The following post contains rare and specific engineering information!

The action principle of framing mechanism is in general as follows: If the framing knob has no rotation, the cam and shutter connected to each other remain the same; if the cam rotates, so it also rotates the shutter at the same angle and the same direction (positive or negative). We will discuss the specific kinds below.

(A) Shutter-compensated framing mechanism

This is a kind of framing mechanism that rotates the connected shutter when framing is done, it’s also called shutter-compensation type. The Songhua River 5501 projector made by HARBIN FILM MACHINERY PLANT (an official Chinese copy of the early Soviet KPT projector models made by LOMO, Leningrad) utilizes this kind.

Fig. A – Songhua River 5501 framing mechanism

1. Structure: The structure of Songhua River 5501 framing mechanism is shown in Figure A. The framing knob shaft (1), is equipped with two framing gears (2) and (7). Gear (2) meshed with the sealed oil bath intermittent movement gear plate (3), pinion gear (7) meshed with the rack of the rack-shaft (8). On the rack-shaft two forks are fixed (9), they connected to two end face of the shutter transmission (fork) gear (10). Gear (10) is a long-type helical gear, it combines with vertical shaft via the sliding key, so it can not only make circular rotation, but also move up and down along the axial. Gear (11) is the shutter gear.

When the framing shaft is twisted, gear (2) will rotate the intermittent movement gear plate (3) and the intermittent gear connected to the framing shaft as well. At the same time gear (7) moves the rack up and down, the (fork) gear (10) causes the longitudinal line up and down displacement, due to gear (10) is a spline gear (having a key slot inside), longitudinal line displacement will move the gear (11) with positive or negative rotation, thus that gives to the shutter additional rotation a number of angles, to achieve the framing purpose.

2. Framing principle analysis due to the carrying capacity on the vertical force of projector, it’s much bigger than the rotational friction of cam, so in the procession of the gear (5) it is displaced around the intermediate-gear (6), the vast majority of cases is to rotate the gear (5) across, almost doesn’t occur intermediate-gear additional rotation phenomenon. When framing, always fixed forks (9) drive the connected gears a number of angles, so it should do the same angle rotation to the connected shutter.

The analysis of framing has mainly two problems: The first one is the compensation amount, namely the relationship between the compensation angles with the additional rotation angle; secondly, the compensation direction, that is, the compensation angle direction related with additional rotation angle direction. For the compensation amount, compensation angle should be equal to the connected rotation, namely equal quantity compensation. If compensation allows greater degrees than the additional rotation angle, it is called “over-compensation”, in reverse, “under-compensation”. Over-compensation and under-compensation, in theory, leads to traveling ghost (image smear). So when designing, if you can’t fully realize the most of compensation, should you make also under-compensation or over-compensation small enough to omit any degree as far as possible. Thus viewers can’t see the traveling ghost. Now let’s make an analysis of the projector compensation:

(1) The amount of compensation: As seen in Figure A, set the gear’s numerical order as the gear’s tooth number matches Z numerical order. When shaft twisted with the θ angle, so the gear (2) rotates with θ angle. According to the Gear Transmission Basic Law, the intermittent movement gear plate (3) rotates angle α is:


In the process of intermittent movement rotation, the gear (5) of cam shaft needs to displacement around the intermediate gear (6), as mentioned earlier, it should rotate one by one tooth along the gear (6). So, the cam additionally rotates itself. Its direction is the same with the intermittent movement’s rotation direction, reverse to the handle’s twisted direction. Auto-rotation angle φ is related to the teeth number of gears (5) and (6) and α. Because when intermittent movement rotate α angle, gear (5) should be rolled across the same angle one by one tooth. So, auto-rotation φ of gear (5) and cam is:


or as substitute the formula for03 becomes:


At the same time the gear (2) makes the intermittent movement rotate, gear (7) rotates angle of θ too, so the gear (7) drives rack (8) longitudinal displacement. Set the displacement as L. L equals to the rotating tooth number of gear (7) and circular pitch of rack (8). Due to t = πm (m is module of the rack), so:


Rack (8)’s move will make the fork moves the gear (10) in longitudinal line displacement L too. Due to both gear (10) and gear (11) are helical gears, so longitudinal displacement of gear (10) will make the gear (11) rotate. According to normal helical gear meshing, their normal module mn should be equal. The sum of the helical angles of the two gears are equal to the angle of intersection characteristics of the two shafts. For this, transverse module of gear (11) should be equal to axial modulus of gear (10). So, when longitudinal line displacement amount L of gears (10) and (11) therefore for that rotation’s circular pitch arc length equal to L too. Set circular pitch diameter of gear (11) as d11, its parameter is πd11, then the shutter’s compensation rotating angle is:


According to the relationship between the geometric parameters of gear,


In the formula, mn is normal module of gear (11), γ is the spiral angle. Then substitute L = Z7θm/2 into that will get:


According to the equivalent compensation’s requirement, β should be equal to φ, therefore:


In the formula, right side of equitation is spline gear’s additional rotating angle, left is shutter’s framing angle. Relative parameter of above-mentioned gear of Songhua River 5501 is: Z2=39,Z3=71,Z5=23,Z6= 36; teeth number of gear (7) Z7=21, module m is 0.8; gear (11) Z11 = 23, normal module mn is 0.6, spiral angle γ = 45˚. Substitute into formula, left side is:


right side is


Obviously left side is greater than the right side because of the over-compensation. The over-compensation’s amount is approx. 0.001θ. When the max. framing amount is 19mm, intermittent movement’s related rotation angle α is 90˚, then the framing handle rotation angle is:


Therefore, the visible max. compensation error is: 0.001θ = 0.173°, so it can basically meets the requirements of framing compensation.

