Photographic apparatus gear train having a unique set of gears

A pair of meshing gears, having unique tooth profiles, are provided in a separable gear train which is usable in a self-developing camera to couple a motor in one housing section of the camera to at least one of a pair of pressure-applying rollers mounted on a second camera housing section that is pivotally coupled to the first section for movement between positions blocking and unblocking access to a film container receiving section in the first housing section. The pair of gears have a speed ratio of other than 1:1 and equal tip spacing to facilitate bringing the pair of gears into operative mesh when the second housing section is moved to the operative blocking position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to photography and, more particularly, to 
photographic apparatus having a gear train for transmitting power from a 
motor to one or more motor-driven components of the apparatus. 
2. Description of the Prior Art 
Compact cameras having motor-driven components are well known in the 
photographic art. For example, highly automated, self-developing cameras, 
such as the SX-70 Land Camera, marketed by Polaroid Corporation, 
Cambridge, Mass., include a motor-driven processing roller (one of a pair 
of pressure-applying rollers), film advance device, camera sequencing or 
timing wheel, and a device for recocking a reflex operator. 
To minimize the size of the camera, the motors are generally of the small, 
high speed, D.C. type with typical loaded operating speeds of 8,000 to 
12,000 rpm when energized by a six-volt battery. Motor power is 
transferred to each of the motor-driven components by a gear train which 
serves to (1) provide appropriate speed reductions for driving the various 
components, and (2) physically couple the motor to components that are 
mounted at various locations within the camera housing. 
The SX-70 camera includes a base housing section having an open ended 
chamber therein for receiving a film container holding a stack of 
self-developing film units. Couple to the leading open end of the base 
section is a loading door and pressure roller mounting section which is 
adapted to pivot between an operative position; wherein the loading door 
section is aligned with and extends forwardly of the open end of the 
receiving chamber to locate the rollers in position to receive a film unit 
advanced from the container subsequent to exposure; and an inoperative or 
open position wherein the loading door section is pivoted downwardly to 
unblock the open end of the receiving chamber for loading and/or 
withdrawing the film container. 
The small high speed motor is located at the trailing end of the base 
section, aft of the receiving chamber. Power is transmitted to the various 
motor driven components by an elongated speed reduction gear train that is 
disposed along the side of the chamber and extends from the motor in the 
rear to one of the pressure-applying rollers mounted on the forwardly 
extending loading door section. 
Since the gear train spans the interface between the leading end of the 
base section and the trailing end of the loading door, two meshing gears 
at the interface must be disengaged to permit the door to be opened and 
then must be easily and reliably brought back into mesh, without binding, 
or being brought into nonmeshing abutment, when the loading door section 
is closed. This type of structure may be thought of as a "hinged gear 
train" having one gear mounted on the base section and its mating gear 
mounted on the loading door section so that these two gears mesh and 
bridge the interface when the loading door section is closed. 
When the loading door is closed, these two gears are in mesh. Upon opening 
the door and disengaging the two gears, the rotational position of the 
gear at the leading end of the base section is fairly stable because it is 
coupled all the way back to the de-energized motor which is rendered 
inoperable by a door switch when the loading door section is opened. On 
the other hand, the mating gear on the loading door section is coupled to 
the top pressure roller (in a low friction bearing) and, because of the 
low gear loading, the mating gear may be thought to be in a "free wheel" 
condition. The angular or rotational disposition of the mating gear will 
most likely change (from its position when in mesh) by (1) the act of 
disengagement, (2) the gear being inadvertently rotated by the user during 
the process of loading a film container or (3) inspecting and/or 
performing maintenance on the rollers. 
To ensure that these two gears will mesh properly when the loading door is 
closed, the two gears are identical. That is they have the equal 
diameters, a speed ratio of 1:1, equal number of gear teeth about the 
periphery or pitch circles, identical gear tooth profiles and equal tip 
spacing between adjacent teeth. To minimize possible tip-to-tip abutment 
when the gears are brought back into mesh, the standard gear blunt tip 
tooth profile may be modified so that the tips are pointed. This, coupled 
with the fact that the gear on the loading door is pivoted into engagement 
with the gear on base section and tends to have a free wheeling rolling 
action when the two gears make initial contact, greatly facilitates the 
ease and reliability of bringing these two gears into mesh without causing 
damage to the gears or inconvenience to the user. 
For representative examples of the "hinged gear train" concept outlined 
above, reference may be had to U.S. Pat. Nos. 3,709,122; 3,714,879; 
3,760,701; and 3,906,527 assigned to the same assignee as the present 
application. Also see U.S. Pat. No. 3,561,340 which discloses a camera 
having a motor-driven roller assembly (which may be removed for 
maintenance) that is coupled to a gear train by a pair of 1:1 gears at the 
point of separation. 
