An oil seal is a sealing element that closes the gap between a rotating shaft and the housing. It prevents the oil inside from leaking out along the shaft and prevents external dust and water from entering. In gearboxes, an oil seal is located at every point where the input and output shafts exit the housing.

The most common cause of oil leakage in a gearbox is wear or hardening of the seal lip, but this is not the only cause. A clogged breather plug, overfilled oil, incorrect mounting position, a worn shaft surface and loose cover bolts can also produce the same result. Therefore, before replacing the seal, it is necessary to determine exactly where the leak is coming from; otherwise, the new seal may also begin leaking in a short time.

How Does an Oil Seal Work?

A standard oil seal consists of three parts: the metal-reinforced outer section that fits tightly into the housing and keeps the seal in its seat, the flexible sealing lip that contacts the shaft, and the thin garter spring that presses this lip against the shaft.

The lip does not rub dry against the shaft. As the shaft rotates, a very thin oil film forms between the lip and the shaft; this film provides sealing while also lubricating and cooling the lip. When this film is disrupted—when the lip runs dry, overheats or the shaft surface is scratched—the seal begins to leak. In most cases, the service life of the seal is determined not by the seal itself but by the conditions in which it operates.

Oil Seal Types and Materials

Two types are most commonly used in gearboxes. A single-lip seal only keeps the oil inside and is sufficient for clean environments. A dust-lip (double-lip) seal has a second small lip outside the main lip; this lip keeps dust, mud and splashing water away from the main sealing lip. Dust-lip seals are preferred in dusty environments such as agriculture, mining and cement plants.

The material is selected according to the operating temperature and the oil used. The temperature ranges below are approximate and may vary depending on the manufacturer.

Material	Approximate operating temperature	Preferred applications
NBR (nitrile rubber)	−30 °C to +100 °C	Standard applications with mineral oils; the most common and economical option
FKM (Viton)	−20 °C to +200 °C	High temperature, high speed, synthetic oils and chemical environments
Silicone (VMQ)	−50 °C to +150 °C	Very cold environments; more sensitive to wear
PTFE (Teflon)	−60 °C to +200 °C	Aggressive chemicals and special applications with a risk of dry running

You can find other sealing elements used together with oil seals, such as labyrinth seals, O-rings and pressure equalizers, on our sealing page.

How to Read Oil Seal Dimensions?

Three numbers are written on the seal: inner diameter × outer diameter × width, all in millimeters. For example, a seal marked 35×52×7 is fitted onto a 35 mm shaft, into a 52 mm housing bore, and has a width of 7 mm. If the markings have been erased, the same information can be obtained by measuring the shaft diameter, housing diameter and housing depth with a caliper. Also note the material (NBR, FKM) and whether the seal has a dust lip; different types are available in the same dimensions.

Causes of Oil Leakage in Gearboxes

Wear or Hardening of the Seal Lip

The seal is a consumable component; the lip wears over time. In gearboxes operating at high temperatures, the rubber loses its flexibility, hardens, cracks and can no longer maintain proper contact with the shaft. In gearboxes that remain unused for long periods, the lip may dry out and stick to the shaft, then tear during the first startup.

Wear on the Shaft Surface

The seal lip, especially in dusty environments, can create a fine groove on the shaft over the years. Once this groove forms, a new seal sits in the same track and begins leaking again in a short time. When replacing the seal, check the shaft surface with a fingernail; if a noticeable groove is present, the seal should be installed a few millimeters away from the groove, a shaft sleeve should be used, or the shaft should be ground.

Clogged Breather Plug

As the gearbox operates, it heats up and the air inside expands. The breather plug releases this pressure. If the plug becomes clogged with dust or paint, internal pressure rises and pushes oil out through the weakest point, usually the seal lip. This is the first point to check on a gearbox that leaks even though the seal is in good condition. On new gearboxes, failing to replace the solid transport plug with the breather plug before commissioning can cause the same problem.

