Hybrid bearings boost turning quality

Houston-A live center is a device with a tapered shank on one end that mates to a lathe or grinding machine. The other end consists of a bearing housing that accommodates a spindle and seal. Together, they dictate accurate rotation or oscillation of a shaft.

Live center bearings must withstand both the axial forces of the tailstock pushing the workpiece into the chuck and the radial forces of the cutting tool on the workpiece. Rotational Technology Inc. (Houston, TX), manufacturer of Rotech(R)live centers, recently completed preliminary testing of a new live center that uses Cerbec ceramic balls in a live-center body having an HSK-standard taper.

While it's been over a decade since the HSK (Hollow Short Kegel-German for taper) toolholder connection first debuted overseas, the automotive and aerospace industries on this side of the Atlantic did not adopt the German standard until just a few years ago. Now, however, the standard's force is helping its proliferation as machinery speeds increase. "This is the first live-center body with a modified quill for manual installation that incorporates an HSK taper," says Rotech's President

Dale McMillian. McMillian claims double the realistic speed and feed rates in tests that ran for seven days.

Programmable tail stocks and workpiece elongation subject the live center bearings to high forces, explains McMillian. "The Cerbec balls used in the tested bearing assembly remarkably enhanced the live center's resistance to compression and thermal expansion. The test unit managed the high temperature with no change in tolerances from static to dynamic speeds and loads." Moreover, after unit disassembly and component inspection, seal failure was discovered that resulted in coolant contamination in the bearing housing. "It's amazing how the ceramic balls held up to abrasives and coolant contaminate in the lubricant for the duration of the tests," he adds.