Hydraulic fittings play an important role in making sure hydraulic systems are leak free for safe and efficient performance. Fittings can be classified as all-metal or O-ring fittings. The first type relies on metal-to-metal contact whereas the O-ring fittings create an elastomeric seal. In both cases, the threads of two separate components (sometimes referred to as male and female parts) must be joined together to form the seal.

Let’s take a closer look at the components of the types of fittings that fall into these two categories of hydraulic fittings.

All-Metal Fittings

1. Pipe Fittings

The threads on pipe fittings are usually tapered so that when the male half is fitted into the female component the stress creates a seal. Usually a sealant for this type of hydraulic fittings is required to help ensure a leak-free seal. However, pipe fittings are not commonly used these days as they have several disadvantages including:

• Distortion of threads due to overtightening, which can lead to leakage.

• Loosening of threads due to vibration and major variations in temperature.

• Leakage in high pressure environments.

• Distortion of threads due to excess assembly and disassembly.

 

2. Flare-type Fittings

Flare-type fittings counter many of these issues associated with pipe fittings with an improved design. When the nut is tightened, the fitting creates a positive seal thanks to the flared tube face. The result is a compact and economical fitting.

 

3. Flareless Fittings

Flareless fittings work by creating a positive seal between a ferrule and the tube when the nut is tightened. They work best with thick or medium tubing because the ferrule actually penetrates the tube. They are also able to better cope with more vibration than the other types of all-metal fittings.

 

O-Ring Fittings

1. O-ring Boss Fittings

In this type of fitting, the O-ring sits between the threads on the male component and creates a seal against a seat on the female component. O-ring boss fittings can be adjustable (need to be positioned in the correct direction) or non-adjustable (no alignment necessary).

 

2. Flat-Face O-Ring Fittings

The O-ring in flat-face fittings is located in a circular groove on the male component and forms a seal between a flat surface on the female component. When you turn the nut, the O-ring is compressed.

 

What is the best hydraulic fitting?

The potential drawbacks of O-ring fittings include:

• O-ring fittings are more expensive than all-metal fittings.

• The O-ring may fall out or get damaged during assembly.

• Not all O-rings can be used for all couplings, and selecting the wrong size can lead to leakage.

• O-rings cannot be reused.

 

Despite these potential drawbacks, O-ring fittings offer two crucial advantages over all-metal fittings:

 

• All-metal fittings are more susceptible to leakage due to thread distortion while the rubber of the O-ring doesn’t cause distortion in the metal.

• Operators may find it hard to know when an all-metal fitting is tight enough and tighten too much causing distortion of the threads. On the other hand, for O-ring fittings it is easier to feel when the connection is tight enough.