Blade type shutter

A blade type shutter adapted for use in photographic cameras and so forth comprising: a front curtain assembly having at least one front blade arranged to travel in linked relationship with two front curtain arms; and a rear curtain assembly having at least one rear blade arranged to travel in linked relationship with two rear curtain arms. This shutter further includes a spring capable of generating a force in the direction in which the spring is opened. This spring presses one of the front curtain arms in the direction in which it travels during an aperture opening operation while applying a load to one of the rear curtain arms in the direction reverse to that in which it travels in a shutter closing operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to blade type shutters for 
photographic cameras, and more particularly to a blade type shutter with a 
mechanism capable of stabilizing the width of an exposure slit formed 
between a front curtain and a rear curtain while they are travelling 
across an exposure aperture. 
2. Description of the Prior Art 
In general, blade type shutters have heretofore been provided with force 
generating means such as springs for generating a force that will act to 
open and close the shutter, shutter driving members to which the force 
generating means are directly secured, pins projecting from the shutter 
driving members and arms having holes or slots. The pins are fitted in the 
holes or slots so that the arms may be interlockingly rotated. Similarly, 
in the conventional arrangement, each blade is pivotably connected to the 
corresponding arms through the engagement between the pins and the holes 
or slots. 
However, the above-described pin-slot (or pin-hole) connection needs a 
small gap to be left therebetween, and this gap is prone to result in 
undesirable looseness during the shutter operation at the points where the 
pins and slots or holes are connected. This looseness further results in 
unwanted variations in the positional relationship between the shutter 
blades. Therefore, such a prior-art blade type shutter involves 
difficulties in that each time the shutter is cocked, the stationary 
positions of the shutter blades are varied, thus making it difficult to 
fix the starting lines of the blades during the shutter operation. In 
particular, this problem leads to fluctuations in shutter speeds due to 
changes in the manner in which the camera body is held. Moreover, as the 
number of shutter operations is increased, the pins and the holes or slots 
gradually become worn, and this accentuates the tendency for fluctuation 
in shutter speeds. 
U.S. Pat. No. 4,231,650 proposes an improved mechanism for solving the 
above-described problems. According to this prior proposal, the shutter 
blade mechanism includes first biasing spring means for urging a front 
curtain assembly with respect to a base plate and second biasing spring 
means for urging a rear curtain assembly with respect to the base plate, 
and each arm is biased in one direction only by the associated springs so 
that each pin may be consistently kept in contact with the corresponding 
hole or slot. Thus the influence exerted by the looseness between the pins 
and the holes or slots is reduced. 
However, since this prior arrangement needs two biasing springs for each of 
the front and rear curtain assemblies, variation between the forces 
generated by the two springs of each pair has an undesirable influence on 
the shutter travelling characteristics. In particular, during high-speed 
shutter operation wherein the exposure slit may be minimized in width, 
there is a risk of causing the front and rear curtains to travel in a 
closed state, that is, without forming any slit. 
SUMMARY OF THE INVENTION 
Accordingly, it is a first object of the present invention to provide an 
improved blade type shutter wherein the looseness between the pins and the 
holes or slots is prevented from adversely affecting the shutter 
operation, thereby improving the accuracy with which the shutter blades 
are positioned. 
It is a second object of the present invention to provide an improved blade 
type shutter capable of positively ensuring the width of an exposure slit 
even for very short exposure times. 
To these ends, the present invention provides a blade type shutter which 
comprises front and rear curtain assemblies and a pressing means arranged 
to apply a force to the assemblies. The front curtain assembly has at 
least one front blade which is arranged to travel in linked relationship 
with a front curtain driving arm and a front curtain auxiliary arm. The 
rear curtain assembly has at least one rear blade which is arranged to 
travel in linked relationship with a rear curtain driving arm and a rear 
curtain auxiliary arm. The pressing means is arranged to apply a force to 
the front and rear curtain assemblies with a pressing force against the 
front curtain assembly in the direction in which it moves in a shutter 
opening operation and a pressing force against the rear curtain assembly 
in a direction reverse to that in which it travels in a shutter closing 
operation. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following description of the 
preferred embodiment thereof, taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIGS. 1A, 1B and 2, the illustrated embodiment of this 
invention comprises the following constituents: a base plate 1; a cover 
plate 2 parallel to and spaced from the base plate 1; a shaft O 
protrudingly mounted on the base plate 1; a front curtain driving arm 3 
supported by the shaft O for rotation about the axis thereof; a shaft P 
protrudingly mounted on the base plate 1; a front curtain auxiliary arm 4 
supported by the shaft P for rotation about the axis thereof; front blades 
5 to 8 pivotably supported on the front curtain driving arm 3 and the 
front curtain auxiliary arm 4 respectively by the engagement between pins 
9a, 9b; 10a, 10b; 11a, 11b; and 12a, 12b and corresponding holes in each 
of the arms 3 and 4. The blades 5 to 8 are linked by the arms 3 and 4, 
which are parallel with the base plate 1 so that the blades overlap each 
other in parallel, thereby consistently enabling the front blades 5 to 8 
to travel parallel with one another and vertically across an exposure 
aperture 30 formed in the base plate 1. 
