Patent Publication Number: US-4059339-A

Title: Brake mechanism for motor driven projection screen

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to projection screen apparatus, and more particularly, to a motor-driven projection screen apparatus. 
     While the size of the projection screen is not of the essence, these screens which are intended for use with the present invention are generally quite large, ranging in width from 5 to 14 feet or more. They are the type of screens which are frequently found in auditoriums, in school rooms or in conference rooms. They are remotely controlled electrically by a small electric motor having a rotatable shaft which is operatively coupled to a large roller. The roller is generally journaled in a casing and the casing houses both the roller and the electric motor. The screen is, of course, rolled upon the roller when the screen is moved to a closed position and unrolled from the roller when it is moved to an exposed viewing position. 
     The electric motors generally employed in this environment are commonly called &#34;gear motors&#34; and include a small electric motor and a reduction gear mechanism coupled to the output shaft of the motor to regulate the rate of rotation of the reduction gear output shaft. The gear motors have to be small in size because of the space limitations and yet they must have sufficient drive to turn the roller which carries the heavy suspended screen. Because the screen is very large and heavy, there is considerable gravitational force tending to unwind the screen from the roller. In order to prevent the screen from peeling from the roller and thereby causing the roller and the shaft of the motor to rotate, the motor must have a braking mechanism which will hold the roller against rotation to prevent unintended movement of the screen whenever the screen is properly positioned. In order to overcome the extreme forces tending to unwind the screen, the braking mechanism must be both reliable and effective. If the braking mechanism should fail to hold the screen in its intended position, the screen will unwind and ultimately be stripped from its adhered position on the roller. 
     In the prior art, motors are often provided with brake mechanisms. These motors have axially movable shafts, the shafts carrying a braking surface within the motor housing which normally is urged into engagement with a braking surface on an internal axial wall of the motor housing. When the motor is operated, the braking surfaces are urged apart to release the motor as the shaft-mounted rotor axially aligns itself in the magnetic field generated by the stator. However, because of the size of the motor, the size of the braking surfaces was limited and, therefore, the holding power of the braking mechanism. 
     Attempts have been made to increase the efficiency of the brake by increasing the surface areas involved. In one such mechanism, the cooperating braking surfaces were formed in a conical shape. However, the capabilities of the brakes were still limited by the size of the motor, or more particularly, by the size of motor housing interior. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the present invention to provide an electric motor for use in a motor-driven screen apparatus having a brake mechanism with increased braking action. 
     It is a further object of the present invention to provide a brake mechanism for an electric motor which may be easily and conveniently repaired, replaced, adjusted or altered without dismantling the motor. 
     An additional feature of the present invention is the improved braking surfaces which increase the friction created therebetween and thereby increase the effectiveness of the brake mechanism. 
     In accordance with the present invention, an electric motor having an axial movable output shaft is provided with a brake mechanism which is mounted externally of the motor housing. The brake mechanism includes an exterior brake support, a brake surface mounted on the support about an end of the motor shaft projecting from the motor housing, a cooperating, confronting brake surface adjacent the first surface carried by the end of the motor shaft extending from the housing through the first surface, and spring means normally urging the brake surfaces into contact. 
     In an exemplary embodiment of the present invention, the second surface is adjustable along the motor shaft to alter the spring force. In addition, both the surfaces may be easily replaced with larger or smaller surfaces so that the friction area is increased or decreased, thereby increasing or decreasing the braking pressure force per unit area. 
