GB2050641A

GB2050641A - Raising and Lowering Flexible Sheets e.g. Roller Blinds, Projection Screens

Info

Publication number: GB2050641A
Application number: GB7920744A
Authority: GB
Original assignee: HARKNESS SCREENS Ltd
Current assignee: HARKNESS SCREENS Ltd
Priority date: 1979-06-14
Filing date: 1979-06-14
Publication date: 1981-01-07
Also published as: GB2050641B

Abstract

A roller mechanism for raising or lowering a flexible sheet such as window blinds, theatre scenery or a projection screen 2 includes a roller 6 around which the screen is rolled. Support cables 7 are wound around the roller and a winding drum 8 coupled by a shaft 9 to an electric motor. A further drum 12 of half the diameter is attached to the winding drum 8 and pays out a safety cable 13 as the roller is lowered. The end of the safety cable is retained in an anchor member (19, 20, 21), attached to the roller 6. In the event of support cable failure, the safety cable prevents the roller from falling and permits it to be lowered in a controlled manner. Alternative methods of paying out the safety cable at half the rate of the support cable are described. E.g. the cables are taken up on drums of equal size, one being rotated at twice the speed of the other. <IMAGE>

Description

SPECIFICATION Safety Mechanism for Flexible Sheet Raising and Lowering Devices This invention relates to roller devices for flexible sheets and, in particular, to safety mechanisms to prevent such devices from falling freely in the event of failure of support cables by which they are suspended. It finds application in devices for raising and lowering photographic projection screens, window blinds, maps, theatre scenery and the like.

In French patent 2 324 561 there is described a projection screen which is raised and lowered by means of a roller device. An electric motor integral with the roller causes the roller to climb up a cable which is fed on to the roller by means of a jockey pulley and lever arrangement. Another projection screen known in the art raises and lowers the rollers on which the screen is rolled by taking in and paying out a cable which is wound around the roller, its end being attached thereto.

The cable is conveniently controlled by an electric motor and winding drum and is brought to the winding roller by an arrangement of pulleys.

Whilst these roller devices are precise and simple to operate, they both suffer from the disadvantage that, if one of the cables supporting the roller should break, the roller will fall freely under gravitational forces, with adverse consequences for a person or object immediately below. We have therefore devised a safety mechanism for roller screens and the like which will permit their being lowered safely in the event of failure of the support cable.

According to the present invention there is provided a device for raising or lowering a flexible sheet comprising roller means attached to one edge of said sheet and on which said sheet may be rolled, a fixed support for the edge of said sheet opposite to said one edge, a first cable for raising and lowering said roller means in relation to said fixed support, first cable winding means for paying out or retracting said first cable, anchor means rotatably fixed to said roller, a second cable attached to said anchor means and second cable winding means for paying out or retracting said second cable at substantially one half of the rate at which said first cable winding means respectively pays out or retracts said first cable.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of the rear view of a photographic projection screen having a safety mechanism in accordance with the invention; Figure 2 is a sectional side elevation taken along the line AA of Figure 1; Figure 3 is a side view of a rolled screen having a safety mechanism in accordance with the invention; and Figure 4 is a partial rear view of the screen of Figure 3 taken along the line BB.

Referring now to the drawings, one edge 1 of a screen 2 is placed between wooden battens 3 and clamped by screws 4. The battens are fixed to a support frame 5. The opposite edge of the screen is attached to a roller 6 around which it may be wound for raising or lowering. Around the ends of the roller are wound support cables 7, one end of each cable being attached to the roller. The other ends of the cables are wound round drums 8 fixedly mounted on a shaft 9 which is supported in bearings 10 attached to the frame 6. The shaft 9 may be caused to revolve by means of an electric motor 11 coupled thereto. The sense of the winding of the cables around the drums and roller is chosen so that if the motor is driven in one direction the roller will be lowered and if it is driven in the other direction the roller will be raised. Further drums 12 are coupled to the drums 8.Each of these further drums has a cable 13 wound around it so that when the first cables are paid out by revolution of the electric motor, these cables will also be paid out. One end of each of the further cables 13 is attached to its respective drum 12 and the other end is attached to an anchor device 14. Each anchor device is rotatably attached to the roller 6 by a bearing 15.

