Blog Layout

3 Common Types of Backup Rings and Their Uses

Cliff • July 27, 2018

The Backup ring likely found its roots in the use of leather packings, where leather was used as the sealing device in glands prior to the use of O-Rings

The O-ring provided a much better seal compared to leather. However, the leather often filled the extrusion gap allowing for larger gaps.

The persistent problem that engineers faced was how to take an O-ring that operated optimally in close extrusion gaps, and extend its service with widening extrusion gaps.

The answer was the Backup ring.

The first Backup ring devices were made from materials like leather.

Leather did two things to help the O-Ring: it filled the gap, and continued to lubricate the O-Ring during dry running conditions. You can still occasionally find some old style hydraulic systems using leather back-up rings.

Modern Construction of the Backup Ring

Our modern hydraulic systems often find O-rings operating at extreme pressures, with the use of Backup rings made from a variety of materials like Teflon® or filled PTFE materials.

Polyester Elastomers like Hytrel®, Nylon , PEEK , and other high modulus materials that are compatible both in pressure and temperature to the application.

These Backup ring devices can take on many forms such as solid rings, split rings and spiral wound Backup rings.

Where there are extreme pressures and high temperatures you may find cammed Backup rings with varying materials to protect the O-ring, while at the same time closing the extrusion gap allowing extreme pressures in excess of 100 KPSI (690 KPA).

3 Common Backup Ring Shapes

There are 3 basic standard shapes for Backup rings: Solid Backup Rings, Spiral Wound Backup Rings and Scarf Cut Backup Rings.

Solid Backup Ring

The Solid Backup ring which, when made of an appropriate material can be forced into solid rod glands, but must be stretched to go into piston glands.

Face seals often find solid Backup rings due to the ease of installation.

Scarf Cut Backup Ring

The Scarf Cut Backup ring is probably the most commonly used Backup ring in today’s modern hydraulic system.

It’s easy to install in solid glands, whether they’re rod or piston. And because it’s split, the Scarf Cut Backup ring can be made of very hard materials to handle extreme pressures.

A note of caution with very high modulus materials: the split can cause nibbling of the O-Ring allowing for premature failure of the sealing system.

In the event of the need for these higher modulus materials, a softer ring may be placed between the O-Ring and the high modulus ring to protect the O-ring from this nibbling.

When applied properly, the Split Backup ring can open or spread to completely fill the extrusion gap when under pressure — something the Solid ring is unable to do.

Spiral Wound Backup Ring

The third Backup ring type is the Spiral Wound.

Modern systems rarely use this type of Backup ring due to the cost and availability. The Spiral Wound Backup ring isn’t commonly found on the shelf, but is occasionally employed for specific purposes.

There are many variations on Spiral Wound Backup ring, device such as the Par-Bak® or Cam-bak, which each serve a more specialized need.

Common Uses of Backup Rings.

The necessity of a backup ring is dependent on the extrusion gap, pressure, and temperature of the system.

Most O-rings from the zero series through the 400 series will survive with gaps ranging from .002 to .007 diametrical.

A typical O-ring will perform within the range of extrusion gap, and series up to 1500 PSI (10 MPA).

An extrusion gap allows for manufacturing to put together a system. Generally, the larger the diameter, the larger the extrusion gap.

Tight tolerance cylinders can be difficult to assemble, especially if you’re trying to maintain very small E-gaps.

Backup rings allow for ease in manufacturing to assemble without the fear of damaging rods or bores during the installation process do to overly tight E-gaps.

Cylinders and valve bodies often use O-Rings as static seals. Pressures exceeding 1500 psi and extrusion gaps exceeding the recommended maximum will require some form of back up ring device.

The shape and material will depend on the pressures and temperatures that the system is operating.

In piston style glands where pressure can be seen from both sides of the O-Ring, it’s common to use two Backup rings to support the O-ring in the extrusion gap from both sides of the O-Ring gland.

Static Face Seals often require Backup rings to support O-rings from entering the extrusion gap.

In extreme pressure cycles a known extrusion gap can be enlarged due to the gland bowing or moving into the extrusion gap.

Under extreme pressures, a Cam-style backup ring Is often used to drive a high modulus material into the ever-expanding extrusion gap closing the gap to zero and allowing the rubber O-ring to fill the void without entering the extrusion gap.

