Pneumatic door operator

An operator system for a counterbalanced door includes a rodless fluid cylinder that has a cylinder body and a rodless piston adapted for reciprocation in the cylinder body. A carriage which is adapted for reciprocation externally along the length of the cylinder body is secured to the piston. A link member connects the cylinder carriage to a door or to a torsion bar for the door. A control circuit is provided for controlling the operation of the fluid cylinder and hence the position of the door.

BACKGROUND OF THE INVENTION 
The present application is a continuation-in-part of application Ser. No. 
029,733 filed on Mar. 24, 1987 now abandon. 
This invention generally pertains to a pneumatic door operator 
construction. More specifically, the present invention relates to a 
sliding door operator system which utilizes a rodless fluid cylinder for 
mechanically opening and closing a door. 
The invention is particularly applicable to sectional overhead doors of the 
multiple panel type in which the door is aided in being raised and lowered 
by a counterbalance means such as a torsion bar and will be described with 
particular reference thereto. However, it will be appreciated by those 
skilled in the art that the invention has broader applications and may 
also be adapted for use in many other environments such as single panel 
overhead lift doors, overhead roller doors, bi-parting doors, and even 
sideward sliding doors or the like. 
Most overhead doors are counterbalanced with a torsion spring or a 
counterweight system for storing energy during door closing so that energy 
may be extracted during door opening. During opening, most of the energy 
for lifting such a door is derived from the energy stored in the spring or 
counterweight system. If the door is counterbalanced fairly well, the 
amount of manual energy required to open the door need only be sufficient 
to make up the frictional and other losses in the system. During door 
closing, the energy required is that for making up the frictional and 
other losses since most of the energy which is transferred to the spring 
or counterweight storage system is derived from the weight of the 
descending door. 
In general, conventional overhead doors of this nature are suspended by a 
cable which is wound around a drum axially driven through a shaft by the 
torsion spring with the shaft being rotated by a chain driven sprocket. 
Generally, the chain is driven by an electric motor. In some environments, 
however, the use of electrical motors is undesirable because of the 
possible danger of a spark causing an explosion or a fire. It has also 
been found that electric motors are disadvantageous since if the door 
becomes jammed the motor will continue rotating and will likely unwind the 
cable between the drum and the door and this could prove hazardous to 
personnel and may damage property. 
Also, when such doors go through a high number of cycles, such as in a car 
or truck wash, or in factories, warehouses or the like, the electrical 
motors, and bearings wear out at a fairly rapid rate and this results in 
frequent breakdowns of the door opening mechanism. Moreover, malfunctions 
of the door opening mechanism sometimes also lead to damage to the torsion 
spring mechanism of the door which can be fairly expensive to repair. 
Some conventional trolley type and jack shaft type pneumatic cylinder door 
operators are known to the art. However, every one of these includes a 
piston rod extending out of the cylinder which increases the length of the 
operator and the amount of room necessary to mount the operator. 
One recent suggestion has been to utilize a pair of pneumatic cylinders in 
a side mounted operator for moving the chain of the chain driven sprocket 
by the use of piston rods thereby rotating the sprocket. However, side 
mounted operators are not recommended when a trolley operator could be 
used in their place since with a side mounted operator any hesitation in 
door travel results in an unwinding of the cable from the torsion bar drum 
which makes the door liable to a free fall that could cause grave injury 
to people as well as damage to objects under the door and to the door 
itself. 
For certain types of doors, such as high-lift type vertical lift doors and 
roll up type doors, a trolley operator cannot be used and thus a side 
mounted so-called jack shaft operator has to be used. The conventional 
electric motor powered jack shaft operators are disadvantageous for the 
reasons previously mentioned. The side mounted dual piston type system is 
also disadvantageous since the design of this system will produce a 
constant downward torque on the main door shaft and also will exert an 
unnecessarily heavy load on the end bearing where the shaft is suspended. 
