Spring Loaded Seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Maintenance-Free Polymer Bearings: PV Limits Are the Start, Not the Spec

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. 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Creep and Stress Relaxation in High-Performance Polymer Seals

Creep and stress relaxation are types of time-dependent deformation that matter in sealing as too many engineers in the field typically see “assembled dry, passed leak test, then seeps later.” Sealing force is not a fixed number: it decays over time. And polymer seals can be affected by factors such as viscoelasticity, temperature sensitivity, and constraint effects. 

This article explores core definitions and concepts related to creep and stress relaxation, then covers how different polymer sealing materials behave and tips for the design and installation of seals to minimize these issues.

Definitions and Concepts for Creep and Stress Relaxation

Creep is defined as the increase in strain under constant applied stress. The constant stress can be, for example,  contact stress from interference, bolt load transferred through a gasket, or differential pressure loading. The results of creep are dimensional change, extrusion growth, reduced interference, and/or a contact pattern shift.

Cold flow refers to creep at moderate or ambient temperature and is controlled by a combination of stress and constraints. As a type of creep, cold flow is dominated by a combination of viscoelastic and viscoplastic deformation under a sustained compressive load.

Stress relaxation is decreasing stress under constant strain as the result of fixed gland volume, captured seal, or fixed squeeze. This can be a problem for static seals, where the gland maintains constant displacement, not constant stress. The results of stress relaxation include clamp-load loss, loss of sealing force, and an increased possibility of leakage.

Polymers can still look like they kept their shape, but they may not be pushing as hard against the metal anymore. In elastomers, “compression set” is mainly about the rubber not springing back. In polymers, the bigger issue is that the internal stress slowly bleeds off over time, so sealing force drops even if the part does not look significantly deformed.

When a polymer is compressed, part of the squeezed portion will spring back right away, but part of it returns slowly, and another part never returns because the material has permanently shifted shape. The longer a seal is under compression, the more the polymer begins to relax and flow, so even after the load is removed, it may not be able to rebound to restore the original sealing force. 

And if you compress a polymer seal and then release it, the force on the way back will usually be lower than on the way in because some energy is lost inside the material. That’s why repeated squeeze-and-release cycles will not bring the seal back to the original force level.

Material Behavior in High-Performance Seal Polymers

PTFE (unfilled): PTFE has extremely low friction and is very chemically resistant, but it gives up the sealing load over time. Virgin PTFE tends to creep and relax under sustained compression, therefore requiring a strong gland support, tight extrusion-gap control, or spring energization.

PTFE (filled): Filled PTFE holds up better because fillers increase stiffness and reduce cold-flow behavior. Filled PTFE can usually retain its sealing force longer than virgin PTFE, but the filler used can also increase friction and may affect counterface wear.

PEEK: PEEK is typically chosen when long-term load retention matters greatly. PEEK’s higher stiffness means better resistance to creep and stress relaxation, though solid gland design and surface control still matter. PEEK is also available in filled variants that can impact its properties.

UHMW-PE: UHMW-PE is excellent for abrasion and low friction, but it can still relax under long compressive dwell, especially if stresses are high or support is limited. It performs best when the design itself minimizes sustained stress and prevents extrusion.

PAI (Torlon): PAI offers the strongest resistance to time-dependent deformation in this group. It retains shape and sealing load well, making it a strong fit for high loads and elevated temperatures where other polymers may drift.

MaterialCreep ResistanceStress Relaxation ResistanceRebound After Long DwellExtrusion Risk (if poorly supported)
PTFE (unfilled)LowLowLowHigh
PTFE (filled)ModerateModerateLow–ModerateModerate
PEEKHighHighModerate–HighLow–Moderate
UHMW-PELow–ModerateLow–ModerateModerateModerate–High
PAI (Torlon)Very HighHigh–Very HighHighLow

Design Variables That Control Creep and Relaxation

Gland constraint is the first major factor. A fully confined gland gives the seal fewer places to move, which cuts down creep flow and helps prevent extrusion. If the gland is only partially confined, any clearance becomes an escape route for the seal, and support has to be both radial and axial. Radial support keeps the polymer from pushing into the extrusion gap under pressure. Axial support helps prevent shifting and uneven edge loading. The small geometry details count as well; add corner radii and lead-in chamfers, and avoid sharp edges that create stress concentrations. Also, remember tolerance stack-up: as the seal relaxes, the “effective” clearance and contact conditions can change even if the metal parts do not.

More squeeze is not going to automatically be safer. Higher initial stress can accelerate creep and stress relaxation, especially with heat. The goal is to start with enough contact stress to seal, then still have enough after the material settles. That means designing around the minimum required contact stress at end-of-life, not just at assembly.

Extrusion gap control is about finding where pressure can escape and blocking it. The gap changes with temperature, pressure-driven hardware deflection, and assembly variation. Backup rings help by mechanically closing off that path. Their details matter, though.

Surface finish can make or break long-term performance. Roughness peaks concentrate stress and encourage localized flow, and surface lay can create leak paths. With filled polymers, counterface hardness matters because wear risk can increase with the wrong pairing. Aim for a finish that reduces stress peaks without creating new friction or lubrication issues.

Hardware stiffness also impacts load retention. Flexible joints can magnify clamp-load loss as polymers relax, so stiffer flanges, spacers, and bolt patterns will significantly assist with stability. For demanding duty cycles, spring-energized seals are an excellent option as they add an additional force to compensate for potential issues, such as relaxation, wear, thermal cycling, and small misalignment. 

Installation Tips for Mitigating Creep and Stress Relaxation

Many issues with creep start at installation, where a small nick, a cut, or a twisted seal can leak early, then get blamed on cold flow. Over-compressing the seal during assembly also makes it worse by driving high stress that speeds up relaxation and can leave permanent deformation. A simple fix is better handling and proper lubrication during installation to reduce the potential for surface damage and help the seal seat without problems due to uneven stress.

Load management matters just as much after assembly. Polymer gaskets and seats often benefit from controlled retorque protocols (when the application allows it) because the initial load can drop quickly during the first dwell. A common approach is initial torque, a short wait, then a retorque and verification check. Keep in mind that if over-torque pushes stress too high, it can accelerate creep and shorten the sealing life.

Finally, storage can quietly pre-load your failure. If a seal sits compressed on the shelf, it may relax before it sees service, starting life with serious issues related to sealing force. Temperature history matters as well, especially if parts are stored near heat sources or in hot warehouses. When possible, ship and store seals uncompressed, and for critical applications, controlled conditioning and careful packaging can protect long-term load retention.

Conclusion

Creep, cold flow, and stress relaxation are not mysterious defects, but rather predictable behaviors that appear whenever polymers sit under load for long periods. For this reason, treat them as design inputs and build a sealing system around them by choosing the right material, controlling deformation with proper gland constraint, relying on stiff hardware to maintain load, and validating the design with tests that match real pressure, temperature, and dwell-time conditions.

Advanced EMC is here to help with all your sealing needs, and our engineers are happy to help you navigate your way through potential creep and stress relaxation issues. Contact us today!