O-rings & Back-up Rings

Overview

The gold standard in sealing

O-rings are found in countless industrial applications. Many remember the faulty O-ring that contributed to the crisis during the Apollo 13 mission — but their reliability in every other system on board is what brought the crew home safely. That's the real story of O-ring engineering.

Their circular cross-section and elasticity create a barrier between two surfaces, preventing leaks of liquids or gases even under extreme pressures and temperatures. Made from nitrile (NBR), silicone (VMQ), fluorocarbon (FKM/Viton), EPDM, HNBR, and more, O-rings adapt to conditions ranging from corrosive chemical environments to deep-freeze cold storage.

At ELASFOR, we supply O-rings in every conceivable size and material — stock and custom. Unusual bore? Metric groove in an imperial world? Specific compound for a fluid your regular supplier can't identify? That's where we come in.

Design Reference

Groove & installation design

Groove dimensions

These are the baseline rules that give an O-ring enough room to function — too tight and it extrudes, too loose and it rolls. Get these four parameters right and the rest follows.

Parameter	Key Design Rule	Why It Matters
Groove Width (b)	Must exceed groove depth (h). Rule of thumb: ×1.35 relative to the O-ring cross-section diameter.	Elastomers are incompressible — the displaced volume must go somewhere. Without lateral room the seal over-stresses and cracks.
Groove Volume	Must be at least 15% greater than the O-ring volume.	Prevents hydraulic locking during installation and stops thermal expansion from destroying the seal at temperature.
Groove Depth (h)	Sets the compression ratio — sized against the O-ring cross-section for the target squeeze percentage.	Under-depth = leak. Over-depth = extrusion under pressure. See Compression Ratio table below.
Radial Clearance	Minimize the gap between mating parts. Maximum allowable gap is a function of pressure and Shore hardness.	Too much clearance allows cold-flow or extrusion into the gap under pressure cycling — nibbling failure. Back-up rings fix large clearances.

Detailed gland dimension tables per AS568, BS1806, and metric standards available on request.

O-ring Gland Calculator ERIKS Technical Manual (PDF)

Compression ratio (squeeze / deflection)

Compression ratio is how much the O-ring cross-section is squeezed between groove depth and mating surface. Too little and you leak at low pressure; too much and you generate heat, friction, and accelerated wear in dynamic applications.

Application Type	Recommended Squeeze	Important Notes
Static — face / radial	15 % – 25 %	Above 30 % creates excessive internal stress and accelerated compression set. Higher squeeze is not always better — it just destroys the seal faster.
Dynamic — reciprocating	8 % – 16 % (max 20 % for cross-sections under 1.78 mm)	Lower squeeze reduces friction, heat generation, and abrasive wear. Over-squeeze in a cylinder is the single most common cause of premature failure.
Dynamic — rotary shaft	5 % – 12 %	Rotary applications generate the most heat per unit time. Consider lip seals or PTFE-encapsulated alternatives at higher RPM.

Material selection & hardness

The O-ring must be chemically compatible with every fluid in the system — including cleaning agents and lubricants, not just the primary process fluid. Hardness (Shore A) governs extrusion resistance and friction.

Factor	Selection Guidance	Typical Application
Hardness (Shore A)	Higher pressure and larger clearance gaps → harder compound (70–90 Shore A). Low-pressure static seals → softer compound (50–60 Shore A) for better surface conformance.	Hydraulic cylinders: 80–90 Shore A NBR or HNBR. Vacuum flanges: 50–60 Shore A silicone or FKM.
Chemical compatibility	Verify compatibility with all fluids present. Swelling >15% or hardness loss >10 points indicates incompatibility.	Petroleum hydraulics → NBR. Phosphate-ester fluids (Skydrol, HyJet) → EPDM. Aggressive solvents / high temp → FKM or FFKM.
Anti-extrusion rings	Required when system pressure exceeds ~100 bar (1500 psi) or radial clearance cannot be tightened. PTFE or hard nylon back-up rings fit standard grooves.	Mobile hydraulics, injection moulding clamps, wellhead equipment.
Temperature range	Match material limits to both operating and peak excursion temperatures. A single 30-second over-temperature event can cause permanent compression set.	−55 °C to +200 °C → FKM. −60 °C to +200 °C → Silicone. −40 °C to +150 °C → HNBR for H₂S / steam.

Surface finish & installation

A perfect O-ring in a damaged or mis-machined groove will fail every time. Surface condition and installation technique account for a significant share of seal failures that get misdiagnosed as material selection problems.

