Hydraulic Essentials – The Evolution of O-ring Boss/Face Fittings

Hydraulic System Fitting Essentials

Pipe Threads and High-Pressure Hydraulics

For generations leakage around threaded pipe fittings was an accepted fact of life in high-pressure hydraulic systems. It wasn’t until WW2, with the advent of airborne high-pressure hydraulic systems in military aircraft that engineers began to reject fluid leakage around pipe thread connections as acceptable. Before the WW2 era, hydraulic pressures were moderately low, with most systems operating in the 800-1000 psi range. Rapid-cycling and high shock pressures in post-war high-pressure industrial and aeronautic applications demanded a solution to eliminate fluid leaks in modern applications where pressures climbed up to 2500-3000 psi.

The perfect ideal hydraulic system would never need components replaced, never require upgrades, and all connections would be welded or brazed permanently for maximum reliability and leak-proof integrity. You’ll forgive us for stating the obvious, but this perfect system does not exist in the real world. Early pioneers in hydraulic engineering dealt with the need for system connection versatility for maintenance and upgrades with the tried and true threaded pipe fitting.

Problems With NPT Tapered Thread Fittings

Threaded pipe fittings rely on applied torque to force two mating surfaces together which form the seal. The tapered threads are actually the weak link. Threads are subject to wear and tear. Overtightening can distort the threads and create a source for leakage. In a demanding industrial environment with high vibration and wide variations in operating temperature, threads are prone to loosening.

Repeated assembly and disassembly can worsen the distortion problem, especially when threading a forged fitting into a cast-iron female port. The obvious countermeasure, sealant compound, is a potential contaminant in hydraulic systems, and, according to this informative report at hydraulics pneumatics.com, is the reason most hydraulic engineers consider threaded fittings to be obsolete. One of our industry-leading suppliers, Parker, rates the 100-year old NPT threaded fitting as “poor” in the three critical categories of :

  1. Seal reliability
  2. Vibration Resistance
  3. Reusability

Flared and Flareless Fittings

37 Degree (JIC Flare)

The first improvements over threaded pipe fittings were the flare type fittings which are still commonly found in hydraulic designs even today. The fitting is drawn into the flared tubing end when a nut is tightened forming a positive metal to metal seal. Flares can be difficult to form in thick-walled tubing, however, so 37-degree flare fittings are limited to thin-wall and medium thickness tubing applications. The JIC (Joint Industrial Conference) Flare is the most commonly used type of hydraulic connection.

Flareless Fittings

Flareless fittings are gaining acceptance for medium or thick-walled tubing, especially for high-vibration applications. Flareless fittings also have the advantage of requiring less tubal preparation. A ferrule is drawn into the fitting body when a nut is tightened, compressing the ferrule around the tube, penetrating the outer circumference of the tube and forming a positive seal. The flareless fitting is suitable only for medium or thick-walled tubing.

Both the flared and flareless type fittings have useful attributes but there was still room for improvement in the quest for a leak-proof high-pressure hydraulic fitting, able to operate at 3000 psi. In the 1950s the SAE recognized this need for improved fittings for high-pressure hydraulics and recruited a group of fittings manufacturers to work under the supervision of the SAE Committee on Tubing, Piping, Hoses, Lubrication, and Fittings. The result was the SAE straight-thread O-ring boss fitting.

The SAE O-Ring Boss Fitting (ORB)

The O-ring is one of those elegantly simple inventions that can revolutionize an industry. The O-ring provides sealing versatility over the all-metal face types which need to be tightened to a high but narrow torque range. Stripped threads or distorted components can be the result when that range is exceeded. Under-tightening produces a weak seal if any at all.

O-Ring Type Fittings

The O-ring solves these difficulties and has gained wide acceptance in a number of fitting styles, providing a superior rubber to metal seal without the risk of distorting metal parts. They also offer an easier to detect torque “feel” for technicians making it easier to determine when the fitting is tight enough to provide a proper seal. The three basic types of O-ring fittings are :

  1. FFOR or Flat-faced O-Ring Fittings also known as “flat seal fittings”
  2. O-Ring flange fittings
  3. SAE straight-thread O-ring boss fittings

O-ring flange fittings are especially suited for large sizes and high pressures, especially in tight quarters. The choice of FFOR or O-ring boss types will depend upon individual designer preference based on application, wrench clearance, and fitting location. All of the O-ring type fittings offer superior sealing characteristics, as rated by Parker in the following categories:

  • Pressure- Excellent
  • Seal Reliability- Excellent
  • Ease of Installation- Excellent
  • Reusability- Excellent
  • Vibration Resistance- Very Good
  • Temperature- Limited by Seal

Adjustable and Non-Adjustable ORB Fittings

When no specific alignment is needed, such as with plugs and connectors, non-adjustable ORBs fit the bill. These simply screw into a port and the O-ring provides the seal without the need of a backup washer or locknut. The flanged area pushes the O-ring into the port’s tapered seal cavity squeezing the O-ring and forming a highly reliable seal.

Elbows and tees require a specific orientation and that’s where the adjustable ORB comes in. Adjustable ORBs are screwed into the mating component until the proper orientation is acquired, then a locknut is tightened forcing a backup washer onto the O-ring thus forming the leak-tight seal. There are some special considerations for installing ORB fittings as opposed to metal face types.

Installing ORB Fittings

  • O-ring fittings are more expensive than the all-metal alternative fittings
  • Care must be exercised to ensure the O-ring remains in place and isn’t damaged during assembly
  • O-rings are not interchangeable between all couplings.
  • The wrong O-ring can cause leaks.
  • O-rings cannot be reused even if they don’t appear to be deformed.

The good news is that manufacturers are now offering ORB fittings which are equal to FFOR styles for leak and weep resistance. And while manufacturer Parker only rates O-ring type fittings as “Very Good” new designs are surpassing all expectations in vibration tolerance testing. No evidence of leakage was detected on these new ORB designs even when subjected to vibration cycles 15 times higher than would normally be experienced on a typical hydrostatic drive.

ORBs and More at Custom Hose Tech

At Custom Hose Tech we provide the hydraulic expertise to keep your plant machinery, forklifts, and farm equipment up and running. Our expert technicians provide new hydraulic installation support and corrective and preventative maintenance 24/7 and we have the Twin Cities region covered with our strategically located mobile units. Our walk-in store carries an enormous inventory of hydraulic system OEM parts. If we don’t stock your rare or discontinued fittings our master machinists can produce small batch custom fittings based on your existing parts or specifications so don’t hesitate to contact us.