Standard O-rings are still the correct answer for the majority of sealing applications. They are simple, low cost, available in thousands of standard sizes, and perform reliably in static and dynamic service across a wide range of temperatures and pressures.
But there are application boundaries where the elastomeric O-ring format fails structurally — not because of the wrong material, but because of the wrong sealing concept. Vacuum service, cryogenic temperatures, aggressive chemical environments that destroy all elastomers, and ultra-low-friction dynamic service are the conditions where spring energized PTFE seals become the technically correct solution rather than an upgrade preference.
Definition Block
Standard O-ring: A toroid of vulcanized elastomer that seals through initial compression squeeze plus pressure energization. Sealing force is generated by the elastic recovery of the compressed elastomer — the squeezed rubber pushes back against the gland surfaces, creating a contact stress that resists fluid leakage. The elastomer must remain compliant and elastic throughout service to maintain sealing contact.
Spring energized seal (SES): A PTFE or filled-PTFE jacket in a U-cup or lip profile, internally loaded by a precision metallic spring. The spring generates the sealing contact force continuously, independent of the PTFE jacket's cold flow behavior. The PTFE jacket provides the chemically inert, low-friction sealing face. The spring maintains sealing force even as the PTFE jacket undergoes slow cold flow (creep) over time, compensating for PTFE's inability to recover elastically.
The Core Problem Spring Energized Seals Solve
Elastomeric O-rings rely on two mechanisms for sealing:
- Elastic recovery — the squeezed elastomer pushes back against the gland surfaces, maintaining contact stress proportional to the residual compression
- Pressure energization — system pressure pushes the O-ring against the low-pressure gland wall, adding to the contact force at the sealing interface
Both mechanisms fail in specific conditions:
- Cryogenic temperatures: Elastomers lose elastic recovery below their TR10 (temperature of 10% retraction). Standard NBR TR10 is −35°C; FKM −12 to −18°C; VMQ −60°C. Below TR10, the elastomer behaves increasingly like a rigid plastic — no elastic recovery means no sealing force. At −100°C, even silicone is essentially glassy.
- High vacuum: System pressure cannot energize the seal from the low side. Only the initial squeeze provides sealing contact — and as the elastomer relaxes under compression set, the contact force drops toward zero. O-ring vacuum seals with significant compression set have zero sealing reserve.
- Universal chemical aggression: Every elastomer has chemical limits. FFKM approaches universal resistance but fails in some fluorinated environments and very strong reducing agents. PTFE, with no polymerizable backbone, has broader chemical inertness than any elastomer.
- Ultra-low friction dynamic service: Elastomers generate friction through their broad contact area and high elastic modulus at the contact zone. PTFE's coefficient of friction (~0.05–0.10 dry vs. 0.15–0.30 for NBR or FKM) and the narrow lip contact geometry of an SES reduce dynamic friction dramatically.
- Wide temperature cycling: Elastomers expand and contract substantially during thermal cycles (NBR thermal expansion: 1.5–2.0 × 10⁻⁴ /°C; FKM: 1.6–1.8 × 10⁻⁴ /°C). At cryogenic plus elevated temperature extremes, the cumulative dimensional change can drop compression below the sealing threshold. PTFE's thermal expansion coefficient (9–11 × 10⁻⁵ /°C) requires spring force to compensate for dimensional changes at extremes.
Spring Types and Their Effect on Sealing Behavior
The metal spring inside a spring energized seal determines the load-deflection curve and the sealing force across the operating range.
Canted Coil Spring (Helical Canted Coil)
The most common type for dynamic seals. Individual coils are tilted (canted) relative to the ring axis, giving the spring a relatively constant load over a wide deflection range — the load-deflection curve is flat in the operating region. Key advantage: consistent sealing force during dynamic stroke, even as the PTFE jacket wears, and even when the gland clearance changes due to thermal cycling.
