2010
Development of “Air Oil Lubrication Ultra-high Speed NU-type Cylindrical Roller Bearing” for Next Generation Machine Tool Main SpindlesNovember 11, 2010
[achieving the highest dmn value*1 of 3.25 million1) and significantly improving bearing rigidity and bearing installation using ceramic inner rings]
(*1: investigated by NTN)
NTN Corporation (hereafter NTN) has developed an “Air oil lubrication ultra-high speed NU-type cylindrical roller bearing” for machine tool main spindles (i.e. machining centers).
The main spindles of high-speed machining centers and combined machine tools typically use duplex angular ball bearings in the front and either duplex angular ball bearings or cylindrical roller bearings in the rear. NTN wanted to increase the speed rating and simplify the main spindle structure at the same time.
In the rear position, the high-speed performance of angular contact ball bearings is better than cylindrical roller bearings. However, the main spindle structure is complex because the ball bushing or preload spring has to absorb thermal expansion of the shaft. On the other hand, when a cylindrical roller bearing is used, a structure absorbing thermal shaft expansion is unnecessary as the bearing itself can move axially but the high speed performance is inferior to the angular ball bearing.
For this reason, the high speed performance of the cylindrical roller bearing has to be improved to ensure both a simple structure and allow for high speed of the main spindle. Moreover, for the main milling spindles of combined machine tools, it is assumed that cylindrical roller bearing will be used also for the front position to enhance rigidity.
In order to improve the high speed performance of the cylindrical roller bearing, the heat generation of the bearing was targeted. NTN focused on two aspects in which allowed the development of an NU-type cylindrical roller bearing able to allow a high-speeds up to “3.25 million dmn”. This is equivalent to an air oil lubricated constant pressure preload ultra high-speed angular ball bearing.
- Constraint of excessive preload: To constrain the heat generation caused by excessive preload from thermal expansion and centrifugal expansion of the inner ring during high speed operation, the inner ring is made from a ceramic material with a low linear coefficient of expansion and low density.
- Constraint of lubrication oil accumulation: To constrain the accumulation of lubrication oil, the outer diameter of the static air oil nozzle works as a guide surface for the cage. With this new design, fresh and low-temperature lubrication oil is constantly supplied to the guide surface and heat generation is reduced.
In addition, the bearing structure is designed so that the outer ring rib is separated from the outer ring and lubrication oil can flow in the clearance between the outer ring and the rib without allowing the accumulation, also reducing the heat generation inside the bearing.
Features
| (1) Ultra high speed performance |
3.25 million dmn under air oil lubrication (40% increase over conventional bearing with air oil lubrication) |
|---|---|
| (2) Improved rigidity |
Improved by approx. 30% from using ceramic rollers and inner ring. Improved by approx. 7% from using steel rollers and ceramic inner ring. |
| (3) Faster bearing installation |
Bearing installation improved significantly.(Interference between ceramic inner ring and steel shaft is reduced by 80% compared to conventional product.) |
Application
Main shaft support bearing of machine tools such as machining center and combined processing machine
Inquiries about this release
Reference
- dmn = dm (Bearing pitch diameter in mm) × n (Rotational speed in min-1)
Example: When a bearing of dm = 100 mm rotates at a speed of n = 50,000 min-1, dmn = 100 × 50,000 = 5 million
Product Photos
Typical Application: Machine Tool Main Spindle
