper mainly discusses the application, adhesion process, and structural configuration of PCTFE in cryogenic valves, aiming to initiate further discussion and offer some useful insights.
Keywords: Ultra-low temperature; Sliding bearing; PCTFE; Bearing liner; Discussion
1 Overview
Cryogenic valves mainly include the following types: cryogenic gate valves, cryogenic safety valves, cryogenic ball valves, cryogenic check valves, cryogenic butterfly valves, and cryogenic globe valves. Currently, they are primarily used in ethylene and liquefied natural gas (LNG) related facilities. The cryogenic liquid media handled by these valves are mostly liquid oxygen, liquid hydrogen, ethylene, and LNG. These substances are not only flammable and explosive but also expand hundreds of times in volume upon vaporization at ambient temperature. Particularly in LNG applications, the material of the valve is even more critical; substandard material quality can easily lead to natural gas leakage and even explosions. Therefore, in the design and development of cryogenic valves, material selection is crucial. For cryogenic valves, especially cryogenic ball valves, the selection of bearings plays a critical role in the reliability of valve sealing.
2 Introduction to PCTFE and Its Composite Sliding Bearings
2.1 PCTFE
PCTFE, namely Polychlorotrifluoroethylene (abbreviated as PCTFE), is an engineering plastic with excellent properties. It has a uniform texture, a smooth and flat surface, and is transparent or translucent in color. With a melting temperature of 213°C, it boasts excellent chemical stability, outstanding airtightness, surface non-stick properties, insulation, and weather resistance. It can be used long-term at temperatures ranging from -196°C to 125°C. Its mechanical strength and hardness are superior to those of PTFE (Polytetrafluoroethylene). Due to its ultra-low temperature resistance, it can be used long-term in cryogenic environments down to -190°C. Furthermore, it possesses a certain degree of self-lubricity, which gives it distinct advantages as a cryogenic bearing material.
2.2 PCTFE Composite Sliding Bearings
PCTFE composite sliding bearings are primarily made by compositing PCTFE with other materials, offering the following application characteristics:
Oil-free or minimal-oil lubrication, suitable for applications where oiling is impossible or difficult; requires little to no maintenance during use.
Good wear resistance, low friction coefficient, and long service life.
Appropriate elastic-plasticity, allowing stress distribution over a wider contact area, thereby enhancing load-bearing capacity.
The static and dynamic friction coefficients are close, which eliminates stick-slip at low speeds, ensuring the working accuracy of precision machinery.
High adaptability; can operate within a temperature range of -195°C to +280°C.
Excellent running-in performance; no scraping is required during assembly.
Reduces mechanical vibration and noise, prevents environmental pollution, and improves working conditions.
Forms a transfer film during operation to protect the shaft, eliminating shaft seizure.
Low hardness requirements for the shaft; unquenched and untempered shafts can be used, reducing the machining difficulty of related parts.
Thin-wall structure, lightweight, allowing for reduced mechanical volume.
Non-hygroscopic and non-oil-absorbing, low coefficient of thermal expansion, good heat dissipation, and dimensional stability.
The steel backing can be electroplated with various metals, allowing for use in corrosive media without rusting.
3 Manufacturing Methods and Discussion of PCTFE Composite Sliding Bearings
3.1 Conventional Manufacturing Methods and Their Pros and Cons
Conventional manufacturing methods in China typically involve sintering spherical bronze powder onto a steel backing, rolling a PCTFE mixed layer, and then roll-forming it into a cylinder.
Advantages: Good running-in performance, convenient manufacturing, short production cycle (especially suitable for mass production); low manufacturing cost, low requirements for supporting facilities; suitable for applications with low geometric and positional accuracy requirements for bearings.
Disadvantages: Since spherical bronze powder cannot be directly sintered onto a cylindrical steel backing (the bronze powder easily falls off during sintering), it can only be manufactured by first sintering onto a flat plate and then roll-forming it. Therefore, the manufacturing process is relatively singular and limited. The rolled sliding bearing has an opening, and it is difficult to keep the opening size consistent, which causes difficulties during assembly. The thickness of the sliding bearing is greatly affected by the diameter; for example, when the diameter is below 100 mm, the thickness should be controlled below 2 mm as much as possible, otherwise, rolling becomes difficult. The cylindricity accuracy of the sliding bearing is difficult to guarantee, making it unsuitable for applications requiring high geometric and positional accuracy.
3.2 Manufacturing Methods and Pros/Cons of Inset Bearings
An inset sliding bearing involves embedding PCTFE, which has good friction performance, into a metal bushing. The specific method is to machine PCTFE tubing into a sleeve with a certain wall thickness; the outer diameter of the sleeve is slightly larger than the inner diameter of the metal bushing. The PCTFE sleeve is soaked in liquid nitrogen to cool it down and then pressed into the metal bushing upon removal to form a specific interference fit, followed by finish machining to the final dimensions.
Advantages: Effectively utilizes the wear resistance of PCTFE and the strength characteristics of the metal bushing; dimensional tolerances and geometric/positional accuracy are easily guaranteed; the wear-resistant layer has a certain thickness; the bushing is easy to install and replace.
Disadvantages: Both the PCTFE sleeve and the metal bushing require a certain thickness, otherwise, the machinability is poor; the interference amount between the PCTFE sleeve and the metal bushing is difficult to control. Under low-temperature conditions, since the linear expansion coefficient of PCTFE is greater than that of metal, the sleeve is prone to slipping; complex manufacturing and high cost; because the elastic modulus of PCTFE is smaller than that of metal, the bearing deformation under compressive stress will be large. Therefore, to reduce deformation, the length of the bushing must be increased.
3.3 Inset Composite Bearings
An inset composite bearing combines the conventional manufacturing method of PCTFE composite sliding bearings with that of inset bearings. The conventional bearing is pre-pressed into a metal bushing, and a calibration die is used to correct the cylindricity of the bearing's inner bore, thereby manufacturing a one-piece bearing, as shown in the figure.
Advantages: Combines the advantages of both conventional composite sliding bearings and inset composite bearings.
Disadvantages: The manufacturing process is relatively complex, and the overall dimensions are larger than those of conventional composite bearings.
4 Conclusion
Both conventional composite bearings and inset bearings have numerous advantages and disadvantages. Applying them individually to cryogenic valves, especially LNG valves, will present various defects. The inset composite bearing can resolve many existing problems with its advantages, offering outstanding operational reliability and replaceability. Therefore, under the condition that spatial dimensions permit, it can be tentatively designed and applied.
2026 October 4 th Week Marginal Product Recommendation
Margianl LCB Plastic Clip Linear Bushings：
Made of high anti-wear material EPB13;
2.Maintenance-free, self lubricating;
3. Chemical resistant;
4. Dust resistant and quit running;
5. Installation by force fitting;
6. The E10 inner tolerance ensues only after the press fit.
