NSK develops Ultra-long-life Ball Bearings for Transmissions
, designed to make bearings more compact and lightweight, with less friction.
The demand for compact, lightweight and more efficient automotive transmissions has risen in recent years in response to the tightening of fuel economy regulations.
Meanwhile, the transmission bearings are always more often subject to harsh lubrication conditions, for example when low-viscosity oil is used, and are expected to withstand surface-originated flaking (flaking that starts at the surface).
As part of the continuous research in material engineering, NSK carried out in-depth analysis of the mechanism leading to surface-originated flaking and discovered that bearing life can be further improved by strengthening the rolling element.
The presence of abrasion particles of hardened steel in the transmission oil is quite usual considering the numerous metal-to-metal contacts between the gearbox pinions. These particles can be source of indentations on the raceway surface when squeezed between the ball and the raceway. In the conventional theory, surface flaking is then due to the stress concentration on the indentation edges firstly leading to cracks and then to flaking. The newly developed theory additionally takes into account the tangential force acting on the edge of the indentations caused by the deteriorated surface roughness of the ball. Inhibiting the deterioration of the rolling element’s surface quality therefore contributes to extend the bearing service life.
The new bearing developed by NSK features a strengthened rolling element (ball) obtained by applying a deposit of hard, minute particles of Si-nitride on the ball and combined with a special carbonitriding heat-treatment technology.
As a result, the life of the developed bearing was extended by at least 2 to 3 times when associated to conventional technology for the bearing inner and outer rings. In combination with NSK’s proprietary long-life ring heat-treatment technology (UR, HTF), the bearing life expectancy can achieve more than 10 times that of a conventional product.
Such a life improvement makes a bearing downsizing possible, leading to a weight reduction of up to 50% and a resulting 12% torque reduction.
The basic technology used in the materials in this product was developed by NSK’s research institute. Going forward, NSK will continue to apply the four core technologies the company has cultivated for over 90 years (tribology, material engineering, analysis technology, mechatronics) to develop products that help create more compact and lightweight automobiles, with less friction, thereby improving automotive fuel efficiency.