This wears the bearing and can cause the bearing plate to cut through the 
shaft. With this type of operator, the chain can jump the sprocket and the 
cable can jump the drum every time the door hesitates during its travel, 
or the chain goes slack, or if the shaft becomes bent at some point during 
door operation. Another difficulty with the conventional side mounted 
pneumatic cylinder system is that it is mounted near the bottom of the 
door and in this location the door track may get hit by a vehicle which 
can also damage the cylinders. In addition, in wet and cold environments, 
moisture in the air can freeze in the cylinders thereby damaging the seals 
on the piston rod and causing air leaks and hence a malfunction of the 
door. In addition, on relatively low doors, i.e. 7 to 10 foot high doors, 
the chain of the conventional dual piston system can be hazardous to 
personnel since the chain is so located that it can entangle the limbs, or 
hair, of personnel. 
Accordingly, it has been considered desirable to develop a new and improved 
jack shaft type door operator system which could be mounted either 
horizontally normal to the closed door or vertically parallel to the door 
and would overcome the foregoing difficulties and others while providing 
better and more advantageous overall results. 
BRIEF SUMMARY OF THE INVENTION 
According to one aspect of the invention, an operator is provided for 
opening and closing a door in which the weight of the door is 
substantially counterbalanced by a torsion spring assembly including a 
drive rod. 
According to this aspect of the invention, a rodless fluid cylinder is 
provided comprising a cylinder body, a rodless piston adapted for 
reciprocation in the cylinder body, and a carriage adapted for 
reciprocation externally along the length of the length of the cylinder 
body. The carriage is secured to the piston. A link means is provided for 
connecting the cylinder carriage to one of the door and the drive rod. The 
link means can comprise a looped chain, which is secured to the cylinder 
carriage, and a sprocket wheel, which is secured to a bushing that rotates 
the torsion bar. The chain is looped around the sprocket wheel. A control 
means is provided for controlling the operation of the fluid cylinder and 
hence the position of the door. 
In accordance with yet another aspect of the invention, an air powered 
operator system is provided for a door that is raised and lowered with the 
aid of a torsion bar. 
More specifically in accordance with this aspect of the invention, the 
system comprises an air cylinder comprising an elongated cylinder body, a 
rodless piston adapted for reciprocation in the cylinder body, and a 
carriage adapted for reciprocation externally along the length of the 
cylinder body and secured to the piston. A looped chain is secured to the 
carriage that rotates the main drive sprocket. A sprocket wheel is secured 
to the torsion bar and the chain is looped around the sprocket wheel. A 
source of pressurized air which is in fluid communication with the 
cylinder is provided. A valve means is interposed between the source of 
pressurized air and the air cylinder for selectively feeding pressurized 
air to the air cylinder to reciprocate the rodless piston therein. 
One advantage of the present invention is the provision of a new door 
operator system which can actuate most types of overhead doors that have a 
counterbalance means and can also actuate selected rolling doors, and 
bi-parting doors. 
Another advantage of the present invention is the provision of a door 
operator system which actuates a door by the movement of a rodless piston 
which reciprocates in a cylinder. 
Still another advantage of the present invention is the provision of a 
pneumatically powered jack shaft type door operator system which can be 
mounted either horizontally normal to the closed door or vertically 
parallel to the door. 
An additional advantage of the present invention is the provision of a door 
system with a rodless cylinder operator which moves a chain that is looped 
around a drive sprocket which is secured to a bushing that is connected to 
and drives the main door shaft of a counterbalanced door. 
A further advantage of the present invention is the provision of a door 
operator system which opens and closes a door with a minimum of force 
thereby preventing injury to persons or damage to objects which are 
inadvertently left in the path of the door when the door is being moved. 
A still further advantage of the present invention is the provision of a 
door operator system with a means for controlling the speed with which the 
door moves. 
A yet further advantage of the present invention is the provision of a door 
operator system with a means for keeping the door stationary when the 
operator stops the door in mid-travel. 