(2) Compensation direction: Compensation direction in principle should be: in framing, if cam occurs additional rotation in reverse direction, then the compensation direction on the shutter should be the same. Here the positive and reverse direction is based on the normal rotating direction, the same with normal direction of rotation is forward, and the reverse is inversion. If the cam does not occur additional rotation, the shutter will occur additional rotation, this time the compensation direction should be connected with rotating in opposite directions. Normal rotating direction of Songhua River 5501 cam and shutter, both are anti-clockwise direction (front). During positive framing, that is, according to the film in normal direction time of framing, the framing handle needs to twist θ angle anti-clockwise (front, same below). At this time the intermittent movement rotates clockwise at α angle, as well as the cam rotates clockwise additionally φ too, due to the additional rotation direction is reverse to the normal rotation direction. So it will make dowel pin delayed with φ angle into the starwheel (Maltese cross) slots. At the same time, gear (7) will anti-clockwise twisted θ angle, make rack (8), gear (10) linear displacement L upwards, thus make shutter gear (11) clockwise rotate β angle, this compensation direction just inverse with the shutter’s normal rotation direction. So the shutter is delayed cutting the light with β angle. Due to βφ, this will keep the relationship in synchronization.

(B) Self-compensated (aka cam-compensated) framing mechanism

Self-compensation is a kind of framing mechanism that shutter doesn’t have contact with the framing device, framing is compensating the cam itself. Figure B shows the framing device structure diagram of the Dongfeng FG-35 projector made by TIANJIN CINE-PROJECTOR FACTORY. The figure shows, only one framing gear (1) on the framing shaft, it meshed with the sector gear (2) installed on the starwheel sector holder. On the sector gear, a fixed gear (3) mesh with rack (4) on the fork holder. There are two forks on the fork holder, located on the intermittent mechanism’s both ends of transmission gear (5). This gear (5) is a long-type helical spline gear too, it can move in a certain range longitudinal line displacement along with the shaft, intermediate idle gear (7) mesh with it then. Gear (8) on the cam shaft mesh with intermediate idle gear (7). Dongfeng’s intermittent movement and sector gear has a rigid connection via the connecting shaft.

Figure B – Dongfeng FG-35 framing mechanism

When framing handle twisted, framing handle gear (1) moves sector gear (2) and the intermittent movement rotate angle of α, cam displaced around the starwheel shaft for framing. Now gear (8) displaced around the intermediate idle gear to roll tooth by tooth, then cam is bound to happen φ angle’s additional rotation. However, at the same time, the gear (3) drives the rack holder (4) in longitudinal line displacement. Certainly, this moves the gear (5) in longitudinal line displacement due to the gear (5) is also a helical gear. Therefore, it should make intermediate idle gear (7) rotate β angle, thus this drives the gear (8) and the cam rotates. The rotational direction of the gear (7) is just reverse to the additional rotation direction of cam. The two cancel each other out, therefore, cam do not occur additional rotation. In fact, just the intermediate idle gear rotates α angle. Due to the shutter and the framing device having no relationship, no traveling ghost made by framing, and this does not occur additional rotation. Both keep the synchronous relationship.

Thus, the compensation structure’s key is: when the gear (8) on the cam shaft displaced around the intermediate idle gear (7), the compensation mechanism equals intermediate idle gear rotation α angle. This will make mesh point of gear (7) and gear (8) remain the same. As a result, the driving cam (connected to the starwheel) do not have additional rotation, thus called the self-compensated type mechanism.

Calculation method of this machine’s framing amount is similar to the Songhua River 5501, the only distinguishment is the framing handle has one framing gear, as a result, rotating angle of gear (2) and gear (3) is the same. Both equal to intermittent movement’s rotation angle α. At the same time, gear (5) displaced longitudinal line L, the intermediate idle gear (7) is moved, thus a one more relationship of transmission transmitted from (7) to (8).

When the handle is twisted, gear (1) make the sector gear (2) rotate clockwise α angle, then the additional rotation direction of cam should be the same with the rotation direction of intermittent movement. Also, the additional rotation angle of the clockwise direction is:


At the same time, gear (3) rolled α angle clockwise, makes the rack holder (4) and gear (5) move upwards, its distance is:


Gear (5) displaced longitudinal line upwards, makes the gear (7) rotate clockwise. As mentioned above, rotation arc length of gear (7) equal to L, its rotation angle θ is:


Gear (7) rotate clockwise θ angle, makes the gear (8) should rotate in invert direction. This is just reverse to the cam’s rotation direction. Its rotation angle β is:


Therefore, if the cam’s additional rotation is totally cancelled, now substitute the known gear parameter given for the proof: Z7=33,mn7=0,8, γ7=42°50′; Z8=13, Z3=45; m4=0.8。




5501 vs. FG-35

Original text in Chinese: Jiang Daliang, Zhuzu Rong, Zhang Zili, et al., Film Projectors, The People’s Liberation Army Publishing House, Beijing, 1988 – 3rd book/3-volume set

Figures: (A) Barbanel, Kachurin, Korolev, Solomonik, Tsivkin, Cine Projection Technology, Iskusstvo Publishers, Moscow, 1966; (B/vs.) Fu Su Yong (Editor), Stationary 35mm Film Projection Equipment – Film Projectors, Science Press, Beijing, 1981

Specialist supervision and additions to the text: Erkan Umut, Istanbul

Chinese translation support: Mr. Peiqun Lin, Chaozhou, Guangdong Province

English-proofing support: Mr. Akif Ergüleç, Istanbul/Los Angeles, CA