NTN Develops Trace Lubrication Mechanism for Grease Lubricated Angular Ball Bearings to Attain the World's Highest Rotational Speeds

NTN Corporation has developed a lubrication mechanism for grease lubricated precision angular contact ball bearings. Suitable for use on machine tool main shafts, these bearings achieve the world's highest speed ratings. NTN's new bearings will be part of the "Challenge to the Future" exhibit at the Japan International Machine Tool Trade Fair beginning November 1.

For maintenance-free machine tool main shaft bearing applications, NTN currently manufactures and sells angular contact ball bearings equipped with conventional grease lubrication seals to provide a service life rating of 20,000 hours at a dmn value of 1.4 million. The key engineering challenge to achieving even higher speed ratings is to extend grease service life, most promisingly through a continuous supply of fresh grease to the rolling surfaces of the bearings. However, simply increasing the amount of the grease packed into a bearing generally causes higher mixing losses and elevated temperatures which can negatively impact machine tool precision and limit operating speeds. Furthermore, any external method of supplying grease requires costly auxiliary equipment, and would therefore lose the chief advantages over air-oil lubrication systems.

NTN has responded to this challenge with the development of a trace lubrication mechanism in which a special grease reservoir is added to the bearing outer ring spacer. A grease thickening agent generates a "capillary phenomenon" in which the lubrication oil is continually fed into the bearing. This system raises the high-speed rating of grease lubrication angular contact ball bearings to a dmn value of 2 million - a 40% improvement over the previous best rating.

NTN's new grease lubrication system effectively achieves the high-speed operation suitable for machine tool spindle applications. Until now, machine tool spindles have almost exclusively used air-oil lubrication, requiring oil feeding devices, an external air supply and other outside equipment. NTN's breakthrough should pave the way for significant reductions in total cost.

(1) Features

- Trace lubrication mechanism
- Total maintenance-free operation
- Improvements in rotating speed rating: dmn = 2 million (40% increase over standard components)
- Lower cost compared to air-oil lubrication

(2) Applications

- Machine tool main shafts (lathes, mills, CNC machining centers, etc.)

1) dmn = D (bearing pitch circular diameter, mm) X N (rotational speed min-1).
When a bearing with D of 100mm is rotated at N = 20,000 min-1, the dmn value will be 100 X 20,000 = 2 million.
Bearing cross section structural view
Bearing cross section structural view

Bearing photograph (cutaway sample)
Bearing photograph (cutaway sample)
Bearing photograph (cutaway sample)