US2922638A - Overhead door operating mechanism - Google Patents

Description

Jan. 26, 1960 s. H. SMITH OVERHEAD DOOR OPERATING MECHANISM 3 Sheets-Sheet 1 Filed June 7, 1956 w w m w.

05M 60 cm To PUSH 8W Y MM 165 MWM @M H a m L wm MW 50\ Jan. 26, 1960 S. H. SMlTH OVERHEAD DOOR OPERATING MECHANISM 3 Sheets-Sheet 2 Filed June 7, 1956 INVENTOR.

534M062 H sM/rw BY 2/0/19, 11/4773, gaaee S. H. SMITH OVERHEAD DOOR OPERATING MECHANISM Jan. 26, 1960 3 Sheets-Sheet 3 Filed June 7, 1956 w s N v MM y W M mm L 2 W a Y m r U t St Pa flm OVERHEAD DOOR OPERATING MECHANISM Samuel H. Smith, Rocky River, Ohio, assignor, by mesne assignments, to The Rado Corporation, Chestertown, Md, a corporation of Maryland Application June 7, 1956, Serial No. 589,945

5 Claims. (Cl. 268-'59) The present invention relates generally to overhead door assemblies and is more particularly concerned with novel power driven overhead doors and with new overhead door power means affording unique and important advantages.

A number and variety of power means have been invented and developed for the purpose of operating over head doors and particularly those of the conventional hinged panel-type used in domestic garages. Several of the best of these devices are presently in general use and in fact the demand for this type of equipment is apparently increasing at a rapid rate. However, this growth in the market is occurring despite certain substantial shortcomings and disadvantages of the heretofore available equipment which have persisted throughout the evolution of thesedevices and overhead door installations incorporating them. In attempts to further stimulate the growth of this market there have been a number of prior efforts to minimize or eliminate these drawbacks; all of which, however, to the best of my knowledge, have fallen well short of their goals at least from the standpoint of net advantage or result. certain drawbacks can be eliminated according to one practice or device or another, other problems are aggravated and still others may be created'in these installations. Thus, for example, power driven overhead door installations can be made to operate automatically and by remote control but then it is essential that these installations incorporate safety features to prevent injury to a person or damage to the door or an obstructing object. The safety equipment and controls may add substantially to the complexity of the equipment and its cost and in installations prior to those of the present invention have imposed undesirable limitations on the operation of the doors.

Another serious shortcoming of overhead door power means available prior to the present invention-is their substantial head room requirement. Where these devices are to be applied to existing overhead door installations, the overhead clearance is frequently insufficient to permit the installation of a motor and drive mechanism. stalled at substantially increased cost by rebuilding the door suspension means or by opening ceilings to provide the clearance necessary for the power mechanism.

By virtue of my present invention these difiiculties Thus while In such cases the equipment may be inand shortcomings of the prior devices of this general v,

class may be eliminated and additional advantages of importance can at the same time be obtained. These results, moreover, are available without offsetting disad- 2,922,638 Patented Jan. 26, 1960 and combinations of elements which are new in the overhead door field. Furthermore, the relative locations and relationships of elements which are common to the prior art and the equipment of this invention are different and the manners inwhich these elements operate in assemblies of this invention are different from the operations of these elements in the heretofore known apparatus.

The overhead door installations of this invention afford an automatic locking feature so that the doors when closed are locked and can be opened from the outside only by radio signal means, while they may be opened from the inside of the house or garage by a switch or push button. Furthermore, unlike certain prior art devices, the door operators of this invention can be 0perated by push button controls to. position doors at any desired point between the fully opened and fully closed positions. A related. important advantage is the independence of these present invention installations from weather limitations which seriously impair the operation of other similar devices.

Another advantage of this invention is that it enables a substantial reduction of motor peak load requirements which in turn means increased safety. Thus by virtue of the fact that door inertia is utilized in operating this system, the clutch setting can be reduced by the amount of. the door inertia force without in any way impairing operation of the system.

