Designer's corner 23321

Radial-load ball screw

Ballnut/screw assemblies accommodate axial loads; simultaneous side loads shorten the assembly's useful life. A second helical raceway, placed parallel to the primary balltrack, overcomes this problem.

As with conventional configurations, the new raceway features its own ball return. Unlike traditional designs, captive balls contact the secondary groove at its bottom, not sides.

Ball nut/screw assembly's axial- and radia-lad bearing circuits function independently of each other.

Result: the nut also accommodates loads perpendicular to the assembly centerline.

Mike Cossentine, Thomson Industries Inc., 2 Channel Drive, Port Washington, NY 11050, (516) 883-8000, Fax (516) 767-4570.

Wafer watch

In general, microcrystal defects measuring 20-60 nm in size, and & 10⁵ defects/cm³ are not considered a problem in today's silicon wafers; they may be, however, when manufacturing next-generation devices such as the 256-Mbit DRAM.

If so, the Optical Shallow Defect Analyzer (OSDA) should be ready. Two laser beams of different wavelengths scan the wafer surface. Because the beams have different absorption coefficients in silicon, OSDA calculates defect depth and size from the difference in scattered light intensities.

Time required to inspect a six-inch wafer is one hour. Defects to 0.5 mum below the surface can be detected with a depth resolution of 0.1 mum.

Hitachi, Ltd.,6 Kanda-Surugadai 4-chome, Chiyoda-ku, Tokyo 101-10, Japan, Fax: +81-3-3258-5217, Ref: Hitachi Review 45, No. 6.

Health ring

A battery-powered light-emitting diode and wireless transmitter, wrapped around the ring finger, may one day monitor patient vital signs. Here's how:

The LED emits light into the finger; a photo diode captures light reflected back from the blood. Because blood vessels expand when the heart beats, the amount of reflected light corresponds to the pulse rate.

Embedded circuitry "cleans" and amplifies the signals, which are transmitted to a remote receiver. Rhythm and shape of the pulse rate, processed on a home computer and available to doctors via the Internet, indicate cardiac condition.

Elizabeth Thomson, Massachusetts Institute of Technology, Room 5-111, 77 Massachusetts Ave., Cambridge, MA 02139-4307, (617) 253-2700.

Sensor bearings

Why bother fitting a separate speed encoder to your drive shaft when an integrated sensor/bearing will do?

Phase-shifted Hall-effect sensors, positioned opposite the bearing seal, measure rotational speed and direction of rotation. Quoted savings: Up to 50% when compared to solutions using a separate encoder.

Sensor bearings operate from a direct voltage of 5V to 24V and are intended for electrical machines, materials handling equipment, and packing machines.

Rocco Docimo, FAG Bearings Corp., 200 Park Ave., PO Box 1933, Danbury, CT 06813-1933, (203) 830-8133.

Convenient coupling

One-piece couplings can be a maintenance headache. Replacement requires pulling drive and driven shafts apart, and that means realignment later.

Removable end caps, clamped in place with socket-head cap screws, eliminate the problem. Cap removal exposes the shaft on its centerline; the drive assembly can be disconnected without moving motors or driven equipment.

Other benefit: Because the end cap fits across a D-shaped flat machined on the shaft, there is no backlash.

Hunt Power Drives Ltd., 2 Freemans Way, Wetherby Rd., Harrogate, North Yorkshire HG3 1DH, +44 1423-889984.