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A Closer Look at O-ring Face Seal, 37° Flare, and 24° Flareless Fittings


Comparing O-ring face seal, 37° flare, and 24° flareless fittings from Parker Hannifin Tube Fittings Division

When a customer asks me which fitting to use, I start by asking for more details about the application. Some of the things I tell them to consider are ease of installation, temperature, reusability, and resistance to over-assembly. 

Depending on what you know about hydraulics and other fluid and gas systems, your experience may suggest different types of tube fittings are more-or-less created equal. While it’s true that many applications can use any one of a number of fittings, and choice often comes down to what’s already in use or what’s most familiar, there are some important differences between them. For this post, I will explain the principal differences between three of the most common types of tube fittings for industrial hydraulic applications. The three types are: O-ring face seal fittings (ORFS), 37° flare fittings (sometimes referred to as JIC fittings), and 24° flareless fittings.


Types of Fittings

An ORFS fitting, as most people know, has a replaceable elastomeric O-ring held in a groove in the face of the fitting. These fittings conform to SAE J1453 and ISO 8434-3 and can be used with inch or metric tube. Sealing is accomplished when the O-ring is compressed between the fitting body and the formed tube or flat face of an ORFS female fitting as the assembly is tightened. These fittings are assembled using the torque method.

                    Sealing for an o-ring face seal fitting is accomplished when the o-ring is compressed between the fitting body and form tube or flat face of the ORFS female fitting.

37° flare fittings meet SAE J514 and ISO 8434-2 and can be used with inch or metric tube. They are designed with a 37° flare that mates up with a 37° flared tube or female flare fitting. The sealing is accomplished by metal-to-metal contact between the fitting nose and the flared tube or internal flare of a female fitting. You assemble these fittings using the Flats From Wrench Resistance (FFWR) method of counting the flats of the fitting nut as it is tightened after resistance.

                    Sealing for 37° flare fittings is accomplished by metal-to-metal contact between the fitting nose and the flared tube or internal flare of a female fitting.

 

24° flareless fittings are offered in inch and metric versions, the SAE J514 version mates with inch tube, and the DIN 3852 version mates with metric tube. Both fitting versions involve a metal-to-metal seal, where the contact between a ferrule/bite ring and both the fitting body and the tube accomplish the seal. For assembly of both versions, the ferrule or bite ring first needs to be preset onto the tube and inspected, and then final assembly is completed using the turns method. 

                    Sealing is a metal-to-metal seal where the contact between the ferrule or bite ring and both the fitting body and tube accomplish the seal.

For more information on the assembly methods for each fitting type, please see Turn vs. Torque? How Making the Right Choice Keeps Your Hydraulic Fitting Connections Leak-Free

 

Ease of Installation

A key consideration for efficient first-time assembly or subsequent repair is how easy is it to install the type of fitting you’re specifying?

O-ring face seal fittings are typically the easiest to install, because they involve a zero-clearance connection: you can just slide the components into place before assembly, and don’t need to maneuver tubes into place as you would with male/female interfaces. 

By comparison, 37° flare and 24° flareless fittings are harder to install because they do involve male/female interfaces, and therefore the need to maneuver tube assemblies into place. For 37° flare the tube needs to sit on the nose of the fitting, and for 24° flareless the tube needs to sit inside the cone of the fitting.

Comparison of ORFS, 37° flare, and 24° flareless fittings
       ORFS Fitting Assembly                  37° Flare Fitting Assembly                 24° Flareless Fitting Assembly

ORFS with its flat face allows for easy installation with zero clearance needed. 37° flare fittings require the tube to sit onto the nose of the fitting, and 24° flareless fittings require tube insertion into the cone of the fitting.


Temperature

Temperature is an important factor to consider as fittings with o-rings will limit the temperature range. So, ORFS fittings can be limited by temperature and media type, whereas 37° flare and 24° flareless fittings do not have the elastomeric material to contend with and thus can function in more extreme temperature environments.

However, recent technological advances in sealing methods for ORFS fittings mean ORFS options now exist that use a metal O-ring in place of the elastomer O-ring, allowing for a much wider temperature tolerance for those fittings. See Parker's Seal-Lok Xtreme metal face seal fittings for extreme temperatures. 

Temperature is an important factor to consider when deciding on what hydraulic fitting type to use for your application


Resistance to Over-assembly

ORFS fittings are resistant to damage from over-assembly and can be torqued up to 200% of the proper assembly torque without damage to the fitting or the sealing surface.

Proper assembly of 37° flare fittings causes some nose collapse, which is further exasperated by over-assembly. This nose collapse leads to reduced flow diameter. 

With 24° flareless fittings, over-assembly can cause the tube to collapse, or the ferrule/bite ring can even shear the tube especially when using thin-wall tubing. The sealing surface between the fitting body and the ferrule/bite ring can also be severely damaged.
 

Reusability

How reusable is each type of fitting? That’s another important question to consider.

Generally speaking, O-ring face seal fittings are reusable due to the flat face connection design. Additionally, the o-ring is both replaceable and highly resistant to over-assembly as mentioned previously.

37° flare fittings experience slight nose collapse during final assembly, so with each subsequent assembly you’ll create a slight additional deformation leading to flow diameter restrictions and potential leak paths. No matter how skilled an assembler is, connections made with these fittings just don’t hold up well to repeated assembly. See How Many Times Can I Reassemble a Hydraulic Fitting for more details on whether you can reuse your 37° flare fitting.

24° flareless fittings have some reusability, because the elastic nature of the ferrule makes it resilient during multiple reuses. However, the metal-to-metal sealing method creates potential for sealing surface damage. With each re-assembly, you increase the chances of collapsing the tube, or causing severe damage to the seal surface—especially during over-assembly or with use of thin-walled tubing.


Conclusion

In most cases, there is no one right fitting type for any given application because there are numerous factors that need to be considered, some of which have been discussed here. The trend in the marketplace is to use fittings with replaceable seals due to many of the advantages discussed in this post. 

If you need help determining what fitting type would be best for your application, remember that we are here to help and want your system to operate safely and efficiently. As well, if you have any questions or comments about this article, please post them and we will respond if warranted. To talk to our techConnect engineer team directly, they can be reached at Parker Tube Fittings Division, 614.279.7070.

Did you find this post helpful? Subscribe to TFD techConnect posts by email. TFD techConnect is a technically-focused monthly blog written for engineers specifically around motion and control engineering challenges. 
 

Nathan Green, engineer at Parker Tube Fittings Division

 

 

Written by Nathan Green, Engineer, Parker Tube Fittings Division

 

 

 

Additional related content about hydraulic tube, hose, and port fitting connections:

Turn vs. Torque? How Making the Right Choice Keeps Your Hydraulic Fitting Connections Leak-Free
How Many Times Can I Reassemble a Hydraulic Fitting?

Factors to Consider When Selecting Hydraulic Fittings and Tube
Tube End Preparation Best Practices for Leak-Free Hydraulic Tube Fitting Connections

 

 

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