Threaded vs Flange Mounting for Rotary Unions
Choose the mounting interface from the machine structure and service plan, not from the rotary union’s outside appearance alone.
Threaded and flange-mounted rotary unions can both provide reliable rotating transfer when the interface is designed correctly. The better choice depends on available space, alignment method, load path, connection access, removal procedure, and how the fixed and rotating sides are supported.
How the two mounting approaches differ
A threaded connection can be compact and direct, but thread engagement, sealing method, installation torque, orientation, and removal access all matter. The surrounding structure must prevent hoses or fittings from imposing bending or torsional load on the union.
A flange can provide a defined bolt pattern and locating feature, which may simplify orientation and repeatable removal. It generally needs more radial space and a suitable mating face. A flange does not correct poor concentricity or eliminate the need for hose support and anti-rotation control.
Compare these machine-level factors
- Alignment and load path Determine how the rotating axis is located and how axial, radial, bending, and torsional loads are kept out of the rotary union.
- Assembly and service access Check tool clearance, fitting access, orientation control, replacement time, and whether adjacent components must be removed.
- Interface definition Specify thread standard and engagement or flange pilot, bolt pattern, fasteners, sealing face, tolerances, and fixed/rotating-side responsibility.
A practical mounting decision process
- Draw both interfaces in section Include the machine shaft, housing, union, fittings, hoses, supports, and removal direction.
- Check assembly tolerances Review concentricity, runout, locating features, thread or flange tolerances, and thermal movement.
- Review the service procedure Confirm how technicians isolate, access, remove, reinstall, and leak-test the union.
- Choose from total integration risk Select the interface that controls alignment and external loads with the fewest special parts and the clearest maintenance procedure.
Pick the threaded interface when envelope is tight and the structure can take installation torque without bending the union. Pick a flange when you need a defined bolt pattern, orientation, and repeatable removal. Neither mounting type corrects poor concentricity.
Compare threaded and flange interfaces for your machine
Send a section of the shaft, housing, fittings, and removal direction. We will compare the two mounting approaches against that layout.