The use of 1:1 gears at the point of separation essentially adds a pair of 
gears to the gear train to facilitate bringing the two sections of the 
train back into mesh. On the negative side, however, is the question of 
gear train efficiency or power loss. As the number of gears in a train 
increases, the power transfer efficiency of the train decreases because 
of, among other considerations, the friction losses at the meshing teeth 
and also at the shafts on which the gears are mounted for rotation. 
It has been found, that when the 1:1 gears (at the point of separation) 
have been replaced with a pair of speed reduction or speed increasing 
gears, in the interest of gear-train efficiency, the problem of getting 
the two nonidentical gears to remesh properly becomes a serious one. 
For example, see U.S. Pat. No. 3,889,280 which discloses a large gear 
mounted at the leading end of the camera base section which is adapted to 
mesh with a smaller gear (pinion) coupled to the top roller to provide a 
speed increase (i.e. the roller rotates at a faster speed than the larger 
driving gear). 
Also, notice should be taken of the following copending applications which 
feature a pair of speed reduction gears at the point of separation. Ser. 
No. 554,777, filed on Mar. 3, 1975, by B. K. Johnson et al.; Ser. No. 
554,778 (now U.S. Pat. No. 3,967,304), filed on Mar. 3, 1975, by B. K. 
Johnson et al.; and Ser. No. 628,486, filed on Nov. 4, 1975, by R. M. 
Augustin et al.; all of said copending applications being assigned to the 
same assignee as the present application. 
When the pair of gears at the point of separation have different numbers of 
teeth (and correspondingly different diameters to produce speed reduction 
or increase), the tip spacing or chordal distance between adjacent teeth 
at the tips will be different (by definitions which will be developed 
later in this disclosure). If standard gear profile teeth (blunt tips) are 
used, the nonmeshing problem is even further compounded because of the 
increased probability of tooth tip abutment. 
When attempts have been made to modify standard gears (for example, a 
12-tooth pinion and 36-tooth gear) by increasing the tooth length to form 
points at the tip, it has been found that the tip spacing on the pinion 
(smaller gear) increases to a larger extent than the tip spacing on the 
larger gear. In some cases, this may cause an interlock condition when the 
two gears are attempted to be pivoted into mesh. That is, the opposite 
outside edges of two adjacent teeth on the large gear may fit into the 
space subtended by the two inside edges of a pair of adjacent teeth on the 
pinion. 
Also, when one of the gears in a set is a small diameter pinion, it is 
common practice in gear technology to extend the addendum or length of the 
pinion teeth to prevent undercut (reduction of the thickness of the tooth 
at its base) and to decrease the length of the teeth on the mating gear. 
Again, this promotes tip spacing mismatch and has a tendency to cause the 
interlock condition. 
SUMMARY OF THE INVENTION 
The present invention provides a pair of gears which have uniquely 
configured meshing tooth profiles that allow the gears to be used at the 
point of separation of a separable gear train in a photographic apparatus 
(preferably a self-developing camera); provide a speed reduction (or a 
speed increase if the application should require it) at the point of 
separation; and yet, unlike standard gears or gears modified according to 
prior art teachings and standard practices in gear technology, may be 
easily and reliably pivoted into mesh without causing jamming, binding, 
tooth interlock, or nonmeshing tooth-tip abutment. 
In the illustrated embodiment, the self-developing camera includes a 
"hinged-type" gear train having a 12-tooth pinion gear and a 36-tooth 
mating gear at the point of separation. The pinion is coupled back to the 
motor and is the driver and the 36-tooth gear is coupled to one of the 
pressure-applying rollers and serves as the follower. 
It will be shown later in this disclosure that it is standard practice to 
modify the tooth profile of such a pinion gear by extending the length 
(and corresponding tooth thickness) of the pinion teeth and shortening the 
mating teeth on the gear to prevent undercutting of the pinion teeth and 
increase their strength. Also, it will be shown that when this standard 
modification (known as long-short addendum gears) is made to such gears, 
the effect on the tip spacing is detrimental to pivoting the gears into 
mesh. 
The present invention provides a pinion and gear which have tooth profiles 
that are diametrically opposed to the teachings of the prior art and 
standard gear technology practices. That is, the teeth of the gear have 
been lengthened and the teeth of the pinion have been shortened to provide 
a pair of gears with a speed ratio of other than 1:1 and yet having equal 
tip spacing to facilitate bringing the gears into mesh in the environment 
of a hinged gear train. While such tooth profile modification is highly 
unusual, the unique gears have been configured to maintain conjugate 
action (that is, the active tooth profiles are based on involute curves) 
and have a standard pressure angle of 20 degrees. In other words, smooth 
running and gear efficiency were not substantially sacrificed to 
facilitate bringing the gears into mesh by providing equal tip spacing. 