Still other advantages and benefits of the present invention will become 
apparent to those skilled in the art upon a reading and understanding of 
the following detailed specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein the showings are for purposes of 
illustrating preferred embodiments of the invention only and not for 
purposes of limiting same, FIG. 1 shows the subject new jack shaft type 
door operator system A which is mounted horizontally and normal to a 
closed door B which closes an opening in a building C. While the door 
operator is primarily designed for and will hereinafter be described in 
connection with a sectional overhead sliding door, it should be 
appreciated that the overall inventive concept involved could be adapted 
for use in many other door environments as well. 
With reference now to FIG. 2, the operator system A includes a cylinder 
member 10 which comprises a tubular body 12 that has a longitudinal bore 
14 extending therethrough. A first end of the bore is closed by a first 
end cap member 16 with a second, and opposite, end of the bore 14 being 
closed by a second end cap member 18. Suitable fasteners 20 secure the 
first and second end caps 16,18 to the tubular body 12. A suitable seal 
means 22 is provided for each of the end caps 16,18 to prevent pressurized 
air from leaking therethrough. 
As is known in rodless cylinders, and is more completely described in Ser. 
No. 029,733, a slot 24 extends longitudinally along one side of the 
tubular body 12 to communicate the bore 14 with the environment. A pair of 
spaced sealing bands or strips 25,26 seal the slot to isolate the bore 14 
and prevent pressurized fluid from leaking out therethrough. The bands are 
secured to the two end caps 16,18. 
Adapted to reciprocate longitudinally in the bore 14 of the cylinder is a 
piston body 30 which is comprised of a pair of identical and opposing 
sections 32,34 that are joined together in a suitable conventional manner. 
A seal means 36 extends peripherally around each section 32,34 of the 
piston to provide a seal between the piston section and the cylinder bore 
14. 
A piston bracket 38 is secured by suitable conventional fasteners 40 to the 
first and second sections 32,34 and is adapted to extend through the 
cylinder slot 24. A section of the bracket 38 is positioned outwardly of 
the tubular body 12 and extends along substantially one face thereof as is 
illustrated best in FIG. 1. This outer section has depending sides 42 to 
each of which can be secured an inwardly extending bearing rod (not 
visible) which slides in suitably configured grooves 44 formed in the 
outer periphery of the tubular body so as to allow a smooth sliding motion 
of the piston bracket 38 as the piston moves. This system allows the 
cylinder 10 to withstand high axial and radial loads and moments while 
eliminating the requirement for external guides and supports. 
Secured to the piston bracket 38 by suitable conventional fasteners 49 is a 
traveler or carriage 50. The carriage has a base wall and a pair of side 
walls (not visible), and a pair of opposing end walls 56. Seal means 58 in 
the form of wiper seals are provided on the end walls 56 of the carriage 
and act to clean the second seal band 26 of the cylinder 10. 
A link means is provided for connecting the cylinder carriage to the 
torsion bar. In the preferred embodiment, the link mean comprises a chain 
60, such as a bicycle chain (as best seen in FIG. 5), which is secured to 
the carriage 50 and is looped around a torsion bar or drive rod 62 of the 
garage door. 
While the instant rodless cylinder apparatus is similar to the construction 
disclosed in the parent application, Ser. No. 029,733, it is of 
considerably different dimensions. More specifically, if, for example, a 
16 ft. high door is meant to be opened by the parent application's trolley 
operator, the operator would require a 16 ft., 6 inch stroke and thus the 
rodless cylinder thereof would need to be at least that long. In contrast, 
the jack shaft operator according to the present invention for the same 16 
ft. door would only require an approximately 8 ft. stroke and thus the 
rodless cylinder's length could be considerably shorter. However, the 
chain drive operator cylinder would likely require a larger bore diameter 
than the trolley operator cylinder to provide sufficient power to move the 
door. It should be evident that both types of operators can be provided 
with cylinders of various bore diameters as necessary. 
