CHOOSING MACHINE TOOL BEARINGS
The machine tool industry represents some of the most demanding applications for rotary bearings. They require high precision, often carry high side loads, and must rotate at thousands of RPM, all while operating for years without replacement or significant maintenance. Achieving those results requires an understanding of how bearings are constructed, specified, assembled, installed, operated, and maintained.
THE BASICS
Bearings consist of an inner and outer race with a set of precision ground balls between them to create a low friction rolling surface. The races are ground with a slight recess to give the balls a raceway, and a lightweight cage often maintains even ball spacing. Bearings can also have lubricant and flexible seals to contain it and keep dust out. With variations on these few parts, there are tens of thousands of configurations.
INSTALLING BALL BEARINGS: GETTING IT RIGHT
Consider a simple gear reducer with a precision ground drive shaft and spur gears stepping down the speed to an output shaft. A mounting hole on each side of the housing captures the shaft bearings, and two bearings are used to minimize shaft runout, since low runout contributes to long life. When the gears mesh, they create forces perpendicular to the bearing axis, which standard ball bearings resist very well.
At least one bearing needs a secure attachment to the housing. Usually the outer race is secured (with epoxy or a press fit) while the inner race holds the shaft. Because there is always small variation in roundness, a preload is applied between the bearing pair, using a spring load, a weight on the shaft end while the epoxy cures, or a precision flat spring. Preload keeps all balls in contact with the races and prevents excessive endplay, which could unload the rolling elements and cause skidding.
Where gear sets are lubricated, a sealing cap outside the bearings keeps the lubricant in the housing and protects against dust.
TAKING ON ANGULAR BEARINGS
In real machine tools, bearings rarely see pure radial loads, and axial forces can reach a 45 degree angle to the central axis. (Pure axial loads are handled by thrust bearings, a specialized topic of their own.) Dual row, deep groove, and roller bearings add some axial load resistance, but should not be relied on beyond about 10 degrees off-axis. For higher angles, you need a true angular bearing.
Angular contact ball bearings use an offset between the inner and outer races so the resulting force “cone” is a composite of axial and radial loads. Tapered roller bearings use rollers in place of balls; their greater engagement with the races makes them more resistant to combined loads. Where compound loads act in both directions, such as a lathe drive shaft, angular bearings are installed as a pair and preloaded against each other.
Multi-axis CNC mills are among the most demanding environments, with tool changes, complex surfaces, drilling, tapping, and high speed finishing passes. Every axis sees a mix of axial and radial loads, making these machines prime users of angular bearings.
SUMMARY
• Calculate load directions and magnitudes based on the planned bearing life.
• If the axial component is essentially zero, a standard ball bearing should suffice.
• Up to 10 degrees off-axis, choose a more load resistant design such as double row, deep groove, or roller bearings.
• From 10 to 45 degrees, select a dedicated angular bearing: an offset ball bearing or tapered roller design.
• Fix one race (usually the outer) solidly to the structure to prevent skidding and race scoring.
• Bearings are usually installed in pairs with a preload. Preload is a must for stability and long life.
• Where lubricant is needed or the environment is dusty, use a sealed bearing.
At Boca Bearings, we don’t just solve problems; we engineer solutions that perform.
For additional assistance, contact one of our bearing experts at Info@bocabearings.com or call (561) 998-0004.