- Best for: Reciprocating seals, rotary face seals, and dynamic applications where consistent spring force across the full deflection range is critical
- Contact force: 0.1–5 N/mm circumference depending on spring geometry and wire diameter
- Available materials: 302/304 SS (to +315°C), 316 SS (corrosion-resistant to +315°C), Elgiloy (−270°C to +550°C), Hastelloy C-276 (corrosive environments)
- Low-temperature limit: Metallic spring — essentially no lower limit down to −270°C; Elgiloy and Hastelloy preferred for liquid helium service
V-Spring (Single-Turn)
A single-turn spring in a V-cross-section profile, often used in lip-style SES configurations. Provides a higher initial sealing force than canted coil springs of the same envelope, with a stiffer load-deflection curve. The V-spring produces a nearly linear load-deflection relationship.
- Best for: High-pressure static seals, face seals, applications requiring high initial contact force for reliable static sealing
- Limitation: Higher friction than canted coil in dynamic service due to stiffer spring response to gland variation; the steep load-deflection curve makes force sensitive to dimensional tolerances
O-Spring (Internal Elastomeric O-Ring)
A small elastomeric O-ring used as the energizing element instead of a metal spring. Provides lower initial contact force, simpler construction, and no metallic spring components that could corrode or fail at extreme temperature.
- Best for: Moderate-temperature (−60°C to +200°C), moderate-pressure service where a full metal spring is unnecessary and where the inner O-ring material is compatible with the service environment
- Limitation: The inner O-ring also loses compliance at extreme low temperature (below its TR10) — the assembly is only cryogenic if the inner elastomer is VMQ or a cryogenic-grade material. Not suitable for liquid nitrogen or lower cryogenic service.
Garter Spring
A helical coil spring formed into a loop (no cant to the coils), commonly used in lip seal configurations. Generates a higher and less consistent contact force than canted coil. Used in simple face seals and some reciprocating seals where cost is a priority over precise load control.
PTFE Jacket Materials and Their Properties
The jacket material determines chemical compatibility, friction coefficient, and wear rate. All PTFE jackets have essentially the same temperature limit (+260°C continuous); the differences are in mechanical performance.
| Jacket Material | Filler Content | Chemical Resistance | Friction (COF) | Wear Resistance | Best Use Case |
|---|---|---|---|---|---|
| Virgin PTFE | 0% | Excellent (near-universal) | Lowest (0.05–0.08) | Low — tends to cold flow | Chemical service, food/pharma, static |
| 15% glass-filled PTFE | 15% glass fiber | Excellent | Low (0.06–0.10) | Good — glass reduces creep | Dynamic reciprocating seals |
| 25% carbon-filled PTFE | 25% carbon | Very good | Very low (0.04–0.07) | Excellent | High-speed rotary, dry-running |
| 15% carbon + 5% graphite PTFE | 20% total | Very good | Very low (0.04–0.07) | Excellent — self-lubricating | Dry or intermittently lubricated service |
| PEEK-filled PTFE | 25% PEEK | Very good | Low (0.06–0.10) | Very good — higher load capacity | High-pressure static, hard surface contact |
| 60% bronze-filled PTFE | 60% bronze | Good | Moderate (0.08–0.15) | Very high | High load, heat-conductive contact |
PTFE cold flow behavior: Virgin PTFE undergoes creep (cold flow) under sustained compressive load. At room temperature under 3.5 MPa: approximately 3–6% thickness loss at 1,000 hours. At +100°C under 3.5 MPa: 8–15% thickness loss at 1,000 hours. This creep is why the metallic spring is essential — the spring compensates for PTFE deformation that would cause contact force to drop to zero in a PTFE seal without energization.
Filled PTFE grades significantly reduce cold flow: 25% glass-filled PTFE shows approximately 50–60% less creep than virgin PTFE at the same load and temperature. For high-temperature static SES applications, specify a filled grade to reduce maintenance frequency.
For pharmaceutical and food-contact applications: Virgin PTFE is required — FDA 21 CFR §177.1550 and USP Class VI compliance is achievable only with pure PTFE or specifically tested filled grades. Verify that any filled grade used in regulated applications has the appropriate compliance documentation.