With reference now to FIG. 4, a rear sprocket housing for the chain 60 
includes a pair of angle brackets 72, 74 which are secured by suitable 
fasteners 75 to the rear of the cylinder body 10. A rectangular tube 76 is 
rotatably secured by a pivot means 78, such as a bolt, between the 
brackets 72, 74. A rear idler sprocket wheel 80 is rotatably mounted on a 
suitable bearing 82 secured to the rectangular tube 76. A chain adjustment 
bolt 84 is provided in spaced relationship to the pivot bolt 78 (i.e. 
above the axis of the rear idler sprocket wheel 80) in order to allow a 
pivotable adjustment of the tube 76 and the rear sprocket wheel 80 in 
relation to the cylinder 10, and hence an adjustment of the tightness of 
the chain 60. 
With reference now to FIG. 3, a front sprocket housing includes a pair of 
angle brackets 92, 94 which are suitably secured by conventional fasteners 
95 to the front of the cylinder body 10. A rectangular tube 96 is 
positioned between the two angle brackets and is suitably secured 
therebetween by conventional fasteners 98. Unlike the rear housing, the 
front housing does not pivot. A drive sprocket 100 is rotatably mounted on 
a suitable bearing 101 secured to the rectangular tube 96 such that a main 
drive rod or shaft 102 of the torsion bar 62 can extend through the 
sprocket. This is accomplished by providing a hollow metal bushing 104 on 
which is positioned a main drive sprocket wheel 106 with the bushing being 
fastened, such as by a key slot for a woodruff key 108, to the main drive 
shaft 102 which is suitably keyed. 
It is noted in this connection that solid drive rods are generally keyed in 
order to allow the drums, for the cables on which the door is suspended, 
to be secured to the drive rods. In most instances, these keyways extend 
the entire length of the drive rod. Thus, the bushing 104 can be readily 
keyed with the woodruff key 108 to the drive shaft 102 without any 
modification needed to the shaft. 
For small and light doors, sometimes a tube drive shaft is used instead of 
a solid drive shaft. In these doors, the drums are pinned to the tube 
shaft. Similarly, the bushing 104 could be secured by conventional pins to 
such a tube shaft (not illustrated) as is well known in the art. 
Also provided on the rectangular tube 96 are a pair of spaced idler 
sprocket wheels 110, 112 around which the chain 60 is looped. These are 
substantially similar and each is mounted on a bearing 114 secured to the 
tube 96. It is noted that two small diameter idler sprocket wheels are 
provided in the front housing 90, in contrast to the large diameter idler 
sprocket wheel of the rear housing. This is done in order to enable the 
chain 60 to achieve more of a grip on the drive sprocket 100. The idler 
sprockets also perform the additional function of spacing the chain 60 
away from the top and bottom surfaces of the cylinder. If desired, the 
front idler sprockets can have a pyramidal shape. 
If desired, the chain 60 can also be spaced from the traveler 50 by the 
provision of guideways 116 secured thereto as is illustrated in FIG. 2. 
The chain 60 is spaced from the back side of the cylinder 10 by a rear 
idler sprocket wheel 118 as shown in FIGS. 1 and 2. 
On advantage of using small sprocket wheels 106, 110, 112 in the front 
housing is that a limited movement of the chain 60 is able to efficiently 
rotate the shaft 102. This enables the door B to be moved with only a 
minimum movement of the actuator A. 
In the design illustrated in FIGS. 1-4, the entire operator can be suitably 
positioned above and behind a door such that the bushing 104 can be 
fastened in the middle of the drive rod of even a wide door, such as an 18 
ft. wide door. 
With reference now to FIG. 5, secured on the traveler or carriage 50 is a 
cap 120 which has suitable elongated apertures 122 located thereon through 
which suitable conventional fasteners 124 can extend into the traveler. 
