Square Rings & Lathe-Cut Square-Section Seals
Square cross-section seals with a wider contact band — molded or lathe-cut — for static face-seal and groove duties.
Overview
Advantages of the Square Profile
Wider Sealing Contact Band
The flat faces of the square cross-section produce a wider sealing contact band than a round O-ring of the same nominal size. The wider band spreads clamp load over more surface area and helps the seal bridge minor surface-finish variation on machined and cast flange faces — the conditions where this profile is most often specified.
Reduced Tendency to Roll or Twist
Flat sides seat against the groove, giving square rings a reduced tendency to roll or twist during assembly and under vibration compared with a round profile. On large-diameter joints where a round seal can be pinched or rolled during installation, the square profile's flat sides make seating more predictable. Sealing ability is comparable to O-rings — the geometry changes how the seal seats, not its fundamental performance.
Molded or Lathe-Cut Manufacturing
Two manufacturing routes cover the full quantity and tolerance range. Compression-molded square rings suit volume production with tighter dimensional control; lathe-cut rings are cut from extruded square-section tube, so custom inside diameters are produced without mold tooling and MOQ starts from 1 piece. On lathe-cut rings, the cut faces carry no mold parting line.
Positioned for Static Flange and Groove Duty
Square rings are primarily specified for static face-seal and groove duties: bolted flanges, covers, sight-glass grooves, and custom rectangular glands. In dynamic service they generate higher friction than X-rings, so reciprocating applications are better served by four-lobe profiles. Specifying the square profile where its contact band and seating behavior matter is the correct engineering use of this geometry.
Materials
| Material | Properties | Best For |
|---|---|---|
| NBR Square Rings | −40°C to +120°C (typical, compound-dependent); excellent petroleum oil and fuel resistance; standard 70 Shore A | General industrial flanges, hydraulic port faces, fuel system joints, OEM replacement seals |
| FKM Square Rings | −20°C to +200°C (typical, compound-dependent); broad chemical and heat resistance | Chemical process lines, high-temperature flanges, aggressive fuels, oil and gas static joints |
| EPDM Square Rings | −50°C to +150°C (typical, compound-dependent); excellent water, steam and ozone resistance; not for petroleum oils | Water systems, HVAC, food-grade drainage equipment, outdoor enclosures |
| VMQ (Silicone) Square Rings | −60°C to +200°C (typical, compound-dependent); wide temperature flexibility; FDA-compliant grades available | Sight-glass grooves, food equipment, medical device housings, low- and high-temperature static seals |
| PTFE Square Rings | Unfilled PTFE for chemical service, lathe-cut; broad chemical resistance and near-universal media inertness | Chemical process flanges, aggressive solvents, semiconductor lines, static duties beyond elastomer range |
Properties & Specifications
| Cross-Section Geometry | Square (four flat sides) |
| Manufacturing Methods | Molded, or lathe-cut from extruded square-section tube |
| Standard Materials | NBR, FKM, EPDM, VMQ (Silicone), PTFE |
| NBR Temperature Range | −40°C to +120°C (typical, compound-dependent) |
| FKM Temperature Range | −20°C to +200°C (typical, compound-dependent) |
| EPDM Temperature Range | −50°C to +150°C (typical, compound-dependent) |
| VMQ Temperature Range | −60°C to +200°C (typical, compound-dependent) |
| Custom Sizes | Manufactured to drawing; MOQ from 1 piece |
Applications
Static Flange & Cover Sealing
Bolted flanges, inspection covers, and manifold face joints in general industrial machinery use square rings where the mating face finish or bolt-load distribution favors a wide flat contact band. The flat sides seat predictably in the groove, and the profile's reduced tendency to roll or twist during assembly simplifies installation on large-diameter joints that are closed by hand.
Glass & Sight-Glass Grooves
Sight glasses, level gauges, and observation windows seal with square-section rings seated in rectangular retaining grooves. The flat sides bear evenly against glass and metal, and silicone (VMQ) or EPDM compounds handle the temperature swings and cleaning regimes these assemblies see. Custom diameters for legacy gauge grooves are lathe-cut to match the existing gland without new tooling.
Semiconductor Clean Process Lines
Ultrapure-water, process-chemical, and drain lines in semiconductor fabrication use square-section seals in PTFE and high-purity elastomer grades. Lathe-cut unfilled PTFE square rings suit chemical service where broad chemical resistance is required, and lathe-cut production supplies the non-standard diameters that specialty manifold and fitting joints demand without mold tooling.
Food-Grade & Drainage Equipment
Food processing equipment, washdown enclosures, and drainage fittings specify EPDM and FDA-compliant silicone square rings for hot water, cleaning chemicals, and ozone exposure. The square profile matches the rectangular grooves common in sanitary equipment, and custom diameters are lathe-cut during maintenance so legacy fittings can be resealed without replacing the housing.
Oil & Gas Static Joints
Static flanged connections, valve body covers, and instrumentation ports in oil and gas service use NBR and FKM square rings where vibration and pressure cycling act on the joint. FKM grades extend service into higher temperatures and aggressive media, while the flat-sided profile holds position in joints that see frequent thermal cycling.
Sizing
Square rings are specified by inside diameter and cross-section (flat-side width). We supply cross-sections to AS568-equivalent sizes, plus custom lathe-cut rings manufactured to drawing. Groove compatibility must be confirmed for each gland — fill, squeeze, and tolerances — before ordering.
