Weight and properties requirements may drive every engineer's choice of metals. But at Honeybee Robotics (www.hbrobotics.com), which makes advanced automation systems for space and difficult earthbound environments, tight mass budgets and the need for robust properties sometimes steer the engineers there toward some unexpected choices. Recently, the company's chief engineer, Tom Myrick, picked beryllium copper for a new system that he believes can serve as a less noisy, cleaner alternative to slip rings.
Like a slip ring, this "conductive wheel" system carries data or power across components that rotate in relation to one another. This new design, however, lacks a slip ring's brushes and other moving components. Instead, it consists of spoked wheels that roll within races integrated into or attached to the rotating components—the inner race on one component, the outer race on the other. A preload compresses the wheels into a mild ellipse, ensuring contact with the races so that the signals and power can pass between them continuously. Most applications would need several wheels to provide enough surface area for a high-power connection, Myrick reports. In this configuration, linkages attached to a hub in the center of the wheels keep them from migrating into a clump.
The main advantage of the system, according to Myrick, is that it drastically reduces slip, particle generation, and noise. "Even a few mils of slip per revolution can cause wear and galling problems over time," Myrick points out, adding that brushes found in most slip rings can be a source of noise and particle problems.
Myrick came up with the idea of the conductive wheels a few years ago while working on a satellite project for NASA. This early design used wheels without any spokes, which limited the degree to which they could be deformed within the races without collapsing. In tests, this design tended to slip a bit, though "it was still an order of magnitude better than a slip ring," Myrick says. He adds that electrical noise likewise proved to be about an order of magnitude less with the conductive wheels.
The latest wheels, however, feature an important design refinement that promise to further boost performance. Myrick came up with spiraled spokes that add stiffness and improve the electrical connection. They do so because they allow the wheels to be "squashed" more without collapsing, increasing the contact area with the races. And the spokes themselves provide an electrical pathway through the wheel. The spokes also allow the wheels to deform without shifting the center hub, which enables the linkages to function and also helps the wheels adjust to any variations in the races.
Myrick admits that BeCu isn't usually his first choice of metals given the difficulties in manufacturing it. Toxic if breathed in particle form, BeCu requires Honeybee to hire a specially equipped machine shop to make these rings—even though the company has its own very capable machine shop in house. But the alloy, which has 2% beryllium content, really shines in the properties department (see box). "It has an extremely high modulus," says Myrick. And that stiffness, along with the spoke geometry, helps him optimize the deformation of the ring—to the point where it maximizes contact area without losing its ability to roll within the races.
It has the right electrical properties too. Honeybee has a contract to evaluate the ring as a way to provide lightning protection for the airborne laser currently under development by the Air Force and Lockheed Martin. The laser would occupy an 11-ft turret integrated into the nose cone of a Boeing 747. Myrick says that conductive wheels would provide ground path against lightening strikes while avoiding particles that could interfere with the laser optic. Experimental versions of these wheels measure 2.5 inches in diameter with 30-mil-thick walls and 20-mil-thick spokes.
To get an idea of just how much juice the wheels can handle, Honeybee recently subjected them to electrical tests in which the wheels were squashed between two 410 stainless steel plates and zapped with 42 simulated lightening strikes. Each strike consists of different current levels and durations, but they all included long, high amperage jolts—such as 3/4 of a second at 400A. "That's the part that scared me the most," Myrick says, and he worried that the wheels might weld to the steel. But they didn't. In fact, the BeCu wheels came through the tests without any signs of damage at all.
Though under development just for the Air Force application, the conductive wheels could ultimately have wider applications in industry. "They could work in any application where slip rings have particle or wear problems," Myrick says.
|Springy and Strong
|Beryllium copper can endure very high bending stresses as a spring material.
|Selected Beryllium (2%) Copper Properties:
|Modulus of Elasticity
|Modulus of Rigidity