Still another significant advantage of the present door installations is the tight weather seal which is assured when these doors are closed. This feature is attributable to the unique manner in which the door operating mechanism hereof is related to the demand also is attributable to the unique manner in which the power means of this invention operates in moving the door from its open position to its closedposition.

As indicated abve the equipment of this invention does not require a special ceiling clearance and can therefore be applied to existing overhead door installations where ceiling clearance is entirely inadequate for available overhead door operating devices of the prior art.

Still another advantage of this invention is the fact that it embodies the installation of garage door operating equipment at a minimum of time and labor cost without the necessity for altering in material aspects any existing overhead door equipment and, in fact, because of the simplicity and the ease of installation of this equipment and the small number of parts to be assembled together at the installation site it is feasible for the home owner himself to install this apparatus. The control mechanism including the unique slip switch feature requires no adjustment of limit stops and it is not necessary for the individual to have special technical knowledge or skill in electrical or mechanical fields to follow simple directions in installing this apparatus.

An additional advantage of my present invention is that it affords automatic operation of a light source so that a garage equipped with one of my door operators is artificially illuminated only when and as long as necessary in the normal and ordinary use of the door operator.

Those skilled in the art will gain a further and better understanding of this invention from the detailed description thereof set out below, reference being bad to the drawings accompanying and forming a part of this specification in which:

Fig. 1 is a plan view of a power unit of this invention including motor and transmission means and a carriage;

Fig. 2 is a side elevational view of the apparatus of Fig. 1, indicating the angle of repose of this unit supported by a pinion shaft;

Fig. 3 is a fragmentary, side elevational view of an I overhead garage door installation of this invention, showing the doorin closed position;

Fig, 4 is a perspective view of the door installation of Fig. 3' with the door in open position, parts being broken away in the interest of clarity;

Fig. 5 is a fragmentary perspective view of a portion of the door suspension means and the drive suspension means of the invention of Fig. 4;

Fig. 6 is a plan view, partly in section, of a slip switch assembly of the Fig. 1 unit; and,

Fig. 7 is a wiring diagram of the unit shown in Fig. 1.

In one of its aspects the present invention comprises, in broad and general terms, an overhead door installationincluding a trackway, a hinged panel door supported on the trackway, a motor to travel along the trackway with the door, motor suspension means independent of the door and located above the trackway, and means to transmit power output of the motor to the door to open and close the door. Even more succinctly, the essence of this phase of my invention is in the concept of providing means for supporting the motor independently of the door, either completely, or to a preponderant degree and to make this motor suspension means eifective over the full length of motor travel course as the motor travels with the door.

In another of its aspects this invention in a typical installation comprises generally the combination of rack meansadjacent to an overhead door trackway, a shaft disposed transversely of the trackway for travel with the door, pinion gear means carried by this shaft for driving engagement with the rack means, power means for driving the gear means to open and close the door, and link means connecting the shaft to an upper portion of the door; Again the essence of this phase of this present invention is in the novel concept of using links to estab- H811 and maintain tight weather seals between the upper parts of an overhead door assembly and the door frames when the doors are closed. This is essentially a matter of using the connections between the drive unit and the door not only for opening and closing the door, but also to hold it in closed, weather-tight condition through the continuous application of a horizontal frce component to. the upper portion of the door.

In still another aspect, this invention is predicated upon the novel concept of'utilizing door inertia forces to assist in: door operation. Thus, an overhead door operator embodying this phase of my invention in general will comprise a motor having a power output shaft, gear means for operative connection to an overhead door, a friction clutch operatively connecting the power output shaft to the gear means, sensing means for measuring clutch slip and automatic motor control means connected to the sensing means and including a switch which will de-energize the motor when the clutch slips to a predetermined extent.

In a preferred embodiment of this invention this overhead door operator includes a power transmission assembly comprising a sheave rotatably mounted on a drive shaft and having a shank which in turn has an externally treated portion. It also incorporates clutch means operatively associated with the drive shaft and the sheave for driving the sheave. A nut is screwed on the threaded portion of the shank and guiding means is provided for the nut, being mounted for rotation with the drive shaft so that the nut is caused to rotate with the. said shaft and relative to the sheave and to move axially along the shaft when the clutch slips under loads in excess of predetermined amounts.