Therefore it is an object of the present invention to provide a 
photographic apparatus having a gear train therein that is characterized 
by the fact that a pair of meshing gears therein will from time to time 
have to be disengaged and brought back into mesh and that said pair of 
gears are uniquely configured to have a speed ratio other than 1:1 and 
have equal tip spacing between adjacent teeth to facilitate bringing the 
pair of gears into mesh when so required. 
It is yet another object to provide such a pair of gears wherein the tooth 
profile of the pinion has a shorter addendum than the addendum of the 
mating gear tooth profile. 
Other objects of the invention will in part be obvious and will in part 
appear hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 3 of the drawings show, respectively, a self-developing camera 
10 which is adapted to receive a film container 12 holding a plurality of 
self-developing film units 14 arranged in stacked relation therein. 
Since the present invention is directed to a pair of uniquely configured 
gears forming part of a "hinged" or separable gear train in camera 10, the 
camera 10, film container 12, and self-developing film units 14 will be 
described only in sufficient detail to provide the necessary background 
for understanding the present invention. 
The self-developing film unit 14 is of the integral "nonpeel-apart" type 
and is best shown in FIGS. 7 and 8 of the drawings. Basically it is a 
multilayer structure or laminate which is diagrammatically illustrated in 
FIG. 8 as including a bottom sheet-like element 16 and a superposed top 
sheet-like element 18. Attached to the leading end of element 18 is a 
rupturable container 20 holding a supply of fluid processing composition 
22 therein. 
In actuality, the multilayer structure includes an opaque bottom support 
sheet, a top transparent support sheet, and a plurality of layers 
sandwiched therebetween and including one or more photosensitive layers 
and one or more image-receiving layers. The laminate is bound along its 
lateral edges by a binding element 24 which also defines the bounds of a 
generally square or rectangular image-forming area 26 on the top 
transparent support sheet through which actinic radiation is transmitted 
to expose the photosensitive layer or layers. Subsequent to exposure, the 
film unit is progressively advanced between a pair of pressure-applying 
members or rollers which discharge the fluid 22 between a predetermined 
pair of adjacent layers within the multilayer structure. In FIG. 8 of the 
drawings, the fluid 22 is adapted to be spread between elements 16 and 18 
which are intended to show the interface between the predetermined pair of 
adjacent layers. In reality, element 18 includes the top transparent 
support sheet in certain chemical layers and element 16 includes the 
bottom support sheet and certain other chemical layers. 
For a more detailed description of film unit 14, reference may be had to 
U.S. Pat. No. 3,415,644 issued to E. H. Land on Dec. 10, 1968. 
Referring to FIG. 3, the film container 12 is generally a box-like 
structure, preferably of molded plastic construction, and includes a 
forward wall 28, a rear wall 30, and a peripheral section joining walls 28 
and 30 and including a pair of lateral side walls 32, a trailing end wall 
34 and a leading end wall 36. Forward wall 28 has a square or rectangular 
exposure aperture 38 therein which is coextensive with the image-forming 
area 26 of the film unit 14. 
The film units 14 are arranged in stack relation within film container 12 
such that the forwardmost film unit bears against the interior surface of 
forward wall 28 and is located in position for exposure through exposure 
aperture 38. Subsequent to exposure, the forwardmost film unit 14 is 
adapted to be advanced from film container 12 through an elongated film 
withdrawal slot 40 in forward wall 36. As will be described later, camera 
10 includes a film advancing mechanism which is adapted to extend through 
an opening 42 near a trailing end corner of film container 12 for engaging 
and advancing the exposed forwardmost film unit through film withdrawal 
slot 40. 
The stack of film units 14 is spring biased toward the interior surface of 
forward wall 28 by a spring platen (not shown) and, preferably, a flat 
battery 44 for powering the electrical equipment of camera 10 is provided 
within film container 12 in overlying relationship to rear wall 30 which 
has openings (not shown) therein providing access to the battery 
terminals. 
Camera 10 is a compact, nonfolding, modular, battery operated, 
self-developing camera. As best shown in FIGS. 1 and 2, it includes a 
housing which is formed by three molded plastic housing sections 46, 48, 
and 50. 
The major housing section 46 is a hollow open-ended structure which is 
adapted to receive a major modular unit 52 mounting most of the operative 
components and subassemblies of the camera on a mounting frame 54 thereof. 