     In a preferred embodiment, the motor housing carries a resilient hard rubber disk and the motor shaft carries a rigid supporting disk which, in turn, carries a medium grade sandpaper which contacts the rubber disk to effect braking action. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the motor-driven projection screen apparatus of the present invention mounted on a wall behind a valance with portions of the valance broken away to show the flexible projection screen in its retracted closed position; 
     FIG. 2 is another perspective view of the apparatus of FIG. 1 showing the screen in its fully extended, exposed viewing position; 
     FIG. 3 is an enlarged exploded perspective view of the motor-driven projection screen apparatus showing an end portion of the modular screen unit and portions of the modular drive motor unit to illustrate the manner of mounting the units together; 
     FIG. 4 is an enlarged broken sectional view of the apparatus taken as indicated on line 4--4 of FIG. 1; 
     FIG. 5 is a broken sectional view taken as indicated on line 5--5 of FIG. 4 particularly showing one of the bolts which detachably or releasably secure the modular drive motor unit to the modular screen unit; 
     FIG. 6 is an enlarged sectional view taken as indicated on line 6--6 of FIG. 4 showing the roller and the pivoted control levers when the projection screen is in its closed position; 
     FIG. 7 is a sectional view of the apparatus similar to FIG. 6, but showing the roller and the pivoted control levers as positioned when the screen is nearly fully extended; 
     FIG. 8 is a sectional view taken as indicated on line 8--8 of FIG. 6 with certain parts broken away and particularly showing the cooperative relationship between the micro switch and the control lever with the control lever being precluded from depressing the micro switch because the free end of the control lever is riding on or engaging the convolutions of screen on the roller; 
     FIG. 9 is a sectional view taken as indicated on line 9--9 of FIG. 6 with certain parts broken away and showing the portion of the screen slat which cooperates with a lever to control a micro switch to deactivate the electric motor with the screen rolled up in closed position; 
     FIG. 10 is an enlarged broken sectional view of the electric gear motor taken as indicated on line 10--10 of FIG. 4 showing the rotor and its shaft shifted axially to operative position to compress the spring means and to shift the brake to released position; 
     FIG. 11 is a further enlarged broken sectional view similar to FIG. 10 but showing the rotor and its shaft shifted to inoperative position by the spring means which in turn shifts the brake to a holding position; and 
     FIG. 12 is a schematic diagram of the electrical circuit which controls the operation of the roller of the projection screen apparatus to move the flexible screen between a closed position and an exposed operative viewing position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Modular Motor Driven Projection Screen Apparatus 
     The modularity of the projection screen apparatus is shown in FIGS. 1-5. The apparatus includes a modular screen unit, generally designated 10, and a modular drive motor unit generally designated 11, mounted adjacent thereto in a manner to be somewhat of a longitudinal extension of the unit 10. 
     The modular screen unit 10 preferably has a conventional outer casing 12 which is constructed to provide an elongated inner chamber which is preferably closed at its opposite ends by a pair of removable end caps 14 and 15 so as to substantially close the modular screen unit 10. The end caps 14 and 15 are secured to the exterior of the outer wall of casing 12 by machine screws 17 passing through inwardly extending flanges 18 carried by the end caps 14 and 15 and threaded into the end portions of the casing 12. Each of the end caps 14 and 15 is preferably provided with a bracket 20 for mounting the screen modular unit 10 on a supporting surface, such as on a wall 21, behind a valance 22, in a recess in the ceiling, or the like. 
     A flexible screen 24 having a conventional slat 25 fixed transversely to its free marginal edge portion is movable between a retracted closed position (shown in FIG. 1) and an exposed operative position (shown in FIG. 2) through a longitudinal slot or opening 29 in the casing 12. The flexible screen 24 is carried on an elongated roller 27, which is preferably hollow. Herein, the roller 27 may be closed at each end by a disk 28. 
     The inner marginal edge portion of the screen 24 is preferably adhered longitudinally of the roller 27, and then two or three convolutions of the screen 24 are preferably wound about the roller 27 so that the adhered marginal edge portion of the screen 24 will be securely fastened to the roller 27. As noted in FIG. 7, a couple of turns of the screen 24 will preferably remain circumposed on the roller 27 when the screen 24 is lowered to fully extended exposed operative position. 
     As more clearly shown in FIGS. 6 and 7, the screen 24 is played out to its exposed position through the longitudinal slot or opening 29 extending between end caps 14 and 15 and defined by a pair of inwardly extending flanges 30 and 31 of the wall of casing 12. When the flexible screen 24 is rewound on the roller 27 to its closed position, the entire length of slat 25 is nested in the elongated slot 29 and abuts the flanges 30 and 31 of the casing. 
     Referring to FIG. 4, the roller 27 has a pair of journals 33 and 34 extending laterally therefrom along its longitudinal axis for rotatably mounting the roller 27 within the modular screen unit 10. The end cap 14 preferably is provided with an internal bearing bracket 35 into which the journal 33 is rotatably mounted. The opposite end cap 15 preferably has a bore with a bearing 37 through which the journal 34 extends so as preferably to provide an exposed end projecting from the casing of the modular screen unit 10. 
     The exposed end of the journal 34 is fitted with a coupling member 38 preferably in the form of a clutch portion. As will be later more apparent, the coupling member 38 is adapted to cooperate with a similar member on the modular drive motor unit 11 to drive roller 27; and the coupling member has the capability of being releasably engaged with the similar member to permit ready separation of the modular drive motor unit 11 from the modular screen unit 10. 
