US1992005A

US1992005A - Screen and cleaning means therefor

Info

Publication number: US1992005A
Authority: US
Publication date: 1935-02-19
Anticipated expiration: 1952-02-19

Description

Feb. 19, 1935. J GOLDSBORQUGH 1,992,005

SCREEN AND CLEANING MEANS THEREFOR I, l lf 'h'hhmg l'l Q INVENTOR g N Johnficoza/lifgszi A TTORNEY Feb. 19, 1935. J. B. GOLDSBOROUGH SCREEN AND CLEANING MEANS THEREFOR Filed Dec. 10, 1932 4 Sheets-Sheet 2 Y mm E T M g 0 WM n o .A 16? BWW M n 3 M J E II III! Feb. 19, 1935.

J. B. GOLDSBOROUGH SCREEN AND CLEANING MEANS THEREFOR Filed Dec. 10, 1952 4 Sheets-Sheet 3 mm m ME 0 V m. dwi BIM H r M J Y B Feb. 19, 1935. J. B. GQLbSBQRQUGH 1,992,005

SCREEN AND CLEANING MEANS THEREFOR Filed Dec. 10, 1932 4 Sheets-Sheet 4 ATTORNEY Patented Feb. 19, 1935 UNITED STATES PATENT oFFICE 'SCREEN AND CLEANINGMEANS THEREFOR John Goldsborough, Croton-on-H-udson,--N. Y.

' Application December.10, 1932, Serial No. 646,639

13 Claims.

7 ;means in suitable coordinated relationship.

One featureof the invention involves the use of an inclined disc screen having substantially radially-arranged screening slots. 'Another feature of the-invention relates toscreen-cleaning means-acting-radially of-the screen to cleansaid screening slots from'the inner to the outer ends thereof. It is also a feature of the invention to have-the cleaning means rotate with the screen about the axis of rotation of the latter while completing the cleaning action from the inner to the outer ends of certain 'of the substantially radial slots. Ajfurther feature of the invention relates to -means whereby the 'cleaning means moves with "the-screen 'while cleaning a radial path 'thereon, is withdrawn therefrom, moved rearwardly toa position over another path on the screen, andmoved into cooperative-relation with thescreen to startcleaning the last mentioned path. provide'a rotary screen having a drum portion with longitudinal screening slots, and screencleaning means travelling with the screenwhile acting from theupper to the lower ends of the slots in a longitudinal path-off thescreen, and then-being'shifted away-from the screen and rearwardly to engage the screen in;a path adjacent to-thefirstand' partially overlapping the same.

The invention also involves the feature of -a rotary screenincluding a discportionand a drum portion, preferably in theform of a frustum of a cone, having longitudinal screening slots. A further feature of theninvention involves the use 'said screening slots.

' of such a screen with cleaning means acting along This cleaning means may 'also move-with' the rotating screen until a path thereon has been completely cleaned, and then bewithdrawn from the screen and brought into engagement with another path to be cleaned. It

is also a feature of the invention to provide arotary screenincluding a discportion with subs tarit ially'radialjslots and a drum portion having longitudinal sl'ots'so that cleaning of both parts may be cleaned by means acting along the slots whereby the screenings from the drum. will be It is also a feature of the invention to deposited on the disc and then moved to the M periphery of the disc and discharged.

A further feature of the invention relates to the use, with a rotary screen having a disc portion with radial screening slots and. a drum portion withlongitudinal slots, of a common cleaning means adapted to wipe material from the upper end of the drum portion downwardly to the disc portion and then outwardly to the periphery .of the disc portionwhere it is discharged. This cleaning means may rotate with the screen, be removed from the disc portion and the-drum portion at the same time, and be shifted rearwardly to be placed in engagement with another portion of thescreen.

chronizing the rotary screen and the cleaning .mechanism, and' to throwing the cleaning mechanism out of operation as it' is withdrawn from the rotary screen, and into operation as it is re- 3 ,1

turned to cooperative relation with the screen.

