The Geneva drive or Geneva mechanism is a

gear A gear or gearwheel is a rotating machine part typically used to transmit rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or ...

mechanism that translates a continuous

rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

movement into intermittent rotary motion. The ''rotating drive''

wheel A wheel is a rotating component (typically circular in shape) that is intended to turn on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machin ...

is usually equipped with a pin that reaches into a slot located in the other wheel (''driven wheel'' that advances it by one step at a time. The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.

History

The name, Geneva drive, is derived from the device's earliest application in mechanical watches, which were popularized in

Geneva Geneva ( , ; ) ; ; . is the List of cities in Switzerland, second-most populous city in Switzerland and the most populous in French-speaking Romandy. Situated in the southwest of the country, where the Rhône exits Lake Geneva, it is the ca ...

. The mechanism is frequently used in mechanical

watch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

es, since it can be made on a small scale and is able to withstand substantial mechanical stress. The ''Geneva drive'' is also called a "

Maltese cross The Maltese cross is a cross symbol, consisting of four " V" or arrowhead shaped concave quadrilaterals converging at a central vertex at right angles, two tips pointing outward symmetrically. It is a heraldic cross variant which develope ...

mechanism" due to the visual resemblance when the rotating wheel has four

spoke A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. The term originally referred to portions of a log that had been riven (split ...

s. In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90 degrees) for each full rotation of the master wheel. If the steered wheel has ''n'' slots, it advances by per full rotation of the propeller wheel. The minimum number of slots in a practical Geneva drive is 3; it is rare to find a mechanism with more than 18 slots. 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 film

movie projector A movie projector (or film projector) is an optics, opto-mechanics, mechanical device for displaying Film, motion picture film by projecting it onto a movie screen, screen. Most of the optical and mechanical elements, except for the illuminat ...

s and movie cameras, where the film is pulled through an exposure gate with periodic starts and stops. The film advances frame by frame, each frame standing still in front of the lens for a portion of the frame cycle (typically at a rate of 24 cycles per second), and rapidly accelerating, advancing, and decelerating during the rest of the cycle. This intermittent motion is implemented by a Geneva drive, which in turn actuates a claw that engages sprocket holes in the film. The Geneva drive also provides a precisely repeatable stopped position, which is critical to minimizing jitter in the successive images. (Modern film projectors may also use an electronically controlled indexing mechanism or

stepper motor A stepper motor, also known as step motor or stepping motor,Clarence W. de Silva. Mechatronics: An Integrated Approach (2005). CRC Press. p. 675. "The terms ''stepper motor'', ''stepping motor'', and ''step motor'' are synonymous and are often u ...

, which allows for fast-forwarding the film.) The first uses of the Geneva drive in film projectors date to 1896, with the projectors of

Oskar Messter Oskar Messter (21 November 1866 – 6 December 1943) was a German Reich, German inventor and film tycoon in the early years of film, cinema. His firm Messter Film was one of the dominant German producers before the rise of Universum Film AG, ...

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

Thomas Armat Thomas J. Armat (October 25, 1866 – September 30, 1948) was an American mechanic and inventor, a pioneer of cinema best known through the co-invention of the Edison Vita Armat studied at the Mechanics Institute in Richmond, Virginia and then ...

's projector, marketed by Edison as the ''Vitascope'', had used a "beater mechanism", invented by

Georges Demenÿ Georges Demenÿ (12 June 1850 in Douai – 26 October 1917 in Paris) was a French inventor, chronophotographer, filmmaker, gymnast and physical fitness Physical fitness is a state of health and well-being and, more specifically, the abilit ...

in 1893, to achieve intermittent film transport. Geneva stop

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

es, but not in a drive, rather to limit the tension of the spring, 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 A plotter is a machine that produces vector graphics drawings. Plotters draw lines on paper using a pen, or in some applications, use a knife to cut a material like Polyvinyl chloride, vinyl or leather. In the latter case, they are sometimes k ...

s, automated sampling devices,

banknote A banknote or bank notealso called a bill (North American English) or simply a noteis a type of paper money that is made and distributed ("issued") by a bank of issue, payable to the bearer on demand. Banknotes were originally issued by commerc ...

counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of

turret lathe A turret lathe is a form of lathe (metal), metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable parts, interchangeable. It evolved from earlier lath ...

s, screw machines, and turret drills; some kinds of

indexing head An indexing head, also known as a dividing head or spiral head, is a specialized tool that allows a workpiece to be circularly indexed; that is, easily and precisely rotated to preset angles or circular divisions. Indexing heads are usually use ...

s and

rotary table A rotary table is a precision work positioning device used in metalworking. It enables the operator to drill or cut work at exact intervals around a fixed (usually horizontal or vertical) axis. Some rotary tables allow the use of index plates ...

s; and so on). The Iron Ring Clock uses a Geneva mechanism to provide intermittent motion to one of its rings. A Geneva drive was used to change filters in the Dawn mission framing camera used to image the asteroid

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...

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

Internal version

A variant exists where the drive wheel is inside the driven wheel. While an external Geneva drive advances the driven wheel one step in less than 180° rotation of the drive wheel, so (assuming a constant-speed drive wheel) the stop is always longer than the motion, in an internal wheel the motion always requires more than 180° rotation of the drive wheel, so the motion takes longer than the stop. The axis of the drive wheel can have a bearing only on one side. Croix malte interne - filled.svg, Internal Geneva drive Internal Geneva wheel ani.gif, Animation showing an internal Geneva drive in operation The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses. A shorter stop time can also be achieved by having more than one drive pin on an external drive wheel.

Spherical version

Another variant is the spherical Geneva drive. Croix malte spherique - filled.svg, Spherical Geneva drive

Kinematics

Chronogrammes croix malte 4 branches complet

The figure shows the motion curves for an external four-slot Geneva drive, in arbitrary units. A discontinuity appears in the acceleration when the drive pin enters and leaves the slot, occurring at the instant the rigid bearing surfaces make contact or separate. This generates an "infinite" peak of jerk ( Dirac peak), and therefore vibrations.

References

External links

* . * . * – ''Quickermittent''. Modified starwheel for fast pulldown. * {{Citation , type = animation and instructions for building , url = http://www.brickengineer.com/pages/2007/10/07/geneva-mechanism/ , contribution = LEGO Geneva Mechanism , title = Brick engineer , date = Oct 7, 2007 Gears Watches Rotating machines Mechanisms (engineering)