How to distinguish and select the connection threads of rotary joint

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How to distinguish and select the connection threads of rotary joints, How to distinguish and select the connection threads of rotary joint

Due to different starting standards for rotary joints, the industry standards for selecting joints were not consistent at the beginning. Of course, due to the gradual standardization of industry equipment, many uncommon thread styles of rotary joints were cleaned.
Threaded connection, flange connection, welding.

High speed rotating joint with cold release inside the spindle

How to distinguish and select the connection threads of a rotary joint

The connection thread of a rotary joint (rotary union) is one of the most important specifications because it determines whether the joint can be installed correctly, sealed reliably, and operate safely under pressure and speed. The correct thread selection depends on thread standard, size, direction, medium, and installation method.

1. Identify the thread standard

Common rotary joint thread types include:

Thread type Full name Common applications Identification
NPT National Pipe Thread (US standard) Hydraulic, pneumatic, industrial equipment Tapered thread, sealing usually with thread sealant
BSPT (R) British Standard Pipe Taper Europe, Asia industrial equipment Tapered thread, similar appearance to NPT but different angle
BSPP (G) British Standard Pipe Parallel Hydraulic systems, rotary unions Parallel thread, requires O-ring/gasket sealing
Metric thread (M) ISO metric thread European/Asian machinery Measured in mm (e.g., M18×1.5)
UNF/UNC Unified thread North American machinery Inch-based thread

Key differences

NPT vs BSPT

  • Both are tapered pipe threads.
  • They are not interchangeable.
  • NPT thread angle: 60°
  • BSPT thread angle: 55°
  • Even if they appear to fit, leakage may occur.

BSPP (G) vs BSPT (R)

  • G thread: straight/parallel thread
  • R thread: tapered thread
  • G thread normally seals with:
    • O-ring
    • sealing washer
    • gasket

    • 2. Measure the existing thread

      If the thread specification is unknown:

      Step 1: Measure outside diameter

      Use a caliper to measure the male thread outside diameter.

      Example:

      Approx. diameter Possible size
      10.3 mm 1/8″ pipe thread
      13.7 mm 1/4″ pipe thread
      17.1 mm 3/8″ pipe thread
      21.3 mm 1/2″ pipe thread
      26.7 mm 3/4″ pipe thread
      33.4 mm 1″ pipe thread

      (Actual dimensions depend on thread standard.)

      Step 2: Measure pitch

      Use a thread gauge.

      Examples:

      • M18×1.5 → 18 mm diameter, 1.5 mm pitch
      • G1/4 → 1/4 inch BSP parallel
      • R1/2 → 1/2 inch BSP tapered

      Step 3: Check thread angle

      A thread gauge can distinguish:

      • 60° → NPT / UN thread
      • 55° → BSP thread

      3. Select thread according to rotary joint application

      Hydraulic oil rotary joint

      Recommended:

      • BSPP (G)
      • NPT
      • Metric

      Consider:

      • Pressure rating
      • Leakage prevention
      • Hose compatibility

      Water rotary joint

      Common choices:

      • BSPP
      • BSPT
      • NPT

      For high-speed operation:

      • Prefer precision-machined BSPP + sealing ring designs.

      Air rotary joint

      Usually:

      • G thread
      • NPT

      Important:

      • Avoid leakage because air loss affects speed control.

      Coolant rotary joint

      Common in:

      • CNC machines
      • Spindles

      Usually:

      • Metric thread
      • G thread

      Example:

      • M16×1.5
      • M18×1.5
      • G1/4

      4. Consider the rotary joint installation position

      Male thread connection

      Example:

      Rotary joint male thread → Machine port female thread

      Advantages:

      • Compact
      • Easy installation

      Female thread connection

      Example:

      Rotary joint female thread → Pipe/hose male thread

      Advantages:

      • Flexible piping arrangement

      5. Check thread direction

      Rotary joints may require:

      Right-hand thread (RH)

      Most common.

      Left-hand thread (LH)

      Used when rotation direction could loosen a normal thread.

      Example:

      • High-speed rotating spindle
      • Rotating shaft applications

      Always confirm:

      • Shaft rotation direction
      • Manufacturer drawing specification

      6. Use the manufacturer’s drawing whenever possible

      For industrial rotary joints, the model number usually defines the thread.

      Example:

      1101-202-651 rotary joint

      The correct connection thread should be confirmed from:

      • Product drawing
      • Installation manual
      • Thread designation on the specification sheet

      Typical markings:

      • G1/4
      • R1/4
      • NPT1/4
      • M18×1.5

      Do not select only by diameter; the thread standard must match.

      Quick selection procedure

      1. Check machine port thread standard
        ↓
      2. Confirm medium (water/oil/air/coolant)
        ↓
      3. Confirm pressure and speed requirements
        ↓
      4. Match thread type:
        • US equipment → NPT often used
        • European/Asian equipment → G/BSP or metric often used
        • CNC spindle → metric/G commonly used
          ↓
      5. Verify sealing method and rotation direction

      If you provide the rotary joint model number (for example 1101-202-651), manufacturer, and the port marking (such as G1/4, NPT1/4, M18×1.5), I can help identify the exact connection thread and replacement specification.