A journal box is a housing unit in a rail truck’s wheel and axle assembly designed to hold the supply of oil for the axle and the axle’s journal bearings. These journal bearings are actually a part of the axle, and are a type of bearing that doesn’t actually move independently from the wheel. The journal bearing simply allows the movement of the wheel to be independent of the axle, which creates a force of friction between the wheel and the axle it is positioned on.
This friction between the wheel and axle must be minimized to keep the wheel from overheating, which can make it brittle and cause it to break into pieces. The journal box is the method of lubrication for the interior of the journal bearing. It acts as the housing for the necessary oil lubricant.
Most of the journal boxes located on the axles of rail cars are machined out of the same material that the axle itself is. This means journal boxes are generally made of thick steel that has been tempered or heat treated to make it strong. The box has to be able to hold lubricant without leaking, meaning it must also be made out of a material that withstands exposure to outside elements. The boxes are mounted on the outside of the wheel. This is another reason it has to be made from a durable material, since it has virtually no protection from the abuses of the rugged travel many freight trains and rail cars have to endure.
Journal boxes are located on the outside of the wheel, directly over the axle’s end, and mounted between the frame rails for the entire wheel apparatus. The location of the box is an imperative factor in its main function, as it provides the lubricant directly to the axle end. This in turn allows for the lubricant to reduce the amount of friction created between the wheel and the axle.
The journal box is also placed in a manner that prevents its interference with any of the moving parts in the rail car’s wheel and brake system. This is another important placement factor. If the journal box were to leak the lubricant inside it, it could end up lubricating the brakes or the wheel itself if it were located anywhere else.
The interior of the journal box is easily accessed from the outside of the wheel. Usually, it is accessible through a hinged door that is pressure locked to avoid leaking. It’s from this point that the bearing and the journal box itself may be serviced or the journal box may be refilled with the bearing lubricant.