     The modular drive motor unit 11 preferably has an outer casing or housing 41 which includes an outer wall constructed to afford an inner chamber. A pair of removable end caps 43 and 44 for the housing 41, substantially close the opposite ends of the inner chamber. The end cap 43 is secured to the housing 41 by machine screws 45 passing through inwardly extending flanges 47 of the end cap 43 which embrace the outer wall of the housing. The other end cap 44 is secured to the interior of the outer wall of housing 41 by sheet metal screws 48 which pass through the end portion of the outer wall and are threaded into the inwardly extending flanges 50 carried by the end cap 44. 
     As best seen in FIGS. 4 and 5, the modular drive motor unit 11 has an internal plate 51 detachably mounted therein by screws 52 passing through the outer wall of housing 41 and threaded into the inwardly extending flanges 54 carried by the internal plate 51. A motor assembly, generally designated 55, is mounted on the internal plate 51 by a plurality of threaded bolts 57 passing through the internal plate 51 and threaded into the motor assembly 55. The motor assembly 55 has a drive shaft 58 to which, at its outwardly projecting free end, is secured a coupling member 60 preferably in the form of a clutch portion. The coupling member 60 is preferably spaced inwardly from the end cap 44 and is shaped to engage the complementary, mating, roller coupling member 38 when one of the coupling member 38 and 60 is moved longitudinally into the other coupling member. In other words, the complementary coupling members each may have male and female portions which interfit to provide a driving relationship between the members. When the coupling member 38 of the roller 27 and the coupling member 60 of the motor assembly 55 are in engagement, the motor assembly 55 will turn the coupling members 38 and 60 and, thus, will also drive the roller 27 which is journaled in the modular screen unit 10. 
     As seen most clearly in FIG. 3, the end cap 44 is provided with a plurality of apertures to permit insertion of any components protruding from the end cap 15 of the modular screen unit 10. The cooperating parts of the confronting end caps 15 and 44 aid in guiding and centering the modular drive motor unit 11 with respect to the modular screen unit 10 so that the modular drive motor unit 11 becomes preferably an extension of the modular screen unit 10. 
     The aperture 61 permits insertion of the coupling member 38 into the modular drive motor unit 11 for engagement with its complementary coupling member 60. A pair of guiding studs 62 carried by the end cap 15 fit into respective guide openings 64 which are formed in the end cap 44, which end cap functions as a guide plate. A male type electric plug 65 carried by the end cap 15 is fittable into a rectangular aperture 67 defined by the end cap 44 so as to engage a female type electric plug 68 secured to the internal plate 51. 
     Another opening 70 in the end cap 44 receives the end portion of a pivoted lever 71 carried by the end cap 15 which detects the fully closed position of the screen 24. Still another opening 72 in the end cap 44 receives the end portion of a second pivoted lever 74 carried by the end cap 15 which detects the fully exposed position of the screen 24. 
     The modular drive motor unit 11 is detachably secured to the modular screen unit 10 by four elongated bolts 77 preferably sharply pointed, each of which extends longitudinally through the modular drive motor unit 11 and is threaded into the confronting end cap 15 of the modular screen unit 10. 
     The heads 78 of the bolts 77 are exposed on the exterior of the end cap 43 of the modular drive motor unit 11. When the modular drive motor unit 11 is separated from the casing, each of the bolts 77 is maintained in a predetermined withdrawn position (shown in phantom in FIG. 5) by an appropriately positioned nut 80 on an intermediate portion of the bolt 77, the nut 80 abutting the interior plate 51. 
     Thus, when it is desired to mount the modular drive motor unit 11 on the end of the modular screen unit 10, the guide portions on the respective end caps 15 and 44 are first utilized to correctly place the modular drive motor unit 11 against the end of the modular screen unit 10. Then, each end of the pointed bolts 77 can be slid longitudinally through their respective openings 81 in the end cap 44 and screwed into their respective threaded apertures 82 on the end cap 15 of the modular screen unit 10. The bolts 77 are easily aligned within the modular drive motor unit 11 because the bolts 77 are guided and directed by aligned passages 84 and 85 placed in the end cap 43 and the internal plate 51, respectively, and through which the bolts 77 pass. 
