Just as “conventional thinking” has changed regarding center distances, ratio combinations, and face widths over the years, the application of rolling element bearings to industrial gearboxes has also evolved. You cannot just put high-performance gears into your refurbished housing on the same old bearings and get optimum results.
As bearing manufacturers improved their product, allowable ratings have increased for the “same” part number. The components that make up that part number are very different than what was found in the thirty-year-old bearing you took out during tear down. The raw material is better, the heat treating is better, the tolerances are tighter, the surface finish is better, and the cage design is different. It fits in the same space so it gets the same basic part number.
After the basic part number is an “alphabet soup” of characters that vary slightly, based upon the manufacturer. You will need to read their catalog to understand what they tell you about the bearing. Some of those letters are critical to getting the life you need. One horror story from a few years ago involved the “E” on a spherical thrust bearing. With “E” design the bearing lasted for several years. Without the “E” features it was only a matter of a few months before all hell broke loose inside that extruder drive.
While you have the catalog out, you need to read and understand the application guide. Some bearing types are no longer “authorized” for the old arrangements. Some applications now require special, internal clearances or interface dimensions.
Just as opinions have changed on acceptable face width to pitch diameter ratios, modern gearbox designs follow different rules on allowable bearing spans. We have a much better appreciation for how shaft deflection effects bearing life. The same goes for whether thrust loads are allowed on some bearings.
A bearing arrangement that provided many years of service may not “calculate out” using today’s methods. Your responsibility as the designer of a “resto mod” is to apply modern methods within the confines of vintage architecture.