{"id":1648,"date":"2026-11-03T17:14:00","date_gmt":"2026-11-03T09:14:00","guid":{"rendered":"https:\/\/www.bosssealing.com\/?p=1648"},"modified":"2026-11-03T17:14:00","modified_gmt":"2026-11-03T09:14:00","slug":"what-is-a-backup-ring","status":"publish","type":"post","link":"https:\/\/www.bosssealing.com\/it\/what-is-a-backup-ring\/","title":{"rendered":"What Is a Backup Ring? Anti-Extrusion Design, Materials and When You Need One"},"content":{"rendered":"

A backup ring is a flat, rigid ring that sits next to an elastomer seal on the low-pressure side and stops the seal from being forced into the gap between the metal parts. It does not seal on its own. It has no squeeze, no lip and no elastic memory. Its entire job is to close the extrusion gap so the O-ring behind it never has to flow into a space smaller than itself.<\/p>\n

Most engineers first meet the part after a failure: an O-ring with one edge chewed off, a gland full of rubber crumbs, and a maintenance team replacing the same seal every few weeks. The fix is usually a ring that costs less than the O-ring it protects.<\/p>\n

What Extrusion Actually Is<\/h2>\n

An elastomer under pressure behaves partly like a liquid. When 200 bar pushes an O-ring against the clearance between a piston and a bore, the material does not simply compress; it flows toward the path of least resistance, which is the gap. The gap on a new cylinder might be 0.15 mm. On a worn one it can reach 0.5 mm. At that size, an 80 \u00b0C NBR O-ring at 200 bar will push a visible tongue of rubber into the clearance within hours.<\/p>\n

Once a tongue forms, the moving parts shear it off. The torn edge becomes a leak path, the leak raises local temperature, and the damage propagates around the circumference. The end state is a nibbled, flattened ring and a leak that no amount of tightening will stop.<\/p>\n

A backup ring interrupts that sequence by occupying the gap with a material that will not flow at the operating temperature. It is a mechanical stop, not a seal.<\/p>\n

Where the Ring Sits<\/h2>\n

Position is not negotiable: the backup ring goes on the side of the primary seal away from the pressure, facing the extrusion gap. In a rod gland pressurised from the bore side, the O-ring is loaded toward the gland outer edge and the backup ring sits between the O-ring and that edge.<\/p>\n

In a double-acting cylinder the pressure alternates, so a single backup ring protects one direction only. Fit two, one on each face of the O-ring. Two rings also mean the groove must be wide enough: a standard O-ring groove sized for the O-ring alone will not accept a second ring without reducing squeeze to the point where the seal leaks at low pressure. Groove width, not ring thickness, is what constrains retrofit options.<\/p>\n

\"Large
A PTFE backup ring on the low-pressure face of a large O-ring: the ring closes the clearance gap, the O-ring does the sealing<\/figcaption><\/figure>\n

The Five Standard Configurations<\/h2>\n

ISO 3601-4 groups anti-extrusion rings into five geometries. The differences are about how the ring gets onto the rod or into the groove without being cut, and how much continuous support it gives once installed.<\/p>\n\n\n\n\n\n\n\n\n\n
Type<\/th>\nCut style<\/th>\nInstallation<\/th>\nWhere it is used<\/th>\n<\/tr>\n<\/thead>\n
T1<\/td>\nSpiral cut (continuous coil)<\/td>\nWraps over a rod or into a groove without dismantling the assembly<\/td>\nField repair, large diameters, retrofits on assembled cylinders<\/td>\n<\/tr>\n
T2<\/td>\nAngle cut (scarf)<\/td>\nOpens and snaps over the rod; ends overlap under pressure<\/td>\nGeneral hydraulic service, both static and dynamic<\/td>\n<\/tr>\n
T3<\/td>\nSolid, uncut<\/td>\nRequires full access; slides over the end of the rod or piston<\/td>\nHighest extrusion resistance, aerospace and high-pressure static joints<\/td>\n<\/tr>\n
T4<\/td>\nConcave angle cut<\/td>\nScarf cut with a profiled inner face<\/td>\nPrecision glands where the ring must mate closely with the O-ring curve<\/td>\n<\/tr>\n
T5<\/td>\nConcave solid<\/td>\nFull access required; profiled inner face<\/td>\nHigh-pressure glands with tight axial space<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

In practice, T2 is what most industrial cylinders use, T1 is what maintenance departments keep on the shelf for emergencies, and T3 is specified where the gland can be fully disassembled and the pressure justifies the assembly difficulty. A T3 ring forced over a rod with a screwdriver is a scrapped ring; the cut styles exist precisely to avoid that operation. Distributor catalogues such as Seal Sales backup rings<\/a> show the same five families with stock sizes, which is a quick way to see what is available off the shelf before ordering a machined part.<\/p>\n

\"L
An L-section ring for a rod gland: the section must match the groove width, which is why retrofit rings are usually machined rather than moulded<\/figcaption><\/figure>\n