The illustrated embodiment of this invention further comprises the 
following constituents: a shaft Q protrudingly mounted on the base plate 
1; a rear curtain driving arm 13 supported by the shaft Q for rotation 
about the axis thereof; a shaft R protrudingly mounted on the base plate 
1; a rear curtain auxiliary arm 14 supported by the shaft R for rotation 
about the axis thereof; rear blades 15 to 18 pivotably supported on the 
rear curtain driving arm 13 and the rear curtain auxiliary arm 14 
respectively by the engagement between pins 19a, 19b; 20a, 20b; 21a, 21b; 
and 22a, 22b and the corresponding holes in each of the arms 13 and 14. 
The rear blades 15 to 18 are linked by the arms 3 and 4, which are 
parallel with the base plate 1 so that these blades overlap each other in 
parallel, thereby consistently enabling the rear blades 15 to 18 to travel 
parallel with one another and vertically across the exposure slit 30. 
A front curtain driving lever 23 is pin-slot-coupled with the front curtain 
driving arm 3, and a rear curtain driving lever 24 is pin-slot-coupled 
with the rear curtain driving arm 4. 
As shown in detail in FIGS. 3 and 4, a coiled spring 25 for preventing 
looseness has a central wound portion fitted onto the shaft R, and one end 
of the spring 25 is engaged with the peripheral edge of the front curtain 
auxiliary arm 4 while the other end is fitted in a hole 14a of the rear 
curtain auxiliary arm 14. The spring 25 functions to apply a remarkably 
small force to the arms 4 and 14 in the direction in which they move away 
from each other. A front curtain driving spring 26 pulls the front curtain 
driving arm 3 in the direction in which the arm 3 travels, namely, 
counterclockwise as viewed in the Figures, and a rear curtain driving 
spring 27 pulls the rear curtain driving arm 13 in the direction in which 
the arm 13 travels, namely, counterclockwise as viewed in the Figures. As 
shown by a dotted line in FIGS. 1A and 2, the base plate 1 includes the 
exposure aperture 30. 
Specifically, the shutter having the above-described construction comprises 
a front curtain assembly and a rear curtain assembly; the former assembly 
including the front curtain driving arm 3, the front curtain auxiliary arm 
4, the front blades 5 to 8 and their respective pin connections to the 
arms 3 and 4, while the latter assembly includes the rear curtain driving 
arm 13, the rear curtain auxiliary arm 14 and the rear blades 15 to 18 and 
their respective pin connections to the arms 13 and 14. 
A shutter operating mechanism does not constitute the subject matter of the 
present application. Hence, for the sake of simplicity, such mechanism is 
not specifically illustrated in the accompanying drawings. Nevertheless, 
for reference, the operation of the shutter of this invention will be 
briefly described below. 
As will readily be understood by those skilled in the art, after a camera 
shutter release has been actuated, various preparatory operations are 
carried out before exposure is made, that is, a deviating mirror is swung 
upwardly and the aperture of a lens is stopped down. Upon completion of 
these preparatory operations, the front curtain assembly first travels 
from the cocked position shown in FIG. 2 downwardly as viewed in the same 
Figure. In consequence, the front blades 5 to 8 are removed from the 
aperture 30 to permit exposure of film positioned behind the aperture. 
After a selected exposure time has elapsed, the rear curtain assembly 
travels downwardly as viewed in FIG. 2 from the cocked position shown in 
FIG. 2, and the aperture 30 is thereby covered by the rear blades 15 to 18 
as shown in FIG. 1A. The exposure of one shot is completed through the 
above-described shutter operation. 
