NIST cranks up an incredible crane.

Though seemingly spindly and precariously balanced, a new crane called SPIDER may prove itself mightier than its more traditional - and bulkier - counterparts. Scalable to a size that can straddle buildings, the crane can lift and position heavy loads precisely, according to its inventor, James S. Albus, at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. The crane also qualifies as a robot that users can fit with sensors, camera "eyes" and motorized platforms.

Albus had fashioned a prototype SPIDER (Stewart Platform Independent Drive Environmental Robol) in his basement. Then last year, he and fellow engineers at NIST decided this robot crane might prove useful in fighting the oil fires that raged in the aftermath of the Persian Gulf War. SPIDER never made it to Kuwait, but the NIST engineers did build a 20-foot model and demonstrate that it can outperform other cranes.

The secret to SPIDER's strenght lies in the Stewart Platform, a device developed 30 years ago for airplane simulators. It consists of upper and lower platforms connected by six pistons. NIST engineers improved on that design by replacing the pistons with cables and winches, says Roger V. Bostelman, an electrical engineer at NIST.

Thus, SPIDER looks nothing like a typical crane, says Bostelman. An octahedron of aluminum tubing forms the crane's support structure. Two pulleys hang down from each of the three corners of this octahedron's top triangle -- the upper part of the Stewart Platform. Each pulley supports a cable. Together, the six cables connect that top triangle to the second, lower level of the Stewart Platform. This second level is a small, triangular work platform to which tools or grabbing devices attach. The winches that control the six cables can work indepedently or in synchrony to tilt, spin or move the platform -- and a load -- up, down or sideways at the desired speed.

In most cranes, the load-bearing cables work in series to pick up a load. SPIDER's cables work simultaneously. With SPIDER, an operator can maneuver an I-beam to whithin a millimeter of its target and can hold an object within a half a degree of the correct angle of rotation, says Bostelman.

Nor does this crane need any counterweight, as a tower crane does, for example. "All the forces are directed back onto themselves," he explains. So the 1,000-pound SPIDER can lift and hold far more than its own weight - up to 6,000 pounds, or 3,000 pounds with a 50 percent safety factor.

Yet "its's very simple to build," Bostelman points out. The NIST engineers envision SPIDERs with 400-foot-high towers supported by guy wires and even larger models for underwater operations, says Albus. They foresee many uses for the robot crane, which they hope will one day become familiar equipment at construction sites here on Earth and even on the moon.