Material Selection<\/h2>\n

The ring has to be harder than the O-ring at operating temperature but not so hard that it scores the bore. Three families cover almost every application.<\/p>\n\n\n\n\n\n\n\n\n\n
Materiale<\/th>\nTemperature<\/th>\nPressure capability<\/th>\nStrengths<\/th>\nLimits<\/th>\n<\/tr>\n<\/thead>\n
Virgin PTFE<\/td>\n\u2212200 to +260 \u00b0C<\/td>\nGood to ~250 bar with tight clearance<\/td>\nLowest friction, wide chemical resistance, lowest cost<\/td>\nCold flow under sustained load; deforms into large gaps<\/td>\n<\/tr>\n
Filled PTFE (glass, carbon, bronze)<\/td>\n\u2212200 to +260 \u00b0C<\/td>\nGood to ~400 bar<\/td>\nHigher compressive strength, better wear, less creep<\/td>\nBronze filler can corrode in some media; slightly more abrasive<\/td>\n<\/tr>\n
PEEK (unfilled or carbon\/graphite filled)<\/td>\n\u221260 to +250 \u00b0C continuous<\/td>\nTo 600 bar and above<\/td>\nHighest stiffness and creep resistance, holds large gaps<\/td>\nHigher cost, less conformable, needs good surface finish<\/td>\n<\/tr>\n
Nylon \/ POM<\/td>\n\u221240 to +100 \u00b0C<\/td>\nTo ~250 bar<\/td>\nCheap, rigid, easy to machine<\/td>\nSwells in water and some glycols; limited chemical range<\/td>\n<\/tr>\n
PU (high hardness)<\/td>\n\u221230 to +80 \u00b0C<\/td>\nTo ~300 bar<\/td>\nAbrasion resistant, tolerant of poor surface finish<\/td>\nPoor at high temperature; not for aggressive fluids<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Two selection traps are worth naming. Virgin PTFE backup rings creep: under a sustained 300 bar static load at 80 \u00b0C, the ring slowly thins and the gap reopens. Filled PTFE or PEEK solves it, at a price. The other trap is nylon in water-based fluid. Nylon absorbs water, grows, and can seize in the groove, which is why water hydraulics and water-glycol systems use PTFE or PEEK instead.<\/p>\n

The elastomer being protected matters as much as the ring. A 70 Shore A compound written to ASTM D2000<\/a> line notation is a different extrusion risk from a 90 Shore A compound in the same groove, and the ring thickness should follow the softer of the two.<\/p>\n

\"Carbon
Carbon-filled PEEK rings hold their shape where virgin PTFE would creep, at two to four times the part cost<\/figcaption><\/figure>\n

When You Need One<\/h2>\n

There is no single pressure threshold in any standard, because extrusion depends on the combination of pressure, clearance, temperature and elastomer hardness. The workable decision rule uses all four.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Condition<\/th>\nRecommendation<\/th>\n<\/tr>\n<\/thead>\n
Below 100 bar, clearance within ISO 5597 limits, 70 Shore A O-ring<\/td>\nNo ring needed<\/td>\n<\/tr>\n
100\u2013160 bar, standard clearance<\/td>\nOptional; fit one on the pressure side as insurance on shock-loaded systems<\/td>\n<\/tr>\n
160\u2013250 bar, or clearance above standard<\/td>\nOne ring on the low-pressure side, filled PTFE<\/td>\n<\/tr>\n
Above 250 bar<\/td>\nOne ring standard; PEEK above 400 bar or where clearance exceeds 0.3 mm<\/td>\n<\/tr>\n
Double-acting, above 160 bar<\/td>\nTwo rings, one on each face of the O-ring<\/td>\n<\/tr>\n
Any pressure with clearance above 0.4 mm<\/td>\nFix the clearance first; a ring alone will not compensate for a worn bore<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The clearance half of that table matters more than the pressure half. A cylinder at 150 bar with a 0.5 mm gap extrudes faster than one at 300 bar with a 0.1 mm gap. If you are choosing between adding a backup ring and replacing a worn guide bushing, replace the bushing. Our guide elements<\/a> exist for exactly that job: they hold the piston concentric so the clearance stays inside the range the seal was designed for.<\/p>\n

Design Details That Decide Whether It Works<\/h2>\n

Groove width.<\/strong> A single backup ring needs roughly 1.2\u20131.5 mm of extra axial space, depending on ring thickness. Two rings need double that. If the groove was cut for an O-ring only, the options are a thinner ring, a machined ring that shares the O-ring’s space, or a different seal profile altogether. The housing conventions behind those numbers are the ones published by Hallite<\/a> for hydraulic rod and piston seals, and they are the reference most European cylinder builders work from.<\/p>\n

Edge radius.<\/strong> A sharp groove corner shaves a PTFE ring during installation and leaves debris in the gland. A 0.2\u20130.4 mm radius on the groove entry is normal practice and costs nothing at the machining stage.<\/p>\n

Surface finish.<\/strong> The bore or rod under the ring should be no rougher than about Ra 0.4 \u00b5m for dynamic service. PTFE tolerates a rougher surface than PEEK; a PEEK ring running on a scored bore will wear the bore.<\/p>\n

Thermal expansion.<\/strong> PTFE expands roughly ten times as much as steel per degree. Rings above 200 mm diameter need a diametral clearance allowance in the groove, or they will close up at 80 \u00b0C and grip the rod.<\/p>\n

Installation Checks<\/h2>\n

Four checks catch nearly every backup ring installation error we see in returned parts:<\/p>\n