Excess Oil and Incorrect Mounting Position

If the oil level is too high, the seal remains continuously submerged in oil, the oil foams and internal pressure increases. A similar problem occurs when the gearbox is installed in a position different from the one specified at the time of ordering: the oil quantity and the positions of the breather and level plugs are determined according to the mounting position. If a gearbox prepared for horizontal operation is installed vertically, the breather plug may remain below the oil level. Details are explained in our article on gearbox mounting positions.

Overheating and Unsuitable Oil

As the temperature rises, the oil becomes thinner and the seal material ages more quickly. Using an oil that is incompatible with the seal material may also cause the rubber to swell or harden. If your gearbox is operating hotter than normal, first determine the cause; our article why does a gearbox overheat provides guidance on this subject. For the correct oil, refer to the gearbox lubrication table.

Shaft Runout and Bearing Clearance

The seal lip can tolerate small misalignments, but a worn bearing or bent shaft may crush one side of the lip while leaving the other side open. Misalignment between the motor and gearbox or between the gearbox and machine can have the same effect. In this case, replacing only the seal will not solve the problem.

Incorrect Seal Installation

Installing the seal backwards, driving it into the housing at an angle, dislodging the spring during installation or cutting the lip on the sharp edge of a keyway are common mistakes. A seal installed dry can also burn its lip within the first few minutes.

Leakage from Covers, Gaskets and Plugs

Not every leak comes from the oil seal. Loose cover bolts, an aged housing gasket, a crushed drain plug washer and, in rare cases, cracks or casting porosity in the housing can also cause oil leakage. Since oil flows downward, the actual source of the leak is often above the point where the stain is visible.

Possible Cause According to the Location of the Leak

For diagnosis, first clean the exterior of the gearbox thoroughly, run it for a short time and observe where the oil first appears.

Location of visible leak	Possible cause	Check to perform
Around the shaft	Worn seal, grooved shaft, internal pressure	Breather plug, oil level, shaft surface
Breather plug	Excess oil, incorrect mounting position, foaming	Oil level and plug position
Cover and housing joint	Loose bolt, aged gasket	Bolt tightness, gasket condition
Drain or level plug	Crushed washer, loose or damaged plug	Replace the washer and tighten the plug
Motor flange side	Input shaft seal; oil may migrate toward the motor	Remove the motor and inspect the input seal
Housing surface	Crack or casting porosity	Check for impact marks; consult the manufacturer

What Should Be Done When an Oil Leak Is Detected?

1. Stop the gearbox and check the oil level. If the level is low, do not operate it with insufficient oil; gears and bearings are far more expensive than an oil seal.
2. Clean the housing and identify the source of the leak according to the table above.
3. Remove and clean the breather plug, and verify that it is located in the correct position for the mounting orientation.
4. If the oil level is too high, reduce it to the correct level; check the cover bolts and plugs.
5. If the leak still comes from around the shaft, replace the seal and also inspect the shaft surface and bearing clearance at the same time.

Leaking oil is a problem not only for the gearbox but also for its surroundings: it makes the floor slippery and, in industries such as food and textiles, may contaminate the product. Even slight sweating should be monitored.

Points to Consider When Replacing an Oil Seal

• Do not scratch the shaft surface or housing bore when removing the old seal.
• Select the new seal with the same dimensions, material and type; an FKM seal may be required instead of NBR in a gearbox operating at high temperature.
• The spring-loaded side should face the oil, meaning toward the inside of the gearbox.
• Before installation, lubricate the lip and shaft with a thin layer of oil or grease.
• Cover sharp edges such as keyways and shaft shoulders with tape or an installation sleeve.
• Install the seal evenly and without tilting, using an installation tool suitable for its diameter.
• If there is a wear groove on the shaft, position the seal so that the lip runs outside the groove.

For replacements that require disassembly, such as the input shaft seal, or if the leak keeps recurring, it is safer to leave the job to a service center. You can find details on the process in our article on gearbox repair and overhaul, and regular inspection steps on our gearbox maintenance page.