Geneva or Maltese cross (starwheel) mechanism is an indexing device that converts the continuous motion into intermittent motion. By means of this mechanism, the rotary motion of the cam is converting into intermittent rotary motion of sprocket. The film passed over the sprocket. Due to the intermittent motion, the film advanced frame by frame for 1/24th second in frequency of 48Hz.

This mechanism is commonly used indexing mechanism where an intermittent motion is required.

The design and manufacturing of a conventional Geneva mechanism is generally simple and inexpensive because there is no specially curved profile on any of the components except straight lines and circular arcs. However, due to the discontinuity of the acceleration at the beginning and ending positions, the shortcoming of using conventional Geneva mechanism is the large impact when the driving pin engages and disengages with the starwheel slot.

Geneva connection

MECHANISM

In this mechanism, for every turn of the cam drive A the starwheel B makes a quarter turn. The pin, attached to A moves in the slots causing the motion of B. The contact between the lower parts of A with the corresponding hollow part of B retains it in position when the pin is out of the slot. A is cut away near the pin as shown, in order to provide clearance for B as it moves. If one of the slots is closed, A can make less than one revolution in either direction before the pin strikes the closed slot and stopping the motion.

Geneva cut-away
Please notice: A is the driver while B is the driven.

This is an external Geneva mechanism. In this type of mechanism, the starwheel is connected with cam drive externally which is the most popular and which is represented.

ADVANTAGES

Geneva mechanism may be the simplest and least expensive of all intermittent motion mechanisms.

They come in a wide variety of sizes, and they have good motion curves characteristics compared to ratchets, but exhibit more jerking or instantaneous change in acceleration than better cam systems.

It maintains good control of its load at all times, since it is provided with locking ring surfaces, however it is not a versatile mechanism.

The ratio of dwell period to motion is also established once the no. of dwells per revolution has been selected.

All Geneva acceleration curves start and end with finite acceleration and deceleration.

This means they produce jerk.

GEOMETRY OF FILM FRAME

In the most common arrangement, the starwheel has four slots and thus advances for each rotation of the cam by one-step of 90°. If the starwheel has 𝒏 slots, it advances by 360°/𝒏 per full rotation of the cam.

COMPONENTS

DRIVER GEAR
The input is given through this cam (driver) gear. It is a one type of continuous motion.

CAM & PIN
It is main part of this mechanism. Because it converts the continuous rotary motion into intermittent motion by guiding the starwheel along its circular path. Then it converts this motion as require for the movement of film frame.

STARWHEEL
It is also take part as vital role in this mechanism. Because the rotary intermittent motion produced in this part only. Starwheel has four slots in it; pin goes into along circular movement of cam.

SHAFT
Shaft holds the all parts of mechanism by horizontally on its threaded portion. There are two shafts are provided in this mechanism.

SPROCKET
It is provided for holding the film frame according to the rotary intermittent motion of the starwheel. Pressure pad rollers also provided for perfect movement of film frame.

FILM FRAME
It is the component, which carries the picture and to slide on the sprockets regularly according to the motion.

WORKING PRINCIPLE

Geneva steps
Drawing courtesy of Eastman Kodak Co., Rochester, NY

According to the principle of this mechanism, the input is given by motor to the cam driver gear of the arrangement. In the most common arrangement, the starwheel has four slots and thus advances for each rotation of the cam by one-step of 90°.

projector head drawing

ELEMENTS

Film frame
A commonly held misconception is that film projection is simply a series of individual frames dragged very quickly past the projector’s intense light source. If a roll of film would merely passed between the light source and the lens of the projector, all that would be visible on screen would be a continuous blurred series of images sliding from one edge to the other. It is the shutter that gives the illusion of one full frame being replaced exactly on top of another full frame.

A rotating shutter interrupts the emitted light during the time the film is advanced to the next frame. The viewer cannot see the transition, thus tricking the brain into believing a moving image is on screen. Modern shutters are designed with a flicker-rate of two times (48Hz) or even sometimes three times (72Hz) the frame rate of the film, to reduce the perception of screen flicker. Higher rate shutters are less light efficient, requiring more powerful light sources for the same light on screen.

Mechanical sequence when image is shown twice and then advanced. The sprockets rotate continuously to advance the film while the intermittent sprocket (connected to the starwheel) is controlled by the mechanism.

Sprockets
Sprockets engage perforations punched into one or both edges of the film stock. These serve to set the pace of film movement through the projector and any associated sound playback system.

Film loop
As with motion picture cameras, the intermittent motion of the gate requires that there be loops above and below the gate in order to serve as a buffer between the constant speed enforced by the sprockets above and below the gate and the intermittent motion enforced at the gate. Some projectors also have a sensitive trip pin above the gate to guard against the upper loop becoming too big.

Pressure plate of film gate
A spring loaded pressure plate functions to align the film in a consistent image plane, both flat and perpendicular to the optical axis. It also provides sufficient drag to prevent film motion during the frame display, while still allowing free motion under control of the intermittent mechanism. The plate also has spring-loaded runners to help hold film while in place and advance it during motion.

Intermittent mechanism
The intermittent mechanism can be constructed in different ways. For smaller gauge projectors (8mm and 16mm), a pawl mechanism engages the film’s sprocket hole one side, or holes on each side. This pawl advances only when the film is to be moved to the next image. As the pawl retreats for the next cycle it is drawn back and does not engage the film. This is similar to the claw mechanism in a motion picture camera.

In all 35mm and 35/70mm, as well as in some 16mm projectors, there usually is an intermittent sprocket immediately underneath the pressure plate. Unlike all the other sprockets in the projector, which run continuously, the intermittent sprocket operates in tandem with the shutter, and only moves while the shutter is blocking the light, so that the motion of the film (frame displacement) cannot be seen.

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