The Geneva drive or Maltese cross is a gear

Gear

A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

 mechanism that translates a continuous rotation

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel advancing it by one step. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps.

Background

The name derives from the device's earliest application in mechanical watches

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

, Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

 and Geneva

Geneva

Geneva In the national languages of Switzerland the city is known as Genf , Ginevra and Genevra is the second-most-populous city in Switzerland and is the most populous city of Romandie, the French-speaking part of Switzerland...

 being an important center of watchmaking. The geneva drive is also commonly called a Maltese cross

Maltese cross

The Maltese cross, also known as the Amalfi cross, is identified as the symbol of an order of Christian warriors known as the Knights Hospitaller or Knights of Malta and through them came to be identified with the Mediterranean island of Malta and is one of the National symbols of Malta...

 mechanism due to the visual resemblance.

In the most common arrangement, the driven wheel has four slots and thus advances for each rotation of the drive wheel by one step of 90°

Degree (angle)

A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

. If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.

Because the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.

Uses and applications

One application of the Geneva drive is in movie projector

Movie projector

A movie projector is an opto-mechanical device for displaying moving pictures by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.-Physiology:...

s: the film does not run continuously through the projector. Instead, the film is advanced frame by frame, each frame standing still in front of the lens for 1/24 of a second

Second

The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

 (and being exposed twice in that time, resulting in a frequency of 48 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

). This intermittent motion is achieved using a Geneva drive. (Modern film projectors may also use an electronically controlled indexing mechanism or stepper motor

Stepper motor

A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...

, which allows for fast-forwarding the film.) The first uses of the Geneva drive in film projectors go back to 1896 to the projectors of Oskar Messter

Oskar Messter

Oskar Messter was a German inventor and film tycoon in the early years of cinema.-Biography:He was born in Berlin, where his father had founded a company selling and manufacturing optical equipment in 1859...

 and Max Gliewe and the Teatrograph of Robert William Paul. Previous projectors, including Thomas Armat

Thomas Armat

Thomas J. Armat was an American mechanic and inventor, a pioneer of cinema best known through the co-invention of the Edison Vitascope.-Biography:...

's projector, marketed by Edison

Thomas Edison

Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

 as the Vitascope, had used a "beater mechanism", invented by Georges Demenÿ

Georges Demenÿ

Georges Demenÿ was a French inventor, chronophotographer, filmmaker, and gymnast.-External links:* digitized by the...

 in 1893, to achieve intermittent film transport.

Geneva wheels having the form of the driven wheel were also used in mechanical watch

Watch

A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...

es, but not in a drive, rather to limit the tension of the spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

, such that it would operate only in the range where its elastic force is nearly linear. If one of the slots of the driven wheel is occluded, the number of rotations the drive wheel can make is limited. In watches, the "drive" wheel is the one that winds up the spring, and the Geneva wheel with four or five spokes and one closed slot prevents overwinding (and also complete unwinding) of the spring. This so-called Geneva stop or "Geneva stop work" was the invention of 17th or 18th century watchmakers.

Other applications of the Geneva drive include the pen change mechanism in plotter

Plotter

A plotter is a computer printing device for printing vector graphics. In the past, plotters were widely used in applications such as computer-aided design, though they have generally been replaced with wide-format conventional printers...

s, automated sampling devices, indexing

Indexing (motion)

Indexing in reference to motion is moving into a new position or location quickly and easily but also precisely. After a machine part has been indexed, its location is known to within a few hundredths of a millimeter , or often even to within a few thousandths of a millimeter , despite the fact...

 tables in assembly lines, tool changers for CNC machines,rupee note counting and so on. The Iron Ring Clock

Iron Ring Clock

The Iron Ring Clock is a clock of unusual design created by four Mechanical Engineering students at McMaster University in Hamilton, Ontario, Canada. The clock was designed and built as a thesis project by Patrick Burton, Braden Kurczak, Michael Paddags, and Peter Whitred...

 uses a Geneva mechanism to provide intermittent motion to one of its rings.

A Geneva drive is used to change filters in the Dawn mission framing camera used to image the asteroid 4 Vesta

4 Vesta

Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 in 2011. It was selected to insure that should the mechanism fail at least one filter would be usable.

Internal Geneva drive

An internal Geneva drive is a variant on the design. The axis of the drive wheel of the internal drive can have a bearing only on one side. The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and always greater than 180° in an internal one, where the switch time is therefore greater than the time the driven wheel stands still.

The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses.

External links

• The Geneva wheel.
• The Geneva stop.
• An animation of an external Geneva drive. - Quickermittent. Modified starwheel for fast pulldown.
• Animation of and instructions for building a LEGO Geneva Mechanism