The mounting frame 54 is a hollow, open-bottomed, cone-like structure of 
molded plastic construction which forms the camera exposure chamber 56 
(see FIG. 4) and mounts; an optical system including an objective lens 58, 
a mirror 60 (on the interior surface of an inclined rear cone wall 62) and 
a viewfinder assembly 64; a camera drive, sequencing, film advance and 
film counting assembly 66 which includes an electrical motor 68, part of a 
gear train 70, a sequencing gear 72, a film advance device 74 and a film 
counter 76; and pressure-applying assembly 78 to which housing section 50 
is attached and includes a mounting bracket 80 and a pair of juxtaposed 
pressure-applying members of rollers 81 and 82 and a roller drive gear 84 
on the end of roller 81. Gears 84 form part of the gear train 70, as will 
be described hereinafter, and is coupled to motor 68 through intermediate 
gears in the train. 
When the camera 10 is assembled, the cone-like frame 54 cooperates with the 
bottom wall 86 of housing section 46 to define an open-ended chamber 88 in 
the base of housing section 46 for receiving film container 12 in the 
position shown in FIG. 4 with the film container exposure aperture 38 
facing the open bottom of cone-like frame 54. Roller mounting bracket 80 
is pivotally mounted on frame 54 so that the roller assembly 78 and 
housing section 50 pivot downwardly to provide access to the open end of 
receiving chamber 88 for inserting and withdrawing film container 12. A 
pair of battery contacts 90, mounted on bottom wall 86, connect battery 44 
to the camera's electrical system. 
In operation, the user views and frames the scene to be photographed 
through the direct viewing viewfinder assembly 64 (enclosed by a 
viewfinder housing 92 integrally molded with housing section 46) and 
focuses lens 58. A cycle of operation is initiated by depressing a camera 
start button 94 on housing section 48 which causes motor 68 to be 
energized thereby driving gear train 70 including roller gear 84 and 
sequencing gear 72. Sequencing gear 72 operates a mechanism (not shown) 
which unlatches a normally closed electronic shutter and latches power 
onto an electronic logic and power circuit (neither of which is shown). 
This circuit controls an automatic exposure control circuit (not shown) 
which includes a photocell to which scene light is directed by a window 96 
on housing section 48. When power is latched on, motor 68 is de-energized. 
Image-bearing light from the scene is transmitted by lens 58 to cross 
chamber 56 where it impinges upon the mirror 60. From mirror 60, the light 
is reflected downwardly through the exposure aperture 38 in film container 
12 to expose the forwardmost film unit 14. Using well known light 
integrating techniques, the exposure control circuit provides an exposure 
termination signal to the logic circuit which in turn provides appropriate 
signals to cause the electronic shutter to close and motor 68 to be 
reenergized. 
Sequencing gear 72 drives the film advance device 74 forwardly along a 
linear path and a hook-like member (not shown) at the trailing end thereof 
extending through film container access opening 42 engages the trailing 
end of the exposed forwardmost film unit 14 and advances it forwardly 
through film withdrawal slot 40 into the bite between rollers 81 and 82. 
Roller 81 is driven in a direction to cause the film unit to be advanced 
therebetween for progressively applying a compressive pressure along the 
length of the film unit 14. The film unit 14 exits from camera 10 through 
a film exit opening 98 in housing section 50. At the termination of the 
processing cycle, the operating mechanisms are reset for the next cycle of 
operation and the cycle is automatically terminated. 
As best shown in FIG. 6, the roller assembly 78 comprises the roller 
mounting bracket 80 and the juxtaposed pair of pressure-applying members 
or rollers 81 and 82 mounted thereon. 
Roller mounting bracket 80 is preferably of stamped metal construction and 
includes a generally planar, horizontal bottom member 102, and a pair of 
integrally formed lateral side members 104 upstanding vertically at the 
lateral ends of bottom member 102. Integrally formed with side members 104 
and extending rearwardly therefrom are a pair of arcuate mounting flanges 
106 having mounting holes 108 therein. Other structural features of 
bracket include an upstanding latch member 110 integrally formed with the 
right hand mounting flange 106 (as viewed in FIG. 6) and a pair of 
locating or positioning holes 112 in horizontal member 102 for locating or 
positioning housing section 50 with respect to bracket 80. 
The pressure-applying members or rollers 81 and 82 are rotatably mounted 
transversely of the side members 104 with their respective lateral ends 
mounted in suitable bearings (not shown) in lateral side walls 104. The 
roller drive gear 84 is mounted on the right hand end of roller 81. 
As best shown in FIG. 2 the component mounting frame 54 includes a pair of 
laterally spaced depending legs 114 having outwardly extended pins 116 
thereon which are adapted to extend through the openings 108 on bracket 
mounting flanges 106 thereby pivotally mounting the roller assembly on the 
leading end of frame 54 for pivotal movement between its positions 
blocking and unblocking the open end of camera receiving chamber 88 as 
shown in FIGS. 4 and 5. When the roller assembly is in its closed 
position, the latch member 110 cooperates with a slidable latch member 118 
on the right hand side of housing section 46 to maintain the roller 
assembly in the closed position wherein the entrance side 120 (the 
horizontal bite between rollers 81 and 82) of the pressure-applying 
members is adjacent the film withdrawal slot 40 of film container 12 and 
the leading end of the forwardmost film unit 14 located therein. 