As shown in the drawings, thesevarious novel aspects or features of this invention are all incorporated in one overhead door installation in order to secure all the special and new advantages and results set forth above. Thus, as shown in Figs. 3 and 4, a typical hinged-panel overhead door installation including a door 10 and a trackway 11 supporting the door for travel between its open and its closed positions is provided with operating apparatus of this invention including a power unit 14 and suspension means 15. Link means 17 serves to deliver directly to the door the power output of the drive unit and also to seal the door and hold it in sealed relation to the jamb when the door is closed, as stated above.

Drive unit 14 may be considered as being made up of motor 20 (Fig. l) with its power output shaft 2]., drive shaft 22 connected to the motor by sheaves 24- and 25 keyed respectively to shafts 21 and 22 and belt 26. It also includes slip switch 28 and clutch assembly 29, sub sequently to be described in more detail, and pinion shafts 30 and 31 with their equalizer shaft 32 extending therebetween and connecting the pinion shafts. Pinion gears 34 and 35, pinion sheave 36 which is keyed to pinion shaft 30, belt 37 and racks 38 and 39 complete this assembly. However, as previously indicated, certain of these elements such as racks 38 and 39, and the pinion shafts and ball bearings 42 and 43 mounted on the free ends of shafts 30- and 31 and disposed in the channels comprising racks 38 and 39 serve dual functions and may be regarded as both guiding and driving means andmotor" suspension means.

As shown in Figs. 4 and 5, racks 38 and 39 are preferably mounted on top of guide rails 11 and securely fastened thereto, and pinion gears 34 and 35 are disposed at the free ends of the pinion shafts in driving engagement with the respective racks over substantially the full horizontal length of guide rails 11. Ball bearings 42 and 43 run in supporting engagement with the rack channels and are dimensioned in respect to these channels and gears 34 and 35 so that the gears are maintained in drivmg relation to their respective racks. Equalizer shaft 32 serves to equalize driving forces delivered to racks 38 and 39, and in cooperation with link means 17 prevents wedging or cocking or misalignment of the door in its travel along trackway 11.

Link means 17 comprises a link arm 40 and a door angle 41 to which arm 40 is pivotally connected at one end. Adjacent to its other end arm 40 is journaled on pinion shaft 31. This assembly also includes motor carriage 44. and a door angle 45 which is the mate to angle 41 and by means of which carriage 44 is secured to door 10 along the side thereof opposite door angle 41 and adjacent to the upper end of the door. Carriage 44 1s journaled on pinion shaft 30, as indicated in Figs. 3 and 4, and its weight distribution is such that before attachment to angle 45. the carriage assumes an angle of 143 50 the vertical, as indicated by the center line in 1g. Motor carirage 44 is of box-like sheet metal construcnon and open at its top. The sides of the carriage are provided with a plurality of openings near one end for shafts 21, 22, and 30. Adjacent to its other end the carriage is provided with a transversely-extending wall portion which together with the side walls provides a small compartment, open at one end at its top, in which 15 mounted a conventional radio signal receiver 46. In the larger compartment where motor 20 is mounted, a snap switch 50 is provided, this element being secured in suitable'rnanner in fixed position to the carriage and havinga follower lead 51 extended for engagement with an element of slip switch 28.

Receiver 46 and snap switch 50 are electrically connected to motor 20 and to a source of electric power (not shown) suitably delivered to the apparatus by means of a switch'53 mounted in carriage 44.

The weight distribution in carriage 44 is such that when the door is fully open, only a negligible load is imposed; upon: the door by motor 20 and the other parts carried by carriage 44. Whatever slight load is thus imposed upon the-door, as indicated by the 42 angle of repose of the carriage in Fig. 2, is diminished as the door is moved toward closed position until just before the door isfully closed when no weight whatsoever is imposed upon the door by this assembly.

