For hydraulic systems, selecting the right type of hydraulic fitting is crucial, as it directly impacts system efficiency, safety, and service life.
In this article, we introduce two commonly used types of fittings: ORFS (O-Ring Face Seal) and ORB (O-Ring Boss). Although both utilize O-ring seals, they differ in terms of application, design, and advantages. This guide will help you choose the fitting best suited to your system.
ORB (O-Ring Boss) and ORFS (O-Ring Face Seal) fittings are both types of hydraulic fittings designed to provide leak-proof connections. The primary difference is in their sealing mechanisms: ORB fittings seal using an O-ring compressed in a boss, or a recessed area, while ORFS fittings seal with an O-ring located on the flat face of the fitting, which mates with another flat face.
ORFS fittings are suitable for high-pressure applications requiring a flat sealing surface, while ORB fittings are ideal for applications with limited space.
ORFS (O-Ring Face Seal) Fittings
In accordance with SAE J1453 and ISO 8434-3 standards, O-Ring Face Seal fittings are known as ORFS. As the name implies, ORFS fittings feature an O-ring installed on the fitting's face. They are typically manufactured from carbon steel, nickel-plated carbon steel, or stainless steel, and utilize O-rings made of Nitrile (Buna-N) or Fluorocarbon (Viton)—with a hardness rating of 90—seated within a groove on the fitting's face.
O-Ring Face Seal (ORFS) thread is a straight thread (UN/UNF) that complies with the SAE J453 standard. The female connector has a flat surface, while the male connector features an O-ring on its face. The seal is achieved when the O-ring on the male connector compresses against the flat surface of the female connector. An outside nut on the female connector secures the connection.
Application
O-Ring Face Seal (ORFS) fittings feature a flat sealing surface and an integrated O-ring design. This configuration ensures a leak-free connection even under conditions of high pressure and vibration. ORFS fittings excel in applications where safety and reliability are paramount.
O-ring face seal fittings are intended for general application in industrial and commercial hydraulic systems, where the use of elastomeric seals is acceptable to prevent leakage. These fittings are capable of operating at working pressures from 28 in. Hg (95-kPa) vacuum to 6,000 psi (41.3 MPa).
They are widely used in heavy machinery, construction equipment, and hydraulic systems that require robust, vibration-resistant seals.
Why use ORFS fittings?
The primary advantage of ORFS fittings is their ability to prevent leaks. The O-ring face seal design ensures the connection remains secure and leak-free under high pressure and vibration.
ORFS fittings are designed for long-term use in harsh environments involving high pressure, vibration, and thermal cycling, making them suitable for both industrial and mobile hydraulic systems.
The design allows for easy installation using standard wrenches, reducing the need for specialized tools and training.
ORFS fittings can withstand high-pressure applications—typically up to 6,000 psi, depending on size and material—making them ideal for applications requiring high performance and reliability.
ORFS Thread Size Specification Chart
| Nominal Tube Size | Thread Size(Notes 1&2) | Working Pressures | |||||||
| Nom. SAE Dash Size | Nom Inch Tube O.D. | SAE J1453 GRFS Tube End | SAE J1926/2 ISO 11926-2 O-Ring Port Thread Size | ORFS Tube Ends, Unions and Bulkheads | Heavy Duty Stud Ends Per SAE J1926/2 ISO 11926-2 | ||||
| Non-Adjustable Studs | Adjustable Studs | ||||||||
| MPa | psi | MPa | psi | ||||||
| -4 | 1/4 | 9/16-18 UNF | 7/16-20 UNF | 41.3 | 6,000 | 63 | 9,000 | 40 | 6,000 |
| -5 | 5/16 | 5/8-18 UNF | 1/2-20 UNF | 41.3 | 6,000 | 63 | 9,000 | 40 | 6,000 |
| -6 | 3/8 | 11/16-16 UNF | 9/16-18 UNF | 41.3 | 6,000 | 63 | 9,000 | 40 | 6,000 |
| -8 | 1/2 | 13/16-16 UNF | 3/4-16 UNF | 41.3 | 6,000 | 63 | 9,000 | 40 | 6,000 |
| -10 | 5/8 | 1-14 UNF | 7/8-14 UNF | 41.3 | 6,000 | 63 | 9,000 | 40 | 6,000 |
| -12 | 3/4 | 1-3/16-12 UN | 1-1/16-12 UN | 41.3 | 6,000 | 40 | 6,000 | 40 | 6,000 |
| -14 | 7/8 | 1-5/16-12 UN | 1-3/16-12 UN | 41.3 | 6,000 | 40 | 6,000 | 40 | 6,000 |
| -16 | 1 | 1-7/16-12 UN | 1-5/16-12 UN | 41.3 | 6,000 | 40 | 6,000 | 31.5 | 5,000 |
| -20 | 1 1/4 | 1-11/16-12 UN | 1-5/8-12 UN | 27.5 | 4,000 | 25 | 4,000 | 25 | 4,000 |
| -24 | 1 1/2 | 2-12 UN | 1-7/8-12 UN | 27.5 | 4,000 | 25 | 4,000 | 20 | 3,000 |
ORB (O-Ring Boss)Fittings
O-Ring Boss (ORB) fittings utilize a threaded connection and an O-ring to create a tight, leak-proof seal. Their compact design makes them an ideal choice for systems with limited space.