For a more detailed description of the previously described structure and 
operation of camera 10, reference may be had to copending applications 
Ser. No. 554,777, filed Mar. 3, 1975 by B. K. Johnson et al.; and Ser. 
No. 554,778, filed on Mar. 3, 1975 by B. K. Johnson et al.; both of these 
applications being assigned to the same assignee as the present invention. 
Attention will now be directed to the gear train 70 which transmits power 
from the motor 68 to the top roller 81 and the camera sequencing gear 72. 
The gear train may be described as a "hinged type" having one portion 
mounted on the modular section 66 in the first housing section 46 and 
another portion (gear 84) mounted on the second housing section 50. 
As best shown in FIG. 9 the modular portion of the gear train 70 comprises 
a cluster of rotationally mounted intermeshed gears to provide appropriate 
speed reductions from motor 68. 
These gears are rotationally mounted on two parallel shafts 122 and 124 and 
from a drive gear 126 on the output shaft of motor shaft 68 power is 
transferred to compound gear 128 on shaft 122. The small segment of gear 
128 is in mesh with the large segment of compound gear 130 on shaft 124 
which in turn has a smaller segment in driving mesh with a compound gear 
132 on shaft 122. The smaller segment of gear 132 is in driving mesh with 
the large segment of compound gear 134 on shaft 124 which in turn has a 
smaller segment in driving mesh with the camera sequencing gear 72. 
Keyed or fixedly coupled to the smaller segment of compound gear 132 is a 
pinion gear 136 which rotates about shaft 122 with gear 132. This pinion 
136 is in operative mesh with the roller gear 84 when housing section 50 
and roller mounting bracket 80 are located in the closed or operative 
position for providing rotational drive to the top roller 81. When housing 
section 50 is opened (See FIG. 5) to provide access to the film container 
receiving chamber 88, gear 84 becomes disengaged from pinion 136. The 
present invention is directed to providing unique tooth profiles for 
pinion 136 and gear 84 to facilitate bringing gear 84 back into operative 
mesh with pinion 136 when housing section 50 is pivoted upwardly into the 
closed operative position. 
In the illustrated embodiment the pinion 136 (smaller gear) has 12 teeth 
and the roller gear 84 has 36 teeth thus providing a speed reduction from 
the pinion 136 to roller 84. It will be noted however that in some 
instances the drawings will not accurately reflect the shape of tooth 
profiles or the correct number of gear teeth because of space limitation 
and the technical difficulty in providing such an illustration. Unless 
otherwise noted, the gears, including some renderings of pinion 136 and 
gear 84 will be diagrammatic in nature to show their relative positions on 
camera 10 rather than to accurately represent the unique tooth profiles to 
be described in detail hereinafter along with selected accurate views of 
such profiles. 
Pinion 136 and gear 84, when in mesh, span or bridge the interface between 
housing sections 46 and 50 and may be thought of as the point of 
separation in a hinged-type gear train. 
As noted earlier, one may facilitate bringing such a pair of gears into 
mesh by using a pair of identical gears (1:1) which have identical tip 
spacing between adjacent teeth. However, such gears do not provide the 
required speed reduction and only add to the power loss of the train. They 
also take up more space, a concept that is at odds with reducing camera 
size to an absolute minimum. 
In order to describe the unique gear teeth profiles of pinion 136 and gear 
84, and, more importantly, to explain how this unique structure departs 
from the prior art and the standard practices of gear technology, some 
basic definitions and gear terminology will have to be presented at this 
point. 
To supplement the following outline, one may wish to consult the following 
references which explore gear technology in detail. Analytical Mechanics 
of Gears, by Earle Buckingham, published by Dover Publications, Inc.; 
Engineering Kinematics, by Alvin Sloane, published by The Macmillan 
Company; selected papers entitled Recess Action Gears, by Eliot K. 
Buckingham, Effects on Size on Gear Design Calculations, by Paul M. Dean, 
Jr., Charts for Designing Long-Short Addendum Spur Gears, by Wayne H. 
Bookmiller published in a book entitled Gear Design and Application, 
published by McGraw-Hill; and The Gear Handbook, edited by Darle W. Dudley 
and published by McGraw-Hill. 