Further features and advantages will appear upon consideration of theffollowing description and of .the drawings in which Fig. lfis a side elevation of the screen, the.

cleaning means and the. operating means whereby both. are operated,'the casing and supports being shown in section; a

Fig. 2 is a fragmentary view of a connection shown in section at the upper part of Fig. 4;

Fig. 3 is. a top plan view of the mechanism I shown in Fig. 1, parts being broken away toshow underlying structure;

Fig. 4 is a longitudinal vertical section on a larger scale, of the structure shown in Fig. 1;

Fig. 5 is a fragmentary top plan view illustrating the .manner in which the screenings are discharged from'the disc screen; v

Fig. 6 is a fragmentary View, partly in section,

illustrating the, means whereby the cleaning de-- s Other features of the invention relate to syn-.

vice is withdrawn from, the rotary screen,- swung rearwardly and returned into engagement with the screen;

Fig. '7 is a section-taken along the line.'7'7 of Fig.6;and e i i Fig. 8 isa section of one of the disc screen sections taken in a plane perpendicular to the screening slots. I

Referring to 'the drawings, 10 represents a well into whichxempties'an influent channel 11 and from which aneflluent channel 12 leads. Extending substantially across the top of the well 10 is' a bridge 13, here shown as'including two iongitudinal I-beams 14 supported above the influent channel/on a. suitable wall and at the efiluent side of the well by another I-beam 15. Toward the influent side of the well 10 the bridge is provided with means for supporting the upper end of a rearwardly inclined screen shaft/16, of

which the lower end is supported in a suitable manner in the well 10.

As illustrated, in Fig. l, the upper end of the shaft 16 is mounted in a bearing 1'7 supported by two I-beams l8 connecting the longitudinal beams 14, and the lower end of the shaft 16 is mounted in a bearing 19 secured between the two inclined I-beams 20. Also the bearing 19 and the corresponding ends of the beams 20 are supported on a wedge-shaped member or casting 21 which in turn rests upon a base 22 projecting upwardly from the bottom of the well, and the other or raised ends of the I-beams 20 rest on a concrete support extending across the effluent channel 12.

Rotatably mounted on the inclined shaft 16 is a rotary screen 23, here shown as including a disc or disc portion 24 and a central drum portion or cone 25 projecting upwardly from the disc portion. If a central drum portion be used, the effective part of the disc portion is in the general form of a flat perforated ring of considerable width which may be made up of a plurality of sections or plates 26 arranged as shown on Fig. 2. Preferably, for the purposes of the present invention, these sections or plates are provided with radial or substantially radial slots 2'7, the central slots of the sections being radial and the slots at the sides of the central slots departing farther from a strictly radial direction as the sides of the sections 26 are approached. 1

According to the illustrated embodiment of the invention the disc portion of the screen is supported by a frame comprising a peripheral ring 28 (Figs. 1 and 4) of channel form supported on the outer ends of radial members 29 in the form of I-beams secured at their inner ends to flanges on a tubular hub 30. Support for the inner ends or edges of the screen sections 26 is obtained by means including members 30 con necting the radial beams 29. To assist in supporting the radial members 29, use may be made of suitable tension members 31 connected at their lower ends to the members 29 and at their upper ends to the hub 30. The inner surface of the hub 30 is spaced from the shaft 16 and the lower end of the hub is rotatably mounted on the shaft by means of a bearing sleeve 32 screwed into the lower end of the hub and mounted on a collar, 33 secured on the shaft 16 and having a flange 34 overlying the top of the sleeve 32. Near its upper end the hub 30 is reduced in diameter thereby providing a downwardly facing shoulder 35 at its interior and an upwardly facing shoulder 36 at its exterior. Extending into the reduced upper end of the hub 30 is a sleeve 3'7 of which the lower edge is flush with the shoulder 35 and is so positioned by means of a flange at the upper end of sleeve 3'7 fitting into the recessed upper end of the reduced portion of the hub. Support of the hub 30 on the shaft 16 may be obtained by suitable means including a ball bearing of which the lower race is provided by a ring'or collar 38 surrounding the shaft 16 and supported by an upwardly facing shoulder on the shaft, and the upper race is provided by a ring 39 engaging the shoulder 35 of the hub and the lower edge of the sleeve 37. Passage of the sewage to be screened, around the lower part of the screen, may be prevented by -means of a member 39a connected with the easing of the well and having a circular edge cooperating closely with the circumferential edge of the disc screen which may be suppo ted by rollers 39b suitably mounted on the wall of the well 10.

Rotation of the rotary screen 23 may be effected by means including a toothed driving ring 40 secured to peripheral ring of the frame of the disc portion of the screen, a gear 41 meshing with the driving ring 40 and mounted on the upper end of a stub shaft journaled in a bracket 42 secured to the I-beams 20, a bevel gear 43 at the lower end of the stub shaft and meshing with a bevel gear 44 on a horizontal shaft 45, a second bevel gear 45a on said shaft 45 meshing with a bevel gear 46 on an upwardly extending shaft 47, a worm gear 48 on the shaft 47 and meshing with a suitable member 49 on a horizontal shaft 50 driven by motor 51 on said bridge 13.

The drum portion 25 of the rotary screen is preferably in the form of a frustrum of a cone and is formed at its periphery of a plurality of screen sections 52 having slots extending longitudinally thereof, that is upwardly and downwardly. These screen-sections may be connected by bolting them to members 53 which are attached at their lower ends to a ring 54 mounted on the disc frame above the member or members 30 to which they are secured. The top of the drum portion is formed of a plurality of sections 55 of sheet metal engaging the hub 30 and having outer flanged edges extending over the upper edges of the'screen sections 52.

An important part of the'invention resides in the formation of the rotary screen with screening slots arranged as hereinbefore described and in the provision of cleaning means whereby the screenings will be wiped or brushed longitudinally of the spots and discharged from the periphery of the screen. Preferably the cleaning operation is effected in the following manner:

Provision is made of a brush carrying frame having two vertically arranged sides 56 pivoted at their inner ends on hubs 5'7 projecting from opposite sides of an annular member 58 ro'tatably mounted on the shaft 16 by means of ball bearings 59, the annular member 58 being supported against downward movement on the shaft 16 by means of suitable thrust bearings 59a in the form of ball'bearings interposed between the member 58 and the upper end of the sleeve 37 at the upper end of the hub 30. Preferably both sets of screen sections are cleaned by the same means which includes a set of rotary brushes 60 fixed on shafts 61 mounted at opposite ends in endless chains 62 and provided at the outer sides of said chains with gears 63 meshing with teeth on continuous racks 64 which also serve as tracks to determine the path of movement of the shafts of the brushes, the lower stretches of the chains 62 being supported by suitable sprockets 641a.

From the showing in Figs. 1 and 3, it will be evident that the brushes are so guided in their movement as to brush down along the drum portion and then outwardly along the disc portion to carry the screenings from the disc por-' tion along the upper face of a discharge platform 6412 to a discharge chute 65 from which they may pass to a suitable container 66 for disposal.

It will be evident that if the chains 62 be driven in the required direction, the brushes will be rotated to advance the material in the same direction that the chains move but at a faster rate. To effect this driving of the chains 62 provision is made of a driving shaft 67 located with the "chains-62. 'benote'd thatthe shafts 61'of the'brushes are sopositioned as not to interfere with'the action the sprocket shaft 67.

the "corner of, the rack -64 near the top of the drum" portion of the screen, and-of sprocket wheels 68 mounted on theshaft :67' and meshing In this connection it should of the'sprocket'wheels on the chains. 7

To drive the sprocket wheel"68, the shaft 67 'is provided with'bevel gears 69 at oppositev sides of "the brush frame and meshing withbevel gears 70-on stub "shafts having at their upper ends bevel gears '71 meshingwith'bevel gears '72 one shaft 73. The-shaft '73'is driven-by means of'a be'vel gear '74 thereon meshingwith abevel gear7'5 ona shaft '76 providedat'its other end with a bevel gear '77 meshingwith a gear ring Wis-secured to an annular member '79 rotatably mounted by suitable anti frictionbearings on a member carried in turn on the hub portion of 'a member 81, said hub portion resting on the upwardly facing shoulder 36 at the exterior of the hub 30 and being connected tothe reduced portion of the-hub-SO as by a key, to turn therewith.

As here shown the upper portion of the member 81 is in the form of a disc to which is secured a gear ring 82 having upwardly facing teeth adapted to cooperate "with teeth on a locking member 83 projecting downwardly from a cross member 84 connecting the sides 56 of=the brush frames. It will be apparentthat when the teeth on thegear ring 82 andthe teeth on the member '83 are in mesh the brush frame will be caused to travel with the hub 30of the screen.

