High-heat-resistant ball bearing unit for automobile turbochargers

Turbocharger with potential for rapid demand expansion

Turbochargers, in their heyday in the 1980s, were installed in many racing cars and sports cars to enhance the speed and maximum output of the automobiles. In those days, there was a trade-off relationship between the turbocharger and fuel consumption. Recently, a turbocharger has attracted attention once again as a device for boosting the insufficient output of engines that are downsized to meet environmental restrictions. Thus, its arena of applications has been expanded to general automobiles and eco-cars as well.

A turbocharger harnesses exhaust gas pressure emitted from the engine cylinder to drive a turbine and a compressor mounted on the same shaft. Air taken in by an automobile is compressed by the compressor and sent to the cylinder, which increases air intake and improves combustion efficiency. The turbine and compressor both play important roles to efficiently send compressed air to the engine. In 2008, MinebeaMitsumi started developing a new type of bearings for a turbine main shaft that directly influences the rotational efficiency.

The bearing for turbine main shafts is used for the turbine driven by ultrahigh-temperature exhaust gases exceeding 800°C as the power source, and it must withstand ultrahigh speed of over 200,000 revolutions while continuously immersed in oil. For this reason, many turbochargers for general automobiles used oil-lubricating bearings that are highly resistant to harsh environments. However, since the friction resistance of oil-lubricating bearings increases at the time of acceleration and deceleration due to its structure, the speed following capability is inferior to that of ball bearings (particularly when an engine starts at the low temperature). This generates a time lag between the increase in intake air pressure and the step-in operation of the accelerator pedal. Consequently, the driver feels the acceleration to be slow, and depresses the accelerator pedal more than necessary, thereby resulting in poor fuel economy. In actual city driving, since acceleration and deceleration are frequently repeated, a series of those steps tend to affect practical fuel economy.

To address these issues, we had to improve the following four main points for the new type of ball bearing unit: Downsizing along with an engine unit downsizing, high durability, high-heat resistance and low torque. In addition, we had to mass produce the bearings so that they would be widely adopted for use in general automobiles and eco-cars. MinebeaMitsumi, which has been continuously improving bearing production technologies since its establishment, has begun to tackle this challenge.