     When the modular drive motor unit 11 is detached from the modular screen casing 10, the motor assembly may then be removed from the outer wall of housing 41. By withdrawing the screws 45 and 52, the end cap 43 and the internal plate 51, respectively, are detached from the outer wall housing 41 thereby permitting withdrawal of the end cap 43, the internal plate 51, the motor assembly 55, and the bolts 77 as a single unit from the housing 41. Thereafter, the screws 57 may be removed to release the motor assembly 55 from its position between the end cap 43 and the internal plate 51 thereby permitting convenient repair or replacement of the motor assembly 55. 
     2. Control Mechanism for Motor Driven Screen 
     Schematically illustrated in FIG. 12 is an electrical circuit particularly suited to control the operation of the motor driven projection screen described herein. 
     The circuit includes a small reversible synchronous electric motor 90, a source of electric current 91, and a three condition master control switch 92. The master control switch 92 is usually conveniently located on a wall somewhere beneath the screen so that it is accessible for manual operation. The lower area enclosed within the dashed line 94 is the portion of the circuit disposed within the modular drive motor unit 11, while the upper area enclosed within the dashed line 95 is the portion of the circuit disposed within the modular screen unit 10. 
     The circuit portion within the modular screen unit 10 includes a pair of normally-closed electric switches 96 and 97 which are connected to the male plug 65 by appropriate electrical lines. The master control switch 92 and the electric motor 90 are connected to the female plug 68 by appropriate electrical lines. Electric current to drive the circuit is supplied through electrical lines 98 and 99. When the male plug 65 and the female plug 68 are mated and electrically connected, the screen 24 may be operatively raised and lowered. 
     Since switches 96 and 97 are normally urged to a closed, electrical conducting condition, then as long as these switches are closed, the rotation of the roller 27 and the raising and lowering of the flexible screen 24 is controlled by the master switch 92. As will be subsequently made apparent, switch 96 is moved to open condition to disconnect the motor 90 only when the screen is fully wound on the roller in closed position (FIG. 6), and switch 97 is moved to open condition to disconnect the motor 90 only when the screen is unwound from the roller to exposed operative position (FIG. 7). 
     a. control Mechanism for Locating Screen in Closed Position 
     Referring in particular to FIGS. 6, 7, 9 and 12, the control mechanism for detecting or locating the screen 24 in a closed nested position and for terminating operation of the motor 90 is shown. The control mechanism primarily includes the mechanical pivoted lever 71 and the electric switch assembly 96 which is anchored against the internal surface of the end cap 15 by sheet metal screws 100. 
     The elongate lever 71 preferably has a pair of upwardly extending spaced flanges 101 which are pivoted to a pair of outwardly projecting fingers 102 of the end cap 15 by a pin 104. The lever 71 is normally urged by gravity (or by suitable spring means) away from the switch assembly 96, as shown in FIG. 7 and in phantom in FIG. 9. 
     The switch assembly 96 has a body portion preferably provided with a plunger or button 107 which is spring urged outwardly of the body portion to normally close the switch assembly (FIG. 7). The switch assembly 96 is also preferably provided with a plunger actuating arm 105 anchored in the body portion. The arm 105 may be of resilient material to afford movement toward and away from the plunger 107 to control the electrical condition of the switch assembly 96. Thus, in FIG. 6, the arm 105 has been caused to depress plunger 107 to open the switch assembly; while in FIG. 7, the arm 105 has been released to allow the plunger 107 to be spring-urged outwardly to close the switch assembly. 
     The lever 71 and the switch assembly 96 are positioned to cooperate with an actuating portion preferably near an end of the slat 25 so that the screen in closed position (FIG. 6) is accurately and precisely positioned with the slat 25 fully and tightly nested in the casing slot 29. In fact, the circuit to the motor is not interrupted, and roller rotation stopped, until the fully nested, closed position of the flexible screen is attained. 
     Thus, when the screen 24 is to be retracted from an exposed position (FIG. 7) to a closed position (FIG. 6), the operator merely shifts the master control switch 92 to position A (FIG. 12) to complete a circuit between the power source 91 and the electric motor 90. The motor 90 drives the roller 27 in a direction to cause the screen 24 to be wound about the roller 27 and thereby effect retraction of the screen 24 into the modular screen unit 10. As the screen approaches full retraction and the slat 25 begins to nest between the flanges 30 and 31 within the casing slot 29, the actuating portion of slat 25 (FIGS. 6 and 9) will contact and pivot the lever 71 upward to move arm 105 which depresses plunger 107, so that the switch assembly 96 opens the circuit and stops the motor 90. 
