When lubricating truck hub bearings think like a Dippy Bird
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When lubricating truck hub bearings think like a Dippy Bird

Jake Schell is the associate product manager for Mitchell 1’s Commercial Vehicle Group.


Not long ago, someone asked how I come up with ideas for these articles. How exciting it would be if I could claim the topics rolled from my mind because of vast personal technical knowledge and experience. Granted, to a degree the article ideas are the result of some 40 years in the industry. Nevertheless, the story ideas generally have their origin in the things I see during an otherwise typical day.

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For example, the Dippy Bird.

The Dippy Bird is this toy bird filled with a dyed liquid and bulbs on either end, and when the fluid shifts back-and-forth, it looks as though the bird is drinking. Variations of this toy have been around for decades, so the odds are high most have at least seen one.  Seeing this unique toy slosh the dyed fluid back-and-forth brought to mind a drive axle, and how the axle lubricating oil flows from the center out to the hubs and wheel bearings.

The rear axle wheel hubs typically are lubricated by the gear oil in the axle assembly. Without that gear oil reaching the hub bearings, overheating and damage can take place quickly. As long as the axle is properly filled, the bearings will be getting the needed lubrication. There is a time particularly available for hub bearing damage to occur, namely when the hubs are being serviced.


When the hubs are off for service, there may be a tendency to pack the rear hub bearings with grease as though they were front axle wheel bearings that do not receive lubrication from the differential. The heavy wheel bearing grease will lubricate the bearings for a while.

Unfortunately, wheel bearing grease, particularly when cold, will block the flow of gear oil needed to fill the hubs and lubricate the bearings. This will also make for a lowered gear oil level once everything warms up and the wheel bearing grease dissipates–provided the bearing is not damaged before everything gets warmed up.


Most service instructions indicate that the bearings should be dipped in the gear oil used by the differential during assemble. Nevertheless, always check the service information specific to the component being serviced.

Once the hubs are assembled it will be time to refill the differential. Some hubs will come equipped with a fill plug to add lubricant at the hub. Be sure to follow the recommended procedure for filling hubs equipped with fill plugs. For hubs that do not have plugs, gear oil in sufficient quantity still needs to make it to the hub.

In comes the inspiration from the Dippy Bird. Once the differential is filled to the proper level, one side of the axle should be jacked-up first, so that gear oil will flow into the hub over a minute or more. Then, the procedure should be repeated for the opposite side. Lastly, recheck the gear oil level and add as necessary. This should provide a good long drink of nourishing gear oil for the nearly dry hub bearings.


Visual cues such as the Dippy Bird and many other mundane things in the world can help remind the  technician to make small but important steps during vehicle service. Thankfully, such visual cues also help me come up with article ideas.      

Additional tips for repair and maintenance of Class 4-8 trucks may be found in the Mitchell 1 ShopConnection Truck blog: https://mitchell1.com/shopconnection/category/truck/

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