Temperature Range Comparison
| Seal Type | Minimum Service Temperature | Maximum Service Temperature | Notes |
|---|---|---|---|
| NBR O-ring (standard) | −40°C (TR10 −35°C) | +120°C | Standard industrial; limited low-temp |
| HNBR O-ring | −40°C | +150°C | Better high-temp than NBR |
| FKM O-ring (standard) | −20°C (TR10 −12 to −18°C) | +200°C | Excellent chemical resistance |
| FFKM O-ring | −20°C (TR10 varies by grade) | +300°C | Near-universal chemistry |
| VMQ O-ring (silicone) | −60°C | +230°C | Best standard elastomer low-temp |
| Spring energized PTFE (metal spring) | −270°C (liquid helium) | +260°C | Defined by PTFE + spring material |
| Spring energized PTFE (Elgiloy spring) | −270°C | +550°C (spring) / +260°C (PTFE) | Spring capable; PTFE limits upper end |
| Spring energized PTFE (O-spring inner) | −60°C (limited by inner elastomer) | +260°C | Inner O-ring limits low-temp |
The −270°C cryogenic capability of spring energized seals with metallic springs is unmatched by any elastomeric format. This is the defining advantage in liquid hydrogen (−253°C), liquid nitrogen (−196°C), liquid oxygen (−183°C), and liquid helium (−269°C) service. No elastomer — including specialty low-temperature VMQ — maintains functional elastic recovery in liquid cryogen.
Pressure and Vacuum Capability
Spring energized seals are used across a very wide pressure range:
- High vacuum: From atmospheric to 10⁻⁹ torr (ultra-high vacuum) and better. The spring maintains contact independent of system pressure — the SES does not rely on pressure energization. It performs identically at hard vacuum and at elevated pressure.
- Moderate pressure (standard dynamic): Typically 0–200 bar for reciprocating service with canted coil spring configurations
- High pressure (static): Static SES designs handle pressures to 700+ bar with appropriate hardware and spring selection — the spring preload must be designed to exceed the separation force at maximum pressure
- Bidirectional: Dual-lip SES designs provide sealing in both pressure directions from a single seal assembly, eliminating the need for back-to-back seal configurations in reversing-pressure applications
Vacuum outgassing comparison: In high-vacuum service, outgassing from the seal material into the vacuum environment is critical. PTFE outgasses significantly less than elastomers — typical PTFE outgassing rate is 10⁻⁸ to 10⁻⁹ mbar·L/(s·cm²) vs 10⁻⁷ to 10⁻⁸ for FKM and 10⁻⁶ for NBR. For ultra-high vacuum service (10⁻⁹ torr and below), PTFE SES is the standard sealing format. Metal gaskets (copper, gold) are the only lower-outgassing option.
