September 4, 2011
NSK Ltd.
Public Relations Department

NSK Develops HST Long-life, High-reliability Cage-equipped Thrust Ball Bearing for Agricultural Machinery

Contributing to the Long Life and High Reliability of Agricultural Machinery by Increasing the Life of Bearings

Tokyo, Japan, August 30, 2011 – NSK Ltd. (NSK; Headquarters: Tokyo, Japan; President and CEO: Norio Otsuka) today announced that it has developed a long-life, high-reliability cage-equipped thrust bearing for application in hydraulic static transmissions (HST*) in agricultural machinery. The new bearing has twice the life of conventional thrust bearings, which will help to lengthen the useful life of agricultural machinery. NSK is targeting 1 billion yen in HST-purposed thrust bearing sales in 2016.

* HST: A continuously variable transmission which combines a hydraulic pump and hydraulic motor. Gear shifting is achieved by changing the angle of the cam plate that contacts the hydraulic pump piston. Thrust bearings are used on the slide surface of the piston

Development background

As the world's population grows, the demand for food and bio fuel is also increasing, which in turn raises the demand for agricultural machinery. In the area of agricultural machinery such as tractors and combine harvesters, power output advancements have been made to increase operability by implementing continuously variable transmissions which provide for smoother operation. HST is the most common transmission used for agricultural machinery. However, as the power output levels have increased, the load on the thrust ball bearings has also increased, which in turn has created an increased demand for long-life, high-reliability bearings.

NSK's latest offering, a thrust bearing designed for HST applications, is designed with the following outstanding features to meet the market need for long life and high reliability in order to bear the increase in power output levels.

New product features

Application of the following long-life technologies has made it possible to more than double the conventional life of the bearing.

Material Technology

Spalling occurs when a large load is applied to conventional parts. The new NSK bearing uses EP steel, which is strong against sub-surface fatigue, for the inner and outer races. It also adopts a special heat treatment, which is resistant to surface fatigue, for the inner and outer races and balls.

Application of a high-reliability cage

The conventional cage incurred large levels of cage load and repeated stress when ball lag occurred, which in turn would sometimes result in fracturing of the steel cage. As a countermeasure to this, the new bearing utilizes a plastic cage, which is resistant to repeated stress, to further increase the reliability of the cage under high load conditions.

What is Needed from the Bearing

HST high power output → increase in bearing load → long life, high reliability

(due to the increase in bearing load, HST thrust ball bearing damage
has been observed several times)

HST Thrust Ball Bearing Usage Conditions

As illustrated on the left, the HST thrust ball bearing is positioned so that the point of contact with the pistons is at a tilted angle

Usage Characteristics

• Since the bearing is used at a tilted angle, it must bear both axial load and component force (radial load).
• The load to the bearing will vary as the piston position changes, which means that the bearing must also bear unbalanced load.

Bearing Problem and Countermeasure

Bearing Specification

This long-life, high-reliability cage-equipped thrust bearing utilizes EP steel for the inner/outer race, special heat treatment on the inner/outer race and balls, and a plastic cage to achieve more than 2x the life of the standard bearing.