The'driving of the brushes is effected through the gear ring '78 which, as already described, is connected by means of a suitable gear train with To "drive thegear ring 78, provisi'on is made of a bevel'gear 85 meshing therewith and fixed on a shaft'86 journaled in a bracket 87 projecting downwardly from the bridge and also in a bracket 88 at the side of the well. Also mounted on shaft 86 is a bevel gear -89me"shing with abevel gear 90 on -ashaft'91 extending upwardly to .the bridge and provided "at its upperend with a bevel gear 92 meshing with a bevel gear I93 on a shaft 94 which is driven by gear 95 thereon'meshing. witha gear or pinion 96 on the shaft 50 of the motor 51.

It will be evident that thebrush frame should notturn with the screen for a longer time than that required for abrush starting at the upper end of the drum to travel down the same and outwardly to the rim of thedisc, and that at the end'of such a period the brushes should be lifted from the screen and released therefrom by disengagement of the toothedmember '83 from the toothed ring'82 and by disengagement of the bevel gear '77 from the toothed ring '78.

It is also a feature of the invention to pro vide means for shifting thebrush device rearwardly over a following and corresponding path of the screen and again putting the brushes into cooperative relation with the screen and locking the brush frame to the screen to turn therewith.

To this end, there is provided a member or yoke 9'7 extending across the'upper sideof thebru'sh frameand serving not only to strengthen the brush frame but also to provide .a convenient means for lifting and shifting the same.

Pivoted to the top' of the yoke 97 to swing about an axis extending longitudinally thereof is a second yoke 98 having arms extending upwardly on opposite sides 'of two superposed I-beams 99 -exte'n'din'g transversely of .the 'I-beams '14 and edges.

"attached to'the lower sides thereof fAt their upper'ends the arms'of the yoke 98g-areprovided "with slots 100 receiving rolls 101 rotatably mounted on a travelling carriagelOZ having wheels 103 running on the lower flanges of channels 104 connectingthe-I-beams l4. Attached to theyoke --98,as"by=-attachment toa cross pin 105, is a flexible member or cable 106 which extends upwardly and'over a pulley 107 on the carriage" 102 and then over'to the side of the well where it passes over a pulley 108; the end of the cable extending downwardly from the pulley IDS- and being attached to a counter weight 109 slidable vertically on guides 110 and checked in its downward movement by a stop 111'.

The counterweight 109 is heavy enough to lift the brush frame and thereby throw the brush mechanism out of operation. As here shown-the brush mechanism is held down for effective action by means-'otwheels 11 2- engaging the lower sides of the upper flanges of the lower beam 99'while the brush device is'advanced with the screen, but is released when the wheels 112 reach openings 113 in the flanges (Fig. 4) so that the counter weight 109 lifts the brush frame and brings the' wheels 112 above the adjoining flanges of the superposed I-beams 99 and then moves the carriage 102 and also the brush frame rearwardly until the counterweight engages the stop 1 11.

The momentum of the brush frame acts to carry it over a spring controlled latch device 114 arranged to permit downward movement of the wheels 112 but to prevent upward movement and to produce enough slack on the cable to enable the brush frame to drop and be locked iii-position to cooperate with the screen to rotate therewith. Thenthewheels 112 will be carried beneath the flanges to hold the frame down and themovem'ent of thebrush frame will lift the counterweight .-until the wheels 112 again reach openings 113 when the brush frame will again be lifted and the actionrepeated. i

fAs indicated'on Fig. 8, successivepaths of the brushes overlap out to' the perimeter of the disc "portion of thescreen so that all parts of thediscf will be cleaned and as a matter of fact certain parts of the screen will be brushed several times in succession.

It is also ir'nportant'to provide'screen sections or plates which offer the least resistance to the flowof sewage and the like and at the same time retain the-solids and also facilitate the cleaning ofthe screening slots or openings. To thisend it is proposed to provide screening openings or slots corresponding'to the vena contracta or tube= between them are from to -2 of an inch. In this connection, it shouldbe understoo'd'that the screen'members or wires may takeother forms.

-For-example, the-wires maybe. oval cross-section or the screening openings maybe formed plate.

