To refurbish the driving boxes, it is necessary to babbitt and machine the hub faces, then machine the crown brasses. The babbitted face, and the crown brass are both sacraficial surfaces that are meant to be consumed and replaced. For now, there is sufficient brass available–we won’t have to replace it this time.
The driving boxes of the Sierra #3 fit over the axles of the driving wheels and are the main bearings for the driving wheels. They bear the weight of the locomotive through the crown brasses and transmit the thrust of the cylinder and rod assemblies to the frame. As the locomotive moves forward, the axles turn inside of the box. This is called radial force.
At the same time, the wheels push against the sides of the boxes as they go around curves. This results in side thrust forces against the face of the boxes which turns the locomotive. Because there are two different bearing forces, there are two surfaces that need to be refurbished, the crown brass and the hub face of the driving box.
These types of bearings are called plain bearings. Over the years there have been many advances made in bearing technology, including ball bearings, needle bearings and many variants. Even so, plain bearings are found in many modern applications. The Millenium London Eye, the world’s largest Ferris Wheel, uses plain bearings. Your car has many types of plain bearings. To put it simply, bearings allow two surfaces to slide against each other more easily. They reduce friction and increase lifespan.
Lubrication is essential to reduce friction in plain bearings. A component of these driving boxes is a cellar which serves as an oil reservoir. As the axle turns, it rolls through the oil, and spreads it over the bearing surfaces. This is an open system, as the oil is used up, it is replaced by the engine crew.