SAE O-Ring Boss (ORB) features a straight thread (UN/UNF). The female component has a sealing face and chamfer, while the male component includes an O-ring. When the O-ring is compressed into the opposing chamfer, it forms a seal. The male and female threads mesh to create a solid connection. ORB threads conform to SAE J1926-1 and ISO 11296-1 standards.
Application
ORB fittings are best suited for applications requiring threaded connections, such as hydraulic cylinders or valves. Their simplicity and efficiency make them essential components in compact hydraulic systems.
Design
ORB fittings feature a cylindrical boss with external threads and an O-ring groove, designed to screw into a corresponding female port.
Why use ORB fittings?
ORB fittings are commonly used in hydraulic cylinders where space is limited and a reliable seal is crucial for maintaining pressure and preventing leaks.
While ORB fittings can withstand high pressures—typically up to 5,000 psi—their ratings are generally slightly lower than those of ORFS fittings. Their threaded boss design ensures a robust, secure connection suitable for high-pressure systems.
They are available in a wide range of configurations—including unions, tees, swivel fittings, and elbows—making them suitable for diverse applications.
The O-ring provides an exceptionally tight seal capable of withstanding high pressure, making these fittings ideal for hydraulic systems and automotive applications. Their design allows for repeated reassembly without the risk of cracking or deformation caused by overtightening.
ORB Thread Size Specifications Table
| Thread Size | Dmaj(in.) | TPI(n/in) |
| #0 - 80 UNF | 0.06 | 80 |
| #1 - 72 UNF | 0.073 | 72 |
| #2 - 64 UNF | 0.086 | 64 |
| #3 - 56 UNF | 0.099 | 56 |
| #4 - 48 UNF | 0.112 | 48 |
| #5 - 44 UNF | 0.125 | 44 |
| #6 - 40 UNF | 0.138 | 40 |
| #8 - 36 UNF | 0.164 | 36 |
| #10 - 32 UNF | 0.19 | 32 |
| #12 - 28 UNF | 0.216 | 28 |
| 1/4" - 28 UNF | 0.25 | 28 |
| 5/16" - 24 UNF | 0.313 | 24 |
| 3/8" - 20 UNF | 0.375 | 24 |
| 7/16" - 20 UNF | 0.438 | 20 |
| 1/2" - 20 UNF | 0.5 | 20 |
| 9/16" - 18 UNF | 0.563 | 18 |
| 5/8" - 18 UNF | 0.625 | 18 |
| 3/4" - 16 UNF | 0.75 | 16 |
| 7/8" - 14 UNF | 0.875 | 14 |
| 1" - 12 UNF | 1 | 12 |
| 1 1/8" - 12 UNF | 1.125 | 12 |
| 1 1/4" - 12 UNF | 1.25 | 12 |
| 1 3/8" - 12 UNF | 1.375 | 12 |
| 1 1/2" - 12 UNF | 1.5 | 12 |
O-Ring Face Seal (ORFS) Vs. O-Ring Boss (ORB)
Both ORFS and ORB fittings utilize a captive O-ring for sealing, but the methods differ. The primary distinction lies in the O-ring's location: in ORFS fittings, the O-ring sits in a groove at the end of the male thread, whereas in ORB fittings, it is positioned at the base of the male thread. When assembling an ORFS fitting, the flat sealing surface of the female component compresses the O-ring on the male component as the threads are tightened, thereby creating a seal. Conversely, the ORB female component features a recessed "boss" area before the first thread; this boss encapsulates the O-ring on the male thread, likewise forming a secure seal. ORFS fittings employ face sealing, making them ideal for high-pressure systems, while ORB fittings utilize a combination of threads and O-ring sealing, making them suitable for compact applications.
Making a Choice
Sealing performance is critical for preventing leaks and maintaining system integrity. ORFS fittings excel in this regard; their flat-face design ensures uniform compression of the O-ring, creating a highly reliable seal. While ORB fittings are effective, they rely on compressing the O-ring between the boss and the mating surface, which may require more precise installation to achieve a perfect seal.
Cost is always a factor when selecting hydraulic fittings. ORFS fittings generally command a higher price due to their advanced sealing design and high-pressure capabilities. However, this investment is justified by their superior performance in leak prevention and high-pressure environments. Although typically lower in cost, ORB fittings offer reliable performance and serve as an economical choice—particularly where space efficiency is a consideration—for many high-pressure applications.
Disadvantages of O-ring fittings
O-rings may melt or corrode if exposed to excessive temperatures or incompatible media. A particular issue to watch for is the O-ring becoming dislodged, especially during assembly. An O-ring can slip out of its groove, leading to poor sealing, extrusion beyond the intended seating surface, or thread damage caused by overtightening. In such cases, both the O-ring and the fitting may suffer irreparable damage, rendering them completely unusable. Therefore, care must be taken to ensure the O-ring is correctly seated prior to assembly; using an adhesive during the assembly process may help mitigate this problem.
Please contact our sales team for more details about our O-ring face seal(ORFS) and O-ring boss(ORB) seal hydraulic adapters.