Slip switch assembly 28 in addition to snap switch 50 and its follower leaf 51 includes a sheave 54 which has a shank 55 provided with an externally threaded portion 56. As shown to best advantage in Fig. 6, it also includes a nut 57 which is screwed on portion 56 and which has an annular groove 58 in its periphery to receive the free end of leaf 51. Abutment 59 and abutment 60 are located on opposite sides of portion 56 and spaced therefrom approximately equal distances to engage and return springs 61 and 62 in position on opposite sides of nut 57. Thus, the nut will be held in engagement with the threaded portion of shank 55. Such axial travel of nut 57 results from the action of guide member 64 attached non-rotatably to shaft 22 and having a pin 65 extending through an opening provided in the nut so as to lock the nut against rotation relative to shaft 22.

Clutch assembly 29 includes in addition to sheave 54, pressure plate 68 and thrust washer or clutch plate 69 which is held by the pressure plate against the outside surface of sheave 54. Pressure plate 68 is keyed to shaft 22 but is adjustable axially thereof so that the clutch can be regulated, a spring 70 and wingnut 71 enabling easy manual adjustment of clutch 29.

In the operation of the illustrated apparatus, door is raised and lowered by motor 20 acting through shafts 30, 31, and 32, racks 38 and 39 and gears 34 and 35.

Actuation for the motor may be by means of a manual switch (not shown) suitably mounted in the house or in the garage, or by a radio signal means (also not shown) mounted in an automobile and operated by a push button, the signal being picked up by receiver 46. Relay 73 is a latching relay and it is connected to motor 20 through switch 53 and controlled by receiver 46. This relay is always used in starting the motor and it may be used to stop the motor, as when it is desired to position the door at a point between its two extreme limits without operating switch 50. Relay 73 is of the double-throw, singlepole type and is so related to switch 50, which is of the three-pole, double-throw type, that the latter never functions to start motor 20 and is not necessarily involved in stopping the motor. Switch 50, however, must be actuated to cause reversal of direction of door travel. Thus, the door is automatically moved toward its open position after its travel toward closed position has been arrested for a sufficient time that switch 50 is thrown by clutch slip-sensing assembly 28. Likewise, the door 10 is moved toward its closed position when the motor 20 is started by means of relay 73 following operation of switch 50 having a period when the door was being moved toward its open position.

In normal operation when there are no obstructions barring travel of the door between its open and its closed positions, the door will be moved through the full length of its travel course each time that the motor is energized. However, the door may be stopped at any intermediate point and maintained at that point for as long as desired simply by operating relay 73 to disconnect the motor from its power source. The operator thus may position the door at a halfway location simply by pressing a push button control and thereby breaking the motor circuit and deenergizing the motor when the door reaches that location.

The door will automatically stop at an intermediate location along its travel course through the operation of slip switch assembly 28 when an obstruction arrests door motion and switch 50 is ultimately thrown to break the motor circuit. The actual force or eifort necessary to arrest the door can be adjusted within normal operating limits by turning wing nut 71 on drive shaft 22 and thereby increasing or decreasing the energy in spring 70 and the pressure exerted by pressure plate 68 on washer 69 in the art that this adjustment will have a direct bearing upon the safety factor. It will also be understood, as stated above, that clutch assembly 29 may be set at a value substantially below that corresponding to the anticipated peak load condition. The reason for this is that door inertia will aid in overcoming such loads as may be imposed in the ordinary normal operation of this installation. Accordingly, by way of example, where a peak load of 50 pounds might reasonably be anticipated in the operation of a given door, its clutch assembly 29 may be pre-set for a 35-pound load with the expectation that the diiferential of 15 pounds will be made up by door inertia. The lag in operation of switch 50 following application of loads in excess of those for which clutch assembly 29 is set enables this unique result without any sacrifice or loss of safety advantage.