By Doug Montgomery February 13, 2025
Learn how Eclipse Seal’s custom spring energized ball seats with angled grooves improve performance
By Doug Montgomery January 17, 2025
Eclipse deals regularly with challenging sealing applications from all industries. High pressures and speeds create unique sets of conditions where seal design and material properties are pushed to the limit. While reciprocating applications can certainly test seals to the edge of capability, often times rotary applications can present the greatest challenge to seal integrity and wear life. Unlike reciprocating configurations where the seal is acting on a different part of the shaft or bore throughout it’s operating range, rotary seals must operate on the same sealing area continuously. This makes things like heat rejection much more difficult, especially in unlubricated or dry running applications. Extreme localized heating can have negative affect on both seal and hardware life. Rotary applications also pose sealing difficulties due to the simple fact that surface speeds can be much higher than in reciprocating systems. A simple electric motor can operate at very high rpm, while long stroke, high speed reciprocating machinery is a major piece of equipment that is far less common (though Eclipse also has sealing solutions in a number of these situations). A customer approached Eclipse with an application that was beyond the scope and capability of any standard, off-the-shelf rotary seal. This sealing system would require a combination of both wear resistance in high-speed rotary, as well as excellent leakage control and sealability. Two factors that, more often than not, work in opposition to each other. The Customer Issue The customer was developing a test system that required an electric motor shaft passed through the wall of a large vacuum chamber. The testing apparatus needed a sizable motor to meet the speed and torque requirements. Adapting the motor to operate inside the chamber would not be practical due to contamination and motor cooling concerns. Therefore, the motor would have to be placed outside the chamber and a driveshaft would have to go through the chamber wall. Which, of course, would need a seal. Operating Conditions:
 Rotary Shaft Seal
 Shaft Diameter: 2.5”
 RPM: 7,500 RPM - unlubricated
 Pressure: Vacuum internal side / 1 ATM external side Temperature: 40° - 90°F The customer knew any kind of off-the-shelf rotary seal with a rubber element would not last any amount of time in the combination of speed and a dry running condition. They also knew a single lip PTFE seal would likely not meet their leakage requirements. Therefore, they turned Eclipse for a custom sealing solution.
By Doug Montgomery November 25, 2024
Eclipse has engineered sealing solutions for applications all over the planet and in a plethora of environments. From the bottom of the ocean to orbiting the earth, Eclipse is challenged by the unique conditions in each application. Whether it be extreme temperature and pressure or severely caustic or abrasive media, Eclipse has a solution for most every sealing problem. One distinct environment presents a particularly challenging set of circumstances for seal design – high radiation. Eclipse’s primary seal material choice for many applications is PTFE and PTFE blends. With all the wonderful attributes PTFE possesses as a seal material, radiation resistance is not one. In high radiation environments PTFE’s properties can degrade to essentially rule it out as a suitable material. The options for effective sealing materials that are also radiation resistant becomes very limited. The seal designer is therefore confronted with creating a seal that is expected to perform in every way a typical PTFE seal operates, out of materials that are not as favorable to sealing. This is where Eclipse’s engineering experience and expertise in seal design come to the forefront. The Client's Issue Eclipse was approached by a customer that was looking for a seal solution for a sensor used in a nuclear application. It would be operating in an environment with both high temperature and high Gamma radiation. Operating Conditions:
 Reciprocating Rod Seal
 Rod Diameter: Ø1.000
 Stroke: 1.5”
Cycle Rate: 2-4 cycles per minute
 Media: Air, Salt Water Mist
 Pressure: 100 PSI
 Temperature: 70° to 450°F
 Gamma Radiation Exposure: 10^6 rads
By Doug Montgomery November 14, 2024
Technological advancements in the area of robotics have led to more and more life-like creations existing only in works of science fiction a few decades ago. Development in autonomous logic processing and sensing allows bipedal robots to walk over uneven ground, up and down stairs, open doors and carry loads. Fast response to dynamic and unpredictable real-world environments is critical for the future use of robots in true-life service and practical employment in the years to come. While software and sensor development remain the primary focus of most research, the physical mechanics of next-gen robotics are also continually progressing. Physical components and control systems such as hydraulic pumps and cylinders, servo motors, and structural members are under pressure to continually be lighter, stronger, more efficient and less expensive. Increased demands on the physical components facilitate the need for innovative solutions in design and material usage. Advancements in construction and technology have spilled into all areas of robotic mechanisms and the many seals located throughout the system need to meet the challenges of tomorrow. Eclipse has been at the forefront of this research and has developed innovative solutions pushing the boundaries of conventional sealing devices. MicroLip™ by Eclipse is a prime example of most demanding applications forging new technologies in the sealing world. The Client's Issue Eclipse was approached by a leading robotics company looking for a sealing solution operating under a challenging set of conditions. While many components of tomorrow’s robotics are now controlled and actuated by servo/stepper motors and various electronic devices, the heaviest and most powerful movements are still driven by traditional hydraulics. The constant demand for more powerful hydraulic actuation in ever deceasing size and weight requirements has put tremendous strain on component design. But if robots are to progress to the point where they are usefully employed in the world, high power in a compact design is necessary. A robot, for example, used to survey and assist in a disaster zone too unstable for normal rescuers, must fit through doorways and over obstacles yet still be physically strong enough to render assistance. Large hydraulic systems are capable of moving extremely heavy loads but size and weight constraints of a humanoid size robot limit potential. The robot’s internal power supply to drive all components is also a limiting factor. Our client was developing a new hydraulic pump to drive all major motion aspects of their robotic systems. Their main objective was to minimize the pump’s physical size as much as possible while increasing output and improving power consumption efficiency. This means higher pressures and speeds on increasingly smaller and lighter components. Application Parameters: Shaft Diameter: Ø9.5mm Seal Housing Envelope: 5mm radial cross-section by 6mm axial width Rotational Speed: 3,500 RPM nominally; 6,000 RPM max Operating Pressure: 125 PSI min, 225 PSI nominal, 350 PSI max Surface Finish: 0.04µm Media: Hydraulic Oil While the above combination of pressure and speed might present difficulties for any conventional seal alone, the client’s extremely small physical envelope to house the seal further complicated the matter. If that wasn’t enough, the application presented the additional sealing challenge of up to 0.003” [0.08mm] of shaft runout. As part of the downsizing of all components in the pump, shaft support bearings were minimized leading to the possibility of runout. The wobbling effect of the shaft creates problems as the sealing lip has follow a moving, uneven mating surface, therefore potential leak-paths are created. Wear life can also be compromised due to higher concentrations of uneven loads. The combination of high pressure, high speed, high runout and minimal gland size present a worst-case scenario for a typical seal. Unsurprisingly, the client faced leakage of hydraulic fluid after only short periods of service with any conventional seal they had tested. Eclipse knew the had the perfect solution for this application. One developed to handle such extreme rotary sealing conditions: MicroLip™.
Share by: