A rotary union on a rotary indexing table started leaking coolant after six months. The union carried 8 bar water-based coolant to a spindle. The customer serviced it — replaced the seals, re-tightened the gland. It leaked again in three weeks. They bought a new union from the same supplier. It leaked after four months. The customer blamed the union manufacturer. The union was fine. The mounting was wrong.
The rotary union is a precision device, not a plumbing fitting
A rotary union has a static housing and a rotating shaft, sealed by carbon-face or mechanical seals. The seal faces must run perfectly parallel. If the housing is misaligned with the rotating axis, the seal faces open on one side and the union leaks. It leaks more as speed and pressure go up.
The indexing table had a runout problem. The spindle shaft that the union mounted on had 0.08 mm TIR at the union location. The union manufacturer allows 0.02 mm TIR at the mounting surface. The spindle was out by 4x the limit. The seal faces were opening and closing 0.08 mm with every rotation. The seal wore out in months. A new union on the same misaligned spindle failed the same way.
The fix that wasn’t a new union
We checked the runout first — that should always be step one. The spindle had a worn bearing at the lower end. The bearing replacement cost $180. The runout at the union dropped from 0.08 mm to 0.015 mm. The existing union (the “leaking” one) stopped leaking. It’s still in service two years later.
The second issue: the union was piped with rigid stainless tube. The tube was forcing the union housing off-axis by another 0.03 mm. The connection should use a short flexible hose between the union and the fixed pipe. The hose absorbs the misalignment and vibration instead of transmitting it to the seal faces. We installed a 200 mm flexible hose. The union ran true.
Third: the union had been over-lubricated during the seal service. Grease was pumped into the bearing cavity until it pushed past the seal. The excess grease contaminated the seal face and caused the initial leak after the service. The grease fitting on a rotary union gets one pump, not ten. The seal cavity fills with grease only when the union is running — the grease expands when warm. A union serviced with a grease gun and left idle will push grease into the seal. Use the specified volume.
The checks before you replace a rotary union
| Check | Limit | Tool |
|---|---|---|
| Mounting surface runout | 0.02 mm TIR | Dial indicator |
| Housing squareness | 0.03 mm over 100 mm | Square + feeler |
| Pipe strain on housing | Zero — use flex hose | Hand test: union should not bind |
| Lubricant volume | Specified pumps only | Greaser with volume stop |
| Coolant contamination | Filter to 50 μm | Particle count |
If you’ve done all five checks and the union still leaks, then look at the union itself. Ninety percent of premature union failures are mounting and piping errors, not seal defects. The seal kit is the last thing to replace, not the first.
The fluid matters too
The coolant had a 10% tramp oil content — the machine’s way oil was leaking into the coolant tank. Tramp oil attacks the union’s elastomer seals. The nitrile seals swelled and lost their sealing force. The coolant was filtered for particles, but nothing filtered for oil. A tramp-oil separator on the coolant tank cost $400. The seal life on the union went from 4 months to 18 months.
Check the runout, check the piping, check the grease volume, check the fluid — in that order. The rotary union that leaked after six months was sitting on a spindle with 4x the allowed runout. Fix the bearing, add a flex hose, stop over-greasing. The union is a witness to the machine’s condition, not the cause of the leak.