Secured on the cap is a keeper element 126 which has a top surface 128 
that is provided with suitable spaced slots 130 into which one or more 
links 132 of the chain 60 can fit. It is noted that the elongated 
apertures 122 allow, upon a suitable loosening of the fasteners 124, a 
downward movement of the metal cap and therefore a disengagement of the 
chain keeper 126 from the links 132 of the chain 60. In this way, when the 
operator is first installed on the door, the traveler 50 can be moved to 
the desired location in relation to the door without having to disengage 
the chain from the drive sprocket secured to the torsion bar and then 
re-engage the chain once the operator and chain are correctly positioned 
in relation to the door. This saves a considerable amount of time and 
effort. 
With reference again to FIG. 1, a front end of the cylinder 10 can be 
mounted on the torsion bar 62. A pair of hangers 146 can be fastened to a 
rear end of the cylinder and to the adjacent ceiling (not illustrated) to 
secure the cylinder in place. 
Preferably, the door B includes a door member 150 which is made of a 
plurality of articulated longitudinally extending slats or planks. The 
door is adapted to slide up and down on a pair of spaced tracks 152, only 
one of which is illustrated in FIG. 1. In general, doors of this nature 
conventionally include a counterbalancing means such as an axial torsion 
spring 154 which is secured on the drive rod 102 above the door B. 
Cooperating with the drive rod 102 is a cable 156 which is secured on a 
first end to the door B (not visible in FIG. 1) and wound at its second 
end on a drum 158 axially driven by the drive rod. 
In order to actuate the cylinder 10, a pressurized fluid circuit 160, 
preferably pneumatic, is provided. The circuit includes a first conduit 
162 and a second conduit 164 each of which is in fluid communication with 
a respective end of the cylinder through the cylinder end caps 16,18. A 
source of pressurized fluid such as a compressor is in fluid communication 
with a respective one of the conduits 162, 164 through an inlet pipe 166 
as directed by a control means 170. The control means can be in the form 
of a valve control in a housing which is interposed between the conduits 
162, 164 and the inlet pipe 166 in order to control the movement of the 
rodless piston in the cylinder. 
With reference now to FIG. 6, the control means 170 can include a valve 
housing 172 which holds a conventional three position four way valve with 
open, stop, and close functions or a conventional two position four way 
valve with only open and close functions (not visible). Of course, it 
should be recognized that a wide variety of other types of valves could 
also be employed in the control means as the specific door application 
requires. Whatever type of valve is used, it can be controlled by 
conventional solenoids 174, as illustrated in FIG. 6, or by the other 
conventional means known in the art. 
The control means 170 can, as is illustrated in FIGS. 6 and 7, also include 
a pair of exhaust lines 175, 176 extending from the valve housing 172, 
which are joined by a T-fitting 177 and vent through an adjustment 
assembly 178, including a conventional pressure relief valve (not visible) 
held in a housing, and a muffler 180 in fluid communication with the 
relief valve, to the atmosphere. The exhaust assembly provides a pressure 
relief on both exhaust ports and will maintain a back pressure when the 
door is held in a mid-travel position. This will help to eliminate cable 
jumping when the operator is stopped in a mid-position of the door. The 
pressure relief valve will dump a certain amount of exhaust unless it 
receives pressure from the other side of the piston. Therefore, the back 
pressure on the piston will, to some extent, balance the forward pressure 
on the piston and reduce the tendency of the door, when stopped in 
mid-travel, to creep up or down as dictated by whichever side of the 
piston has the higher pressure on it. 
An adjustment knob 182 is provided on the assembly 178 in order to regulate 
the rate at which the exhaust is vented to the atmosphere by the pressure 
relief valve and thus control the speed with which the piston will travel. 
In other words, the adjustment knob will enable one to adjust the rate 
with which the pressure relief valve will vent pressurized fluid from the 
exhaust conduits 175, 176 and thereby control the speed of piston travel. 
