In addition to manufacturing ball bearing and plain spherical rod ends, NHBB Astro markets a series of roller bearing rod ends (RBREs1) manufactured by MinebeaMitsumi’s NMB Rod End Division, Karuizawa, Japan.
The NMB brand of roller bearing rod end products, which meet SAE Aerospace Standard AS8952, are specified in flight control linkages where rolling elements are necessary but load requirements exceed the capacity of equivalent sized ball bearing rod ends.
RBREs enable rolling and sliding contact between bearing and shaft, plus they handle a specific degree and range of misalignment.
Typical components include a shank connecting the bearing to the airframe assembly, a head that functions as a housing and integral outer raceway, an inner race with a bore, one or two rows of cylindrical rollers, and shields and seals for protecting the rolling elements from contaminants. Optional lube fittings are available for MinebeaMitsumi’s roller rod ends as well.
RBREs possess a complex internal geometry. The compound radii of the inner and outer raceways support cylindrical, hourglass-shaped rollers. This design maximizes the contact areas, thereby increasing the bearing’s load capacity.
To increase the bearing’s fatigue resistance, MinebeaMitsumi carburizes (case-hardens) and polishes the raceways. MinebeaMitsumi makes both single and double row RBREs; the double row rod ends include a retainer that separates the rollers to reduce rolling friction.
MinebeaMitsumi has designed an optional retainer for double-row RBREs that helps prevent fretting corrosion. Fretting corrosion occurs when motion within the bearing is so minute that lubricant migrates away from the bearing’s load zone. This phenomenon can lead to a roughening of the contacting surfaces due to highly localized frictional heating.
Fretting damage usually occurs in flight control applications on high-speed aircraft, where the flight control surfaces controlled by the roller rod ends are subject to small amplitude, high-speed dither motions.
MinebeaMitsumi’s precessing retainer counteracts the effects of dithering by forcing the rollers to index and rotate around the rod end body. This action, which is caused by the specialized geometry machined into the retainer, maintains enough lube thickness to prevent fretting.
bodies, which are typically made from steels that lack stainless properties, are susceptible to corrosion caused by environmental conditions (rain, salt spray, etc.). Since the races are case-hardened, the body material needs specific elements within its alloy matrix to facilitate the carburization process.
Unfortunately, most stainless steels cannot be carburized due to their unique heat-treatment process.2 To prevent corrosion, MinebeaMitsumi uses galvanic protective coatings, such as cadmium plating3, on the outer surfaces.
MinebeaMitsumi manufactures roller rod ends that can be re-lubricated in the field. These bearings come with an optional flexible seal and lubricator fitting. Since grease protects the components from corrosion and wear, this feature can extend the operating life of this product. Provisions for re-lubrication must be evaluated carefully, as lube fittings are a potential source of weakness in high-stress regions of the rod end housing.
While special features like precessing retainers and lube fittings offer unique performance enhancements, they often come with significant tradeoffs. That is why it is essential to consult the factory before making your final selection. To ensure that the bearing you select conforms to the requirements of your particular application, contact NHBB Astro Division, your “One Stop Shop” for rod end bearings.
Scott McNeil, Product Engineering Manager, NHBB Astro (firstname.lastname@example.org).
1. Since the term “roller bearing rod end" is used repeatedly throughout this article, for brevity's sake, I have chosen to shorten it to the acronym "RBRE.”
2. Research is being conducted on the carburization of stainless steels, and a new family of Nitrogen steels for rolling elements and inner races referred to CREN steels (Corrosion Resistant Nitrogen Steels), may eventually be available for these products.
3. Zinc nickel plating is also in the early phases of testing as a replacement for cadmium. Zinc nickel is typically dichromate sealed and presents a hexavalent chromium hazard concern. Zinc nickel is not a totally "green" alternative to cadmium plate.