Geneva drive

The Maltese cross transmission is a transmission that is named after the characteristic, similar in form to the Maltese cross member. Other names are the star wheel (differing by four arm number ) and Maltese Cross ratchet as more general term.

With it, continuous rotary motion can be converted into intermittent rotary motion, or the number of rotations limit ( ratchet ).

The transmission has a star wheel with four or more slots. The drive has a pin which engages into a slot at each rotation of the star wheel and this entrains until the pin exits from the slot. After that engages a circular sector shaped locking disk of the drive wheel and keeps the Maltese Cross at the periphery firmly so that it can not rotate.

Geneva drives have some disadvantages; For example, the sliding friction of the driving rod and the blocking circle sector must be mastered. In particular, the pin is subject to wear. The mechanism must be reliably lubricated, run well greased or oil bath.

Technical application

Film projector

The most important application of the Geneva gear is in movie projectors. The film is not here continuously before image window and look pass ( except Mechau projector with mirror ring ), but will gradually ( intermittent) transported. The film is driven for example via a so-called switching roller, which is seated on the cross shaft.

The application of the Maltese cross in movie projectors dating back to 1890, when it was used in projectors by Louis Le Prince, Oskar Messter and Max Gliewe (1896 ) and in " Teatrograph " by Robert William Paul ( 1895).

Previous projectors, including invented by Thomas Edison Armat and marketed as a " Scope Vita " had another, by Georges Demenÿ 1893 invented feed mechanism. The film advance is often also grapple with.

Modern projectors have to control the image sequence a stepper motor.

Watches

Geneva drives are used in watchmaking. Here they do not serve as a drive, but to limit the lift -revolution number of clock spring (the so -called " position "). To this end, missing one of the slots or the sector-shaped recesses in the Maltese cross, so that the number of possible rotations of the lift is limited. The clock spring thereby operates in only one area in which its spring force is nearly linear, and they can not be raised excessively.

In the lift mechanism of watches a star wheel with five slots is usually used. This type of position is an invention of the 17th or 18th century.

Other Applications

Geneva drives were also used in the pen change mechanisms of plotters in automated laboratory equipment and step gears in assembly plants. Also in letterpress machines, such as the long -built crucibles of Heidelberger Druckmaschinen, the Geneva mechanism is used.

Internal pin gear

A rarer form is the inner pin gear, which is larger and less resilient by design, since the drive shaft can be mounted only on one side. The angle by which the drive wheel must rotate in order to cause a step of the driven wheel, the outer pin gear is always less than 180 °, the inner greater than 180 °. In the latter, the time for a switching movement is therefore always longer than the time in which the output gear is. The internally -driven indexing is not locked against rotation during his arrest.

Version with lock is achieved.