Each screen section 26 is shown as made up with atsurroundingxframe and bars or members ina 12:1 extending transversely. with respect to the Heretofore it has been thegeneral wires and provided with wire-receiving rerearwardly until the stop 111 is engaged by the cesses in which the wires fit, the recesses being open at the'top .but extending above the middle thereof so that the material at the sides of the recesses overlies the sides of the Wires and keeps them from being lifted therefrom. Also the inner end of the frame of the screen section is provided with corresponding recesses extending only part way from the inner edge of the end toward the outer edge thereof which is adjacent to the cone 25, thus limiting the movement of the wires in this direction. The outer end of the section is provided with recesses of the same sort extending completely across the same, so that the wires 120 may be inserted longitudinally through these recesses and the recesses in the members 121 until stopped by engagement with the ends of these recesses. After insertion in this way, the wires may be secured against withdrawal, as by placing a member 122 across the outer ends of the recesses in the outer end of the screen section. The sections of the drum portion 25 are formed in substantially the same manner but are curved instead of lying in the same general plane.

Recapitulating briefly, the operation of the apparatus is substantially as follows: The liquid from the influent channel 11 passes through the lower part of the disc screen and the larger solids are deposited on the screen. It will be evident that the liquid of the sewage will be held back to such an extent by the screen and the accumulation of solids thereon that the level in the influent channel will be substantially higher than in the eiiiuent channel.

With the illustrated arrangement of parts, the

screen is turned relatively slowly in a counterclockwise direction (Fig. 3) and the solids thereon gradually raised above the surface of the sewage at the influent side of the screen. Assuming that the wheels 112 have just been dropped through the openings normally closed by the latch 114, the brush frame will be carried with the screen and the brushes 60 will travel along and clean the screening slots in a path ex tending from the top of the cone to the inner edge of the screening portion of the disc and then outwardly to the edge of thedisc and out over the support or trough 64 to the chute 65 for 1 discharge into a container 66. At the beginning of this brushing action the solids willbe deposited on one side (Fig. 5) of the support 641), and at the end of this brushing action the solids will be deposited on the other side of the support. It should be understood that the timing is such that the brushes starting at the top of the cone when the brush device is thrown into operation will have passed off the disc screen before the brush device is withdrawn fromthe screen.

At the time the wheels 112 drop through the openings controlled by the latch 114 which permits downward movement but prevents upward movement, the toothed member 83 is brought into mesh with the toothed ring 82 thus locking the brush frame to the hub 30 of the screen and causing the brush frame to travel with the screen; and the bevel gear 77 is brought into mesh with the toothed ring '78, thus throwing the brushes into operation. Then as the screen advances it lifts the counterweight 109 from the stop 111 and continues to lift it until the wheels 112 reach the openings 113 and the counterweight 109 lifts the brush frame, thereby breaking the connection which causes the brush frame to rotate with the screen and also breaking the connection through which the motor 51 actuates the brushes.

The counterweight then moves the brush frame counterweight. Although the movement of the counterweight is thus stopped, the momentum of the brush frame at that time is sufficient to carry used without others without departing from the.

true spirit and scope of the invention.

Having thus described my invention, I claim: 1. In an apparatus of the class described, a

disc screen rotated about an inclined axis and having screening slots radially arranged, cleaning means for said'disc acting radially and outwardly thereon, and means whereby relative rotation of the cleaning means and screen will be prevented during the cleaning of one radial path and then effected to enable the cleaning means.

3. In apparatus of the class described, a disc screen with radial slots rotating about an inclined axis, disc-cleaning means, and means whereby the disc cleaning means will act radially and outwardly on said disc and-will rotatewith saiddisc until one radial path has been completely cleaned.

4. In an apparatus of the class described, a rotary screen including a barrel portion provided with longitudinal screening slots, and means for cleaning said barrel portion in successive longitudinal paths including means to act along the entire length of said slots, and means whereby" the cleaning means will turn with said barrel during the cleaning action along one of said paths.

5. A rotary screen comprising a disc portion,

6. A rotary screen comprising a central barrel.

having longitudinal screening slots and a disc and means' surrounding one end of said barrel and formed of sectors each of which has a central radial 7. In an apparatus of the class described. a?

rotary screen comprising a disc'portion having substantially radial screening slots and a central drum portion having longitudinal screening slots,

and brushing means acting to brush along said longitudinal slots and outwardly along said radial:

slots to discharge the screenings at the periphery of the disc.

8. In an apparatus of the class described, a rotary screen having its axis of rotation inclined with respect to the vertical and comprising a disc portion having substantially radial screening slots and a central drum portion having longitudinal screening slots, a discharge platform arranged to receive screenings from the rim of the disc portion, and brushing means acting to brush downwardly along said longitudinal slots and outwardly along said radial slots and over said platform.

