Maltese cross gear
The Maltese cross gear consists of a coupling gear to drive a star-shaped wheel and a locking mechanism , both of which are active alternately.
The name Maltese cross gear comes from the design of the gearbox with a four-point star-shaped wheel that resembles a Maltese cross .
The gear part is a swinging crank loop with the special feature that the coupling between the crank and swing arm is temporarily interrupted and is then re-established with the next swing arm (next slot in the Maltese cross). In the resting phase, the star wheel is blocked in its position by a cylinder lock .
Both functional parts together form a step or ratchet gear because of the temporary rest of the output member (star wheel) .
The star wheel has four or more slots. The pin on the crank wheel engages in a slot in the star wheel with every turn and takes it with it until it emerges from the slot again. A locking disk in the shape of a sector of a circle then engages in a circular cavity in the star wheel on the crank wheel so that it cannot twist. After one revolution, the next step begins with the engagement of the drive pin in the next star wheel slot.
A frequent application of the Maltese cross gear is the film transport in a cinema projector .
Technical application
Movie projector
The most important application of the Maltese cross gear is in film projectors . The film should not run continuously in front of the picture window and optics (exception of the Mechau projector with mirror rim), but should be transported step by step (intermittently). The film is driven, for example, by a so-called switch roller that sits on the cross shaft.
The use of the Maltese cross in film projectors dates back to 1890 when it was used in projectors by Louis Le Prince , Oskar Messter and Max Gliewe (1896) and in the "Teatrograph" by Robert William Paul (1895).
Earlier projectors, including the one invented by Thomas Armat and marketed by Edison as "Vitascope", had a different advance mechanism invented by Georges Demenÿ in 1893.
Modern projectors have a stepper motor to control the sequence of images .
Clocks
Maltese cross gears are also used in watchmaking . Here they are not used as a drive, but to limit the number of winding revolutions of the clock spring (the so-called "position"). For this purpose, one of the slots or the circular sector-shaped recesses in the Maltese cross is missing, so that the number of possible revolutions of the elevator is limited. The clock spring only works in a range in which its spring force is almost linear, and it cannot be excessively wound.
A star wheel with five slots is usually used in the winding mechanism of watches. This kind of posture is an invention of the 17th or 18th century.
Other uses
Maltese cross gears were also used in the pen change mechanisms of plotters , in automatic laboratory devices and index drives in assembly plants. The Maltese cross gear is also used in letterpress machines - such as the long-built pans from Heidelberger Druckmaschinen .
The framing camera of the Dawn spacecraft uses a Maltese cross gear with 8 steps to change the filter.
Inner pin gear
A rarer form is the inner pin gear, which is larger and less resilient because the drive shaft can only be supported on one side. The angle by which the drive wheel must rotate in order to effect a step of the output wheel is always less than 180 ° for the outer pin gear and greater than 180 ° for the inner one. The time for a switching movement is therefore always longer in the latter than the time in which the output gear is stationary. The internally driven ratchet wheel is not locked against turning while it is stationary. A design with a lock is possible.
Web links
Individual evidence
- ^ Siegfried Hildebrand : Feinmechanische Bauelemente. Hanser, Munich 1968, p. 636.
- ↑ Step or ratchet gears are also referred to as switching mechanisms or step switches , cf. Siegfried Hildebrand, p. 751
- ↑ a b H. Bock: The clock - basics and technology of time measurement . BGTeubner, Leipzig 1908, p. 48.
- ↑ Christopher Russell: The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres. Springer Science & Business Media, 2012, ISBN 978-1-4614-4903-4 , p. 287 ( limited preview in Google book search).