C. HUBS

The hub provides the mounting surface for the wheel. The hub spins on the bearings which slip over the spindle. The bearings, although sepa- rate parts, are usually associated with the hub. When brakes are used, the brake drum is attached to or is integrally a part of the hub and spins with it. Figure 3.11a shows the elements common to hubs as they slide onto a spindle. Coordinating the capacities and dimensions of all these elements is an arduous task, but must be done to ensure a reliable system. The load capacity of the assembly is dependent on many factors: bearing size, bearing position, cross sectional area of the hub and of the spindle, and design of the hub flange. Fortunately, the capacities of hubs, brakes and bearings are usually determined by the manufacturer and all you need to do is specify your desired capacity and abide by it.

The automotive and trailer industries use tapered roller bearings in hubs almost exclusively. Prior to tapered rollers, ball bearings were quite common. For a given size, tapered roller bearings have much greater load capacity and have become the preferred choice for wheel bearings. A process of elimination has resulted in a relatively standardized set of hubs and brakes as shown in Figure 3.11. Common ratings for hubs are 1800- lbs, 3500-lbs and 5200-lbs/pair as shown here.

A bearing is never designed to last forever, even under ideal conditions. Bearing Life, or Rating Life, is subject to many criteria but is basically the specific time that 90% of a group of identical bearings will last before a fatigue spall or pit damages the surfaces of either the cup or the cone. Life, load and speed are interrelated; for instance, doubling the load, reduces life expectancy to approximately one-tenth.

Doubling the speed cuts the life in half. In addition, the amount of lubrication and temperature are a part of the Life Adjustment Factor. Here less lubrication and higher temperatures tend to reduce a bearing’s life.12

Thus bearing capacity and life are a function of applied load as well as the bearing’s environment. In normal use, depending on tightness of the nut, trailer bearings heat up just from rotation. This heat can soften the grease which, depending on temperature and type, can then leak from a worn or weak seal gradually reducing the amount of grease in the cavity. After a trip hot hubs cool, at which time condensation can occur.15 Trailers left sitting exposed to the elements are also vulnerable to moisture, as are boat trailers backed into the water for launching their cargoes. Any moisture in the hub creates rust-small particles which can then disrupt the required roller action of the bearing. Rust filled grease creates more friction and the potential for more heat. Hot bearings with little or no grease get even hotter. . . and so the cycle continues.15 Proper re-greasing at appropriate intervals is one of the best solutions for longer bearing life and retained load carrying ability-two thousand mile intervals is a common suggestion in trailer owner’s manuals. After this discussion, you can see why it’s not such a bad idea.

Bearing cavities should be kept full of grease. To assist with this and keep the elements out, bearing protectors, which replace the dust cap, are available. Dutton Lainson’s version provides a zerk fitting for quick and easy re-filling of the hubs, a 4-psi pressure relief valve and a higher quality rear grease seal. Bearing Buddy uses a 3-psi pressure inside the hub as well as the refiller zerk fitting and claims the standard seal is adequate; however, a very high quality seal is available as an accessory.

The Sure- Lube system provides for complete refill of the cavity without removing the bearings, since the pressure relief valve is at the end of the special drilled spindles. Properly filled and maintained bearing protectors reduce or eliminate the need to re-pack bearings. Be sure to check the instructions included with each product. Figure 3.12 shows cross sections of two kinds of bearing protectors in place.

It is important to note that specs for some hubs state a maximum wheel size. For instance, some BT hubs (see Figure 3.11) are rated for a maximum 12-in wheel. Others will take a maximum 13-in wheel. To place a 13-in wheel on a 12-in rated hub is mere folly and a request for future problems. For example, a BT hub rated at 1800-lbs but with a maximum 12-in wheel requirement effectively reduces capacity to that of a 12-in wheel, where ratings commonly range from 800 to 1100-lbs-not the 1800-lbs of the hub. The flange design is often the deciding factor as to this type of capacity. Notice the gusseting on the back side of the BT and the 13 HRM hub flanges in Figure 3.11. This style hub results in a much stronger unit for the amount of material used.