AS568-Equivalent Cross-Sections
- • 1.78 mm (CS)
- • 2.62 mm (CS)
- • 3.53 mm (CS)
- • 5.33 mm (CS)
- • 6.99 mm (CS)
Custom Lathe-Cut
- • Any ID to drawing
- • MOQ from 1 piece
- • No mold tooling required
Frequently Asked Questions
What is a square ring?
A square ring is a seal with a square cross-section — four flat sides of equal width — used primarily in static face-seal and groove duties. Where a round O-ring presents a curved sealing surface, the square ring presents flat faces, producing a wider sealing contact band against the mating surface. Two manufacturing routes are used: compression-molded complete rings, or lathe-cut rings cut from extruded square-section tube. Cross-sections are offered to AS568-equivalent sizes, and custom inside diameters can be lathe-cut to drawing without mold tooling.
Square ring vs O-ring — what is the difference?
The difference is cross-section geometry. A round O-ring seals along a narrow curved contact band; a square ring presents flat faces, producing a wider sealing contact band on the mating surface. The square profile also shows a reduced tendency to roll or twist during assembly and under vibration. Sealing ability is comparable to O-rings, not inherently superior — square rings are chosen for static face-seal and groove duties where the wider contact band matters, or where a custom-dimension seal is needed without mold tooling. In dynamic service, square rings generate higher friction than X-rings and are rarely the first choice.
Square ring vs X-ring — which is better for my application?
They solve different problems. X-rings (four-lobed seals) are designed for dynamic reciprocating service, where their geometry resists spiral failure and lowers friction. Square rings generate higher friction than X-rings in dynamic service and are primarily specified for static face-seal and groove duties, where a wide flat contact band is the advantage. If the seal reciprocates — hydraulic cylinder rods, pneumatic pistons — specify an X-ring. If the seal is stationary — bolted flanges, covers, sight-glass grooves, custom rectangular glands — a square ring is the appropriate profile, and it can be lathe-cut to any inside diameter.
Are square rings interchangeable with O-rings in the same groove?
Not automatically. Square rings are produced with cross-sections to AS568-equivalent sizes, but groove compatibility must be confirmed for each gland: fill, squeeze, and tolerances all differ between a square profile and a round O-ring. A square cross-section carries more cross-sectional area than a round section of the same nominal size, so gland fill increases, and the flat sides seat differently against the groove walls. Before substituting a square ring into an existing O-ring groove, measure the groove, check fill and squeeze, or send us the gland dimensions and we will confirm fit.
What is the difference between molded and lathe-cut square rings?
Molded square rings are compression-molded as complete rings in dedicated tooling — suited to production volumes and tighter dimensional control. Lathe-cut square rings are cut from extruded square-section tube: any inside diameter can be produced without mold tooling, which makes custom sizes economical at low quantities, with MOQ from 1 piece. Because the profile is cut rather than molded, the cut faces carry no mold parting line. Lathe-cut rings produced from cord stock may show a vulcanized butt joint; molded rings are seamless.
What materials are square rings available in?
Five standard families. NBR from −40°C to +120°C (typical, compound-dependent) for petroleum oils and fuels. FKM from −20°C to +200°C (typical, compound-dependent) for chemical and heat resistance. EPDM from −50°C to +150°C (typical, compound-dependent) for water, steam, and ozone — not petroleum oils. VMQ silicone from −60°C to +200°C (typical, compound-dependent) for wide temperature swings, with FDA-compliant grades available. And unfilled PTFE for chemical service — lathe-cut, with broad chemical resistance. Compound selection depends on media, temperature cycling, and pressure; send your operating conditions for a recommendation.
What sizes are available in square rings?
Cross-sections are offered to AS568-equivalent sizes: 1.78, 2.62, 3.53, 5.33, and 6.99 mm, matching the standard AS568 cross-section series. Inside diameters span the corresponding size range, and any non-standard inside diameter can be lathe-cut from extruded tube. When ordering, specify inside diameter, cross-section (the flat-side width), material, and hardness. When replacing an existing seal, measure the inside diameter and cross-section of the old ring and the groove it seats in — groove compatibility must be confirmed for each gland before ordering.
Do you offer custom square rings from drawings?
Yes. Custom square rings are manufactured to drawing by two routes: lathe-cut from extruded square-section tube for quick-turn custom inside diameters with no mold tooling and MOQ from 1 piece, or compression-molded in dedicated tooling for volume and tighter tolerances. Send inside diameter, cross-section, material, hardness, and quantity — and include groove dimensions if the gland is new or being revised, so gland fill and squeeze can be confirmed as part of the review. Dimensioned sketches and formal drawings are both workable.
How do I design the groove for a square ring?
A square ring is normally specified for a dedicated rectangular gland with flat, parallel sides; follow manufacturer groove tables for width, depth and corner radii rather than assuming O-ring gland dimensions transfer directly. For face seals where the ring must stay in place during assembly, a dovetail-style retaining groove can work, but retention should be verified against the ring's weight and the assembly direction - dovetail contours are dimensioned for round profiles, so check the fit with your seal supplier. Keep gland fill conservative (around 75% nominal is a common design target, with margin for thermal expansion and fluid swell), specify sealing surface finish per the groove design tables (32 uin Ra on sealing faces with more allowance on groove sides, 63 uin Ra a common upper bound), and for horizontal assemblies a compatible grease can temporarily hold the ring in a plain groove during assembly. When in doubt, send the gland drawing to your seal supplier for review.