Soft Seal Connectors – The ultimate hydraulic connectors

Soft Seal Connectors – The ultimate hydraulic connectors

Soft seal connectors are a modern development in hydraulic connectors, intended to address the most common issues with older designs, including leakage and the effects of vibration.

They’re also designed to work in much higher pressure systems, to meet the growing needs of a wide range of applications across different industries.

Soft Seal Connector Definitions


noun (Chemistry)

An elastic substance occurring naturally, as a natural rubber, or produced synthetically, such as butyl rubber or neoprene.


adjective (Chemistry)

A part that has an elastic quality about it, or one that uses an elastomer.

Soft seal, or elastomeric connectors use a soft seal to complete the joint, resulting in a leak free and vibration resistant joint.

The seal is usually an O-ring.

Advantages of soft seal connectors

Soft seal connectors have a number of advantages, including:

  • They are positive sealing
  • Resistant to vibration
  • Very high pressure capability
  • Easy and quick to assemble, which is important in high-volume production

Disadvantages of soft seal connectors

The O-ring in soft seal connectors may harden with heat or soften with incompatible system fluids, so it is important that the seal material is suitable for the application.

Fluids that can affect the elastic material in soft seal connectors include fuels, oils, solvents and other substances, depending on the composition of the soft seal.

For this reason, selecting the right soft seal connector for the intended application is essential to ensure that the connector continues to perform as expected.

Practical Applications of Soft Seal Connectors

Examine the hydraulic system of this loader.

RYCO Hydraulic Hose Soft Seal Connectors

It has been designed to be leak free. It employs elastomeric sealing with SAE flange, ORFS, UNO ports and O-ring sealing inside the pumps.

It has been in service for a period of time already, and remains clean and leak-free.

Elastomeric couplings are a modern, leak-free alternative to some older design hydraulic couplings.

New hydraulic systems can be designed and sold on the basis of being leak-free.

All it takes is a little understanding of the connection options available.

Soft Seal Connectors In Industry

Soft seal connectors, also called elastomeric connectors, use a soft seal to complete the joint, addressing both leakage and vibration effects.

Types of Soft Seal Connectors

Here are the main types of soft seal connectors available:

  • DKOL (Dicht Kegel O Ring Leicht)
  • DKOS (Dicht Kegel O Ring Schwer)
  • SAE Flange (Society of Automotive Engineers)
  • Quick release couplings
  • BSPP port – British Standard Pipe Parallel
  • UNO port – Unified National O-ring
  • ORFS – O-Ring Face Seal

For an overview of the different hydraulic connector types, check out our introduction to 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.

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.