     This control mechanism described above is actuated independently of the master control switch 92. Thus, a person operating the screen can hold the master control switch 92 toward position A (screen closing condition), but when the screen 24 reaches its closed position, its movement will automatically stop even though the master control switch 92 remains engaged. Hence, the operator need not carefully regulate the master control switch 92 since operation will automatically cease as soon as the slat 25 is fully nested. 
     When the screen 24 is to be lowered from a closed position, the master control switch 92 is shifted to position B (FIG. 12) to complete a circuit between the power source 91 and the electric motor 90. The motor 90 then runs in reverse to cause the screen 24 to be unwound from the roller 27 and thereby effect extension of the screen 24 from the modular screen unit 10. When the slat 25 is unnested, the lever 71, by gravity and by spring action of the switch assembly 96, returns to its downward position so that the plunger 107 is urged outwardly and the switch 96 is closed once again to condition the circuit again for retraction of the screen 24. 
     Of course, the screen 24 may be stopped at any intermediate position between its fully extended exposed and fully retracted closed position by appropriate flipping of the master control switch 92. 
     B. control Mechanism for Locating Screen in Viewing Position 
     Referring in particular to FIGS. 6, 7, 8 and 12, the control mechanism for detecting that the screen 24 has unwound to a fully exposed viewing position and for terminating the operation of the motor 90 is shown. The control mechanism primarily includes the pivoted lever 74 and the electric switch assembly 97 which is secured to the inner surface of the end cap 15 by machine screws 108. 
     The lever 74 preferably has a pair of upwardly extending spaced flanges 109 which are pivoted to a pair of outwardly projecting fingers 110 of the end cap 15 by a pin 111. The lever 74 is of a length to overlie the screen convolutions on the roller 27 and is normally urged toward the roller by a U-shaped leaf spring 114 embracing the pin 111 and having its opposite ends bearing against the end cap 15 and the web portion joining the flanges of the lever (FIGS. 6, 8). The lever 74 terminates in a free end portion 112 extending transversely of the lever toward the roller. 
     The electric switch assembly 97 is positioned preferably immediately below the web portion of the lever and is provided with a spring-urged plunger or button 120 which is normally urged outwardly of the body of the switch assembly 97 to normally close the switch assembly (FIG. 8). When the spring-urged lever 74 is permitted to move toward the roller 27 (counterclockwise in FIG. 8) then the web portion of the lever 78 exerts sufficient force to depress the plunger 120 to open the circuit through the switch assembly 97. 
     The free end portion 112 of the lever 74 is preferably tapered to a rounded screen contacting tip (FIGS. 6, 7). The portion 112 is also of such length as to maintain the lever 74 out of engagement with the plunger 120 as long as the free end portion bears against the screen 24, as shown in FIGS. 6 and 8. 
     Thus, the free end portion 112 is positioned to ride on the screen 24 and will circumscribe an imaginary ring about the roller 27 and the convolutions of screen 24 thereon as the roller 27 is rotated about its axis. When the screen is unwound to its fully extended open viewing position, there are two or three convolutions or turns of screen material still encircling the roller 27. The remaining turns as well as the adherence of the transverse margin edge of the screen 24 to the roller 27 firmly attach the screen 24 to the roller 27. These remaining two or three turns have a cut-out slot 115 along the imaginary ring so as to expose a depression 117 formed in the roller 27 along the imaginary ring. 
     When the screen 24 is fully extended, the end portion 112 of lever 74 is urged through the slot 115 in the screen material and into the depression 117 of the roller 27. When the end portion 112 is spring-urged into the depression 117, the web portion of the lever 74 moves against and depresses the plunger 120 of the switch assembly 97 to open the circuit between the power source 91 and the motor 90 so that the operation of the motor 90 is terminated and the roller 27 ceases rotation. 
     It is also apparent that the end portion 112 abuts and engages an edge 118 of the drpesssion 117 in the roller 27 when the end portion rides downwardly into the depression. This engagement effects an auxiliary braking action further to prevent rotation of the roller 27. 
     Just as in the control mechanism for locating the screen in closed position, as described above, the cessation of unwinding of the screen 24 from the roller 27 is automatic when the screen is at its fully-exposed operative viewing position even though the master switch 92 is closed. It is independent of operator control at the master control switch 92 so that there is no need for the operator to play the screen 24 up or down to its proper position because the screen automatically stops in its proper position. 
