Article is written by Erkan UMUT, Istanbul, Turkey
1. MALTESE CROSS MECHANISMS
PRINCIPLES OF MALTESE CROSS MECHANISMS
The Maltese mechanisms (intermittent or Geneva movement) are used, when the conversion of uniform rotary intermittent motion is necessary. It consists of two main parts: the cam, pin and the Maltese cross. Crosses are divided into three-slot, four-slot and multi by the number of faces. Commonly, four-slot Maltese crosses are used in the projection equipment.
Cam is shown in Fig. 1, and, where 1 – disk with stopping cam A, 2 – pin, 3 – shaft. Maltese cross (Fig. 2) has the following parts: 1 – head, 2 – heading portion of the shaft, 3 – tail portion of the shaft (it’s attached to the intermittent sprocket), 4 – pin nest, 5 – slot blade, 6 – face, 7 – slot.
Successive phases of the Maltese mechanism are shown in Fig. 3.
Phase I – the Pin entrance into a slot of the Starwheel – the center of the Starwheel, O1 – cam center, O2 – the pin center. Combining these points with straight lines, we obtain the triangle O-O1-O2, which is called the base.
Phase II – Pin passes the slot this way in which the Starwheel turned to one face.
Phase III – the idle cam. Starwheel stationary, the frame is projected on the screen.
Thus, the principle of the Maltese mechanism is as follows (Figure 4). Cam 1 is rotated with uniform velocity from the drive mechanism. Cam Pin 2 periodically enters into the slots of the Maltese Cross 3, later resulting in rotation with the intermittent sprocket, pulling the film from the film gate in one frame size. Hence, with continuous rotation of the cam, Maltese cross together with sprocket carries intermittent rotation.
Intermittent movement will also drive the film in film gate. For uniform rotation of the cam shaft, assembled to the flywheel 4, which was idle during the cam (Phase III) to store energy, due to its inertia preventing change in the velocity of cam. When the pin enters into the slot (Phases I and II) the stored energy is expended to perform work.
PRECISION OF MALTESE CROSS MECHANISMS
For vertical stability, it requires high precision manufacturing and assembly of the main elements of the Maltese mechanism. The most crucial dimension in the Maltese cross is the angle E between the stopping faces (see Fig. 2). On the deviation angle E of the nominal value (90 ° at four-slot Starwheel) is given by close tolerance (less than ± 1′). The accuracy of the film movement also depends on the stopping hits. Sometimes the error of manufacturing and assembly of the Maltese mechanism consists of algebraically and therefore cancel each other out. However, it must proceed from the fact that Maltese is a multiphase mechanism, because at every turn, a Starwheel on one blade will extend their mistakes, and hence, the transition from blade to blade errors will be different. The number of phases is determined by the number of the blades of the Starwheels. Therefore, in cases where the requirements of precision movement is particularly important, as for example, in film shooting equipment, Maltese mechanisms are being not applied. In the projection apparatus, particularly in equipment for 35mm and 70mm film, the accuracy of Maltese mechanisms, provided for showing the film, allows to ensure a satisfactory vertical stability of the frame for normal equipment operation.
Design-operational performance of the Maltese cross mechanisms
Maltese cross device is an independent mechanism part of projector. The mechanism is distinguished by high durability major parts, which explains the relatively good dynamic characteristic, lubricant friendly, the choice of wear resistant materials and their heat treatment. Maltese cross is made of chrome-nickel or tool steel, head and shaft of the Starwheel are hardened by high frequency currents. Cam pin is made of steel, locking cam of anti-friction cast iron or steel, the cam shaft sleeve of anti-friction cast iron or bronze. The service life of the Starwheel and cam without pin is about 2000 – 2400 per hour, pin and cam sleeve 500 – 1500 per hour, the cam shaft bushing 1200 hours.
