A bearing housing that leaked oil from the shaft seal. The housing was a cast iron pillow block, running a 30 mm shaft at 1500 RPM. The seal was a double lip with garter spring. The housing was filled with oil to the center of the lowest ball. After 2 months, oil dripped from the seal. The customer replaced the seal. It leaked again. The issue wasn’t the seal — it was the housing building internal pressure. The housing had no vent. As the oil warmed, the air inside expanded. The pressure pushed oil past the seal. This is about bearing housing pressure balance and the often-overlooked vent plug.
The thermal pressure rise
A sealed bearing housing contains oil and air. At startup, the air inside is at ambient pressure (1 bar). As the bearing runs, the oil and air warm to 60°C. The air expands: P2 = P1 × T2/T1 = 1 × (333/293) = 1.14 bar. The internal pressure rises 0.14 bar. That’s 1400 N/m². The oil seal lip is designed to hold oil, not air pressure. The air pushes oil past the lip. The seal leaks.
The failed housing had no vent plug. The customer had installed a solid plug in the fill hole. The housing was completely sealed. The thermal pressure had nowhere to go. Every warm-up cycle pushed oil past the seal. After 2 months, enough oil had leaked to lower the level. The bearing started to starve. The seal kept dripping.
What was changed
1. Installed a vent plug. The solid plug was replaced with a vented breather (a small plastic cap with a filter). The vent equalizes internal and external pressure. The air can escape as it expands. The pressure doesn’t build. The seal stopped leaking. The vent plug costs $5. It’s the cheapest fix in maintenance.
2. Used a labyrinth seal. For shafts running at over 1500 RPM, a contact lip seal wears out in months. A non-contact labyrinth seal has no rubbing surface. It doesn’t leak oil (the oil must fight gravity through a tortuous path). The labyrinth seal is rated for 10,000 hours. The double-lip seal was replaced with a labyrinth. The oil stayed in the housing. The seal lasted 3 years.
3. Added an oil sight glass. The housing had no way to check the oil level. The customer over-filled it (thinking more oil = better). Over-filling causes churning heat. The oil temperature rose 10°C. A sight glass was installed. The oil was filled to the center of the lowest ball (not over). The churning stopped. The oil ran cooler. The seal didn’t leak.
The housing design rules
Every bearing housing needs three things:
- Fill plug: for adding oil. Must be removable. Location should be at the correct oil level.
- Drain plug: at the bottom. For changing oil. Must be accessible.
- Vent/breather: on top. Equalizes pressure. Must have a filter to keep dust out. Never plug the vent.
- Sight glass: on the side. Shows the oil level. Must be readable while the machine runs.
A housing with only fill and drain plugs is a pressure vessel. It leaks oil. A housing with a vent breather runs at atmospheric pressure. The seal holds.
The vent filter
A bare vent (open hole) lets dust in. The dust mixes with the oil and grinds the bearing. A vent filter (5 μm paper element) keeps dust out while letting air pass. For dusty environments (cement, woodworking), the vent filter needs monthly replacement. For clean environments (machining), annual replacement. The filter clogs over time — if the vent is blocked, the pressure builds again. A clogged vent filter causes the same leak as no vent.
| Housing type | Vent required | Seal type | Oil level |
|---|---|---|---|
| Pillow block (cast iron) | Yes | Lip seal | Center of lowest ball |
| Flange bearing (integral seal) | No (sealed) | Contact seal | Pre-lubricated |
| Tank-mounted bearing | Yes (tank vent) | Labyrinth | Below the oil seal |
| Gear reducer | Yes (breather) | Lip or mechanical | Oil sight glass |
The housing rule: every oil-filled bearing housing needs a vent breather. The leaking seal wasn’t defective — a sealed housing built 0.14 bar thermal pressure that pushed oil past the lip. Install a $5 vent plug. Use a labyrinth seal for high-speed shafts. Add a sight glass. Never plug the vent — it turns the housing into a pressure cooker.