     When the master control switch 92 is shifted to position A (FIG. 12) to rewind the screen 24, rotation of the roller 27 causes the end portion 112 to ride up a ramp or cam 121 formed as part of the roller defining the depression 117. The end portion 112 thereafter rides from the cam to the back surface of the screen 24 which is being rewound on the roller 27 so that each convolution covers the cut-out slot 115 (FIG. 6). The position of the lever 24 thereafter has no effect on the electric circuit, since the plunger 120 is released once again and closes the switch 97 and re-establishes an operative circuit between the master control switch 92 and the motor 90. 
     3. Electric Motor Brake for Motor-Driven Movie Screen 
     The electric motor and brake mechanism are particularly shown in FIGS. 4, 10 and 11. The motor assembly 55 generally comprises a gear motor including the small reversible synchronous electric motor 90, a reduction gear assembly, generally designated 124, and a brake assembly, generally designated 125. The motor is of the type which automatically applies a braking action when the circuit to the motor is broken. 
     The motor 90 has a stator or electromagnet 127 and a rotor 128 which is mounted on a shaft 130. One end of the shaft is preferably journaled in a bearing 133 of a reduction gear housing 138, and this end has a spline 136 which engages a gear 137 in the gear train in the reduction gear housing 138, which gear 137, in turn, through well-known means, regulates the rotative turning of the output drive shaft 58 of the assembly. 
     As earlier referred to, the coupling member 60 is fixed to one end of the drive shaft 58. The gear reduction housing 138 is mounted to the internal plate 51 by machine screws 57 and, in turn, the electric motor 90 is mounted to the gear reduction housing 138 by screws 140 threaded through a four-sided motor supporting frame 135. 
     The other end 148 of shaft 130 is journaled in bearing 134 which is carried by a cup-shaped member 131 of generally octagonal shape which affords a brake-supporting outer face 131a. The member 131 is affixed to the end of stator 127 in any convenient manner as by bolts passing through the stator and threaded into the frame 135. The addition of member 131 enables the braking surfaces to be moved from within the motor housing to an accessible location outside such housing; it enables the area of braking surfaces to be greatly increased; and it enables the ready adjustment of spring pressure and thus of braking forces. 
     A coiled spring 141 preferably encircles the other end 148 of the shaft 130, and the spring bears at its opposite ends against a pair of friction-reducing nylon washers 143 and 144 which are in turn disposed between the rotor 128 and a bearing bracket 145. A fiber sleeve 154 loosely encircles spring 141 and loosely bears between nylon washers 143 and 144. 
     Mounted on or adhered to the supporting face 131a of member 131 is a resiliently flexible rubber disk 147, which is slightly hard and acts as a first braking surface. Mounted on the end 148 of the shaft 130 is a supporting disk 150, which may be made of plastic carrying a medium grade sandpaper on its surface 151. Surface 151 confronts the exposed surface of the rubber disk 147 and acts as a second braking surface. A set screw 152 permits axial adjustment of the supporting disk 150 on the shaft end 148, thereby to adjust the compression in the spring 141 and the force between the braking surfaces. 
     The shaft 130 and its attached rotor 128 are arranged for limited axial movement. The spring 141 normally urges the shaft toward the gear reduction housing 138 so that the second braking surface is urged against the first braking surface, as shown in FIG. 11, to prevent rotation of the shaft 130. When the motor 90 is actuated, the rotor 128 automatically is pulled into alignment with the lines of force generated by the stator 127 (to the right as shown in FIG. 10). By its movement into alignment with the magnetic field, the rotor 128 compresses the spring 141 to move the supporting disk 150 outward so as to release the braking surfaces and permit rotation of the shaft 130, which drives the reduction gear train and output shaft 58, and ultimately rotates the roller 27. 
     Thus, when the screen is to be stopped at any position by opening the circuit between the motor 90 and the master control switch 92, the spring 141 urges the shaft 130 axially toward the reduction gear housing 138 (FIG. 11) so that the disks 147 and 150 are frictionally engaged to prevent further rotation of the shaft 130 to retain the screen 24 against movement. 
     It can be appreciated that the brake pressure may be altered by resetting the tension in the spring 141 to increase or decrease the force per unit area by shifting the position of disk 150 on the shaft end 148. In addition, the braking surface area is readily accessible and can be easily altered by changing the disks 147 and 150. Furthermore, by affording an outer enlarged brake supporting surface 131a, the effective area of braking action can be greatly enlarged to prolong the effective life of the braking surfaces.