When motor 20 is energized following actuation of switch 50 by assembly 28, nut 57 will be at one end of threaded portion 56, i.e. at one of the two extreme posi tions it occupies to actuate switch 50. Drive shaft 22 is turned in the opposite directions on alternate actuatio-ns of switch 59 and nut 57 thus is caused to reverse its direction of travel each time the motor is started following actuation of said switch 50. However, nut 57 may be started and stopped several times and on each incident of clutch slip between motor starts, the nut will move along shank 55 in one direction until switch 50 is finally thrown to stop motor 20. Normally, nut 57 will move through a full turn in the starting operation as door inertia is overcome and the door is started along its travel course. As the door continues to move and there is no further slip in the clutch, nut 57 remains stationary relative to the threaded portion 56 and sheave 54. When the door reaches an obstruction so that the clutch is caused to slip and sheave 54 comes to a stop, nut 57 is driven toward its other extreme position through rotation of the nut with shaft 22 through the action of guide member 64 and pin 65, shank 55 of sheave 54 remaining stationary so long as the obstruction bars travel of the door. If, however, door inertia serves to overcome the obstruction condition, clutch assembly 29 will again drive pinion gears 34 and 35 through sheave '54 and rotation of shaft 22 with respect to shank 55 of the sheave will cease. On meeting another obstruction, as for instance when the door comes to rest against an abutment at the end of its travel course, the clutch again will slip and since door inertia cannot remove or overcome this obstruction, nut 57 will be moved to its other extreme position through rotation of the nut relative to shank 55, actuating snap switch 50 through its leaf 51. Nut 57 is re tained against disengagement from threaded portion 56 of the sheave by springs 61 and 62. Furthermore, nut 57 is self-starting on threaded portion 56 so that actuation of switch 50 each time the door operator assembly is used is assured.

The wiring diagram of Fig. 7 serves to illustrate the, relationships between the various components and ele ments of a typical garage door operator of this invention as illustrated and described above.

The schematic wiring diagram of Fig. 7 serves to illustrate the relationship between the various components and elements of a typical garage door operator of this invention as illustrated and described above. The system is designed for use with the customary, domestic volt, 60 cycle, electrical power supply as supplied through lines designated L1 and L2. The radio receiver 46 receives such electrical power directly from the power lines through a pair of branches thereof and motor 20 receives power from these lines under the control of a latching relay 73 and reversing snap switch 50. One terminal of a winding 20a of the motor is directly connected to line L1 of the power supply.

Latching relay 73 includes an arm 73a pivotal to conductively engage either of a pair of contacts 73b or 730 and the arm is movable under the magnetic influence of a relay solenoid 73d which in turn isenergizable from step-down transformer 74 under the control of radio re ceiver 46 or a manually actuable push button 75. The primary winding 74a of transformer 74 is directly connected across the line wires L1 and L2 and the secondary winding 74b is connected in series with solenoid 73d and parallel connected tenninals 46a of radio receiver 46 and contacts 75a of push button 75. Accordingly, closure of contacts 46a as may occur by the reception of an appropriate signal by the receiver or the manual closure of contacts 75a of push button 75 completes the electrical circuit through winding 74b and solenoid 73d whereby the arm 73:; of relay 73 is flipped to its alternate position.

.Deenergization and reenergization of the solenoid 73d is effective in each instance to flip arm 73:: to its alternate position.

Line L2 is connected to arm 73a of relay 73 and the contacts 73b and 73c are connected, respectively, to contacts 50c and 50b of one section of snap switch 50. Arm 50a of this section of snap switch 50 is connected to the other terminal of running winding 26a of motor 20 whereby line power is applied to this winding when the arms 73a and 50:: are connected with contacts 73b and 50c, respectively, or with contacts 730 and 50b, respectively.

A starting winding 20b of motor 20 is provided for creating a starting field for the motor together with winding 20a and two sections of switch 50 are provided for controlling the direction of current flow through the winding to consequently control the direction of rotation of the motor armature. The arm 50d of the second section of the motor is connected to the arm 50a and a first contact 50c of this section of the switch is connetced to the terminal of a normally closed centrifugal switch 200 which has an arm connected to one terminal of winding 20b for automatically disconnecting the winding 2% from the motor circuit when the motor reaches a certain speed and a contact 50) of the second section of the switch is connected to the other terminal of winding 2%. A third section of switch 50 includes an arm 50g connected to line L1, 21 first contact 5011 connected to contact 50 of the second section of the switch and a contact 50: connected to contact 502 of the second section of 'the switch. Each of the arms 50a, 50a and 50g is connected to arm 51 for unitary movement of the switch arms.

