Views: 0 Author: Site Editor Publish Time: 2026-04-22 Origin: Site
A rotary union (also known as a rotary joint, rotating coupling, or swivel) is a precision mechanical sealing device that transfers fluid or gas from a stationary source into a rotating component. In CNC machining centers, the rotary union is mounted at the rear of the spindle. Together with the through-spindle coolant (TSC) system, it delivers coolant from a stationary hose to the high-speed rotating spindle and finally to the cutting edge. This effectively flushes chips, cools the workpiece, and improves machining efficiency.
Water‑based coolant → Requires corrosion‑resistant materials (stainless steel or hard‑anodized aluminum). Coolant filtration should be at least 40–60 µm.
Hydraulic oil → Use seals compatible with petroleum‑based fluids. Pressure can reach up to 210 bar.
Oil mist / MQL → Choose a full‑flow‑path design to prevent chip accumulation.
Compressed air → Must support “dry running” (sealing designed for unlubricated rotation).
Standard: 500 – 3,000 RPM
High speed: 5,000 – 20,000 RPM (common for high‑speed machining centers)
Ultra‑high speed: 12,000 – 40,000+ RPM
At high speeds, look for:
Balanced seal design
Low‑friction bearings (ceramic or hybrid)
Dynamic sealing technology (e.g., lip seals or non‑contact magnetic seals)
Note: Maximum pressure and maximum RPM cannot usually be used simultaneously – trade‑offs must be made.
Pressure Level | Range | Typical Application |
|---|---|---|
Low | 0 – 10 bar | Compressed air, low‑pressure cooling |
Medium | 10 – 70 bar | Standard CNC coolant |
High | 70 – 140 bar | High‑pressure coolant (HPC), hydraulic clamping |
Ultra‑high | 140 – 210 bar | Hard milling, extreme TSC conditions |
Consider both pressure spikes (instantaneous peaks) and the pressure‑RPM balance.
Single‑passage – Simple, low cost, one medium.
Multi‑passage – Several independent channels for different media (coolant + oil + air) simultaneously.
Closed seal – Minimal leakage, stays closed without media.
Controlled leakage seal – Small leakage lubricates the seal faces (standard for most machining).
Pop‑off seal – Designed for TSC; seal faces separate during tool changes to drain residual coolant.
All‑media seal – Compatible with many different fluids.
AutoSense seal – Automatically adjusts sealing condition based on operating parameters.
Many high‑end rotary unions use silicon carbide (SiC) seal faces – extremely hard, wear‑resistant, and thermally conductive.
Bearing‑supported type – Integrated bearings, easy to install, heavier.
Bearingless type – Compact, lightweight, suitable for very high speeds, but requires precise mounting.
Stainless steel (304/316L) – Best corrosion resistance for water‑based coolants.
Hard‑anodized aluminum – Lightweight for high‑speed spindles.
Carbon steel – For oil‑based media, lower cost.
Seal material: FKM (Viton) full‑encapsulated seals offer excellent resistance to cutting fluids and abrasive particles.
Before finalizing your choice, verify:
Coolant type (water‑based or oil‑based)?
Need multiple media simultaneously?
Max spindle speed & typical operating speed?
[ ] Does the union support “dry run”?
System working pressure & possible pressure spikes?
Connection thread type (NPT / BSPP / metric) and size?
Available mounting space at spindle rear?
Spindle runout (should be ≤0.05 mm at the mounting interface)?
Compatibility with your spindle model or brand (e.g., Deublin, RIX, Kadant)?
Installation
Measure runout at the union mounting face – keep ≤0.05 mm.
Use flexible hoses, never rigid piping directly to the union.
The drain port must point downward and have no back pressure.
No side load on the union from hoses.
Maintenance
Maintain coolant filtration ≤50 µm (finer for abrasive materials like glass or cast iron).
A small amount of leakage is normal. Replace only when leakage exceeds the manufacturer’s limit.
Consider a leakage monitoring sensor to warn before coolant enters the spindle bearings.
Regularly inspect hoses for aging and bend radius.
If you prefer or require a locally produced rotary union for CNC spindles, the following companies are well‑regarded in China.
Shandong Focus Machinery Co., Ltd. is located at No. 988 Xingteng East Road, Tengzhou Economic and Technological Development Zone, Shandong Province, China. it is a well-known rotary joint manufacturing enterprise in China.
The company has 12 R&D technicians, more than 60 high-precision and high-efficiency CNC machine tools, more than 20 testing equipment, and an annual production of more than 30000 rotary joints.
The rotary joints produced by our factory not only have excellent quality, but also have a very competitive price.
Support OEM/ODM outsourcing production; Short delivery cycle; Produce according to your drawings or provide samples for customization.
