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A Molecular Differential Gear

This differential gear was designed by K. Eric Drexler while he was a research fellow at the Institute for Molecular Manufacturing. Related designs, such as a molecular planetary gear, can be found in Nanosystems: Molecular Machinery, Manufacturing, and Computation (Wiley Interscience) or at IMM’s web site in the section on molecular machine parts designs: Further information on molecular manufacturing is available at

Macroscopic machines often use rotating shafts and gears to drive motion. Molecular machines can do likewise, sometimes using parts in ways that follow conventional engineering practice.

A differential gear links two shafts through a casing, constraining the sum of the rotational angles of the shafts to equal the rotational angle of the casing. In a car, a differential gear lets the engine drive the wheels while letting them roll different distances in cornering. A molecular differential gear like that shown above can serve this vital automotive function, but in a volume a few nanometers on a side, containing mere thousands of atoms.

different gear cutaway view

Larger version (139K, 770 x 775 pixels)
A cross-eyed stereo pair (105K) is also available.

Designs for molecular machine parts, produced by K. Eric Drexler, or by K. Eric Drexler and Ralph C. Merkle, that appear on the Web sites of the Foresight Institute and the Institute for Molecular Manufacturing (IMM) are copyrighted by IMM. It is not necessary to obtain permission to use IMM-copyrighted images for either commercial or non-commercial purposes. Permission to use the images is granted on the condition that credit is given and our copyright notice appears in the publication. Images should also contain the URL for IMM (

This cutaway reveals the two cylindrical shafts and their facing bevel (conical) gears, along with two of the four casing-mounted side-gears that mesh with both shaft-gears. Holding the casing fixed, clockwise rotation of the top shaft drives the side-gears, in turn driving counterclockwise rotation of the bottom shaft. Relatively smooth motion (despite the atomic granularity of the building blocks) is ensured by geometry and symmetries. For example, the shaft-gears have 14-fold symmetry, while the casing has 4-fold symmetry; if one side-gear is exactly opposite a shaft-gear tooth, its 90-degree partners will be opposite shaft-gear grooves. Thus, energy fluctuations at the tooth-meshing frequency will cancel, leaving only higher-frequency, lower-amplitude fluctuations as barriers to rotation. The shafts rotate in the casing on standard sliding-interface bearings, using the same principle to achieve smooth motion.

The structure is designed to be built chiefly of hydrogen, carbon, silicon, nitrogen, phosphorus, oxygen, and sulphur. The larger size of second-row atoms helps in constructing tapered gears and reduces the number of atoms needed to construct the outer cylinder of the casing. Such structures are far beyond the state of the art of chemical synthesis today, but their design and modeling is becoming straightforward.

K. Eric Drexler

Atomic Coordinates Available

776K PDB file of differential gear.

Information on free tools for visualization of molecules defined by PDB files is available. However, although these tools work well with smaller files, such as those for the fine motion controller and the neon pump, they may be less reliable with this much larger file, and may cause system crashes.