Schaeffler has developed an angular contact ball bearing that prevents the premature failure of ballscrews in short-stroke applications, particularly those resulting from the phenomenon known as “false brinelling”.

Short-stroke operation of screw drives can lead to the premature failure of bearing supports if the lubricant film that should separate the rolling elements from the raceways is absent – due, for example, to a lack of relative motion between the surfaces or where the pivot angle is small. This leads to localised fretting corrosion and unprotected steel-to-steel contact points can start to corrode. Vibrations cause the corroded (rusty) parts to detach and become embedded in the lubricating grease. Over time, the rolling elements fuse with the raceway, eventually leading to bearing and machine failures.

With screw drive bearings, this false brinelling can occur if the axes are stopped for extended periods or if they perform small strokes. Depending on the pitch of the ballscrew spindle, the stroke distances are typically less than 2mm. False brinelling is a particular problem in medical equipment and when producing moulds, because of the small workpiece sizes involved.

Schaeffler’s new ZKLF..-HC hybrid angular contact thrust ball bearing (above) has ceramic rolling elements that avoid any metal-to-metal contact. In addition, the limiting speed is increased and there is a threefold increase in grease operating life.

Previously, hybrid bearings have not been used for short-stroke ballscrew applications because their basic load rating is around 30% lower than that of standard bearings, due to the higher modulus of elasticity of ceramic balls. As a result, the contact zone in the raceway is smaller and the pressure under comparable loading is higher. However, field experience with hybrid bearings has shown that their life ratings are at least comparable with those of standard bearings.

As a result, DIN standards working groups have now decided to withdraw the theoretical reduction of the dynamic basic load ratings. This means that the positive results from using hybrid bearings in the field have now been incorporated into the basic load rating calculations. There is still a reduction in the basic static load rating, but this is not normally a limiting factor in screw drive support bearing applications. In terms of lifecycle costs, there is now nothing to prevent the use of hybrid bearings in ballscrew applications.