O-ring vs backup ring in hydraulic systems: when to use each and why

An O-ring closes the space between mating surfaces through being able to deform under pressure. A backup ring cannot seal by itself—it is a support for the O-ring and blocks it from expanding into the gap in the clearance at high pressures. Use O-rings by themselves at low to moderate pressures. Add backup rings if system pressure is higher than about 1500 psi or when the clearance gaps are huge.

What is an O-ring?

An O-ring is a toroidal (doughnut-shaped) seal made of elastomer that is designed to fit into an engraved groove, creating an airtight seal between mating surfaces. When the unit is set up and pressure is applied on the structure, the O-ring gets stretched and deforms to fit the groove's walls as well as the mating surfaces, filling tiny imperfections and preventing the flow of fluid.

O-rings for hydraulic systems are generally made of nitrile (NBR) used for general mineral oil services or fluorocarbon (FKM/Viton) for high-temperature or extremely corrosive fluid environments or EPDM when phosphate- or water-based fluids are required. The choice of the material affects the compatibility of the fluid and the temperature range, and the physical shape determines how the ring is sealed.

The process is quite simple: the O ring operates because the elastomer can't be compressed. When squeezed radially, it will cause it to flow outwards and fill the space and fill the gap. The same principle applies regardless of whether the use will be stationary (face seal fittings, port fittings, and port plugs) as well as dynamic (cylinder rods and the piston grooves). 

The main drawback of O-rings that operate on their own is extrusion driven by pressure. When the system pressure is elevated, the elastomer will begin to flow into the diametric gap between the bores mating. In the event that pressures are sufficient or the gap between the clearances is too wide, the ring will be physically pushed into the gap, which causes damage to the seal, causing leakage. This is the type of failure whose backup rings are designed to stop.

What is an alternate ring?

An additional ring (also called an anti-extrusion ring) is a solid semi-rigid ring that is set inside the groove that is the O-ring and located on the side that is downstream, or low-pressure, of the seal. Its function is structural: It bridges the diametric gap in clearance and stops the O-ring from expanding into it during pressure.

Backup rings are made of materials that are much more durable than O-ring elastomers:

• PTFE (Polytetrafluoroethylene): The most common choice. PTFE is extremely low friction, superior chemical resistance to virtually every type of hydraulic fluid, and has a broad temperature range for service. Its slight cold-flow tendencies at extremely high compression are typically acceptable for backup ring applications.
• Nylon (polyamide) is harder and more durable than PTFE, employed when mechanical load is excessive. It is slightly more sensitive to absorption of moisture and can cause the swelling of water-based systems.
• Leather is a more traditional material that is still used in older equipment. Leather backup rings have good conformability; however, they are limited in compatibility with water-based or synthetic fluids.

Backup rings are available in two different geometric shapes. The solid backup ring can be described as a complete circle without a split; it is an axial fit into the groove. It can be utilized when disassembly is not frequent and the groove is accessible. An elliptical (or cut scarf) backup ring is one with a helical cut, which allows it to be inserted into the groove, which makes it easier to install, especially in grooves with blinds or on cylinder rods when assembling.

It is important to clarify that a backup ring without an O-ring has no value. It is not a sealant on its own. The O-ring is the sealant element. The backup ring is simply keeping its geometry intact when pressure is applied.

The threshold for pressure—what is the right time to use both?

The most commonly accepted guidelines for hydraulics are

• Below about 1,000-1,500 PSI (70-105 bar), an O-ring alone is generally sufficient if the dimensions of the groove are accurate as well as the gap for clearance is within the tolerances of the standard.
• 1,500-3,500 Psi (105-210 bar) Backup rings are suggested especially for applications that are dynamic or when clearance gaps are on the high end of tolerance.
• More than 3000 Psi (210 bar) backup rings are required in the majority of configurations. For extreme applications or big bore sizes, two back-up rings (one on either end of the O-ring) can be specified.

They are guidelines but not rigid rules. The size of the clearance gap is the second major variable. A large-bore cylinder that has an ample diametric clearance is likely to have O-ring extrusion even with lower pressures than a smaller bore fitted with a tight tolerance. This is the reason hydraulic cylinder manufacturers typically specify backup rings, even in moderate-pressure designs in which the bore's diameter is big and the gap between the rod gland and the piston land can cause extrusion risks even if the pressure is considered conservative.

The temperature of the system also plays into the calculation. Higher temperatures decrease the Shore hardness of O-rings and make the elastomer susceptible to flow under load. In high-temperature circuits such as hydraulic presses and die-casting equipment, steel mill hydraulics backup rings are typically utilized at pressures where they would otherwise be ineffective due to the fact that the softened elastomer requires additional mechanical support.

