Choosing a rotary union for a CNC machine tool involves more than comparing maximum RPM, working pressure, or purchase price. The internal support structure affects installation, spindle loading, vibration behavior, and the way the assembly responds to external forces.
Two common configurations are bearing-supported rotary unions and bearingless rotary unions. Both can transfer coolant or other compatible media into rotating machine components, but their advantages depend on how they are integrated into the spindle and installed in the machine.
Understanding these structural differences helps machine tool designers avoid unnecessary cost, prevent installation-related problems, and select a rotary union that matches the actual operating conditions.
1. Bearing-Supported Rotary Union: Pros and Cons
A bearing-supported rotary union uses internal bearings to support its rotor relative to the stationary housing. Its integrated construction can simplify installation and replacement, although the bearing arrangement also introduces additional mechanical constraints.
Advantages of Bearing-Supported Rotary Unions
Easier installation and replacement
A bearing-supported rotary union is generally installed as a complete assembly. This reduces the number of separate components that must be positioned during installation and can simplify replacement during machine maintenance.
However, the one-piece design does not eliminate alignment requirements. The spindle mounting surface, rotor interface, and hose connections must still be installed according to the manufacturer’s instructions.
A defined route for leakage drainage
Many bearing-supported rotary unions use a housing drain to direct leakage away from the spindle and surrounding machine components.
This arrangement is useful when leakage needs to be collected and discharged through a dedicated line. However, the drain only works as intended when its position, routing, and connection match the product’s installation requirements.
A poorly routed drain hose can restrict gravity drainage or allow liquid to accumulate inside the housing. Therefore, drainage should be considered during machine layout, rather than treated as a minor plumbing detail after installation.
Potential reduction in spindle axial thrust
One important structural difference concerns the axial force generated by coolant pressure.
A rotor-mounted, bearing-supported rotary union can be designed to absorb most of the pressure-induced axial thrust, reducing the force transmitted to the spindle.
This can be an advantage when the spindle’s axial load budget is limited. However, it should not be assumed that every bearing-supported design provides this benefit. In a bore-mounted configuration, the pressure-induced thrust may be transmitted to the spindle, depending on the design.
Disadvantages of Bearing-Supported Rotary Unions
Higher component cost
The internal bearings add parts and manufacturing requirements. Compared with an otherwise comparable bearingless design, this can increase the rotary union’s initial cost.
The relevant comparison, however, is not simply bearing-supported versus bearingless. Seal technology, operating ratings, dimensions, materials, and mounting arrangements also affect the final price.
Additional mechanical constraints at high speed
At high rotational speeds, the bearings introduce considerations such as friction, heat generation, lubrication requirements where applicable, and bearing operating limits.
A bearing-supported rotary union may therefore be unsuitable for some very high-speed applications. Nevertheless, maximum RPM depends on the complete design, not on the bearing arrangement alone.
Sensitivity to mounting errors and external loads
A common selection mistake is to assume that an integrated assembly is automatically tolerant of imperfect installation.
In fact, incorrect centering, contamination on the mounting interface, or excessive lateral force from a coolant hose can affect operation. Depending on the design, these conditions may contribute to runout, vibration, bearing wear, or premature failure.
The installation arrangement is therefore part of the engineering decision, not merely a maintenance concern.
2. Bearingless Rotary Union: Pros and Cons
A bearingless rotary union eliminates the internal bearings used to support the rotor within the union itself. Typically, the rotor connects to the spindle while a separate stationary housing contains the non-rotating sealing element and fluid connection.
Removing the internal support bearings changes the unit’s dimensions, cost structure, and response to external loading. It also places greater importance on installation accuracy and protection of the sealing surfaces.
Advantages of Bearingless Rotary Unions
Lower component cost and high-speed potential
Removing the internal support bearings reduces the number of components and can lower manufacturing cost. It also removes the speed constraints associated with those bearings, potentially making the design attractive for high-speed CNC spindles.
However, this does not mean that every bearingless rotary union can operate at a higher speed than every bearing-supported model. The seal system, rotor geometry, pressure, and rated operating conditions must still be considered.
Reduced risk of vibration originating from the union’s bearing-supported housing assembly
Only the relatively small rotor is directly connected to the rotating spindle, while the housing remains stationary.