When the front and rear curtain assemblies complete travelling and reach 
the uncocked position shown in FIG. 1A, these assemblies are cocked in 
association with the ensuing camera sequence, such as film winding. The 
assemblies travel upwardly as viewed in FIG. 1A and are restored in the 
position shown in FIG. 2 where the shutter is completely cocked. 
The following description particularly concerns the operation of the slit 
forming front blade 5 of the front curtain assembly during the 
above-described operational sequence. 
As shown, the front blade 5 has one end movably coupled with the arms 3 and 
4 through the pins 9a and 9b fitted into the holes of these arms, and the 
arms 3 and 4 are pivotably fitted onto the shafts O and P, respectively. 
Each of the fitted portions has a slight tolerance for smooth operation. 
In some cases, this slight tolerance causes the horizontal positioning of 
the front blade 5 to be inaccurate, whereby the blade 5 is inclined; and 
this results in variations in shutter speeds on both sides of the aperture 
30. The illustrated embodiment succeeds in eliminating the above-described 
drawback by adding the coiled spring 25 for the purpose of preventing any 
unwanted looseness from being produced by the tolerance. Specifically, the 
front curtain auxiliary arm 4 is pressed slightly counterclockwise by one 
end of the coiled spring 25, thereby consistently biasing the 
above-described shafts O, P, and the pins 9a, 9b forming part of the 
fitted portions. Since the shafts O, P and the pins 9a, 9b are biased in 
constant directions, irrespective of the manner in which the camera body 
is held, undesirable looseness can be absorbed by the coiled spring 25. In 
such a biased state, if the position of the front blade 5 is slightly 
adjusted or the blade 5 is designed to be accurately held in position, it 
is possible to solve problems such as variation in shutter speeds. On the 
other hand, the rear curtain auxiliary arm 14 of the rear curtain assembly 
is pressed slightly clockwise by the coiled spring 25. Therefore, the 
above-described problems of prior shutters can be successfully solved in 
the same manner. 
As will be evident from the above description and the drawings, the sole 
coiled spring 25 for eliminating looseness presses the front and rear 
curtain assemblies in the direction in which they are caused to move away 
from each other. Specifically, for the purpose of effecting exposure, the 
spring 25 presses the front curtain assembly lightly in a downward 
direction as viewed in the drawings. On the other hand, the spring 25 also 
applies a slight load against the rear curtain assembly in an upward 
direction, as viewed in the drawings, for the purpose of determining the 
exposure time. In particular, in cases where the slit width formed between 
the front and rear curtains is made especially narrow for short exposure 
times, the coiled spring 25 applies a force in such a way that the slit 
width is widened, thereby eliminating the above-described problems of the 
prior proposal, such as the curtains travelling in a closed state. 
In the illustrated embodiment, since the sole coiled spring 25 for 
eliminating looseness presses the front and rear curtain assemblies at the 
same time, there is no material change in the angle formed between both 
ends of the spring 25 during a high-speed shutter operation in which the 
accuracy of shutter speeds greatly influences correct exposure, that is, 
the slit width is especially narrow. Accordingly, the coiled spring 25 
applies a substantially constant force to the front and rear curtain 
assemblies during the period from the start of travelling to the end 
thereof, whereby it is possible to essentially eliminate the adverse 
influence exerted on shutter running performance in the prior art by 
looseness at each of the fitted portions of the shutter. 
The blade type shutter according to the present invention, which features 
the arrangement and operation mentioned above, offers the following 
various advantages. 
As described above, since the shutter of this invention is equipped with a 
single spring for pressing the front curtain assembly and causing it to 
travel while acting as a load upon the travel of the rear curtain 
assembly, it is possible to eliminate looseness at each of the fitted 
portions. Therefore, improvements in the accuracy of positioning the 
shutter blades can be efficiently achieved with low production costs, and 
correct exposure can be attained with high precision. Moreover, the slit 
width can be positively obtained even during high-speed shutter operation. 
In particular, when using high-speed shutters in which the slit width 
greatly affects correct exposure, the spring acts to apply a substantially 
constant force to the front and rear curtain assemblies. Hence, the 
present invention successfully provides an exposure-slit stabilizing 
mechanism capable of eliminating adverse influence on shutter travelling 
performance. 
While the above provides a full and complete disclosure of the invention, 
various modifications, alternative constructions and equivalents may be 
employed without departing from the true spirit and scope of the 
invention. Therefore, the above description and illustrations should not 
be construed as limiting the scope of the invention, which is defined 
solely by the appended claims