A pair of meshed gears may be visualized in terms of a pair of rolling 
cylinders in frictional engagement, one being the driver, the other the 
follower. Ignoring slippage the speed ratio--or the angular velocity of 
the follower divided by the angular velocity of the driver--is a constant 
quantity. Upon adding gear teeth to the two cylinders, this constant 
quantity or ratio is maintained. 
Looking only at the ends of the cylinders (See FIG. 10), one may reduce the 
analogy to a two dimensional view and think of two rolling circles called 
pitch circles. The line joining their axis of rotation is the line of 
centers, and the point of contact is the pitch point. The diameters of the 
pitch circle are called pitch diameters. 
The pitch circles fix the speed ratio 
EQU .omega..sub.f /.omega..sub.d = D.sub.d /D.sub.f 
where D.sub.d is the pitch diameter of the driver and D.sub.f is the pitch 
diameter of the follower. 
Using the pitch circles as a basis, the gear teeth are formed as shown in 
FIG. 11. The outermost circle of the gear is called the addendum circle 
and the radial distance from the pitch circle to the addendum circle is 
the addendum. The circle found at the bottom of the space between teeth is 
the dedendum circle and the radial distance from the pitch circle to the 
dedendum circle is the dedendum. When another gear is in mesh with the 
gear of FIG. 11, its addendum will project into the space between teeth of 
that gear to the working depth circle thereby providing a certain amount 
of clearance. 
Circular pitch is the distance between adjacent teeth on the pitch circle 
and diametral pitch is the ratio of the number of teeth T to the pitch 
diameter D. For example, if a 3-inch pitch diameter gear has 30 teeth, the 
diametral pitch equals 10. But if the 3-inch gear had 60 teeth, the 
diametral pitch would equal 20. 
The face of the tooth is the tooth surface between the pitch and addendum 
circles. The flank of the tooth lies between the pitch and dedendum 
circles. The illustrated gear tooth of FIG. 11 is of the standard gear 
profile and has a blunt or flat tip called the face of the gear. 
Without going into extensive detail, mating gear teeth are designed for 
conjugate action. That is the tooth profiles are based on complementary 
curve to maintain a substantially constant driving force or pressure as 
corresponding teeth approach the pitch point, come into initial contact, 
engage in sliding contact, pass through the pitch point, and then begin 
disengagement as they pass beyond the pitch point. 
Over a long period of time gear tooth profiles have evolved to the point 
where it is more or less standard practice to base the tooth profiles on 
involute curves of circles to provide the necessary conjugate action. 
An involute of a circle is a definite property of that circle. It may be 
best visualized by considering FIG. 12. If a cord, fastened to a cylinder 
at point A and kept constantly taut, is unwound from the circumference, 
any point, like B, describes a curve called the involute of the circle. 
When two teeth of mating gears are in contact as at point a in FIG. 13, a 
line may be drawn from the point of contact to the pitch point P. This 
line is called the pressure line and the angle formed by the pressure line 
and a normal to the line of centers of the gears is called the pressure 
angle. For reasons that are beyond the scope of this disclosure, it is 
desirable to design a set of gears having a standard pressure angle, one 
of the most common being 20.degree.. 
When the pitch circles and pressure angle for a given set of gears have 
been chosen, then two circles, concentric with the pitch circles and 
tangent to the pressure line at a and b may be drawn as shown in FIG. 14. 
It is these circles whose involutes will become the tooth profiles and 
they are called the base circles of the gears. 
As noted earlier, there are standard gear tooth profiles that have evolved 
over a long period of time. The tooth profile is based on involute curves 
and the teeth are characterized by their blunt tips. Standard profiles 
allow one to select a pair of gears with the desired number of teeth 
without worrying whether the gears will mesh. For example, if the desired 
speed ratio is 1:2 one may select a 20-tooth driver and a 40-tooth 
follower. The speed ratio may be changed to 1:3 simply by selecting a 
60-tooth (standard profile) gear to replace the 40-tooth gear. 
FIG. 15 is a diagrammatic illustration of a pair of standard profile gears. 
The bottom gear is the driver and the top gear is the follower. By 
definition, these gears will be blunt tipped and have equal addendums and 
corresponding dedendums to receive the addendums. We will assume that the 
driver is a 12-tooth pinion and the follower is a 36-tooth gear. 
Because the gears mesh, it is not readily apparent that the tip spacing 
between the midpoints of adjacent teeth on the 36-tooth gear and the 
12-tooth pinion is not equal. The spacing between teeth is equal at the 
pitch circle. When equal addendums are added radially, the tip spacing of 
the 12-tooth gear increases to a larger extent than the tip spacing of the 
36-tooth gear because of the different angles subtended by adjacent teeth 
on each gear. For example, adjacent teeth on the 36-tooth gear subtend an 
angle of 10.degree. while adjacent teeth on the 12-tooth gear subtend an 
angle of 30.degree.. 
