What Are Axle Seals?

Jeremy Laukkonen

Axle seals have two primary functions in the drive trains of cars and trucks. The first purpose of an axle seal is to keep gear oil from escaping. Without this sealing function, the gear oil would leak out and contaminate various components, such as brake shoes, and the transmission or differential could overheat. The other important function performed by axle seals is to keep dirt and other contaminants from entering the system. If these types of contaminants are allowed inside a transmission or differential, the delicate gears and other components can become severely damaged.

A mechanic can determine if a seal can be repaired or the axle needs to be replaced.
A mechanic can determine if a seal can be repaired or the axle needs to be replaced.

There are many different types of axle seals, but they all perform the same basic function. These seals can be found in front, rear, and four wheel drive vehicles. All axle seals consist of a circular component with a hollow center and rigid outer surface. Seals are typically pressed into place and remain stationary, while an axle is allow to spin freely within the hollow portion. The inner diameter of an axle seal is typically made of a pliable substance that is capable of allowing this movement.

Many different factors can lead to axle seal failure. One common issue is pressure within the transmission or differential, which must be vented properly. If this pressure builds up and has nowhere to go, the point of failure is typically one or more axle seals. After a seal has been damaged in this way, fluid will typically leak out until a repair has been made. It is also possible for a seal to be damaged if care is not taken during the removal or installation of an axle.

Another issue sometimes associated with axle seals is damage to the outer surface of an axle. The pliable inner diameter of an axle seal is much softer than the metal surface of an axle, but seals often harden over time. In some cases the constant rotation of an axle within a hardened seal can carve a very shallow groove in the surface of the metal, which then allows oil to leak out. One way to deal with this type of issue is to obtain a replacement seal with a different inner lip placement so that the point of contact on the axle is shifted slightly in or out. If that is not possible, replacement of the entire axle is often the only viable option.

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