Dynamic vs. static. Dynamic applications

The distinction between dynamic and static sealing conditions influences the choice of ring for backup significantly.

The static seals (face seal fittings, port connections, flange joints) have no movement between the surfaces of the seals. O-rings for static applications can perform reliably even when pressure is higher than that of dynamic seals because they're not affected by the abrasion and mechanical strain that come with sliding contacts. A lot of static SAE O-ring face seal (ORFS) fittings function without backup rings at pressures of 5,000 to 6,000 psi because the groove's geometry is managed and the elastomer doesn't move in relation to metal.

Dynamic seals—piston seals, cylinder rod seals, or rotary shaft seals—they are a completely different scenario. The O-ring is subjected to constant sliding contact with the metal surface and, simultaneously, hydraulic pressure. The combination of heat, friction, and pressure loads significantly increases the risk of extrusion. Particularly in hydraulic cylinders, backup rings are common in designs with piston grooves with pressures of more than 1,500 PSI and are a standard feature for the design of rod seals to stop the rod's seal from getting forced into the rod-to-gland clearance in retract and extend strokes.

Installation: Making it work

Backup rings must be put on the right side of the O-ring, the downstream or lower pressure side in relation to the pressure seal. If the pressure is acting from above the backup ring, it will be below. For bidirectional applications of pressure (common for hydraulic cylinders) A backup ring is required on both sides, creating an arrangement of grooves: backup ring, O-ring, backup rings.

Groove dimensions should be accurate for the stack. Incorporating a backup ring into the groove designed for an O-ring by itself will fill the groove, which can result in excessive O-ring compression and increased friction in dynamic applications and the possibility of damage to the seal. Parker Hannifin and Trelleborg, as well as other major seal makers, provide dimensioned groove guidelines for backup ring and O-ring combinations in relation to the O-ring's cross-sectional diameter.

During installation, do not stretch O-rings to more than 50% of their diameter, and do not place a backup ring on sharp edges or threads. Nylon and PTFE can cut and lose their ability to prevent extrusion. Utilize a blunt-tipped installation tool or a chamfered guide sleeve.

Common errors in the field

Utilizing a backup ring replacement for a designated O-ring that is properly specified. If an O-ring has failed due to chemical incompatibility, the backup ring won't aid, as it will only slow the process down a bit as the root cause persists.

The backup ring should be placed on the opposite side. A backup ring that is placed on the high-pressure side of an O-ring does not provide any protection. This error is quite common in field repairs, when technicians are working off of memory, not sketching a cross-section.

Assuming that backup rings will always be needed. Over-specification can cause friction and cost. In return line applications with low pressure, including backup rings in every O-ring groove can increase assembly complexity but does not provide any benefit.

Inattention to the gap in clearance after component wear. A bore or cylinder that has deteriorated over time could have larger diametric clearance than what was designed. O-rings that are sealed properly in the initial stages of equipment could begin to expand after wear has opened the gap. Adding additional rings in the event of a rebuild is the best option.

Both the O-ring and the backup ring aren't two different technologies—they're complementary. The O-ring seals while the backup ring maintains the shape of the O-ring, so it will continue to do its work under the tensions modern hydraulic systems require. Knowing which combination you require for your application is among the most simple decisions to make regarding hydraulic sealing when you understand the function of each component.

Can I put a backup ring that does not have an O-ring?

No. A backup ring does not have the ability to seal by itself. It should always be used by an O-ring.

Do I require a second ring that is on each side of my O-ring?

In bidirectional pressure applications, for example, hydraulic pistons in cylinders. Yes. A single backup ring safeguards against the extrusion of one pressure direction.

If I fail to put in an additional ring?

When pressure is high, the O-ring begins to expand into the gap. This can manifest as leakage as well as a snipped or torn O-ring upon inspection, and in the most severe instances, the seal will fail quickly.

Are PTFE back rings compatible with hydraulic fluids of all kinds?

PTFE has wide chemical compatibility and can be used with natural oils, synthetic liquids, biodegradable fluids, and water-based fluids. It is the most secure option when the type of fluid is not certain.

Can I reuse backup rings during a seal kit replacement?

Backup rings must be replaced with the O-ring on a regular basis as a matter of routine. PTFE rings are particularly vulnerable to cold flow and micro-cracking with time, and the expense of an extra ring is not much in comparison to the cost of a second seal failure.