NTN Develops Oil-Lubricated Angular Contact Ball Bearing that Attains
5 Million dmn value through New Oil Jet Lubrication Mechanism

The NTN Corporation has developed a new oil jet lubrication mechanism for oil-lubricated precision angular contact ball bearings used in machine tool main shafts. These bearings have attained the world's highest speed operation and will be exhibited for reference positioned as a "Challenge to the Future" at the Japan International Machine Tool Fair to be held from November 1.

In response to demands for higher speed machine tools, the NTN Corporation is manufacturing and selling air-oil lubricated angular contact ball bearings. For machine tool bearings, not only higher speeds, but also minimal power losses are required. With air-oil lubrication, dmn values of 2.6 million have already been attained. With oil jet lubrication and under-race lubrication, larger amounts of lubricant can be provided by forcing oil into the inside of the bearing, achieving still higher speeds. However, parasitic mixing losses increase, requiring higher drive motor output. These conditions result in thermal expansion of the rotating shaft due to heat generated by the bearing, sometimes requiring a cooling system for the lubricant.

In machine tools, heat generated by the motor and bearings is typically dissipated from the outside with bearing jacket cooling. NTN has developed a new oil jet lubrication mechanism that uses this machine tool cooling oil for bearing lubrication, injecting the cooling oil jet on the end face of the inner ring to supply part of the lubricating oil to inside the bearing, and using much of the rest for inner ring cooling. The amount of lubricating oil supplied to inside the bearing is optimized to attain the same level of power losses as for air-oil lubrication even though running at higher speeds. Also, because this system has an inner ring cooling function, it minimizes the temperature difference between the inner ring and the outer ring, thus reducing the variation in preload. In addition to these functional improvements, this system eliminates the need for the individual lubricating oil supply systems required by conventional lubrication methods and can therefore contribute to machine tool cost reduction.

1 Features

- New oil jet lubrication mechanism(lubrication using cooling oil + inner race cooling mechanism)
- Improved rotation speed limit:dmn value of 5 million (90% improvement compared to air-oil lubrication and 30% improvement compared to under-race lubrication)
- Power losses: Same as for air-oil lubrication
- Pre-load variation: Ultra-minimal

2 Applications

- Machine tool main shaft (CNC machining center for metal mold fabrication and aluminum processing)

Note) dmn value = D (bearing pitch circular diameter in mm) x N (rotation speed in min-1)
When a bearing with a D of 100 mm is rotated at N=20,000 min-1, the dmn value will be 100 x 20,000 = 2 million.

Bearing lubrication mechanism
Bearing lubrication mechanism
Bearing photograph (cutaway sample)
Bearing photograph (cutaway sample)
Bearing photograph (cutaway sample)