How to select a rotary union

In short:Start with the media and their pressures, because they decide the seal type and materials. Then count the passages, check pressure and speed together (not each at its maximum), confirm temperature and filtration, and plan the installation with flexible hoses. If electrical circuits must also cross the rotation, specify one combined unit rather than stacking separate parts.

The decisions, in order

A rotary union carries fluid from a stationary supply into something that turns: air to the grippers on an indexing table, hydraulic oil to clamps on a rotating fixture, coolant through a spindle, water to a rotating drum, or vacuum to suction cups. It is a seal, a bearing system and a set of passages in one part, and it fails when any of those is asked to do something it was not designed for.

Select one in this order:

1. Media in each passage: air, hydraulic oil, water or coolant, steam, vacuum, other gases.
2. Pressure per passage, including peaks, and flow.
3. Number of passages, and whether electrical circuits must cross the same rotation.
4. Speed and duty: continuous rotation, indexing or oscillation.
5. Temperature of the media and surroundings.
6. Seal type and materials that suit all of the above.
7. Installation: shaft or flange mounting, hoses, drains and filtration.

If you are still deciding which kind of rotating interface you need, our slip ring selection guide compares rotary unions with the electrical and optical types.

Step 1: media decides almost everything

Write down exactly what flows in each passage and how clean it is.

• Compressed air: usually 6 bar or so on factory pneumatics. Dry and filtered air extends seal life. If the air is oiled, say so.
• Hydraulic oil: much higher pressures, often tens to hundreds of bar. Mineral oil suits nitrile (NBR) or fluorocarbon (FKM) seals.
• Water, water-glycol and coolant: watch temperature, additives and any abrasive particles. EPDM suits hot water and glycol but not mineral oils.
• Steam and hot oil: specialist unions with carbon seals and high-temperature bearings.
• Vacuum: the pressure acts the other way, so the seal must be designed to seal under vacuum, not only under pressure.
• Flammable, hazardous or corrosive media: treat as a special case. Deublin’s general-purpose instructions, for example, state that its unions should not be used for flammable media with a flash point at or below 60 °C, and prohibit hazardous or corrosive media (Deublin 040-517). Specialist designs exist, but ask for them explicitly.

Seal materials

Parker’s O-ring handbook gives a useful starting point for elastomers (Parker):

Rotary seals see sliding, heat and wear on top of chemical attack, so confirm the supplier’s rating for the specific seal, not only the base polymer.

Step 2: pressure and flow

Give the working pressure and the peak pressure on each passage, including pressure spikes when valves switch. A union rated for steady pressure can still be damaged by hydraulic shock.

Pressure and speed interact. Seal friction and heat rise with both, so maximum pressure and maximum speed are rarely available together. Deublin’s general-industry catalogue shows this clearly: one air and hydraulic oil series is rated 1,000 psi on oil at 3,500 rpm, while another reaches 3,000 psi on oil but only at 250 rpm (Deublin catalogue). Maier’s machine tool catalogue states that combinations of maximum values should be avoided (Maier).

Flow sets the passage bore. As a rule of thumb, make each passage at least as large as the bore of the hose or tube feeding it, so the union is not the restriction in the circuit. For pneumatic grippers, check the cycle time with the union in the circuit; a small passage on a long hose run can slow actuation noticeably.

Step 3: number of passages and hybrids

Count every independent circuit:

• separate pressure and return lines for double-acting cylinders
• clamp and unclamp lines on hydraulic fixtures
• one passage per vacuum circuit you need to switch independently
• coolant in and out, if it must return through the union
• a spare, if the machine may gain a function later

Multi-passage unions add length and seal friction with each passage. Where electrical power and signals must also cross the rotation, a hybrid unit combining a rotary union and an electrical slip ring is usually the better answer: one housing, one bearing system, and no risk of two separately supported rotors fighting each other.

Step 4: speed and duty

State maximum speed, normal speed and duty: continuous rotation, indexing (stop and start) or oscillation through a limited angle. Indexing tables and positioners often turn slowly but start and stop many times, which matters for seal wear and for the slip ring section of a hybrid.

High speed is a specialist area. Machine tool coolant unions run at tens of thousands of rpm: Maier lists models from 10,000 to 42,000 rpm with 50 µm filtration required on its K range (Maier). General automation unions are usually rated at a few hundred rpm or less.

Step 5: temperature, filtration and dry running

• Temperature: both media and ambient. Grease life, seal materials and bearing type all change with temperature. Deublin’s catalogue, for example, uses pressure-type carbon seals with bushing bearings above about 160 °C (320 °F), where ball bearings are not suitable.
• Filtration: particles score seal faces. Use the filtration level the union maker specifies and fit the filter before the union.
• Dry running: a liquid-lubricated face seal can be destroyed in a short time without liquid. Maier notes that classical mechanical seals allow dry running only very briefly, if at all, and offers specific models for coolant, minimum-quantity lubrication and dry air (Maier). If a passage may run empty, say so.

Step 6: seal types

Deublin describes four main seal approaches in its catalogue (Deublin catalogue):

• Balanced mechanical seals: two lapped faces, one rotating and one stationary, with the load on the faces reduced regardless of media pressure for longer life.
• Soft (elastomeric) seals: simple and compact, but only for very low speeds.
• Pressure-type carbon seals: loaded by media pressure, used with steam and hot oil.
• Hydrostatic seals: a small fluid-filled gap with no contact, which needs a drain line back to the reservoir.

