Patent GB-532852-A (GB)
Title: Improvements in or relating to focal plane shutters for photographic cameras
IPC: G03B9/34
Filed: 1939-09-22
Abstract: 532,852. Photographic shutters. KODAK. Ltd. Sept. 22, 1939, No. 26423. Convention date, Sept. 24, 1938. [Class 98 (i)] In a focal-plane shutter of the type comprising a pair of curtains each supported at opposite ends by a spring-actuated wind-up roller and unwinding roller, the unwinding rollers are mounted on a common shaft and are adapted to be coupled together during an exposure to rotate with it, and in order to obtain uniform exposure as the slit moves across the exposure aperture the unwinding rollers are of different diameters such that one curtain is unwound at a greater speed than the other and the width of the slot increases proportionately to the acceleration of the blind, or decreases proportionately to the deceleration, as the case may be. In the shutter shown, a front curtain FC is wound in set position on a roller 36 rotatable on a shaft 34, and the tapes RR attached to a rear curtain RC are wound on spools 33 attached to the shaft. The other ends of the blinds are attached to spring rollers in the usual manner. The shutter is set and the film wound by mechanism similar to that described in Specification 520,954, the upper end of the film sprocket shaft (not shown) being in driving engagement with a stub shaft 30, Fig. 2, which carries a collar 37 engaging by a shoulder 38, Fig. 6, a spring-pressed pawl 40 pivoted on a gear 42 loose on the shaft 30. The gear is thus rotated, and through gears 45, 46 rotates the shaft 34 to wind the shutter. The rear curtain is first drawn across the exposure aperture by the rotation of the spools 33, and a lost-motion drive then rotates the spool 36 to wind the front curtain. This drive comprises a loose washer 52, Figs. 2 and 5, having an upstanding lug 51 which is engaged by a pin 50 projecting from a collar 49 secured to the shaft, and also a lug 53 which engages a pin 54 projecting from the end of the roller 36. A ratchet and pawl prevent anti-clockwise rotation of the stub shaft 30, and so maintain the blind wound. Depression of the release rocks a bell-crank lever 56, Fig. 6, which through a link 58 rocks a lever or release member 59 about its pivot 60 so that a lug 63 on it engages the end of the pawl 40 and frees it from the shoulder 38 of the collar 37, so allowing the gear 42 to rotate and the rear curtain to move. Until the rear curtain has moved sufficiently to establish a slot of width pre-determined by the setting of a speed indicator, movement of the front curtain is prevented by a pawl 67, Fig. 9, engaging a shoulder 36a on the roller 36 ; this pawl also engages a pawl 64 on the roller 36 and lifts it from engagement with a ratchet 66 on the spindle 34, so that the rear curtain may move independently of the front curtain. When the front curtain has moved to establish the desired slit, the pawl 67 is moved to the position shown in Fig. 9 in which the roller 36 is released and locked to the spindle 34 by the pawl 64, so that both blinds now move together. For this purpose a cam 75, Figs. 6 and 9, secured to the gear 42 is engaged by a pin 79 on a bell-crank lever 77 pivoted at 76, so that as the rear curtain moves the lever turns clockwise, carrying with it a trip lever 80 which is pivoted to it at 81. When upstanding lugs 87 on the right-hand end of the trip lever strikes the end of a timing lever 82, so arresting movement of this end, the left-hand end continues to move until it engages the upturned end 73 of a lever 70 carried by the spindle 68 of the pawl 67, so moving the pawl to the position shown in Fig. 9, when both curtains move together. The lever 82 is moved about its pivot by operation of a speed-setting disc (not shown) so varying the point at which the end of the trip lever 80 is arrested by it, and hence the initial width of the slit between the curtains. In order that this width may increase during passage of the blinds across the exposure opening to compensate for their increasing linear speed, the diameter D33 of the spools 33 is greater than the diameter D36 of the roller 36, so that the rear curtain is reeled off at a greater speed than the front curtain. To prevent the roller 36 from overrunning the spools 33 during exposure (the pawl and ratchet 64, 67 preventing only the reverse relative movement), a pawl 98, Figs. 2 and 4, secured to a stud 97 passing through the lower spool 33 engages slots 96 in the lower flange 95 of the roller. When the slit has passed beyond the exposure aperture the pawl is withdrawn, to allow the front curtain to complete its travel, by a hook 99 secured to the stud 97, which overrides a semi-circular collar 105 and so rocks the stud 97. The collar 105, during the first part of the movement of the curtains rotates under the action of a spring 104, with the hook 99, and only when it is arrested by the engagement of a lug 102 on it with a pin 103, which occurs when the slit has passed the exposure aperture, does the hook override it to release the pawl 98. The spacing of the slots 96 and the teeth of the ratchet 66 corresponds to the establishment of slits of the necessary widths for definite exposures. Time and slow automatic exposures may be provided for by the means described in Specification 529,297.