by Christopher D. Small
Compressor operation relies heavily on the mixture of refrigerant and lubricant in refrigeration and air conditioning systems. Speeds and loads theoretically result in hydrodynamic lubrication for most compressor applications. However, boundary and mixed-film lubrication occurs frequently under conditions of startup, shutdown, oil wash-out from gas migration when compressors sit idle under elevated temperatures, and decreased lubricant viscosity due to refrigerant dilution. The duration of these conditions varies with the ability of the system design to obtain steady-state flow.
Lubrication conditions have a direct impact on the performance and reliability of traditional compressor bearings, such as rolling element, leaded-bronze and BI-metal Mixed-film and boundary lubrication allows contact between mating surfaces, which can result in higher friction and excessive wear. Because of their self-lubricious nature, metal-polymer bushings are less subject to wear and friction, offering more reliable performance under the same conditions.
Metal-polymer bushings have been used in commercial compressor applications for the past 20 years, but are still a relatively new concept for most of the industry. Their success in commercial applications required overcoming particular design challenges arising from differences in structure, system design and performance between metal-polymer and traditional compressor bearings.
METAL-POLYMER STRUCTURE
Metal-polymer bushings typically consist of a multi-layer composite structure, beginning with a rigid steel backing, plus an intermediate porous or mesh layer, typically bronze, and a polymer layer overlaid and impregnated into the intermediate layer. This polymer overlay, for most compressor applications, consists primarily of polytetrafluoroethylene (PTFE) and a combination of fillers. The PTFE and fillers determines the inherent performance properties of the bushing. Figure 1 shows the cross-section of a metal-polymer bushing with an intermediate layer of porous bronze material.
 Figure 1: Cross-section of a typical metal-polymer bushing
Metal-polymer bushings are typically manufactured in two steps from material that is produced in strip form. The process involves adhesion of the intermediate layer to the steel backing and impregnation of the polymer material. From strip form, the material is slit and wrapped into a bushing. The final dimensions are calibrated for press-fitting into a housing to obtain a predetermined inner diameter after installation. Because it is not a continuous cylinder like a cast bronze or all-metal component, this type of bushing is sometimes referred to as a split bushing.
The precision of the final dimensions will vary with the production and measurement capabilities of the manufacturer. Typical total tolerance on the final wall thickness of a standard bushing can range from 20µm to 45 µm, depending on the thickness of the strip, but better tolerance control is achievable in certain cases.
TYPICAL CLEARANCE RANGE
In most applications, bearing system design is influenced primarily by the clearance between the shaft and bearing surface, which can affect operational efficiency. The polymer surface is a critical factor in bearing performance. Because of its composite structure, processing capabilities limit the amount of tolerance that can be controlled. Accurately forming and measuring a bushing comprised of steel, bronze and polymer can be difficult, and is continuously being studied for improvement. At present, machining is not an option, since removing the polymer overlay would leave a bronze-polymer matrix, adversely affecting bearing performance compared with a full polymer surface.
Table 1 shows the difference between typical metal-polymer bushings and standard drawn-cup needle bearing clearances. Both were calculated based on ISO h7 and N6 shaft and housing fits. Exact clearance-range values will vary by manufacturer.
Bronze or BI-metal bushings were not included, because they typically can be machined after installation so clearance can be held within a much tighter range. Higher-precision needle bearings are available if the clearance range has to be reduced. Rolling-element and BI-metal bearings can maintain tighter tolerance ranges, but this may not be a factor in applications using metal-polymer bushings.
COMMERCIAL COMPRESSOR APPLICATIONS
Tables 2 and 3 compare typical clearance ranges for traditional compressor bearings and the metal-polymer bushings that replaced them in reciprocating and axial-plate compressors. Additionally, Table 4 shows examples of commercial scroll clearances for a given shaft size to demonstrate the range of clearances for metal-polymer bushings. Every manufacturer's compressor design is unique, therefore the clearance range required for the same size may vary.
Clearly, metal-polymer bushings can operate effectively and efficiently at higher clearance ranges than BI-metal or needle bearings. In some cases, the range must be adjusted for proper performance. Replacing a traditional compressor bearing with a metal-polymer bushing requires investigation to determine the clearance range for optimal performance.