In the quiescent condition of the door opener, the components may be as illustrated in Fig. 7 wherein the power flow in line L2 is interrupted at switch 50, since arm 50a is in engagement with contact 50c and at latch relay arm 73a is not connected to contact 73b which is connected to contact 50c. In response to energization of solenoid 73d, however, as may be accomplished by the reception of a signal at receiver 46 or by manipulation of push button 75, arm 73a is flipped to its alternate position in engagement with contact 7312. A circuit is completed through line L1, winding 20a, arm 50a, contact 50c, contact 7312, arm 73a and line L2. Additionally, under these circumstances, switch 20c is closed and another circuit is simultaneously established through line L1, arm 50g, contact 501', switch 200, winding 2012, contact 50 arm 50d, arm 50a, contact 50c, contact 735, arm 73a and line L2. Accordingly, the windings of motor 20 are supplied with line power and after the motor reaches a certain speed, switch 20c is actuated to disconnect winding 20b and the motor operates until the flow of power is automatically interrupted by the action of arm 51 flipping the arms of switch 50 to their alternate positions in a manner described hereinabove. At this occasion, line L2 is interrupted since arm 50a engages a contact 50b which is connected to the terminal 73c of relay 73 which is not engaged nor energized by arm 73a. It should be observed that at all times, line L1 is applied "to the top of winding 20a as seen in Fig. 7 and that during the period of starting of motor 20, as just described,

switch 50 is positioned to apply line L1 to the bottom .of the winding 20b as seen in Fig. 7 and line L2 to the top of this winding to establisha first direction of current flow through the winding and therefore afirst direction of rotation of the motor.

If the positions of the arms of relay 73 and switch 50 are in their alternate positions as shown in dotted lines in Fig. 7, the circuit is again in the quiescent condition. Re-application of energizing current to solenoid 73d, however, flips arm 73a to the position shown in solid in this figure to again supply line voltage to the motor. The

circuit through winding 20a is completed through arm 50a, contact 50b, contact 73c and line L2 and the circuit through starting winding is completed through line L1, arm 50g, contact 50h, winding 20b, switch 20c, contact 50c, arm 50d, arm 50a, contact 50b, contact 73c, arm 73a and line L2. Again the motor increases in speed until switch 20c automatically disconnects winding 20b from the circuit. Under these circumstances, however, the arms of switch 50 are positioned to apply line L1 to the top of winding 20b as shown in Fig. 7 and line L2 to the bottom thereof while lines L1 and L2 are applied to the top and bottom, respectively, of winding 20a. Accordingly, the current flow through winding 20b is reversed from the direction of flow when switch 50 was in its alternate position and since the current flow through winding 20a is in the same direction, the direction of rotation of the motor is reversed. The motor drives the door until the flow of power to the motor is interrupted by the actuator of switch 50 by arm 51 in a manner hereinabove set forth.

From the foregoing description it is apparent that as solenoid 73d of latch relay 73 is successively energized, the motor 20 is alternately driven in opposite directions to raise or lower the door.

Having thus described this invention in such full, clear, concise and exact terms as to enable any person skilled in the ,art to which it pertains to make and use the same, and having set forth the best mode contemplated of carrying out this invention, I state that the subject matter which I regard as being my invention is particularly pointed out and distinctly claimed in what is claimed, it being understood that equivalents or modifications of, or substitutions for, parts of the above specifically described embodiments of the invention may be made without departing from the scope of the invention as set forth in what is claimed.

What is claimed is:

l. A power transmission assembly for an overhead door operating apparatus enabling unique use of door inertia during operation to assist in opening and closing the door, which comprises a drive shaft attached to the door, a sheave r-otatably carried on the drive shaft and having a shank with an externally threaded portion, clutch means operatively associated with the drive shaft and the sheave for driving the sheave, a door propelling shaft on the door, belt means driven by the sheave and operatively associated with said propelling shaft for driving the same, a nut screwed on the said threaded portion of the shank, and guiding means secured to the drive shaft for rotation therewith and engaging the nut whereby the nut is caused to rotate with the drive shaft and relative to the sheave and to move axially of the drive shaft along said threaded portion of the shank when the driving force applied to the sheave exceeds a predetermined value and the clutch is caused to slip.

