The most common material for ball carriers in ball bearings and ball screws is bearing steel. The use of steel balls in steel raceways is a proven, low-cost solution that offers high load capacity and long service life in a wide range of conditions. And when an application is not suitable for steel, the bearing housing or nut exterior can be made from a different material, such as stainless steel or aluminum, or coated to provide special properties such as corrosion resistance. But rolling elements (balls) are more difficult to modify, as they must have the strength and material properties necessary to withstand high loads and/or reach high speeds, without excessive wear or premature failure.
Ceramic balls can be used in linear recirculating bearings and ball screws when steel is unacceptable.
This is why you will find a variety of material options for a bearing housing or spherical nut, while spherical bearings in most cases remain bearing steel. But for applications where any steel in the bearing is unacceptable, several manufacturers now offer linear bearings and spherical nuts with ceramic balls (specifically, silicon nitride: Yes 3 No 4 ).
I need speed
Recirculating linear bearings with ceramic balls were originally introduced for applications requiring higher operating speeds. Traditional recirculating bearings with steel balls are normally limited to a maximum speed of 5 m/s due to the forces that the balls exert on the bearing heads during recirculation.
Remembering that force = mass * acceleration, the best way to reduce the forces on the heads without compromising the speed capacity is to reduce the mass of the recirculating elements, and ceramics do exactly that, with a lower density and mass than steel. Recirculating linear bearings with ceramic balls can reach maximum speeds of up to 10 m/s.
Vacuum and ESD sensitive applications
From a materials perspective, ceramics are non-porous and non-conductive, making them ideal for vacuum and static-sensitive applications. These requirements arise frequently in semiconductor and electronics manufacturing, as well as in aerospace applications. The aerospace and military industries also face frequent requirements for components that can withstand extremely high or low temperatures or large temperature fluctuations. In these applications, ceramic is also a better choice than steel due to ceramic's lower coefficient of thermal expansion.
Hybrid ball nuts combine steel and ceramic
Snail manufacturers offer two models that incorporate ceramic balls. In the first – as in most linear bearings – the traditional steel balls are completely replaced by ceramic versions. But some manufacturers offer a hybrid design with a combination of ceramic and steel balls.
Ball nuts can use all ceramic balls or a “hybrid” design where smaller diameter steel balls alternate with ceramic versions.
Image credit: Steinmeyer Inc.
The first option – all ceramic balls – replaces the metal-to-metal contact between the ball and the raceway with ceramic-to-metal contact, thus eliminating adhesive wear and microwelding. In the hybrid design, steel balls often act as “spacers”, which alter the kinematics of the nut, reducing friction and wear while maintaining good load capacity.
- Contact and friction between steel balls cause adhesive wear and microwelding.
- Hybrid designs, which alternate between steel and ceramic balls, provide good load capacity with less friction and wear.
Image credit: Umbra Group
Note that although it is possible to operate all-ceramic bearings (both ceramic races and balls) without lubrication, linear bearings and nuts have metal races (steel or stainless), therefore, even if the balls are all ceramic, they still require lubrication. However, ceramic balls are less sensitive to changes in lubrication conditions and are therefore less likely to experience extreme wear or failure when lubrication runs out or the lubricant's properties change with use.
