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What are Hydraulic Connection Types?

What are Hydraulic Connection Types?


As the name suggests, hydraulic connectors are used to connect hydraulic circuits by joining two different end terminations together and creating a flow passage to ensure fluid transfer.

Hydraulic connectors can be grouped into three main types:

  • Internal Thread
  • External Thread
  • No Thread

Internal and external threaded connectors are also known as female and male, respectively.

There are many thread standards in the industry primarily based on the country / region of origin. BSP (British Standard Pipe), NPT (National Pipe Tapered), UN (Unified Inch Screw Threads) and Metric to name some of the most common.

To avoid damage to hydraulic systems, selecting the right type of thread is very important.

Common variation in fitting designs are outlined below that is useful in thread identification.

Tapered / Parallel Threads

Some threads have a tapered thread surface, such as BSPT, NPT (denoted by letter T) whereas others are parallel. BSPP, UN are examples for parallel threads.

Tapered threads are designed to seal on the thread surface.

Sealing of Hydraulic Connectors

Along with tapered threads, other sealing methods include:

  • Metal Seal
  • Soft Seal

Metal seal type was developed to maximise sealing of a connector under pressure. Connectors that have this sealing method will include a machined cone seat (convex or concave) that will seal with the corresponding male / female connector.

Common seat angles include:

  • 30° cone seat (BSPP, some Metricfittings)
  • 45° cone seat (SAEfittings)
  • 37° cone seat (JICfittings)
  • 24° cone seat (Metricfittings)

Some connectors are designed to seal using elastomeric seals, natural or synthetic to prevent leakage. This type of sealing method is common with adaptors intended for port use, where a soft seal on the base of the adaptor will seal with the flat surface of a port.

Soft seal connectors can be seen on the face of male fittings and adaptors (such as ORFS) which will establish a seal with a flat surface on a female fitting of the same size.

Thread Size, Thread Count & Pitch

Thread size is designated by the external major diameter of a thread that can easily be measured using a calliper. The actual measurement is generally slightly lesser than the nominal thread size. Imperial thread sizes are expressed in inches and metric thread sizes in millimetres paired with “M” (e.g. M6 for 6mm).

Thread count (also known as Threads Per Inch – TPI) and pitch are also important factors when identifying threads and are usually stated along with the thread size.

TPI is common with imperial connectors which basically is the number of threads within an inch measured lengthwise.

For metric threads, pitch is used instead which means the distance between two crests of a thread in millimetres.

How to Identify Connectors?
RYCO’s Thread ID Mate application provides an intuitive and simple identification process to help you find detailed information about threads specifications, sizes and more.

Hydraulic threads can be identified using a few simple steps using the Thread ID Kit and RYCO Thread ID Mate application.

  1. Determine the thread type:
    Visually inspect to determine if the connector has and internal or an external thread (if any).
  2. Is the thread tapered or parallel?:
    Tapered threads can be identified by visual inspection or placing a straight surface, such as a ruler, next to the thread to compare the difference in diameter at the shoulder of the fitting to the diameter at the tip.
  3. What type of sealing face / method does the connector have?:
    Check if the connector has a conical sealing face or a flat sealing face, and whether a soft seal is present.
  4. Using Vernier Calipers included within the RYCO Thread ID Kit, measure the outer / inner diameter of the thread:
    Measure the ID / OD of the connector across flats of the Vernier Calipers for better accuracy.
  5. Determine the sealing face seat angle (if any):
    Align the seat gauge of the Thread ID Kit with the seat of the connector and identify the angle that matches with the seat.
  6. Find the pitch / TPI:
    Use the thread pitch gauge to find out the exact pitch / TPI and verify the thread.