It was late on a Wednesday evening when the message appeared in Chris Napoleon’s email. The question was straightforward: “Can you support the manufacturing of a bearing cage made from Teflon-coated glass fabric?”
The simple answer was yes, absolutely. Napoleon Engineering Services had perfected the process years ago to support the high demand for custom turbopump bearings used by the commercial space industry.
The email indicated that the bearing selection process was complete; the bearing type and size were locked in. Napoleon’s first thought was, given the cage material choice, it is likely for a high-speed application operating in a cryogenic liquid environment. This material has proven to perform in these applications, but it’s very difficult to machine and has been used exclusively in angular contact ball bearings.
But then NES’ president and chief engineer read the words “deep-groove ball bearing” and said, “Oh my, that might be a problem. (This individual) probably didn’t think about the manufacturing challenges related to this material when selecting a deep- groove ball bearing for this application.” Quite often, cage design and manufacturing constraints do not drive the bearing selection process. Typically, direction and magnitude of the applied loads, operating speed and understanding if the bearing is locating or non-locating all have a significant impact on selecting the bearing type and size. Once the bearing type is known, the cage material and design can be determined, which is often influenced by speed and lubrication method. These are all important application-dependent considerations that ensure success. But when considering short-run production of custom bearings optimized for specific application conditions, the cage can be the deciding factor on bearing type selection.
STAMPED AND molded cages of unique pitch diameters, rolling element complements and configurations are often unrealistic considerations due to high tooling costs, regardless of suitability for the application.
This is easy for the typical designer to understand.
But what about understanding the risks associated with manufacturability, strength and running accuracy of the cage? Certain bearing types have more inherent risk associated with manufacturing of the cage. A deep-groove ball bearing (DGBB) is a great bearing design. Individually, it can support radial loads and axial loads in both directions. It can even be used in pairs functioning as the locating bearing with preset preload or endplay in the set. As such, it’s often referred to as the “poor man’s” angular contact ball bearing. But because of the assembly method for DGBBs, the cage is either a two-piece riveted or one-piece side entry snap-on design. There are constraints on each design depending on material choice, application speed and load conditions. High-speed applications above 500,000 dN require a switch from riveted stamped steel construction to riveted machined land riding designs. A variety of materials can be used depending on the lubrication or operating environment.
Unfortunately, riveted cages possess some degree of error in balance and running accuracy due to manufacturing and assembly variation. There is some amount of cage halve mismatch in both the radial and circumferential directions created by true position error in pocket and rivet hole location. Furthermore, rivet head forming is an art that requires special tooling and attention to detail to ensure heading that does not create rivet buckling, bulging, cage halve separation or asymmetrical head formation. When lightweight plastic or phenolic material is used, special reinforcement plates need to be bonded to the cage for strength and enabling of the rivet heading process. At some point, you’re working so hard to win the battle against the manufacturing constraints that changing the bearing type to fit an easier cage design might become a necessity.
SPLIT-RING angular contact ball bearings, angular contact ball bearings and fractured outer ring ball bearings each have different assembly practices that enable the use of a one-piece fully pocketed cage that eases the burden on the manufacturing process, regardless of material, and provides better running accuracy at higher speeds. Napoleon often sees equipment designs progress too far without consideration of the cage options, as in the case of the emailer, and stresses overall cage risk reduction from a manufacturing standpoint early in the selection process.
Cage material, style and type can play a critical role in bearing selection and, in some cases, be the driving force to ensure the highest manufacturing quality, running accuracy, strength and lower product cost.
A cage-first mindset is important when considering optimized bearing design and manufacturing.
(Courtesy of J.P. Butler, Marketing Coordinator, Napoleon Engineering Services)
This article is published in BearingNews March 2025 issue