PERFORMANCE
The performance of metal-polymer bushings in compressors depends not only on clearance, but also on the material's ability to withstand the speeds, loads, temperatures and corrosive attack in these applications. Typical bearing failures in compressors result from wear, fatigue and cavitation. By combining fillers in PTFE, metal-polymer bushing manufacturers have been able to produce materials that resist wear and fatigue as well as, and in most cases, better than traditional compressor bearings. This is due to their self-lubricious nature in marginal and boundary lubricated conditions. Cavitation resistance does not match that of BI-metal bearings, but is adequate for compressor conditions.
Figure 2 gives the results of a test comparing the wear resistance of a PTFE-based metal-polymer bushing with leaded bronze bearing. Two samples of each type of bushing were tested. Both leaded bronze bushings and one metal-polymer bushing were tested under identical conditions, while the load and speed were doubled for the second metal-polymer sample.
Figure 2. Lubricated wear of leaded bronze vs. metal-polymer bushings
The results show that the wear resistance of a metal-polymer bushing is vastly superior to that of a leaded bronze, even at twice the load and speed.
HYDRODYNAMIC LUBRICATION
Conventional hydrodynamic calculations in journal bearings can be used as a guideline to determine film generation, along with other factors in bearing design. However, a unique phenomenon can occur with metal-polymer bushings, the bearing surfaces of which have a lower modulus of elasticity than bronze, BI-metal or steel bearings. The effect at higher loads can cause elasto-hydrodynamic lubrication (EHL) conditions that produce greater film thicknesses compared with traditional journal bearings. It is generally accepted that the compliance of the polymer surface distributes loads more effectively and increases film thickness. However, even under conditions of poor film generation, metal-polymer bushings still provide a performance advantage because of their lubricious PTFE surface.
MAINTAINING EFFICIENCY
Because of the increased range of installed tolerances available with metal-polymer bushings, the challenge of maintaining or improving efficiency lies primarily in compressor design. These increased ranges often can cause mating parts within the compressor to function improperly due to the accumulation or stack-up of all the tolerances within the system design, including the housing, shaft, bushings and other components. This phenomenon can result in increased power consumption or reduced output.
However, this has been overcome with proper testing and investigation into bearing design and changes within the compressor. For instance, shaft and housing dimensions can be manipulated to decrease the range of tolerance to the limits of a manufacturer's capabilities. Moreover, ongoing studies by metal-polymer bearing manufacturers are seeking ways of decreasing tolerance ranges.
Another method of adjusting tolerance range in metal-polymer bushings is sizing, or burnishing, after installation. This involves pushing through a sizing tool of a specified diameter. If the inner diameter of the bushing is less than the sizing tool, the polymer overlay will be compressed, thereby reducing the tolerance range. However, compressing polymer material can adversely affect performance if the interference is too high. Each PTFE/filler material will react differently to sizing operations, and most manufacturers can provide more information for their particular materials.
The results of an investigation into the effect of clearance upon efficiency are shown in Figure 3. It was conducted by a manufacturer of scroll compressors that plotted the performance of a metal-polymer bushing against a machined BI-metal bushing over a range of clearances. Results show an increase in efficiency over larger clearance range than that required for the BI-metal bearing.
Figure 3. Metal-polymer vs. bi-metal efficiency over a specific clearance range
The nature of metal-polymer bushing performance in relation to compressor system design is still not fully understood. This and other types of testing are required to comprehend the mechanisms that drive and maintain performance in compressor applications. Each design is unique, and therefore will require its own testing and investigation.
Properly designed, metal-polymer bushings can provide a more reliable bearing system than traditional journal bearings. PTFE is a natural lubricant that offers better resistance to heat and friction during periods of marginal and boundary lubrication. The compliance of the polymer surface accommodates misalignments better than traditional compressor bearings, allowing for better system compliance with tolerance stack-ups in the compressor design. However, metal-polymer bushings should not be viewed as direct replacements for traditional compressor bearings in all cases, as they tend to function differently at various clearances.
Continuous advances in polymer science may lead to the development of metal-polymer bushings that can be machined at installation to help maintain control of clearances. Additionally, new material technologies can provide for development of more robust, reliable bushings that can keep pace with the changes facing the compressor industry, such as lead-free, 13 SEER, CO2 and others, better than traditional BI-metal, bronze or rolling element bearings.
For more information, contact Christopher D. Small, Applied Technology Group, GGB Bearing Technology, e-mail: chris.small@ggbearings.com.
The author is an application engineer at GGB Bearing Technology's Applied Technology Group in Thorofare, N.J.
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