FEATURE MATERIALS IN DESIGN & PROTOTYPING The right finish for improved performance

Black oxide finish is a cost-effective way to prolong the service life and/or performance of rolling element bearings, especially in critical

applications. David Schaljo, head of application engineering at NKE Austria, looks into the benefits


lack oxide finish has a long tradition in corrosion protection and decorative

purposes. For improving the properties of surfaces, the process itself as well as the requirements and testing methods have been standardised in DIN 50938 and ISO 11408, and further refined by the bearing industry for its specific application to rolling element bearings. On steel parts, the finish forms a

Right: Full complement

cylindrical roller bearing NCF1880-SQ94D from NKE

protective layer. In a multistage chemical process, the surface layer of the treated parts is converted into a 1-2 micrometer thin mixed ferrous oxide layer containing FeO, Fe2

O3 and Fe3 O4 . Because the

black oxide finish contained in the conversion layer does not represent a coating as such, this treatment yields only minimal dimensional changes of the component. The characteristic black appearance results from the mixed ferrous oxides contained in the so-called conversion layer. Black oxide finish is often used purely as a corrosion protection measure. However, due to the thin and porous layer, corrosion protection is mainly achieved by the application of suitable (corrosion protection) lubricants, which are retained to some degree in the structure of the porous conversion layer. As a result, the layer provides only limited corrosion protection that depends on proper maintenance; and, from today’s perspective, more suitable methods are available to achieve effective and long lasting corrosion protection.

THE BENEFITS FOR ROLLING BEARINGS Black oxide finished bearing components feature a set of special technical characteristics, especially in the case of components that move relative to each other. Multiple protective effects can be achieved if only one functional element (typically the rolling elements) is treated. For best effect, however, all functional surfaces of a rolling element bearing, including the inner and outer ring as well as the rolling elements, should be black oxide finished. Benefit 1: Improved run-in and wear characteristics. The conversion layer of the black oxide finish exhibits a marked resistance against wear and bending, resulting in favourable run-in characteristics and run-in periods for rolling element bearings, and a subsequent trouble-free and long- lasting operation. In addition, the porous nature of the conversion layer aids in the adhesion and retention of lubricant on the functional surfaces. As an example, NKE supplies black

oxide finished cylindrical roller bearings (full complement or with cage) to various OEMs for large sized and wind turbine main gearboxes. Besides verifiably lessening the occurrence of WEC (white etching cracks), black oxide finish has been proven to provide additional protection especially during the run-in and testing phase of these gearboxes. Benefit 2: Improved adhesive

Surface damage to a rolling element without black oxide finish


wear characteristics. Rolling bearings operating in low-load situations, and those that are subjected to rapid speed changes during operation, can be susceptible to adhesive wear (slippage damage and smearing). Under these conditions, slippage occurs at the contact area between the rolling elements and most commonly the inner ring raceway, especially when the difference between the rolling speeds between raceway and rolling elements is very high. Slippage can cause lasting damage to the rolling element and raceway’s surfaces and material structure, and premature bearing failure can be the result. Such damage often occurs where lubrication is insufficient or an unsuitable lubricant is used. A special type of slippage damage occurs on full complement cylindrical roller bearings where the counteracting surface speeds of the contacting rolling

elements prevent the formation of a stable lubricant film to separate the contact surfaces. Here, direct contact between adjacent rolling elements cannot always be avoided so that, as friction increases, smearing and damage to the rolling elements’ lateral surfaces occur. In both cases, the black oxide finish can act as a protective layer, which delays the onset of wear and smearing. If steel on steel contact occurs due to a short breakdown of the separating lubrication film, the risk of cold shut and other types of severe surface damage is reduced considerably. Depending on the severity of the operating conditions in an application, the conversion layer will eventually wear off, not least because of its limited thickness. Cylindrical roller bearings with a bore

diameter smaller than 560mm are designed to operate at an operating load of approximately 1% of their dynamic load carrying capacity and with a rotation speeds of up to 1200rpm. Despite design measures, the formation of the characteristic arrow or diamond shaped smearing marks in the raceway and corresponding damage to the rolling elements cannot be entirely avoided. NKE has resolved this issue effectively by using rolling elements with a black oxide finish and

introducing a lubricant chosen specifically for

this application. Benefit 3: Protection against

environmental effects. Black oxide finish also provides protection

against environmental effects, including to help prevent fretting corrosion, and forms a barrier on the bearing steel against harmful hydrogen diffusion and chemical reactions with aggressive lubricant ingredients.

FINISHED BEARINGS NKE offers a large number of black oxide finished rolling bearings, in particular for cylindrical roller bearings, which then carry the suffix SQ94. The most common variants are SQ94B (rolling elements black oxide finished) and SQ94-D (all bearing components except for the cage black oxide finished).



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