Direct Comparison: O-Rings vs Spring Energized Seals
| Factor | O-Ring | Spring Energized Seal |
|---|---|---|
| Standard size availability | Highest (369+ AS568, 400+ ISO sizes) | Limited standard sizes; mostly custom |
| Unit cost | Low to moderate ($0.01–$50 typical range) | 10–100× higher than equivalent O-ring |
| Temperature range | −60°C to +300°C (VMQ to FFKM) | −270°C to +260°C (spring-energized PTFE) |
| Cryogenic service (< −60°C) | No — elastomers become rigid | Yes — metallic spring maintains contact |
| High vacuum (10⁻⁶ torr+) | Marginal — compression set reduces contact | Yes — spring maintains contact force |
| Universal chemical resistance | Marginal — FFKM approaches it | Yes — PTFE jacket |
| Dynamic friction (COF) | 0.15–0.30 (NBR/FKM) | 0.05–0.10 (virgin PTFE) |
| Wear in high-cycle reciprocating | Moderate to limited | Better with filled PTFE grades |
| Groove dimensions | Standardized (AS568, ISO, SAE gland tables) | Typically custom per seal design |
| Lead time (custom) | 7–15 days | 3–6 weeks |
| Installation complexity | Simple — stretch onto piston/rod | Moderate — spring alignment required |
| Pressure energization required | Yes — for low-squeeze or vacuum service | No — spring provides independent contact |
| Compression set dependency | High | Low — spring compensates |
| RoHS/REACH compliance | Compound-dependent | PTFE: RoHS compliant; spring alloy-dependent |
PTFE vs Elastomer Sealing Force Generation
Understanding the quantitative difference in sealing force generation helps explain why each technology has its application domain:
Elastomeric O-ring contact stress (typical):
- At 15% squeeze (standard dynamic): contact stress ≈ 0.3–0.8 MPa (30–80 Shore A NBR)
- At 25% squeeze (standard static): contact stress ≈ 0.5–1.5 MPa
- With 100 bar system pressure energization: contact stress increases by ≈ 0.5–2 MPa additional
Spring energized PTFE contact stress:
- Canted coil spring preload: typically 0.05–0.3 MPa contact stress from spring force alone
- With system pressure: contact force increases as pressure acts on the PTFE jacket's inner face
- At very low pressures (< 5 bar) or vacuum: spring maintains minimum 0.05–0.15 MPa contact stress
The elastomeric O-ring generates higher contact stress at moderate pressure — beneficial for hydraulic sealing. The spring energized seal generates lower but reliable contact stress at any pressure including vacuum — beneficial for vacuum and cryogenic service where pressure energization is unavailable.
When O-Rings Win
For most industrial, automotive, hydraulic, and process applications:
- Standard O-rings in appropriate materials (NBR, FKM, EPDM, FFKM) cover the temperature, pressure, and chemical requirements
- The cost differential of SES is not justified (10–100× higher per piece)
- Standard groove dimensions are already in the hardware
- Lead time and availability favor O-rings
An application does not need spring energized seals simply because it is demanding or uses aggressive chemicals. FFKM O-rings handle near-universal chemical resistance at temperatures to +300°C — for most "extreme" applications, FFKM is the more practical and less expensive solution than SES.
When Spring Energized Seals Win
The SES is technically justified when at least one of the following is true:
1. Cryogenic service below −60°C: Liquid nitrogen (−196°C), liquid oxygen (−183°C), liquid hydrogen (−253°C), or liquid helium (−269°C). No elastomer remains functional at these temperatures. PTFE with metallic Elgiloy or Hastelloy springs does.
2. High vacuum requiring independence from pressure energization: At pressures below ~10⁻³ torr, there is insufficient pressure differential to energize an O-ring from the vacuum side. Only the initial elastic squeeze provides contact force — and as compression set accumulates, the O-ring loses contact with zero sealing reserve. A spring energized seal maintains contact force from the spring alone, independent of pressure.
3. Universal chemical aggression that defeats all elastomers: Fuming nitric acid, chlorine trifluoride, concentrated hydrofluoric acid, some strong reducing agents — environments where even FFKM has limited life. Virgin PTFE withstands a wider range of these extremes than any elastomeric compound.
4. Ultra-low friction in precision dynamic service: Semiconductor wafer handling equipment, medical surgical robots, precision analytical instruments, laboratory microfluidic systems — applications where O-ring friction would cause stick-slip, inconsistent force output, or vibration in precision motion systems. PTFE COF of 0.05–0.10 vs elastomer 0.15–0.30 is a 2–5× friction reduction.
5. Very wide temperature cycling combined with compression set sensitivity: Applications that cycle between −100°C and +200°C exceed the reliable range of any single elastomer compound. A PTFE SES with metallic canted-coil spring maintains consistent contact force across the full range.