This avoids the rotary union’s own bearing-supported housing assembly becoming a vibration source through that particular mechanism. It does not eliminate vibration caused by spindle imbalance, poor centering, unsuitable mounting, or external forces.
The distinction matters when evaluating vibration-related problems: removing the union’s bearings removes one possible source, but does not make the entire installation insensitive to alignment.
No internal support bearings to receive hose-induced side loads
A supply hose can exert a lateral force when it is too short, bent too tightly, or placed under tension. In a bearing-supported union, this force can load the internal bearings.
A bearingless design eliminates that particular bearing-loading mechanism because it has no internal rotor-support bearings. However, hose forces do not disappear. They can still act on the connections, housing restraints, or spindle, so the prescribed hose routing and support arrangement remain important.
Compact construction for multi-spindle machines
Removing the bearing assembly allows a bearingless rotary union to be made compact in suitable designs.
This can be especially useful in multi-spindle machining centers, where several closely spaced spindles compete for limited installation space. A smaller envelope can simplify machine integration without requiring the designer to enlarge the surrounding structure.
Disadvantages of Bearingless Rotary Unions
More demanding installation
A typical bearingless rotary union is installed in two parts: the rotor and the stationary housing.
The two components must be positioned and assembled correctly. Unlike a one-piece bearing-supported unit, the assembly process exposes the sealing interface before the components are fully installed.
This makes handling and alignment particularly important.
Precision sealing faces are vulnerable during installation
The sealing faces in a precision rotary union may be finely lapped. Contamination, scratches, or accidental impacts during installation can compromise their condition.
A practical mistake is to focus on mechanical fit while treating the sealing surfaces as ordinary machined faces. Cleanliness and careful handling are just as important as dimensional accuracy.
The correct installation procedure should specify how the sealing faces are protected, how the two components are positioned, and what inspection is required before operation.
Pressure-induced spindle thrust still requires evaluation
A bearingless rotary union does not automatically eliminate the axial thrust generated by coolant pressure.
Depending on the design, this force can be transmitted to the spindle. If the spindle’s bearing arrangement or allowable axial load is already constrained, the force path must be evaluated before selecting the rotary union.
A bearingless design should therefore be chosen for its actual structural benefits, rather than on the assumption that eliminating bearings removes every load-related concern.
3. Bearing-Supported vs. Bearingless Rotary Unions
The following comparison summarizes the main differences that matter during machine tool design.
Design consideration | Bearing-supported | Bearingless |
Construction | Integrated assembly with internal support bearings | Typically a separate rotor and stationary housing |
Installation | Generally simpler as a complete assembly | Requires careful two-part assembly |
Component cost | Includes bearing components | Can reduce component cost |
High-speed potential | Subject to bearing-related constraints | Eliminates the union’s internal bearing speed constraints |
Hose-induced side loading | Can load internal support bearings | No internal support bearings to receive that load |
Vibration considerations | Depends on alignment and complete assembly design | Avoids vibration associated with the union’s internal bearing-supported housing arrangement |
Installation space | Depends on the specific model | Can be particularly compact |
Pressure-induced axial thrust | Rotor-mounted designs can absorb most of the thrust | Thrust transmitted to the spindle must be evaluated |
Sealing-face handling | Depends on the specific construction | Exposed sealing faces require particular care during assembly |
Neither configuration is universally better. The choice depends on the spindle interface, operating requirements, available space, and installation conditions.
4. Installation Mistakes and Selection Errors to Avoid
The most useful way to compare rotary union designs is to consider what can go wrong when the unit is installed or selected without accounting for its structure.
Mistake 1: Treating Runout as a Product Defect Before Checking the Mounting Interface
A rotary union may run eccentrically or vibrate if the spindle pilot or mounting surface contains chips, burrs, or dents. Incorrect centering or damaged interfaces can also affect alignment.
Replacing the rotary union without checking the mounting interface may therefore fail to solve the underlying problem.
Before commissioning the spindle, inspect the specified mounting surfaces, verify the relevant dimensions and tolerances, and confirm that the unit is centered as required by the product drawing.
This is particularly important in high-speed applications, where small alignment errors can become more consequential during rotation.
Mistake 2: Ignoring Coolant Hose Routing
A hose that appears acceptable when the machine is stationary may exert additional force when pressurized. A tight bend or tensioned hose can generate lateral loading.