The shutter is designed to allow the light flux at the time when the frame is stopped in the aperture frame. When designing the shutters, it must be ensured that it provided the required frequency of the light flux and at the same time, the greatest transmittance. It is necessary to consider the following factors influencing the critical frequency of flicker:
1. If the critical flicker frequency is greater, the brightness of the image on the screen will be higher. Usually the silver screen brightness does not exceed 150-200 lux. In this case, the brightness of the image on the silver screen does not exceed 75 – 100 apostilb with the brightest images of the film. This brightness is at the critical frequency of flashing about 48 per second (with equal angles of the shutter blades and cuts).
Thus, the frequency of projection of 24 fps is sufficient to block the flow of light twice in one period of the intermittent mechanism, since the frequency of blocking would be equal to 48 per second. If the shutter makes one revolution during the period of the mechanism, it should have two blades, and if it turns twice, then it should have one blade.
2. The lowest critical frequency of flickering occurs in the case where the open and closed blades are of equal size and symmetrically placed. From this, it follows that the closed blade cannot do much less than the open one.
3. Critical flicker frequency is greater than an angular size of the screen to the viewer, therefore, flicker more visible to spectators sitting in the front rows. For the same reasons, critical flicker frequency increases slightly in wide-screen film projection.
The projection equipment primarily uses two-blade shutters with a symmetrical blade design, rotating at the same speed as the cam (eccentric).
The single-bladed shutter rotates twice with the number of revolutions.
Consider the most common case where the shutter makes one complete revolution during one period of the intermittent mechanism. Before the film starts moving, shutter should completely block the luminous flux (light). To do this, the shutter must turn at some angle, called the angle of pre-closure. After this, the movement should move the frame just after the shutter blocks the luminous flux. The shutter angle corresponding to the timing of the frame movement is called the angle of motion. Thus, the central angle of the open blade, called the working shutter angle, must be equal to the total of the angle of motion and the angle of pre-closure.
Typically, the distance between the shutter and the aperture frame must be a little.
Pre-closure angle also depends on the location of the shutter rotation axis on the aperture frame. Since their axes are parallel, shutter axis could be located in different places in relation to the aperture. The axis of the shutter is positioned laterally and lies in a horizontal plane passing through the middle of aperture frame. The greater the distance from the axis of the shutter to the aperture frame, the greater the diameter of the shutter.
The cylindrical shutter (drum shutter) is a hollow cylinder with two symmetrically placed open areas on the surface and rotating around an axis located in the same horizontal plane with the center of the aperture frame perpendicular to the optical axis of the cine projector.
The cylindrical shutter differs from the disk and conical shutters, so that the light flux is blocked by the two blades above and below at the same time (see below).
In the cylindrical shutter, each blade has an angle of pre-closure: for the first blade and for the second blade.
The angle is quite significant, since a large section of the beam and the diameter of the shutter, and relatively small.
Conical shutter is a cone.
The axis of rotation of shutter located in the same horizontal plane with the center of the aperture frame, and the aperture forms with the optical axis of the cine projector at an acute angle. When the angle becomes equal to 90°, the angle becomes equal to zero and the conical shutter acts like a disk shutter.
The main advantage of conical shutters is that the blocking blade is structurally easier to place in close proximity to aperture frames, i.e. making the distance very small (see below). Consequently, the angle of pre-closing in a conical shutter may be less than the disk one. This leads to an increase in light transmittance of the conical shutter.
Some experiments are showed that cylindrical shutters are superior to the conical ones due to image steadiness.
3. ARRANGEMENTS FOR MATCHING FRAME with aperture frame (FRAMING)
In the process of projecting the film, it could be a frame difference between image boundaries and the boundaries of the screen, i.e. it can be visible out-of-framing phenomenon (out-of-phase) and the lines of two adjacent frames on the screen.
This may be due to: 1) improper film threading in a projector, and 2) improper film splicing, and 3) an inaccurate out-of-framing location on the perforation caused by printing errors, 4) film slip jumping on the sprocket, caused by, for example, the lack in some perforations areas.