Spring Material Selection Guide
| Spring Material | Temperature Range | Corrosion Resistance | Best For |
|---|---|---|---|
| 17-7 PH Stainless Steel | −196°C to +315°C | Moderate | General industrial, mild chemical |
| 316 Stainless Steel | −196°C to +315°C | Good | Mild corrosive environments |
| Elgiloy (Co-Cr-Ni) | −270°C to +550°C | Excellent | Cryogenic, high-temperature, corrosive |
| Hastelloy C-276 | −270°C to +430°C | Excellent (Cl⁻ resistant) | Seawater, chlorine, acid service |
| Inconel 718 | −270°C to +650°C | Excellent | Very high temperature; aerospace |
| Titanium | −196°C to +300°C | Excellent (HF resistant) | Hydrofluoric acid; weight-sensitive |
For liquid helium and liquid hydrogen service: Elgiloy or Inconel 718 are specified — both maintain adequate spring force characteristics at cryogenic temperatures where 302/304 stainless may partially embrittle.
Groove Design Differences
Spring energized seals require significantly different groove geometry than standard O-ring grooves:
| Dimension | O-Ring Groove | SES U-Cup Groove |
|---|---|---|
| Groove depth | 0.84–0.90 × CS (dynamic) | Specific to seal OD/ID — not CS-based |
| Groove width | 1.25–1.35 × CS | Wider — must allow lip deflection + spring travel |
| Corner radius | 0.1–0.2 mm (radius not sharp) | Larger — 0.3–0.5 mm to avoid PTFE jacket damage |
| Surface finish (dynamic bore) | Ra 0.1–0.2 µm | Ra 0.1–0.2 µm (same) |
| Surface finish (groove base) | Ra 0.4–0.8 µm | Ra 0.4–0.8 µm (same) |
| Lead-in chamfer | 15–20° over 1.5–2 mm | 20–30° over 2–3 mm (PTFE less stretchable) |
Critical note: PTFE has approximately 250–400% elongation at break — significantly lower than most elastomers (NBR 300–500%, FKM 150–300%). PTFE jackets must be assembled over hardware by compressing/folding, not stretching. SES groove lead-in chamfers must be generous enough for assembly without damaging the jacket.
O-ring grooves already in hardware cannot typically be used for SES without modification — the wrong depth, width, and surface geometry will cause SES contact force to be wrong and may damage the jacket during installation.
Cost Analysis
A spring energized seal costs 10–100× more than an equivalent O-ring. When is this justified?
| Cost Factor | O-Ring | Spring Energized Seal |
|---|---|---|
| Unit cost (typical) | $0.10–$50 | $10–$2,000+ |
| Lead time (custom) | 7–15 days | 3–6 weeks |
| Service life in cryogenic application | Hours to days (elastomer becomes rigid) | Years (no elastic recovery required) |
| Service life in vacuum application | Limited by compression set accumulation | Long-term (spring maintains contact) |
| Replacement cost per maintenance event | Low | High |
| Consequence of failure | System-dependent | Often very high (vacuum, cryogenic, precision) |
Break-even scenario: A cryogenic valve in a liquid nitrogen system requires quarterly O-ring replacement at $5/O-ring + $2,000 labor/downtime per event = $8,020/year. A spring energized PTFE seal at $200/unit with annual replacement = $200 + $2,000 = $2,200/year. The SES is 73% cheaper annually despite the 40× higher unit price — driven entirely by the service life and maintenance labor differential.
Procurement Notes
Spring energized seals are not catalog items in the same way standard O-rings are. Most configurations are custom-manufactured to the customer's groove dimensions and service conditions. Standard lead time is 3–6 weeks for custom configurations; urgent delivery 1–2 weeks for standard envelope sizes. MOQ is typically 5–10 pieces minimum for custom sizes.
Specify: groove OD and ID (or rod diameter and bore diameter), seal width, operating temperature range, pressure, fluid media, motion type (static/reciprocating/rotary), and required spring material. Provide groove dimensions rather than nominal shaft/bore sizes — the groove geometry determines fit and sealing force.
NBR, FKM, HNBR, and EPDM O-rings for standard grooves are available from stock with 3–7 day delivery. Spring energized seals require custom manufacturing with 3–6 week lead time. For cryogenic or vacuum service, contact our engineering team to confirm spring type, PTFE grade, and groove dimensional verification before ordering.
