How to Choose the Right Rotary Joint: A Complete Selection Guide
Why Rotary Joint Selection Matters
A rotary joint — also called a rotary union — is the critical link between a stationary supply line and a rotating machine component. It allows steam, water, thermal oil, coolant, or hydraulic fluid to pass through while the equipment spins.
Select the wrong joint, and you get leaks within weeks. Select the right one, and it runs maintenance-free for years.
This guide covers the six factors every engineer should evaluate before specifying a rotary joint.
Factor 1: Media Type
The medium flowing through the joint determines everything — seal material, housing material, and internal geometry.
| Media | Recommended Seal Material | Notes |
|---|---|---|
| Cold water / Coolant | Carbon, ceramic, or silicon carbide | Most common application |
| Hot water (80–120°C) | Silicon carbide or tungsten carbide | Higher thermal resistance |
| Steam (saturated or superheated) | Carbon with anti-scaling treatment | RTH series excels here |
| Thermal oil | Carbon or specialized graphite | Avoid rubber seals |
| Hydraulic fluid | Nitrile or Viton O-rings | High-pressure compatible |
| Compressed air | Standard carbon seals | Low-pressure application |
| Corrosive media | Stainless steel internals + PTFE seals | Requires anti-corrosion treatment |
Tip: If your media contains particles or scale, choose a joint with easily replaceable friction pairs — like the RTH series with its external multi-spring balanced seal.
Factor 2: Operating Pressure
Rotary joints are rated for specific pressure ranges. Exceeding the rating causes seal failure and potential safety hazards.
- Low pressure (up to 1.0 MPa): Suitable for most water, coolant, and air applications. Standard joints work well.
- Medium pressure (1.0–4.0 MPa): Requires balanced seal design to prevent uneven wear. The RTH series handles up to 1.6 MPa.
- High pressure (4.0–100 MPa): Demands specialized hydraulic joints with multi-passage design. Ruitus RT-GY series handles up to 100 MPa for steel mill hydraulic circuits.
Always specify: Maximum operating pressure, not just nominal pressure. Include pressure spikes if your system has them.
Factor 3: Temperature Range
Temperature affects seal life more than any other factor.
- Below 80°C: Standard carbon seals perform well. Most water and coolant applications fall here.
- 80–120°C: Requires silicon carbide or tungsten carbide seal faces. Hot water and low-pressure steam.
- 120–250°C: High-temperature seals needed. The RTQ series is specifically designed for thermal oil and high-temperature steam up to 250°C.
- Above 250°C: Custom engineering required. Contact the manufacturer with your specific conditions.
Critical point: Always measure the actual temperature at the joint, not just the system temperature. Friction generates additional heat at the seal interface.
Factor 4: Rotational Speed
Speed capability depends on the seal design, bearing type, and joint diameter.
| Speed Range | Typical Application | Joint Type |
|---|---|---|
| 0–200 rpm | Steel mills, slow equipment | Large-bore joints, multi-passage |
| 200–1,000 rpm | General industrial | Standard balanced-seal joints |
| 1,000–3,000 rpm | Machine tools, roll cooling | RTH series (patented design) |
| 3,000–10,000 rpm | CNC machining centers | RT-DE CNC through-spindle |
| Above 10,000 rpm | High-speed spindles | Specialized high-speed joints |
Key insight: The RTH series uses a patented external multi-spring design that maintains balanced pressure at speeds up to 3,000 rpm — significantly higher than conventional H-type joints of the same size.
Factor 5: Housing and Internal Material
Material selection balances corrosion resistance, strength, and cost.
- Brass housing: Standard for water and coolant. Good corrosion resistance, easy to machine. The RT-DE series uses forged brass for superior strength.
- Stainless steel housing: Required for corrosive media, food processing, and pharmaceutical applications. The JL-DD/JL-DC series offers precision-cast stainless steel alternatives.
- Carbon steel: Used in high-pressure hydraulic applications where strength matters more than corrosion resistance.
- Internal components: Always specify stainless steel or corrosion-treated internals, regardless of housing material. Ruitus treats all media-contact components with anti-corrosion coatings as standard.
Factor 6: Connection Type and Size
The joint must match your piping and equipment接口.
- Threaded connections (G, NPT, BSP): Most common for sizes up to 2 inches. Easy installation.
- Flanged connections: Preferred for larger sizes and higher pressures. More secure, easier to align.
- Bayonet or quick-connect: For applications requiring frequent disconnection.
- Custom connections: Send your drawing and specifications — Ruitus manufactures to match.
Pro tip: Specify both the inlet and outlet connection sizes. They may differ depending on your flow requirements.
Quick Selection Checklist
Before contacting a supplier, have these answers ready:
- ☐ What medium flows through the joint?
- ☐ What is the maximum operating pressure?
- ☐ What is the temperature range (min and max)?
- ☐ What is the rotational speed?
- ☐ What connection type and size do you need?
- ☐ What is the expected duty cycle (continuous vs intermittent)?
Getting Expert Help
Even with this guide, rotary joint selection can be complex. If you’re dealing with extreme conditions — high temperature, high pressure, corrosive media, or unusual mounting space — it’s worth consulting with an engineer.
Ruitus offers free technical consultation for rotary joint selection. With 20+ years of sealing engineering experience and 10 standard models covering most industrial applications, we can help you find the right solution.
Contact us or reach Frank Cao directly at frankcao@ruitus.com / +86 177 9883 6850 (WhatsApp).
Send Us Your Drawing or Duty Conditions
Tell us the medium, temperature, pressure, speed and connection size. Our engineers reply with a selection or a custom proposal — usually within one working day.