For a bearing-supported rotary union, this can increase the load on its internal bearings. For a bearingless design, the lack of internal support bearings does not remove the need to control forces on the connections and spindle.
Use the specified flexible connections, provide adequate hose clearance, and avoid unnecessary tension. The correct arrangement depends on the product’s mounting and anti-rotation requirements.
Mistake 3: Treating Drainage as a Secondary Detail
Where a rotary union relies on gravity drainage, the drain line must be routed to allow liquid to flow away continuously.
An upward section or an incorrectly oriented drain outlet can obstruct drainage and cause liquid to accumulate in parts of the assembly that are not intended to remain flooded.
For a horizontal spindle, some designs specify that the drain outlet face downward. The applicable orientation must be verified in the installation manual for the selected model.
Drainage should be incorporated into the machine’s layout from the beginning, especially when the rotary union is installed in a confined space.
Mistake 4: Assuming All Bearing-Supported Designs Manage Axial Thrust in the Same Way
The distinction between rotor-mounted and bore-mounted designs is easy to overlook.
A rotor-mounted, bearing-supported rotary union can be designed to absorb most of the axial thrust generated by coolant pressure. A bore-mounted or bearingless configuration may transmit some of this force to the spindle.
The correct question is not simply whether the rotary union has bearings, but how the pressure-induced load travels through the complete assembly.
For a new spindle design, confirm the mounting geometry and load path before finalizing the rotary union selection.
Mistake 5: Choosing a Bearingless Design Solely Because It Is Cheaper or Faster
A bearingless rotary union can reduce component cost and remove the speed constraints associated with internal support bearings. These are meaningful advantages, but they do not automatically make the design the better choice.
If the application requires straightforward replacement, has limited installation access, or depends on a particular axial-thrust management arrangement, other structural considerations may be more important.
Likewise, a bearing-supported design is not necessarily the better option merely because it is easier to install as a complete unit.
The decision should balance speed capability, mechanical integration, pressure-induced loading, installation complexity, and maintenance requirements.
Mistake 6: Comparing Maximum Speed Without Checking the Actual Operating Conditions
Maximum RPM is an important specification, but it should not be considered in isolation.
The selected rotary union must be suitable for the actual combination of speed, pressure, medium, temperature, and operating cycle. The permissible operating envelope varies by model, and the highest listed speed should not automatically be assumed to apply at every pressure or operating condition.
Applications that involve periods without coolant flow also require confirmation that the seal design permits the intended operating sequence.
Checking the rated limits and installation instructions before purchase can prevent an apparently suitable product from becoming unsuitable in service.
5. How to Choose Between the Two Designs
A bearing-supported rotary union may be the more practical choice when integrated installation, easier replacement, a defined leakage-drain arrangement, or a rotor-mounted thrust-management feature is important.
A bearingless rotary union may be preferable when the spindle requires a compact arrangement, the application has high-speed requirements, or multiple spindles must fit within a restricted space.
Before making the final selection, confirm:
- The actual spindle speed and working pressure.
- The coolant or other operating medium.
- The rotor connection and mounting dimensions.
- The available installation space and alignment tolerances.
- The axial-thrust load path.
- The required drainage and hose-routing arrangements.
- Whether the intended operating cycle permits periods without fluid flow.
These factors should be assessed together. Choosing a rotary union solely by maximum RPM or purchase price overlooks important differences in how the unit interacts with the spindle.
Conclusion
Bearing-supported and bearingless rotary unions offer different advantages because their internal structures solve different mechanical problems.
Bearing-supported designs can simplify installation and replacement, and certain rotor-mounted configurations can absorb most of the axial thrust generated by coolant pressure. Bearingless designs eliminate the union’s internal support bearings, potentially reduce cost, offer high-speed potential, and provide a compact solution for multi-spindle equipment.
The practical differences become clearer during installation. Mounting-surface contamination can cause runout, incorrect hose routing can introduce external loads, and inadequate drainage can undermine reliable operation. Bearingless designs also require particular care to protect their precision sealing faces during assembly.
For machine tool designers, the best choice is the configuration that matches the spindle’s load path, rated operating conditions, installation space, and maintenance requirements.
When requesting a rotary union quotation, provide the required RPM, operating pressure, fluid type, spindle interface dimensions, and installation constraints. This information allows the supplier WayDun Technology to evaluate the appropriate structure instead of recommending a unit based on speed or price alone.