In brief
The seal is what separates clean lubricant from everything else, which is why it decides bearing life more than bearing quality does. It is selected on four conditions: shaft surface speed, working temperature, differential pressure and what sits on the other side — abrasive dust, pressurised water, chemicals. In dirty environments one seal is rarely enough: an outer barrier that stops the coarse material is combined with a lip seal that retains the lubricant. And fitting matters as much as selection: a folded lip or a seat scored during installation cancels out a correct seal from the first minute.
A quality bearing, correctly mounted and properly lubricated, can still fail early for a reason that never appears on its data sheet: dirt getting in. The ISO 15243 failure classification calls that mechanism abrasive wear, and in mining, aggregates and construction it is the normal working condition, not the exception.
The seal costs a fraction of the bearing, and a far smaller fraction of the stoppage. Even so, it tends to be the last thing specified and the first thing replaced with whatever is on the shelf.
What a seal has to do
Two opposite things at once: hold the lubricant in and keep contamination out. Both require contact, and contact produces friction, heat and wear on the seal itself. All sealing design is a negotiation between those forces.
| Type | How it works | Where it fits |
|---|---|---|
| Non-contact seal (labyrinth, cover) | Geometric barrier, no rubbing | High speed and temperature; holds little against pressurised water |
| Radial lip seal | Elastomer lip spring-loaded against the shaft | The general case: retains lubricant, stops dust and splash |
| Double lip or dust-lip seal | One lip retains, the other excludes | Dirty environments; the outer lip protects the inner one |
| Factory-sealed bearing (2RS) | Seal integrated, greased for life | Inaccessible or lightly loaded points; cannot be relubricated |
| Shielded bearing (ZZ) | Integrated non-contact protection | Clean environments; stops neither moisture nor fine dust |
The four figures that decide the choice
- Shaft surface speed. That is what the lip sees, and it rises with diameter as well as with rpm. Above the material limit, the lip burns.
- Real working temperature, not ambient. The lip material — NBR, FKM, polyurethane — is chosen for it, and a seal outside its range hardens or swells.
- Differential pressure. A standard seal is designed to work with almost none; where there is pressure, use a specific design or vent the housing.
- What is on the other side: abrasive dust, pressure-wash water, steam, chemicals or plain humidity. Each calls for a different barrier.
Fitting: where a good seal is ruined
- 01
Inspect the shaft seat
The lip runs on a surface that must be smooth and free of circumferential scoring. A shaft carrying a worn groove from the previous seal will leak through it: fix it with a repair sleeve or by shifting the seal a few millimetres.
- 02
Protect the lip past keyways and threads
Any sharp edge cuts the lip. Use a fitting cone or cover the edge with thin tape, and remove it before seating.
- 03
Lubricate before fitting
Coat the lip with the same grease or oil as the system. A seal fitted dry starts with friction and can burn before the lubricant arrives.
- 04
Enter square and fully home
Push with a tool that bears on the whole face of the seal, never with a point blow. A seal tilted a few degrees leaks from day one.
- 05
Check the orientation
The main lip faces the side that must be retained. Fitting it backwards turns a correct seal into an inlet.
When the seal is the symptom, not the cause
A leaking seal may be reporting something happening further in. Before replacing it a third time, look at the whole arrangement.
- Leaking with a new, correctly fitted seal: usually excess lubricant or a housing with no breather; internal pressure pushes the lubricant out.
- Fast, uneven lip wear: misaligned or unbalanced shaft, or seat run-out.
- Hardened, cracked lip: the material does not withstand the real working temperature — review the selection and, above all, why that point runs so hot.
- Fretting corrosion in the seal seat: there is micro-movement where there should be a firm fit.
Sealing as part of the plan
In a reliability programme, seals deserve the same attention as the bearings they protect: stock of the critical ones, a written replacement criterion, and a check at every intervention. Replacing a bearing and reusing the old seal is saving on the wrong component. Rodacaribe distributes SKF industrial and automotive seals alongside the bearings and the Q8 Oils greases compatible with each application.
Frequently asked questions
- Can I reuse a seal that looks fine?
- It is not advisable. The lip takes the shape of the shaft in service and develops its own contact track; on refitting, even in the same position, that track rarely lines up and leaking follows. The seal is single-use.
- Can a factory-sealed bearing be relubricated?
- No, and it does not need to be: it comes with the grease calculated for its service life. Removing the seal to grease it turns it into an open bearing with no protection. Where the application requires relubrication, specify an open bearing with external sealing.
- Why is grease purging recommended in dirty environments?
- Because grease coming out past the seal carries with it the particles trying to get in. It is an active barrier as well as a mechanical one. That is why relubrication intervals in mining and aggregates are shorter than in general industry.
- Does a non-contact seal protect less?
- It protects differently. It does not rub, so it neither wears nor heats, and it allows speeds a lip cannot take. In exchange it does not close tightly: against pressurised water or very fine dust it needs to be combined with a contact seal.
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About this contentThese guides collect industrial maintenance good practice and widely used standard references. They do not replace the technical documentation of the equipment or component manufacturer, which always prevails.