The pressure relief valve, which may for the sake of quiet operation, be of 
the diaphragm type as is conventionally known, can be set to vent fluid 
which is at a pressure of greater than a predetermined amount. If desired, 
this vent pressure can be the fluid inlet pressure minus, for example, 20 
psi. In this way, when the piston is moving exhaust fluid pressure can be 
vented but when piston movement stops, a substantial amount of pressure is 
maintained on the face of the piston opposite to the direction of motion. 
With reference again now to FIG. 2, a bore 190 in the first end cap 16 
enables pressurized fluid from the first conduit 162 to enter one end of 
the cylinder behind the piston first section 32 to urge the piston 30 
toward the second end cap 18 of the cylinder. The second conduit 164 
extends longitudinally down the cylinder 10 and is secured thereto by 
suitable hose clamps 192 (FIG. 1). The second conduit 164 communicates 
through a bore 194 in the second end cap 18 with the second end of the 
cylinder. A suitable conventional adjustable valve means such as a needle 
valve, of which a bore 196 is illustrated, can be provided in each end cap 
16,18 to cushion the movement of the piston 30 adjacent the two ends of 
the cylinder. 
If desired, suitable conventional micro switches 200 can be secured to the 
end caps 16,18 to activate additional electronic functions in conjunction 
with the opening and closing of the door. The micro switches 200 are 
connected to the control means 170 by suitable wiring 204. Alternatively, 
proximity switches can be utilized at the ends of the cylinder. Also, 
conventional magnetic reed switches could be positioned alongside the 
cylinder for position sensing of the piston between the ends of the 
cylinder. Such micro switches or the like can be useful, for example, to 
disengage an electrically controlled safety edge on the door when the door 
is in the closed position. A switch can also trip a timer when the door 
reaches the open position so that when a predetermined amount of time has 
elapsed the door will begin to close. 
When a pressurized fluid such as compressed air or another suitable 
compressed gas is supplied by the pressurized fluid source 166 and the 
control means 170 is actuated to a door open position, the rodless piston 
30 and its attached carriage 50 will be urged by pressurized fluid flowing 
through the first end cap 16 to move from a front end of the cylinder 10 
towards a rear end thereof. At this time, the bore 194 and the second 
conduit 164 act as an exhaust means for exhausting fluid from a section of 
the cylinder between the second section 34 and the end cap 18 through the 
control means 170 and the adjustment assembly 178 to the environment. The 
carriage 50 thus moves thereby moving the chain 60 which rotates the 
torsion bar 62 and exerts a pull on the door B to open the door. The 
piston 30 slows down a few inches before contacting the cylinder body 
second end cap 18. The speed of movement of the piston 30 adjacent the end 
caps 16, 18 can be regulated by means of the adjustable valve means. 
Generally, the piston's speed of movement can be regulated through the 
directional valve in the control means 170 by use of the adjustment knob 
182. 
The piston 30, and hence the carriage 50, remains under pressure while the 
door B is in the open position until the control means 170 receives a 
signal electrically or manually to shift to another position. 
When the valve 170 shifts to the closing direction, the bore or port 194 in 
the rear end cap 18 becomes a fluid supply while the bore or port 190 in 
the front end cap 16 becomes the exhaust. Therefore, the speed with which 
the door closes can be different from the speed with which the door opens. 
In some cases, for example in automated car washes, the door B requires a 
rapid opening cycle. This can be regulated by the adjustable knob 182. On 
the other hand, for safety reasons, the closing cycle should be at a 
normal speed, which is, at. a maximum, approximately 1 second per foot as 
is recommended by the Canadian Door Institute. 
However, the door's speed could be set to slow, normal, or rapid speed to 
suit the particular door application involved. This setting can be done 
through an adjustment in the combination of the fluid flow and the exhaust 
restrictors. To stop the door at any intermediate position, one merely 
needs to provide a three position control valve which is actuated to a 
neutral or stop position in which it will block the flow of pressurized 
fluid to either end of the cylinder 10. Also, the motion of the door can 
instantly be reversed by having the directional valve shift from one 
direction to the other. This can be done either manually or electrically 
depending upon the type of controls used. 