The framing of projector aperture with film frame is necessary, without interrupting the work of the projector, to combine the frame with the aperture. This is done by a special mechanism, which must meet the following basic requirements:
1) The mechanism should be simple in design and convenient in operation;
2) The combination of an aperture with frame should occur in the shortest time;
3) The mechanism should not cause a spontaneous shift of the aperture frame, i.e., be reliable in operation.
Combining the aperture frame with the film frame can be achieved in two ways: an additional movement of frame on the film aperture, or an additional movement of the film in the gate.
However, this method has a number of serious shortcomings like vertical shift in the image, clearly visible to spectators, is unacceptable, vast loss of light, and the quality of the image is much more impact due to aberration of the lens. For amateur usage, when the film was shot on the reversal film and printing error is excluded, this method of combining is more acceptable.
In order to eliminate the first and third drawbacks of this method, the aperture frame and the lens move together. In this case, the movement of frame and aperture of screen image will be the same, which is practically a spectator could not perceived.
In cine projector, which uses Maltese mechanisms can be used four ways of combining the frame with the aperture frame:
1) Displacement Maltese mechanism in the vertical plane relative to the film gate;
2) Additional film travel on film gate through roller located between the film gate and intermittent sprocket;
3) Rotation about the axis of the cam;
4) Rotate about the axis of the Maltese cross mechanism.
The first method is not found wide application, because of the complexity. The second and third methods were used relatively wide, but they cause increased wear on the film perforations and increase the vertical smear of the image.
In the stationary equipment the fourth way is used almost exclusively.
When the Maltese cross mechanism assembly is rotated, the Starwheel rotates at an angle, and consequently intermittent sprocket 4 rotates at the same angle. In this mechanism all the shortcomings of the previous ways are removed, but a new flaw: In fact, when the Starwheel turns around the cam shaft gear 5, on the intermediate gear 6 (Fig. 7) that transmits the rotation of the drive mechanism receives an extra turn with respect to the rest of the gears of the drive mechanism, and that causes the Maltese mechanism will be out-of-phase with the shutter.
This leads to the fact that the shutter will sooner or later (depending on the direction of rotation of the Maltese mechanism) to block recruiting aperture, causing the appearance on the screen “traveling ghost” of the image (image smearing).
Solve this in two ways:
1) At the turn of the Maltese mechanism, give the shutter an extra turn in either direction or for an additional angle of cam rotation;
2) Give a rotation to the cam gear shaft in the opposite direction from the running of the intermediate gear.
In the first case, it requires an additional mechanism for compensation of the shutter (Fig. 7), and the gaps in the links which will inevitably affect the accuracy of the shutter. In the second case, it is excluded an additional mechanism thanks to some complicating construction of the mechanism (Fig. 8).
Some European designs
It’s a very complex mechanical design of the German engineering. This design is also applied in the Yugoslavian-made Iskra, Polish-made prexer and Japanese-made New Star projectors which are the clones. The rotation timing changes, so shutter adjustment is necessary in this system.
The rotation timing changes, so shutter adjustment is necessary in this system, too. The shutter compensation is done by a gear having the key slot locating in the midst of the fork. It is a common simple solution.
A new type of framing system works with a split cross shaft is seen in Fig. 13.
Whereas previously cross and sprocket wheel was rigidly on an axis, the axis of this system has been interrupted.
Both shaft ends are provided with counter-rotating helical gears, which are connected by a coupling tube of nylon together. The coupling tube is by means of ball bearing mounted in a metal shell, which follows the non-rotational movement of the shafts and the clutch, but this leads to the direction of the shaft axis. Shifting in the axial direction to this shell, then this is one of the two against each other rotating shaft ends, so that the sprocket wheel undergoes an additional rotational movement without causing a change in the function of the other transmission parts. The movement in such force acting in axial direction is offset by two pins on the hardened outer shaft ends. In this system, an automatic adjustment of the shutter is not necessary. This system, used in Philips FP 20 projector allows a particularly simple transmission structure and is a very unique solution.
