Wheel/Brake/Bearing Question

OK.... What is preload?.... Just what it says, a forced load imposed on the bearing. The actual load in lbs. plus the preload in lbs equals the total load on the bearing. As long as that total does not exceed bearing capacity, it's fine. However, calculating how many lbs are being applied with that extra turn of the wrench us near impossible. Preload instructions on automobile is probably to eliminate any wheel looseness and is taken into consideration when selection was made by the engineers. It's common to apply a very measured preload in machine applications such as precision grinding and cutting, to eliminate deflection when the cutting tool contacts the material. These preloads are very precise and done so with shims and/or from the bearing mfg when the bearing is ordered. Also, these bearings are running at much higher speeds and require special lube, much thinner and frequently an oil mist due to that high speed and preload. Trailers do not need a preload and that extra gap allows the lubricant to fill that gap and do its job. The bearing rollers actually do not make contact with the bearing raceways. They ride on a cushion of oil from the grease. Exceeding the bearing rated load (preloading could cause overloading) would cause metal to metal contact and damage the bearing. The same thing can happen if the wrong grease is applied. Too heavy and it won't flow properly into the microscopic gap between roller and raceway. Another item you may be interested in, grease does not lubricate - it's the oil in the grease. Grease is made with a carrier, imagine a sponge, that holds the oil in place. The oil finds its way and does the work.