The method of construction is shown in more detail in Figures 3 and 4. The bearings 10 for the drive shaft 9 are held by clamps 16 on supports 17 on the main frame 5 which is of box section tube.

Mounted on a bearing 18 on the end of the roller 6 is the anchor device 14 comprising a steel tube 19 having a closed end 20. The further cable 13 passes freely through a hole 21 in the closed end and is attached to a cylindrical anchor member 22 which retains the cable end within the steel tube 19. The diameter of the drums 12 around which the further cables 13 are wound is one half of the diameter of the drums 8 for the support cables 7. The length of the further cables is chosen so that, with normal operation, all of the strain of holding the rollers is taken by the support cables 7.

Optionally a limit switch 23 may be provided for the electric drive motor.

In operation the velocity ratio of the drum and roller arrangement is two, so that for the roller to be lowered by one metre, two metres of the support cable 7 must be paid out. Since the further cable drums 12 are one half of the diameter of the support cable drums 8, during this time one metre of the further cables will be paid out and the spacing of the anchor member 22 and the end 20 of the steel tube 19 remains constant.

If, therefore, a support cable breaks the roller will fall only a short distance before the anchor member meets the closed end and the strain is taken by the further cable. The roller may then be lowered safely and evenly by continued rotation of the drive shaft.

It will be apparent to those skilled in the art that various modifications may be made within the ambit of the invention. For example, it is not necessary that the respective cables be rolled on separate drums. Common drums may be provided for the support cables and for the further cables, the cables being suitably located by means of guide pulleys. An alternative to the use of drums of different diameters may be employed to ensure that the support cables are paid out at twice the speed of the further cables. For example, the respective drums may be of equal diameter with one being driven at twice the speed of the other by an epicyclic gear train.

A strain-sensitive switch may be provided in the further cables to cut off the power supply to the electric motor in the event of cable failure.

The ends of the support cables need not be fixed to the roller, but can be taken to a remote fixing point, for example attached to the main frame.

Other flexible sheets such as window blinds or theatre scenery may replace the projection screen.

Claims

1. A device for raising or lowering a flexible sheet comprising roller means attached to one edge of said sheet and on which said sheet may be rolled,.a fixed support for the edge of said sheet opposite to said one edge, a first cable for raising and lowering said roller means in relation to said fixed support, first cable winding means for paying out or retracting said first cable, anchor means rotatably fixed to said roller, a second cable attached to said anchor means and second cable winding means for paying out or retracting said second cable at substantially one half of the rate at which said first cable winding means respectively pays out or retracts said first cable.

2. A device as claimed in claim 1 wherein said first and said second cable winding means are drums on which said first and second cables are respectively wound.

3. A device as claimed in claim 2 wherein said drums are of substantially equal diameters and said device incorporates means for rotating the drum for the first cable at twice the speed of the drum for the second cable.

4. A device as claimed in claim 2 wherein the diameter of the drum for the first cable is substantially twice that of the drum for the second cable and said device incorporates means for rotating the drums at substantially equal speeds.

5. A device as claimed in claim 5 wherein said drums are rigidly coupled together.

6. A device as claimed in any one of the preceding claims wherein said anchor means comprises a tubular member having means for access of an end of said second cable and means attached to or integral with said end to retain said end within said tubular member.

7. A device as claimed in claim 6 wherein said means for access is a hole in a closed end of said tubular member.

8. A device as claimed in claim 7 wherein said means attached to or integral with said end is a cylindrical anchor member.

9. A device as claimed in an one of the preceding claims incorporating switching means coupled to said second cable to interrupt the power supply to said first cable winding means in the event of failure of said first cable.

10. A device for raising or lowering a flexible sheet substantially as herein described with reference to and as shown in the accompanying drawings.