9. The combination with a rotary screen comprising a disc portion having substantially radial screening slots and a central cone portion having longitudinal slots, means for rotating the screen, screen-cleaning means including brushes for brushing the screenings downwardly along the longitudinal slots of the cone and outwardly along said slots brush-carrying frame mounted to swing about the axis of said screen and to be separated from the screen or moved into cooperative relation therewith, means whereby said frame will be caused to turn with said screen when in cooperative relation therewith for cleaning purposes and will be freed when separated from the screen, means whereby the brushes will be actuated when the frame is down and will be at rest when the frame is raised, a traveling carriage connected with said frame to travel therewith but to permit vertical movement of the frame with respect thereto, a pulley on said carriage, a vertically movable counterweight to the rear of the frame when referred to the direction of rotation of the screen, a pulley above said counterweight, a flexible connector attached to said counterweight and to said frame and extending over said pulleys, and means for controlling the action of the counterweight on the frame including means to hold the frame down to travel with the screen and then, after releasing the same to permit the counerweight to lift a frame and start it rearwardly, to hold the frame in raised position until after the counterweight is stopped in its downward movement and slack is formed in the flexible connector to permit the frame to drop, and a spring catch permitting such dropping of the frame but preventing upward movement thereof.

10. The combination with a rotary screen comprising a disc having substantially radial screening slots and a central drum having longitudinal slots, means for rotating the screen, screen-cleaning means including brushes for brushing the screenings downwardly along the longitudinal slots of the drum and outwardly along said slots in the disc, a brush-carrying frame mounted to swing about the axis of said screen and to be separated from the screen or moved into cooperative relation therewith, means whereby said frame will be caused to turn with said screen when in cooperative relation therewith for cleaning purposes and will be freed when separated from the screen, and means for moving the brush-carrying frame into cooperative relation with the screen, holding the frame in such relation to the screen while the screen rotates through a predetermined angle, withdrawing the frame from the screen, and swinging the frame rearwardly preparatory in the disc, a

to again moving it into cooperative relation with the screen.

11. The combination with a rotary screen comprising a disc having substantially radial screening slots and a central drum having longitudinal slots, means for rotating the screen, screencleaning means including brushes for brushing the screenings downwardly along the longitudinal slots of the drum and outwardly along said slots in the disc, a brush-carrying frame mounted to swing about the axis of said screen and to be separated from the screen or moved into cooperative relation therewith, and means causing the brushes to effect complete cleaning along successive longitudinal paths on the drum and corresponding successive radial paths on the disc, said last mentioned means including means whereby the frame will be caused to turn with .said screen while in cooperative relation there with for cleaning purposes and will be freed upon separation from the screen.

12. The combination with a rotary screen comprising a disc portion having substantially radial screening slots and a central cone portion having longitudinal slots, means for rotating the screen, screen-cleaning means including brushes for brushing the screenings downwardly along the longitudinal slots of the cone and outwardly along said slots in the disc, a brush-carrying frame mounted to swing about the axis of said screen and to be separated from the screen or moved into cooperative relation therewith, and means efiecting complete cleaning by the brushes along successive longitudinal paths on the drum and corresponding radial paths on the disc, said last mentioned means including means whereby the brushes will be actuated when the frame is down and will be at rest when the frame is raised.

13. The combination with a rotary screen comprising a disc having substantially radial screening slots and a central drum having longitudinal slots, means for rotating the screen, screencleaning means including brushes for brushing the screenings downwardly along the longitudinal slots of the drum and outwardly along said slots in the disc, a brush-carrying frame mounted to swing about the axis of said screen and to be separated from the screen or moved into cooperative relation therewith, means whereby said frame will be caused to turn with said screen when in cooperative relation therewith for cleaning purposes and will be freed when separated from the screen, a traveling carriage connected with said frame to travel therewith while permitting vertical movement of the frame with respect thereto, a pulley on said carriage, a vertically movable counterweight to the rear of the frame when referred to the direction of movement of the screen, a pulley above said counterweight, a flexible connector attached to said counterweight and to said frame and extending over said pulleys, and means for controlling the action of the counterweight on the frame including means to hold the frame down to travel with the screen and then, after releasing the frame to permit the counterweight to lift it and start it .rearwardly and to hold the frame in raised position until after the counterweight is stopped in its downward movement and slack is formed in the flexible connector to permit the frame to drop.