The unequal tip spacing of standard gears is of no importance when the 
gears are used in an application where they are in permanent mesh. However 
when one attempts to use standard gear tooth profiles for the pinion 136 
and the roller gear 84 in a hinged gear train, it is readily apparent that 
the unequal tip spacing will tend to cause jamming and binding. 
Also, the blunt tip of the standard profile is detrimental because of the 
tendency to promote, rather than minimize, nonmeshing tooth tip abutment. 
One way to minimize the tip abutment problem is to extend the standard 
profile teeth until they come to a point. This means that the addendums of 
both gears will be increased and their line of centers will have to be 
extended for proper mesh. 
When this approach has been tried, it was found that the tip spacing 
problem was more serious. Again, the tip spacing of the 12-tooth pinion 
increased radially at a faster rate because of the different angles 
subtended by the adjacent teeth on the two gears. This caused an interlock 
condition when the gears were attempted to be brought into mesh. That is 
the outside edges of two adjacent teeth on the 36-tooth gear fit into the 
space subtended by the inside edges of two adjacent teeth on the 12-tooth 
gear as shown in FIG. 21. 
From the above, one skilled in the art will appreciate that gears 136 and 
84 cannot have standard tooth profiles or even standard profiles that have 
been extended to a point without causing jamming, binding, tip abutment, 
or even tooth interlock. 
It is also common practice in gear technology to design a set of meshing 
gears, for certain applications, having unequal addendums. This is 
especially true in gear sets that include a small diameter pinion. In 
order to provide clearance for the addendum of the mating larger gear, the 
dedendum of the standard profile pinion extends downwardly into the base 
circle of the pinion causing a condition known as undercut. That is, the 
base of the pinion tooth is thinner than if the standard involute curve 
defined the base of the tooth. In effect, to get the necessary clearance, 
the pinion tooth base is weakened. 
To overcome this problem, the gears are designed with unequal addendum so 
that the pinion has a longer addendum and the mating gear has a 
correspondingly shorter addendum. From FIG. 16 one will see that as the 
addendum is increased, the tooth becomes correspondingly thicker thereby 
adding strength to the tooth. In FIG. 16, the innermost tooth is the 
standard form. The next two are known, respectively, as semirecess action 
and full recess action tooth forms. 
Modification of the addendum of the pinion to avoid undercut, and in most 
cases decreasing the addendum of the gear, is recommended when either or 
both gears have small numbers of teeth; the gears are used in speed 
increasing drives; the gears carry maximum power for a given weight 
allowance; or an absolute minimum of energy loss through friction is to be 
achieved. 
The amount the addendum of small gears should be enlarged to avoid undercut 
has been standardized and such information is available in the gear 
technology references cited previously herein. 
FIG. 17 shows a 12-tooth driver pinion having an extended addendum in mesh 
with a 36-tooth follower gear having a reduced addendum to compensate for 
the increased addendum of the pinion. It will be noticed that the pinion 
teeth are both longer and thicker than the standard form pinion teeth in 
FIG. 15. The semirecess action teeth pinion teeth shown in FIG. 17 avoid 
the undercut problem, are stronger than standard tooth profiles, and tend 
to be more pointed at the tip. But this gear set is not well suited for 
the hinged gear train application because of the unequal tip spacing. 
Again, the addendum has been extended on the small gear having adjacent 
teeth that subtend an angle of 30.degree.. Since the thicker teeth are 
even longer than standard teeth that have been extended to form a point at 
the tip, the mismatch of tip spacing is even greater and if applied to 
pinion 136 and gear 84, tooth interlock will occur when these gears are 
attempted to be brought into mesh. 
To solve the meshing problem of the hinged gear train, pinion 136 and gear 
84 have been designed to have equal tip spacing and pointy teeth profiles. 
This has been accomplished by proceeding in direct contradiction to the 
prior art and standard pract..es of gear technology-- namely providing a 
long-short addendum gear set where the addendum of gear 84 has been 
extended and the addendum of the small pinion gear 136 has been decreased 
accordingly. While standard practice tells us to extend the addendum of 
the pinion to avoid undercut, pinion 136 has had its addendum shortened to 
provide equal tip spacing with the mating gear 84. The fact that the 
pinion does have undercut is secondary to the requirement for equal tip 
spacing. For this application, any loss of inherent strength due to the 
undercut and the reduced thickness of the short addendum pinion tooth can 
be compensated for by properly selecting the material from which the gear 
is made. For example, in this application it is preferable to form the 
gears of sintered stainless steel for reasons of strength and other 
benefits such as the ability to get a better interference fit of gear 84 
with the shaft on which it is mounted. 
