Axle grease is a substance used to lubricate wheel bearings. Manufactured from a thick, petroleum-based grease, axle grease is often waterproof, allowing the substance to survive in the harsh environment of a wheel bearing. Unlike many grease types, axle grease is not applied through the use of a grease gun; instead, it is typically packed into the bearing by hand. In some applications, such as boat trailers, the grease is pumped into the wheel bearing by a grease gun through a hub fitted with a grease fitting.
Some applications, such as a wheel bearing, cannot use oil as a lubricant. The oil would simply leak out of the wheel hub and the bearing would dry out. This would result in a burned bearing, which would require replacing. By placing axle grease in the bearing, the grease warms to a thick liquid when the wheel is rotating, thus lubricating the bearing. The typical method for greasing most wheel bearings is to pack the grease into the bearing by hand.
When packing a wheel bearing, the axle grease is placed into a rag or the bare hand and the bearing is physically pushed into the grease until grease fills the entire diameter of the bearing. Once the bearing is slipped back into the bearing race in the hub, additional grease is packed in and around the bearing. Once the mechanic is satisfied the bearing is thoroughly filled with grease, the bearing cover is reinstalled. The axle grease is formulated with a higher-than-average grease melting point. This provides extra protection for the bearing by allowing the thick grease to remain on the bearings for a longer time without melting and running off.
Some types of axle grease are also formulated to withstand water. Used on boat trailer axles, waterproof axle grease is used to displace the water that commonly enters the trailer's wheel bearings as the wheels are often submerged in the water when launching or loading a boat. Special additives are manufactured into this waterproof grease to avoid contaminating the grease with water. Water introduced into regular wheel bearing grease will turn the lubricant a milky color and can rust and pit the bearings, even when greased. The special waterproof grease is formulated to prevent water from breaking through the molecular structure of the grease.
The typical grease used for bearings is commonly packaged in a small tub. The tub allows the bearing to be placed directly into the product to be packed. The waterproof version of the grease commonly comes in a tube that can be pumped into a trailer hub, thereby forcing any water out and providing lubrication for the bearings.