2. A power transmission assembly for an overhead door operating apparatus enabling unique use of door inertia during operation to assist in opening and closing the door, which comprises a drive shaft attached to the door, a sheave rotatably carried on the drive shaft and having a shank with an externally threaded portion, clutch means including a pressure plate keyed to the drive shaft for movement axially thereof a clutch plate held against the sheave by the pressure plate, means including a spring for adjusting the pressure between the pressure plate and the clutch plate, a door propelling shaft on the door, belt means driven by the sheave and operatively associated with said propelling shaft for driving the same, a nut screwed on the said threaded portion of the shank, and guiding means secured to the drive shaft for rotation therewith and engaging the nut whereby the nut is caused to rotate with the drive shaft and relative to the sheave and to move axially of the drive shaft along said threaded portion of the shank when the driving force applied to the sheave exceeds a predetermined value and the clutch is caused to slip.

3. A power transmission assembly for an overhead door operating apparatus enabling unique use of door inertia during operation to assist in opening and closing the door, which comprises a drive shaft attached to the door, a sheave rotatably carried on the drive shaft and having a shank with an externally threaded portion, clutch means operatively associated with the drive shaft and the sheave for driving the sheave, a door propelling shaft on the door, belt means driven by the sheave and operatively associated with said propelling shaft for driving the same, a nut screwed on the said threaded portion of the shank, guiding means secured to the drive shaft for rotation therewith and engaging the nut whereby the nut is caused to rotate with the drive shaft and relative to the sheave and to move axially of the drive shaft along said threaded portion of the shank when the driving force applied to the sheave exceeds a predetermined value and the clutch is caused to slip, and means for positioning and starting the nut including a pair of springs disposed on opposite sides of the nut to engage the nut when it reaches either end of the threaded portion and to resiliently urge the nut into engagement with said threaded portion.

4. In an overhead door operating apparatus including an electric motor to be connected to an overhead door, gear means to be operatively connected to the overhead door and a snap switch to connect the motor to an electric power source, the combination of a transmission assembly enabling unique use of door inertia during operation to assist in opening and closing the door which comprises a drive shaft to be attached to the overhead door, a sheave rotatably carried on the drive shaft and having a shank with an externally threaded portion, clutch means operatively associated with the drive shaft and the sheave for driving the sheave, a door propelling shaft, belt means driven by said sheave and operatively associated with said propelling shaft for driving the same, a nut screwed on the said threaded portion of the shank, said nut having an annular peripheral groove in which to receive a follower leaf of the snap switch and guiding means secured to the drive shaft for rotation therewith and engaging the .nut whereby the nut is caused to rotate with the drive shaft and relative to the sheave and to move the snap switch follower leaf axially of the drive shaft when the driving force applied to the sheave exceeds a predetermined value and the clutch is caused to slip and to open the snap switch and de-energize the'motor when the drive shaft has completed a predetermined number of revolutions relative to the sheave.

5. A power assembly comprising an electric motor, an electric power source for connection to the motor, a snap switch to disconnect the motor from the power source, a drive shaft, a sheave rotatably carried on the drive shaft and having a shank with an externally threaded portion, slip clutch means operatively associated with the drive shaft and the sheave for driving the sheave, a driven shaft, belt means driven by the sheave for driving said driven shaft, a nut screwed on the said threaded portion of the shank, said nut having an annular peripheral groove in which to receive a follower leaf of the snap switch, and guiding means secured to the drive shaft for rotation therewith and engaging the nut whereby the nut is caused to rotate with the drive shaft and relative to the sheave and to move the snap switch follower leaf axially of the drive shaft when the driving force app-lied to the sheave exceeds a predetermined value and the clutch is caused to slip and to open the snap switch and de-energize the motor when the drive shaft has completed a predetermined number of revolutions relative to the sheave.

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