For multi-passage automation unions, elastomeric or PTFE-based lip and O-ring style seals between passages are common at low speed. Ask whether a small amount of leakage is normal (Deublin notes some leakage may occur in the starting period) and whether the design has a weep or drain port so leakage from one passage cannot reach another, or the electrical section of a hybrid.

Step 7: installation

• Never pipe solid. Deublin’s instructions say rotary unions must not be piped solid and that a flexible connection is imperative to absorb installation eccentricity (Deublin IG-004-U). Route hoses without strain, twist or sharp bends (Deublin 040-517).
• Anti-rotation, not bracing. The stationary housing needs a torque arm or pin that stops it turning without clamping it rigidly.
• Concentricity. The union rotor must run true with the shaft; run-out loads the bearings and seals.
• Access. Seals wear. Plan for replacement without stripping the machine.

Worked example: a rotating hydraulic welding fixture

Requirement: A welding positioner turns a fixture at up to 5 rpm, in both directions, with occasional continuous rotation. The fixture has hydraulic clamps on mineral hydraulic oil at 80 bar working pressure, double-acting (clamp and unclamp lines), plus one 6 bar compressed air line for a purge, eight proximity sensor signals and two 24 V power circuits for a valve manifold on the fixture.

Passages: two hydraulic (clamp, unclamp), one air, plus one spare: four fluid passages.

Pressure check: 80 bar is 8 MPa. A simple pneumatic/hydraulic rotary joint rated around 1 MPa would not do; the QR Series in our range, for example, is listed at 1 MPa and suits the air but not the hydraulics. A hybrid rated for 10 MPa leaves 2 MPa of headroom at a very low speed, so the pressure and speed combination is comfortable. Clamp valve switching can produce spikes, so confirm the peak with the hydraulic designer and ask for the union’s rated peak.

Seals: mineral hydraulic oil points to NBR or FKM seals on the hydraulic passages, not EPDM. The air passage carries dry, filtered air.

Electrical: eight sensor signals plus two power circuits, with the fluid passages on the axis and the electrical rings around them. Request a drain or weep path between the fluid and electrical sections.

Choice: a pneumatic/hydraulic integrated electrical slip ring with four fluid passages, at least 12 electrical circuits (including two spares), 10 MPa rating, mineral oil compatible seals, and flexible hose connections on the stator side. The specification also states welding current must not pass through the union: a separate earth return is required on welding positioners.

Products in our range to consider

• QR Series pneumatic/hydraulic rotary joint: 1 to 20 air or fluid channels, up to 1 MPa, vacuum to −100 kPa, optional electrical circuits, to 100 rpm.
• Pneumatic/hydraulic integrated electrical slip ring: up to 16 gas or liquid channels at 10 MPa or more, with up to 48 electrical circuits at up to 30 A.
• ER Series through-bore slip ring: through-bore electrical ring, bores 12.7 to 100 mm, where a separate union passes through its centre.
• BHR Series frameless carbon brush slip ring: heavy-duty rings to 500 A per circuit, with pneumatic and hydraulic options for cranes and marine plant.
• Waterproof slip ring: IP64 to IP68 sealed rings for wet and washdown areas where the electrical section must stay dry.
• High and low temperature slip ring: rated −55 to +250 °C for hot rollers and heated plant.

What else the build needs

• Electrical slip ring for sensors, valves and power. See slip rings, the rotary union type page and how to specify a slip ring.
• Drive and motor for the rotating axis, sized for the union’s seal friction torque as well as the load. See servo and stepper motors and servo drives.
• Gearing for slow, high-torque positioners and tables. See gearboxes.
• Feedback for indexing position. See encoders.
• Hoses, filters and valves matched to each passage, with filtration to the union maker’s figure.
• Controller to sequence valves with rotation. See motion controllers.

Things that catch engineers out

• Rigid piping to the stator, which loads the bearings and seals.
• Maximum pressure and maximum speed assumed together.
• Pressure spikes from valve switching that exceed the rated peak.
• Seal material chosen for one passage and applied to all, such as EPDM meeting mineral oil.
• Dry running a liquid-lubricated face seal.
• Vacuum treated like pressure. Specify the vacuum level and acceptable leak.
• No drain between fluid and electrical sections of a hybrid.
• Seal friction torque left out of the motor sizing on small indexers.
• Welding current allowed to return through the union or slip ring.

Checklist

Send these to our engineers, or enter them in the spec builder:

• Media in each passage, including additives, cleanliness and whether oiled or dry
• Working and peak pressure per passage; vacuum level if any
• Flow rate or hose bore per passage
• Number of passages, plus spares
• Media and ambient temperature
• Speed (rpm) and duty: continuous, indexing or oscillating
• Electrical circuits: number, current, voltage, signal types
• Mounting: shaft or flange, bore, envelope
• Port sizes and thread types on rotor and stator
• Filtration available; any dry-running periods
• Acceptable leakage and drain arrangements
• Environment: IP rating, washdown, hazardous area
• Quantities and timing

For the wider family, see our slip rings overview.