Most Century projectors, as well as patented U.S.-made Westar, British-made Westar, along with its clones the Indian-made Cinecita and Monee, Mexican-made Cinemex, Peruvian-made Centree projectors have the same system. Rotating the Century framing knob employs rack and pinion gearing to move the intermittent movement up or down. Thus the intermittent sprocket does not pivot. The rotation timing remains the same, so no shutter or cam adjustment is necessary in this system. It is a very simple mechanism which makes the Century projectors service friendly and famous, and thus most copied in the world.
Simplex projectors use the shutter compensated system, and correct framing by rotating the intermittent movement, and thus repositioning (pivoting) the intermittent sprocket.
This system is based on keeping the Starwheel and cam position exactly the same while framing, so no shutter out-of-phase occurs.
When the framing knob is turned, the intermittent movement turns the same way, so the gears A move and the gear holder (fork) goes up-and-down according to the direction, the gear holder moves the gears B, by doing this, every movement, that the intermittent movement does, is transferred to the position of the Starwheel and cam precisely in-sync.
The gear B locating in the midst of the fork is a gear having the key slot. The pin for gear seen acts as a key.
This system is a very practical solution mainly used in the Japanese projectors.
(For more information please visit my Technical corner page on my other specific Blog.)
4. The wear resistance of the Maltese cross drives
For the practice of the projection technology, the study of the wear resistance of Maltese cross drives is important. The most exposed edges and surfaces of the cross are the slot channels, as well as the continuously running part of the pin transmission.
In numerous experiments were made in this regard, which have led to the following requirements:
I. The life of the cross gear for projectors depends primarily on the wear resistance of the pin as it is claimed most, a worn pin cannot be easily replaced, since the individual parts of the gearbox is well matched.
II. The clearance between pin and slot guide can be in the assembly from 0.005 to 0.01mm. Wear is only permitted to a maximum of 0.05mm.
III. For the production selected steel grades are used, which lead to good results if treated in the gear manufacturing. (In the Soviet Union following materials were used: Steels У10A for the cross, PФ-1 for the pin, hardening steel for the locking disc (cam), a heat treatment is subjected.)
For the comparison, the moment of the pin inertia and the moment of the cross inertia of the Soviet designs are only slightly above, while the U.S. ones are lower, which may well be advantageous, however, leads to other serious disadvantages.
Soviet KP-30 FRAMING SYSTEM
When turning Maltese mechanism, gear 4 of the eccentric shaft is driven around to gear 3, but at the same time sleeve 9 turns, so screwed up the bush 6 (or unscrewed) the sleeve 9 and moves along axis 7, carrying gear 2 and 3.
So that the helical gears 2 and 3 will receive an additional rotation, as two gear meshes with the worm 1. Gear 4 also receives additional rotation in the opposite direction. As a result, the out of the phase operation of the Maltese mechanism and the shutter will not happen.
Pin 11 has a certain eccentricity. When you turn the pin axis 7 will also rotate and shift the sleeve 6 with gears 2 and 3 along the axis 7 regardless of the framing mechanism. Thus, turning the pin 11, you can eliminate the shutter compensation in the projector driving process.
K. Röwer, Die Technik für Filmvorführer, VEB Wilhelm Knapp Verlag, Halle (Saale), 1953
Ing. K. Enz, Filmprojektoren Filmprojektion, VEB Fachbuchverlag, Leipzig, 1965
R. Barbanel, I. K. Kachurin, Kinoproyektsionnaya tekhnika, Izdatelstvo Iskusstvo, Moscow, 1966
Century “C” Parts List, Century Projector Corp., New York, N.Y., 1940
Tokiwa TSR Instruction Manual, American Theatre Products, Inc., Hollywood, CA, 1982