Since the cylinder 10 operates on a low pressure fluid (such as gas or air 
at 40 to 150 PSI), the cylinder does not move with enough force to cause 
damage to the door. More importantly, the door does not move with enough 
force to cause damage to objects or be hazardous to personnel. 
The cylinder 10, carriage 50, and end caps 16,18 can all be made from any 
suitable material such as anodized aluminum. The piston 30 and the piston 
bracket 38 can be made from a suitable conventional material such as 
aluminum or steel. The sealing bands 25,26 can be made from a high density 
oil resistant plastic, or another suitable material and the various seals 
can be made from Buna N or another suitable rubber. One such suitable 
cylinder assembly is sold by the Norgren Martonair Co. under the 
designation LINTRA C/45000. 
FIG. 8 shows a second preferred embodiment of the invention as a side 
mounted or jack shaft-type operator for a garage door. For ease of 
illustration and appreciation of this modified construction, like 
components are identified by like numerals with primed (') suffix and new 
components are identified by new numerals. 
In this FIGURE, the operator A' similarly includes a cylinder member 10' 
which is utilized on one side of a door B' that closes an opening in a 
building C'. The operator system A' is identical with that disclosed in 
FIGS. 1-7 above. In this embodiment, one or more braces 210 are used to 
secure the system A' to a wall 212 of the building C' adjacent a door 
opening 214. In this embodiment, a front housing of the system A' includes 
a bushing which slips over an end 216 of a drive rod 102' outside a drum 
158' on which is wound a cable 156' attached to the door B'. 
One advantage to using a non-electrically powered operator is that a magnet 
(not illustrated) can be inserted in the rodless cylinder piston so that 
the magnet can, by means of magnetic fields only, trip a magnetic switch 
218, of the so-called Hall effect type, to actuate a function in relation 
to the door. Obviously, such magnet type remote actuators for magnetic 
switches were not possible with electric motor driven operators. 
The operation of the system is the same as described above with regard to 
the first preferred embodiment. 
An advantage of the second preferred embodiment over the conventional dual 
cylinder side mounted operator is that such operators are usually 
installed adjacent the floor of the building. Many times the door track 
gets hit at this location by vehicles also damaging the cylinders. The 
possibility for such damage is greatly reduced in the present invention by 
the positioning of the system A' near the top end of the door. 
Additionally, the conventional dual cylinder air operator utilizes a single 
sprocket connected to the main door shaft and linked with a chain hanging 
down to the drive of the operator. This conventional system will apply a 
constant downward torque on the main door shaft and delivers an 
unnecessarily heavy load on the end bearing where the shaft is suspended. 
This wears the bearing and in many cases causes the bearing plate to cut 
through the shaft thereby necessitating a replacement of at least this 
portion of the torsion rod assembly utilized on the door. In contrast, in 
the present invention, a three sprocket arrangement is provided in the 
front housing and the cylinder is secured to the building wall adjacent 
the front housing. Since a small drive sprocket is utilized and the 
cylinder is secured to the wall adjacent the front sprocket housing, there 
is less of a tendency to deliver large loads to the drive rod 102' which 
would put a strong downward moment on the end 216 of the drive rod 102'. 
The present invention thus provides a door operator system which minimizes 
frequent breakdowns, hazard to personnel, and damage to the door or 
objects which might be in the way of the door. Such a door operator is 
also believed to have a greatly improved life cycle in relation to the 
conventional electrically driven trolley door operators and to the 
conventional side mounted compressed air powered dual piston operators. 
The invention has been described with reference to a pair of preferred 
embodiments. Obviously, modifications and alterations will occur to others 
upon the reading and understanding of this specification. It is intended 
to include all such modifications and alterations insofar as they come 
within the scope of the appended claims or the equivalents thereof.