Earlier, it was shown that as the addendum on the 12-tooth pinion was 
extended, the tip spacing increased at a greater rate than if the same 
addendum was added to the 36-tooth gear because two adjacent teeth on the 
pinion subtend the larger 30.degree. angle than the corresponding 
10.degree. angle on the mating gear. In other words, by extending the 
addendum on the pinion, which is standard practice for small gears, the 
tip spacing mismatch became even more exaggerated. 
By following the opposite procedure of lengthening the addendum of gear 84 
having adjacent teeth that subtend the smaller 10.degree. angle, and 
decreasing the length of the addendum of the matching pinion 136, the tip 
spacing mismatch moves in the opposite direction, and finally becomes 
equal when the proper ratios are reached. 
The unique tooth profiles for pinion 136 and gear 84 are shown separately 
in FIGS. 18 and 19. Portions of these two gears are shown in mesh in FIG. 
20. The tooth profiles are based on involute curves and have pointy tips 
to minimize nonmeshing tip abutment. The spacing between adjacent tips is 
substantially equal despite the fact that the two gears provide a speed 
ratio of other than 1:1. It will be noted that the tooth profile on the 
12-tooth pinion gear 136 is short and rather thin when compared to the 
thicker and longer tooth profile of gear 84. 
Unlike the diagrammatic illustration of gears 136 and 84 in some of the 
earlier drawings, the tooth profiles of pinion 136 and gear 84 in FIGS. 
18, 19, and 20 are accurate representations. 
The following table will provide numbers for selected parameters: 
______________________________________ 
Pinion 136 Gear 84 
______________________________________ 
Number of Teeth 
12 36 
Diametral Pitch 
48 48 
Pressure Angle 20.degree. 20.degree. 
Pitch Circle Diameter 
.250 in. .750 in. 
Base Circle Diameter 
.235 in. .705 in. 
Addendum .0195 in. .0445 in. 
Dedendum .0398 in. .0115 in. 
Tip-to-Tip Spacing 
.0722 in. .0722 in. 
______________________________________ 
It will be noted that the addendum of pinion 136 exceeds the addendum of 
gear 84 by a ratio in excess of 2:1. 
Despite the fact that the unique tooth profiles shown in FIGS. 18, 19, and 
20 depart from standard practice, it should be noted that the desirable 
20.degree. pressure angle has been maintained and that the contacting 
surfaces of the gear teeth are based on involute curves. In other words, 
equal tip spacing and pointy tips have not been designed into gears 136 
and 84 at the expense of gear efficiency. If there is any penalty to pay 
for insurance that these gears will mesh when the loading door section 50 
is moved from the open inoperative position to the closed operative 
position, it is in terms of material selection for pinion gear 136. That 
is, because of the undercut and their tooth structure, one would fabricate 
pinion 136 from a stronger material than one would normally select for 
this application and load if standard profile or long addendum teeth were 
to be used on pinion 136. 
From the above table it will be seen that the ratio of the addendum of gear 
84 to the addendum of pinion 136 is at least 2:1. 
FIG. 20 shows a standard pitch circle and an operating pitch circle for 
each gear. The measurements in the preceding table are taken relative to 
the standard pitch circle. The operating pitch circle indicates that in 
this particular camera these two gears are configured to be mounted on 
centers that are just slightly further apart than standard center spacing 
to adapt these gears to the particular dimension and spacing of the camera 
components. However, the inventive concept of providing a short addendum 
pinion and long addendum mating gear with equal tip spacing may be applied 
to gears spaced on standard lines of centers. 
From the designation of the number of teeth in each gear segment of the 
gear train shown in FIG. 5, one skilled in the art will appreciate that 
the speed reduction from motor 68, which preferably has a nominal 
operating speed of 11,500 rpm, to the top roller is very close to 1:25 
and, of course, the speed ratio of the follower gear 84 to the driver 
pinion 136 is 1:3. 
It will be noted from FIGS. 4 and 5 of the drawings that gear 84 moves 
along a pivotal path between the inoperative and operative positions. This 
coupled with the fact that roller 81 and therefore gear 84 coupled thereto 
are in a free wheeling condition will facilitate mesh of gear 84 even if 
the pointy teeth of these gears illustrated in FIGS. 18, 19, and 20 should 
initially come into tip-to-tip contact. The pivotal movement enhances the 
ability of the free wheeling gear to roll into the proper mesh engaging 
attitude relative to pinion one 136 once initial contact is made by the 
equally spaced tips (as measured between the radial end centerlines of 
adjacent teeth). 
Since certain changes may be made in the above apparatus without departing 
from the scope of the invention herein involved, it is intended that all 
matter contained in the above-described description and shown in the 
accompanying drawings shall be interpreted as illustrative and not in a 
limiting sense.