Should you make the switch?
By Dave Osmolski
The use of oil-filled hubs on boat trailer wheels has increased during the past few years, as owners convert grease-lubricated hubs or buy new trailer packages with the new lubrication system.
Transportation companies have employed oil-filled hubs on semi-trailers for a long time. They are reliable and work well.
Oil-filled hubs have a port through which you can visually inspect the oil level. You can also see if there has been water incursion by checking the clarity of the oil in the hub. If water seeps through a seal or condensation has formed, the oil becomes cloudy. Even a small amount of water causes cloudiness, indicating that you need to disassemble the hub, inspect for bearing damage, and either replace the oil or both the bearings and the oil if there is bearing damage.
As the oil in trailer hubs heats up, it becomes less viscous. Heat expands the air above the oil, creating a positive pressure. If a leak occurs under these conditions, all of the oil will likely leak out quickly, resulting in catastrophic bearing failure and a call to your favorite towing company. The saving grace is that in semi-trailers the risk of leaks has been small.
When comparing the performance of oil-filled hubs on a semi-trailer to those on a boat trailer, however, you need to consider that a semi-trailer is on the road and running most of the time, while a boat trailer is off the road and sitting most of the time. Oil fills about 50 percent of the hub, leaving the top of the bearing relatively dry. If the trailer is standing for a long period of time, the wheel should be turned periodically to wash the entire bearing in lubricant. This will prevent condensation inside the hub from reacting with the “dry” part of the bearing and causing rust. Keep in mind that oil floats on water. Any condensation that forms will float the oil off the bearing surface.
Another consideration is that a semi-trailer is never intentionally backed into the water deep enough to submerge the hubs. Boat trailers are backed into the water a lot and while the hubs are hot. If the seals are weak, the rapid cooling of the hub in cold water will cause a negative pressure inside the hub, potentially drawing in water and contaminating the oil.
Grease-filled hubs come with their own special problems. The grease in a hub can only be inspected by removing the cover. Some greases make it difficult to tell if there has been water incursion. The milkiness readily apparent in oil is much less evident in grease. Air pockets can potentially leave parts of the bearing unlubricated, opening up the possibility of damage and failure.
Grease-filled hubs have positive points that can’t be ignored, however. The loss of a hub cap or a leaky seal will rarely cause catastrophic bearing destruction. Enough grease usually remains to allow you to reach your destination.
I once made a makeshift hub cap from a soft drink can held on with a tie wrap. I ran with this cap from central Georgia to southwest Florida, where I parked my trailer, removed the hub, replaced the bearing and installed a proper hub cap.
Many hub-capping systems used with grease-filled hubs have a spring mechanism that puts mechanical pressure on the grease filling. When a warm or hot hub is immersed in cold water, the mechanical pressure overcomes any negative pressure caused by rapid cooling and prevents water incursion.
Although oil-filled hubs have the potential for better-than-average bearing lubrication and reliability, you must address many concerns when choosing the lubrication system for your trailer hubs.
D/Lt/C David H. Osmolski, SN, of Charlotte Power Squadron, has been repairing boats since high school when his first boat, a canvas-covered canoe with cedar ribs, leaked in gallons per minute and required constant repair.
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