Maintenance-free polymer bearings live or die by four factors that PV just does not capture. PV limits are often treated as the defining metric for polymer bearings, and while PV is an excellent quick screening tool during the early stages of the design process, there is more involved. In fact, there is a disconnect between betweeb the published PV ratings and real-world performance. And that is the topic of this blog post.
What Maintenance-Free Polymer Bearings
One of the common reasons that polymer bearings made from materials like PTFE are used for bearings is their maintenance-free aspects. PTFE leaves a transfer layer on the counterface materials that serves as a dry lubricant. However, maintenance-free does not mean that the bearing has an infinite life, experiences zero wear, or is immune to environmental contaminants.
The Limits of PV Limits in Maintenance-Free Polymer Bearings
The PV limit is a thermal ceiling for bearings, representing the maximum combination of load and speed a material can withstand before it is destroyed by friction and heat. The PV limits for materials are determined using test rigs in highly controlled operating conditions that usually do not represent actual working conditions. In addition, PV values represent average conditions and do not capture material reactions to transient or local events.
Counterface Material for Maintenance-Free Polymer Bearings
A key aspect of maintenance-free polymer bearing performance is the counterface material, including its hardness, roughness, and metallurgy. For example, the counterface metallurgy and hardness must be sufficient to resist corrosion and scratching. Additionally, surface roughness must fall within a specific range. A shaft that is too rough acts like a file, while one that is too smooth generates excessive heat and friction because the polymer is not able to achieve a transfer film on the counterface.
Before going into the details of which counterface material will be used with the polymer bearings, the general material category must be determined. Note that a polymer bearing may meet PV limits on one shaft material and fail early on another. PV ratings are almost always derived using a polished, hardened carbon steel shaft, and if you change the shaft material, then you change how the system will handle beat and abrasion. This means that a bearing that works on steel might not work well on stainless steel, even when the pressure and speed are the same. Because of this, counterface selection should be treated as part of the bearing specification.
Edge Loading and Real Geometry Effects
In the real world, assemblies will rarely achieve a perfect load distribution, and that makes edge loading an issue. Edge loading leads to localized pressure spikes that exceed the normal PV, even though the average PV might look acceptable. Edge loading can be caused by several factors, including shaft deflection, housing tolerances, thermal distortion, and misalignment. One solution is to use shorter bearings to reduce the risk of edge loading, but that is not always possible. Other strategies can focus on geometry mitigation efforts, such as chamfers, lead-ins, and housing geometry.
Thermal Path: The Hidden Limiter
If there is any sliding contact, frictional heat will be generated. One difference between metals and polymers is polymers’ insulating nature. This means that polymer bearings depend on surrounding hardware to remove the heat that is generated. This problem can lead to accelerated creep, wear, and loss of dimensional stability. For that reason, it might be wise to compare metal housings to polymer housings in terms of thermal performance. Again, PV ratings assume ideal conditions with adequate heat dissipation, which may not occur in service.
Contaminants: The Variable That Breaks Assumptions
Bearings of all types can be compromised by physical contamination, and this can be especially true for maintenance-free polymer bearings. Common contaminants include dust, grit, process debris, and even fibers. And such contaminants can lead to serious abrasive damage if measures are not taken to prevent them — and PV testing rarely reflects contaminated environments. Fortunately, there are numerous measures that can be taken to mitigate the ingress of contamination, including shielding and seals.
Conclusion
PV limits are an entry condition, not a design guarantee. They must be designed as part of a system, with consideration given to load distribution, thermal effects, and the operating environment. Maintenance-free polymers must be designed with the counterface material, edge loading effects, geometric effects, thermal issues, and contamination all accounted for to truly take advantage of the host of benefits that maintenance-free polymer bearings provide.
If you are working on a design that can benefit from the use of maintenance-free polymer bearings, contact the experts at Advanced EMC today.