FAQ
Q1: Can O-rings be used in cryogenic service?
Silicone O-rings (VMQ) are rated to −60°C and can function at liquid CO2 temperatures (−78°C) with reduced performance. Below −60°C, all practical elastomers lose functional elastic recovery — they become rigid and cannot maintain sealing contact. Spring energized PTFE seals with Elgiloy or Hastelloy metallic springs operate to −270°C (liquid helium temperatures) and are the standard seal technology for liquid cryogen service.
Q2: Why can't standard O-rings seal high vacuum?
In high vacuum (below ~10⁻³ torr), there is no meaningful pressure difference across the O-ring to push it against the low-pressure gland wall. The O-ring relies entirely on its initial elastic squeeze for sealing contact. As compression set accumulates over time (particularly at elevated temperature), the contact force decreases toward zero and the vacuum seal fails. A spring energized seal maintains contact force from the internal spring, independent of system pressure — it seals identically in hard vacuum and at elevated pressure.
Q3: What spring material should I use for cryogenic service?
For liquid nitrogen (−196°C) and liquid oxygen (−183°C), 17-7 PH stainless steel or Elgiloy springs are standard. For liquid hydrogen (−253°C), Elgiloy or Hastelloy C-276 are preferred. For liquid helium (−269°C), Elgiloy is the standard — it retains adequate spring force characteristics at this extreme temperature. Avoid standard 302/304 stainless steel springs in liquid nitrogen and colder service — these alloys may partially embrittle and lose spring force consistency.
Q4: Are spring energized seals suitable for high-cycle reciprocating service?
Yes, with the correct PTFE jacket fill. Filled PTFE grades (15% glass-filled, 25% carbon-filled, or carbon/graphite blends) provide significantly better wear resistance in reciprocating service than virgin PTFE. The canted coil spring maintains consistent sealing force as the jacket wears — unlike an O-ring where wear creates a gap. Spring energized seals routinely outlast elastomeric O-rings in high-cycle reciprocating service when friction and wear are the primary failure mechanisms.
Q5: Is FFKM a better choice than spring energized seals for aggressive chemical service?
For most aggressive chemical applications, FFKM O-rings are the simpler and often more economical first choice — they provide near-universal chemical resistance in standard groove geometries at 10–100× lower cost than SES. Spring energized PTFE seals are justified when FFKM is also chemically attacked (specific fluorine environments, strong reducing agents, ClF₃), when cryogenic temperatures exceed FFKM's elastic range (below −20°C), or when friction must be lower than any elastomer can achieve.
Q6: Do spring energized seals work in rotary applications?
Yes. Canted coil springs are particularly well-suited to rotary service because their constant-force characteristic maintains consistent lip contact force throughout rotation, even as the PTFE jacket wears asymmetrically. PTFE's very low friction (COF 0.05–0.10) generates significantly less heat in rotary sealing than elastomers (COF 0.15–0.30), which is critical for high-speed rotation where friction-generated heat can be the life-limiting factor.
Q7: Can spring energized seals handle both directions of pressure?
Single-lip SES (one seal lip) seals in one direction — the system pressure loads the spring, and the spring maintains contact. For bidirectional pressure, dual-lip SES configurations are used — each lip seals one pressure direction. This eliminates the need for back-to-back elastomeric seal arrangements in reversing-pressure applications. Bidirectional SES is common in semiconductor process equipment, analytical instruments, and precision hydraulic systems where O-ring back-to-back pairs generate unacceptable friction.
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Need spring energized PTFE seals for cryogenic, vacuum, or low-friction service? Contact our engineering team with your groove OD/ID, width, temperature range, pressure, fluid media, and motion type — we specify spring type (canted coil, V-spring, garter), PTFE jacket grade, and spring material for your application, and supply custom SES with 3–6 week lead time at MOQ 5 pieces. O-ring alternatives in NBR, FKM, FFKM, and EPDM are available from stock with 3–7 day delivery for standard applications.