The three types of hydraulic connectors and their two key functions

The three types of hydraulic connectors and their two key functions

This article is an overview of the theory of hydraulic connectors, the three main types of connectors and the two key functions that all hydraulic connectors must fulfil.

Three Main Types of Hydraulic Connectors

It provides basic information about the three main classes of hydraulic connectors:

  • Tapered Threads
  • Metal seal connectors
  • Soft seal connectors

Two Key Functions of Hydraulic Connectors

It’s important to understand that there are two functions that a hydraulic connector must fulfil:

  • Holding
  • Sealing

Sometimes the holding and sealing functions are provided by the same mechanism, and sometimes by two different parts.

Hose connectors may seem very confusing because of the many different types available.

However, when looking objectively, connectors will always simply do two things. Hold, and seal under pressure.

Development of Hydraulic Connectors

Hydraulic connectors were developed in this order:

  • Tapered threads
  • Metal seal connectors
  • Soft seal connectors

Tapered Threads

Tapered pipe threads were one of the first ways to connect pipes together.

They involve cutting the pipe thread in a tapered manner, so that the diameter of the pipe changes over the length of the fitting.

Tapered Pipe Thread

These kinds of connectors require some form of sealant, either a tape sealant, which can block the pipes, or a chemical sealant, which is preferred.

Tapered pipe threads can also be difficult to orient correctly, leading to poor sealing and leakage.

Tapered threads can also cause damage to other components, including deformation of aluminium and cracking of cast iron, due to the pressure of the steel pipe thread within the component.

They also can’t be connected to hydraulic tubes without soldering or welding of the tube to the part.

Metal Seal Connectors

Metal seal connectors were developed to address some of the limitations of the tapered pipe threads.


A metal seal connector is basically a tapered pipe thread with the addition of a 30 degree machined face, to form a seal.

This feature is generally only in hydraulic systems, because of the high pressures that need to be dealt with.

The addition of the “chamfer” on the face of the thread split the holding and sealing features of the connector.

The thread does the holding and the machined face does the sealing.

This modification also makes the connector easier to orient, better sealing, and eliminates the need for a sealant.

However, metal seal connectors still can’t accept hydraulic tube unless the tube is soldered or welded to it, and they can also have issues with vibration.

The JIC connector is a further development. It has a 37º metal seal and can accept hydraulic hose and tube. BSP Thread, BSPT, BSPP and NPT are common types of threads used in metal seal connectors.

Soft Seal Connectors

Soft seal connectors are the most recent development in the evolution of hydraulic connectors.

They make use of an elastomeric seal – natural or synthetic – to prevent leakage.

Soft Seal Hydraulic Connector

The soft seal also helps connectors resist the effects of vibration, as well as the high-pressures used in many hydraulic systems today.

The seal in a soft seal connector usually takes the form of an O-ring.

Another advantage of soft seal connectors is their rapid assembly, which can be important in high-volume production environments.

The main limitation of soft seal connectors comes from the fact that performance of the o-ring can be affected by temperature and the system fluids, if they’re not compatible with the elastic substance.

Want More Information?

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Hydraulic connector thread forms and their applications

Hydraulic connector thread forms and their applications

If you’re new to the world of hydraulics, it can seem difficult to gain an understanding of the many hydraulic connectors available today.

In this series of articles, you’ll learn about the different types of hydraulic connectors and their key differences, which will assist in making some sense of this complex subject.

Factors affecting development of hydraulic connectors

Two factors have influenced the development of hydraulic connectors:

1: A reluctance to adopt industrial standards between countries
Many of these connector types were developed during the 20th century; a time of fierce industrial competition.

Co-operation between the major industrialised countries was limited for many years due to conflict.

2: The evolution of hydraulic systems
Hydraulic systems have changed a great deal in a short period of time.

Driving this change is the gradual increase of the desired machine output force, which is seen as a pressure increase.

In the previous series of articles introducing the three types of hydraulic connectors we discussed three principle types of connectors:

  1. Tapered threads
  2. Metal seal connectors
  3. Soft seal connectors

Each major industrial region has developed threads and connectors which fall into these categories.

How a thread is defined

There are three main elements that define a thread.

  1. The thread angle
  2. The thread pitch
  3. The thread size, or outside diameter

Other dimensions such as the depth of the thread and the minor diameter are calculated from the above sizes.

The following sections describe some basic fundamentals of threads.

Standards are used in engineering to ensure that parts from two manufacturers will fit together.

Most threads are made to a standard, and have set engineering dimensions.


The thread angle of a BSPP thread is 55 degrees.

Different threads have different thread angles.

For example NPT is 60 degrees.

Thread Anatomy Angle


The pitch of a thread is the length between two crests or two roots.

A ⅜” BSPP thread has a pitch of 19 threads per inch (TPI).

1” divided by 19 is 0.0526”.

Thread Anatomy Pitch

Outside Diameter

The outside diameter of a thread is nominated in the standard.

BSPP is a NOMINAL thread size.

(The diameter is the not the size as stated, but instead, suits the walls of a pipe of that bore size.)

Thread Anatomy Outside Diameter


The depth of a thread is based on a ratio of the pitch. (d = 0.61 x P).

In this case: 0.61 x 0.526 = 0.032”

Thread Anatomy Depth

Minor Diameter

The minor diameter of a thread is calculated as follows:
md = OD – (2 x depth)
md = 0.64” – (2 x 0.032”)
md = 0.64” – 0.064”
md = 0.576”

Thread Anatomy Minor Diameter

Tapered Pipe Threads

Tapered pipe threads have an outside diameter that is cut on a cone, and the sides are not parallel.

Tapered pipe thread types include BSPT, NPT, UNS, JIS, Metric Taper and NPTF.

Metal Seal Connectors

Metal seal hydraulic connectors use a profiled seat to form a seal.

Common types of metal seal connectors include JIC, DIN, BSPP, BSPP Flat Face, JIS, NSPM, 30 Degree Flare, Komatsu, Metric French GAZ, Metric French Millimetric and SAE (Automotive).

Soft Seal Connectors

Soft seal connectors use an elastomeric soft seal to ensure a leak-free joint, and may use O-rings, encapsulated seals and bonded washers.

Soft seal connector types include BSPP, UNO, DIN, ORFS, RYCOLOK, SAE Pilot O-Ring and SAE Flange.


The available hydraulic connectors can be defined by two sub groups: country of origin and sealing method.

The table below shows how each region has developed solutions for each of the sealing methods.

Thread Forms Sealing Methods by Country

For a quick introduction to hydraulic connectors check out our series on hydraulic connectors.

Want more information?

Make sure you select the right hydraulic hose for your next project. Download our 15 Keys To Selecting The Right Hose guide today, or read about BSP Thread here.