May 2007 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was edited by Associate Editor Alan S. Brown.

Technology Focus part 2:
Materials and Assembly

Good to the Core
by Peter Easton

Looking for a rugged slide bushing? Then consider the flanged bushing under development for the 2009 Mars Science Laboratory mission. To be space qualified, it must not only withstand the 62 g produced by takeoff, but also vacuum, vibration, fine Martian dust, and temperatures down to -70°C.

The bushing is part of a device called the Mini-Corer, which will drill into the Martian surface to collect soil and rock samples for chemical analysis. It will account for 85 percent of all the science the Mars Science Laboratory will perform.

The Mini-Corer itself consists of a fist-size box of motors, gears, and bearings. It sits inside a shoebox-size enclosure made of a lightweight aluminum-beryllium alloy that stiffens the structure and seals the interior from dust.

A double-wide bushing provides more rigidity to the Mini-Corer than do two standard-width bushings, and also has a more distributed load.

When it comes time to drill, the smaller box slides down 9 centimeters inside the larger box on an acme screw and two linear bearings. This brings the coring bit, 5 centimeters long and 8 millimeters in diameter, into contact with the ground. The gearbox then moves back up the slides with samples and to replace dulled tool bits.

To make the unit lighter and stiffer, Mini-Corer designer Honeybee Robotics Spacecraft and Mechanisms Design Co. of New York opted for double-wide linear bearings from NB Corp. of America in Hanover Park, Ill. Compared with using two standard-width bushings, the double-wide bearings offer greater stiffness and rigidity and fewer weak points for failure.

"We wanted to have a stable linear slide," said Honeybee systems engineer Mike Maksymuk.

"The forces and torques that the two NB Corp. double-wide linear bearings can handle were enough that we didn't need to look further.

"That's because, being a double linear slide, it gives us more distributed load. Imagine coming down on a rock with your drill moving forward without cutting rock perpendicular to its surface. That's going to put a lot of side loads back into the tool. We needed something really rigid to prevent the tool from flexing."

Cars to Get Smarter
by Harry Hutchinson

Smart materials that can change from one shape to another and back again may replace some of the small motors that actuate devices on vehicles in a few years, according to General Motors.

The company says that it has been issued or applied for more than 175 U.S. patents involving applications of so-called smart materials, which change properties under various influences—temperature, stress, charge, or magnetism.

Magnetic Ride Control, introduced by GM a few years ago, is a case in point. Available today on the Corvette and other high-end GM vehicles, it is a suspension system that makes use of magnetorheological materials, which change stiffness under the influence of a magnetic field.

These automotive shape-memory alloy-activated louvers remain closed to reduce drag and speed warmup during cold starts, then open to allow more air to cool the engine.

Next on the agenda are shape-memory alloys and polymers. These are materials that can be deformed under the influence of heat or electric charge and then resume their former shapes when the energy is withdrawn.

According to GM, actuators and sensors made from materials of this sort can replace some of the electric motors or hydraulics in current vehicles. The materials may also allow the addition of movable features that would be impractical applications for motors. For instance, shape-memory materials could be used on the surfaces of vehicles to make louvers that open wider to let in more air as the engine heats up.

The use of the shape-changing materials instead of motors could reduce mass, as well as the size and complexity of components, thereby saving fuel and cost. Other applications suggested by GM include use of the materials in hood, door, and glovebox releases, or in a grab handle that appears when needed.

The automaker said it is working with the University of Michigan and with HRL Laboratories in Malibu, Calif., to develop possible applications for smart materials. HRL Laboratories is a corporate R&D lab that's owned by GM and Boeing.

According to GM, some of the new applications could be in production model vehicles by 2010.