Second Day with Fittings: Common Terms and Definitions

Learn the basic terminology and definitions that go with the world of hydraulic fittings.In our first post, we covered the first day basics - in a fittings 101 article. Now you’re on day two of the job and you at least know what fitting is - a great start. However, you are now wondering what all these other terms are. Co-workers are talking, you’re listening, but they seem to be speaking a whole other language. This post should help you add a little more fittings vocabulary into your tool belt. These will be very basic definitions, and some of these terms might be more complicated than we are going to give them credit. For now, we will give you a list of definitions that will help fill in the blanks you might still have regarding the basics of fittings. Even if you aren't new to the industry, you might learn a thing or two from the "pro tips" we included for many of these.


Fluid conveyance: Allows for the flow of fluid through a system without leakage. This term is used broadly over multiple industries.  

Tube: Hollow and cylindrical shaped, the outside diameter (OD) is not standardized for threading. It is dimensionally classified by its OD and wall thickness. Fittings are the connection piece between tubes and components in a system, allowing flow of media (fluids or gases). (see Figure 1)

Tube dimensions are classified by tube O.D. and wall thickness.







Figure 1. Tube dimensions 

Hose adapter: Connects to a hose assembly. For these, a nut and a sleeve are not required. (see Figure 2) 

A hose adapter connects to a hose assembly.





Figure 2. Hose adapter connecting to a hose assembly

Internal and external threads: Sometimes referred to as female and male threads. Female refers internal threads, and male refers external threads. (see Figure 3) 

Illustration showing the both internal and external threads on a fitting.






Figure 3. Internal and external threads 

Union fitting: A fitting where the ends are the same connection style but can be different sizes.  Ex. a straight fitting with both ends that are O-ring Face Seal (ORFS), or an elbow fitting where both ends are a 37-degree flare, etc. (see Figure 4) 

Union fittings have the same end on all sides of the fitting.







Figure 4. Straight ORFS union fitting (left) and 37-degree elbow union fitting (right)

Tee fitting: A fitting that has three ends, resembling the shape of a “T”.  

Run: The two ends that are opposite of each other on a tee fitting. (see Figure 5) 

Branch: The end of a tee fitting perpendicular to the run ends on a tee fitting. (see Figure 5) 

Union tee: All three ends are the same connection style. (see Figure 5) 

All three ends of a union tee are the same connection style.







Figure 5. Union tee also illustrating the runs and branch ends

Run tee: One of the run ends has a different connection style than the other two ends. (see Figure 6) 

Run tee has a different connection style on one of the run ends.





Figure 6. Run tee 

Branch tee: The branch end has a different connection style from the run ends. (see Figure 7) 

A branch tee has a different connection style on the branch as the run ends.







Figure 7. Branch tee 

Conversion adapter: The ends on these fittings have different tube end connection styles, so they are converting a connection from one style to the other. (see figure 8) 

ex: 37-degree flare end on one side and O-ring Face Seal end on the other like shown in Figure 8. The Parker equivalent of this configuration can be seen here.

A conversion adapter has different tube end connection styles on each end.






Figure 8. Conversion adapter 

Adjustable fitting: Allow shaped fittings to be threaded into a port, oriented for the needed connection, and then tightened to create a seal. These usually use an O-ring, back-up washer, and locknut to achieve the seal. (see Figure 9) 

Pro tip: ISO 1179/DIN 3852-2 and ISO 9974/DIN3852-1 adjustable fittings use the addition of a retaining ring to create a cavity for the O-ring to form a seal. See our previous blog post for more information on this - Metric Ports: Which Fitting Goes with Which Metric Port

Adjustable shaped fittings can be threaded into a port, oriented to the needed connection, then tightened down for final assembly.






Figure 9. Adjustable elbow and adjustable elbow assembled into a port

Swivel nut: Swivel nuts are attached to fitting bodies that have a female seat (sealing surface). They connect with fittings that have external threads using the same sealing style. The swivel aspect of the design allows for the fitting to be oriented and held in position while the nut is tightened. (see Figure 10)  

Pro Tip: These do not move once tightened, unlike “live swivels” which can move while pressurized without leakage.

A swivel nut allows fittings to be oriented and the nuts to be tightened without changing the orientation.




Figure 10. Union swivel nut assembly 

Bulkhead: Allows a connection to be made through a panel. This panel could be a structural element on the equipment or a plate that is joined to the equipment. Bulkhead fittings can also be used as a transition point in a hydraulic system, such as the connection of tube lines to hose lines. (see Figure 11) 

Bulkhead fittings allow for connections to be made through a panel.






Figure 11. Fitting with bulkhead end and locknut

Plugs: Used to block flow in ports, hose ends and tube ends with internal threads. (see Figure 12)  

Pro Tip: You cannot select a plug based on threads alone. Plugs for ports are different than those for fittings - the difference is the sealing method. If the plug is for a fitting, it will have the same fitting style with which it is mating. Be careful which style of plug you choose for ports. The plug needs to be compatible with both the mating connection thread and the sealing method. 

A plug fitting blocks flow in ports with internal threads.







Figure 12. Plugs

Caps: These are available for various fitting styles to stop flow through a fitting end connection that has external threads. (see Figure 13) 

 Caps block flow through a fitting that has external threads.   




Figure 13. ORFS cap 

Dynamic pressure ratings:  A system where operating pressure fluctuates, in accordance with load, up to a maximum pressure limited by a relief valve. These systems can experience shock, vibration, and temperature fluctuations. Parker, for example, uses a design factor of 4 to 1 to rate fittings for dynamic applications. Meaning, with a dynamic pressure, the system can withstand 4 times the rated pressure before failure or leakage.  An example of a dynamic system would be a hydraulic system for a backhoe. 

Static pressure ratings:  A system where once pressurized, it is essentially free from pressure fluctuations. These systems should be free from shock, vibration, and temperature fluctuations. Parker, for example, uses a design factor of 3 to 1 to rate fittings for these systems. The static pressure ratings can be calculated by multiplying 1.33 by the dynamic pressure ratings (static pressure rating= 1.33 x dynamic pressure rating).  An example of a static system would be a hydraulic jack. 

For more detailed information on pressure ratings, see The Truth About Pressure Ratings for Hydraulic Fittings and Adapters blog post. 

For details on different types of tube end connection styles, see A Closer Look at O-ring Face Seal, 37° Flare, and 24° Flareless Fittings blog post.

Now that you have the basics and the terminology in your fittings tool belt, you can feel more confident in talking the talk. To further jump-start your depth of knowledge, check out the past posts of techConnectRefer to our Tube Fittings Division Frequently Asked Questions page,, for a wealth of information and resources. 

If you have any questions or comments, 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. See Parker Tube Fittings Division's full line of tube fittings and hose adapters


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Contributed by Emily Alexander, senior design engineer at Parker Tube Fittings Division

Contributed by Emily Alexander, senior design engineer at Parker Tube Fittings Division






Additional related content about hydraulic tube, hose and port fitting connections:
First Day with Tube Fittings: Fittings 101 the Basics
Metric Ports: Which Fitting Goes With Which Metric Port?
Proper Assembly Steps for Parallel Thread Adjustable Style Port End Fittings

Have a question about Parker products or services? We can help: Contact Us!

Comments for Second Day with Fittings: Common Terms and Definitions

Chuck Lovett
Great job TFD.
Super teaching tool for all levels.
Keep 'em coming.
Kerem Cetinkaya
Very good explanations for new starters.
Would be great if it continues.

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