Element	Requirement	Installation Notes
Groove edges	All edges contacting the seal must be radiused — no sharp corners, no burrs.	Sharp edges are the leading cause of cut seals at assembly. A radius of 0.2–0.4 mm is sufficient. Inspect with a fingernail before installing any seal.
Surface finish (Ra)	Static: Ra 0.8–1.6 µm. Dynamic reciprocating: Ra 0.2–0.4 µm. Dynamic rotary: Ra 0.1–0.2 µm.	Too rough abrades the seal; too smooth can hydroplane lubricant film in dynamic applications.
Lead-in chamfers	15°–20° chamfer on the entry edge of piston bores, ports, and gland covers. Length ≥ 1× O-ring cross-section.	Always lubricate the O-ring — system fluid, or compatible silicone grease for static seals.
ID stretch	Inner diameter stretch during installation should not exceed 50% for standard elastomers. PTFE-based O-rings have much tighter limits.	Over-stretching causes immediate micro-cracking. Use an O-ring cone or mandrel for shafts — never sharp tools.

Material Quick Reference

Common O-ring elastomers at a glance

First-pass filter only — dozens of compound grades exist within each family. Confirm against your specific fluid and temperature profile before specifying.

Back-up Rings

Anti-extrusion rings — shapes, capabilities & when to specify them

A back-up ring is a rigid or semi-rigid ring installed in the same groove as an O-ring, on the low-pressure side (single back-up) or both sides (dual back-up). Its job is to bridge the diametric clearance gap between mating parts and prevent the O-ring from cold-flowing or extruding into that gap under pressure. Without a back-up ring, that gap is the limiting factor on system pressure — with one, the O-ring operates well above what the elastomer alone could handle.

Solid (endless) back-up ring

One-piece continuous ring — no joint, no gap. Maximum extrusion resistance because the O-ring has zero path through which to cold-flow. Requires full disassembly to place in the groove; not usable in blind grooves without removing the shaft or piston.

Extrusion Resistance	Installation	Best For
Maximum — zero gap	Full disassembly required. Shaft or piston must be removed to place ring in groove.	High-pressure static seals, piston seals assembled before bore installation, applications above 400 bar.

Split back-up rings — scarf cut (30°) & butt cut

Both types feature a single cut across the ring cross-section that allows the ring to open and be installed without full disassembly. The scarf cut is diagonal (typically 30–45°), leaving a small helical gap under pressure. The butt cut is a straight perpendicular cut — slightly simpler to produce but leaves a straight-through gap that provides marginally less extrusion protection than the scarf. Both are the practical field choice when disassembly is not possible.

Type	Extrusion Resistance	Installation	Best For
Scarf cut (30°)	Good — small helical gap remains. Gap size depends on ring stiffness and groove fit.	Opens and winds into groove without disassembly. PTFE scarf cuts snap into place easily.	Standard field-installable back-up ring for most hydraulic and pneumatic cylinder applications.
Butt cut	Good — straight-through gap slightly less favorable than scarf under pressure cycling.	Same ease of installation as scarf cut. Simpler geometry — easier to produce in harder materials.	Where scarf geometry is unavailable in the required material; lower-pressure split-ring applications.

Spiral (multi-turn) back-up ring

A strip of PTFE wound in multiple overlapping helical turns — typically 1.5 to 2.5 turns — into the groove. No single gap runs through the full cross-section, giving extrusion resistance that approaches a solid ring while being the easiest of all types to install. Exclusively made in PTFE.

Extrusion Resistance	Installation	Best For
Excellent — overlapping turns eliminate any continuous gap from face to face.	Wind the strip into the groove by hand, no tools required. Works in blind grooves and any groove geometry.	Large-bore static seals, flanges, and oversized cylinders where solid or step-cut rings are impractical. Go-to when adding a back-up ring to an existing design without modifying the groove.

Standard back-up ring dimensions follow the same groove as the O-ring they protect. Parker, Trelleborg, and similar manufacturers publish standard groove dimension tables per AS568 and ISO 3601 for O-ring + back-up ring combinations.

Back-up Ring Materials

Anti-extrusion ring compounds at a glance

Back-up ring materials are rigid or semi-rigid — they do not seal, they support. The key properties are hardness (to resist deformation into the clearance gap), chemical compatibility with the system fluid, and enough flexibility to install into the groove without cracking. Material choice also sets the temperature ceiling and determines whether a scarf or step cut is practical.