GB2312716A

GB2312716A - Overload clutch

Info

Publication number: GB2312716A
Application number: GB9608943A
Authority: GB
Inventors: Derek Jorgensen
Original assignee: APPEAL BLINDS Ltd
Current assignee: APPEAL BLINDS Ltd
Priority date: 1996-04-29
Filing date: 1996-04-29
Publication date: 1997-11-05
Also published as: GB9608943D0

Abstract

A pulley wheel 2 carried on a shaft 4 comprises a ball 22 slidably received in a radical bore 24 and biased radically inwardly towards the shaft 4. A recess (34, Fig 4) is formed in a side wall of the shaft 4 in which the ball 22 engages to transmit drive between the shaft and the pulley wheel 2. The ball 22 is disengaged from the recess (34) when the applied drive torque overcomes the biasing force on the ball. The pulley wheel 2 may be provided with surface formations or a grit coating to increase friction between the pulley wheel and a draw cord used to operate blinds.

Description

OVERLOAD CLUTCH FOR A PULLEY ARRANGKMJT This invention relates to a pulley arrangement and particularly, although not exclusively, relates to an overload clutch for a motorised pulley used to draw or retract blinds by means of an endless cord.

In its simplest form, a motorised blind comprises a blind which is suspended substantially vertically, a pull cord attached to an end of the blind and a motor to retract or extend the blind against the force of gravity. In certain locations, such as the roof of a conservatory, the blinds may not be able to hang vertically and hence it may be necessary for the blind to be extended as well as retracted under power. This is generally achieved by use of an endless cord wound around the drive pulley of the motor.

In order to reduce the overall cost of the motorised blind assembly, it is desirable to operate several blinds using a single motor. In more complex applications, these blinds may be of different draw length, so that the pulleys, rigidly attached to the drive shaft of the electric motor, must be of different diameters to achieve the required draw length for each blind. The use of pulleys of different diameters considerably increases the complexity of the installation process, since it is necessary to carry out calculations based on the required draw length for each blind to identify what sizes of pulley which are required. Furthermore, if the blinds contract or stretch over time, one must adjust the draw length of all of the blinds together, which may not be appropriate. Alternatively, new pulleys of different diameter must be installed on the drive shaft of the motor. This is an expensive and time consuming process.

According to the present invention therej,-Zs provided a pulley arrangement comprising a putiey*heel carried on a shaft which extends through an axial opening in the pulley wheel, the pulley arrangement comprising an engagement element slidable received in a bore which extends radially through the pulley wheel from a wall of the opening, the engagement element being resiliently biased radially inwardly towards the shaft, a recess being formed in a side wall of the shaft in which the engagement element engages to transmit drive from the shaft to the pulley wheel or from the pulley wheel to the shaft, the engagement element being disengaged from the recess when the applied drive torque overcomes the biasing force on the engagement element.

The engagement element may comprise a ball which may be resiliently biased by a compression spring situated in the bore. The spring may be situated between the engagement element and a movable plug which is provided with an external thread and is retained on a corresponding thread in the bore. The biasing force applied by the engagement element may be adjusted by screwing the plug into or out of the bore.

In a preferred embodiment the bore passes completely through the pulley wheel and the end of the plug remote from the engagement element is adapted to cooperate with a tool which is used to rotate the plug.

The tool may for example comprise a screwdriver or hexagonal key.

A plurality of radial bores may be provided at equidistant spacing around the pulley wheel, each bore housing a respective engagement element. Preferably two engagement elements are provided on opposite sides of the pulley wheel, so that the biasing force applied through the engagement elements to the shaft are equal and opposite and so that the axis of rotation of the pulley wheel is aligned with the longitudinal axis of the shaft.

The shaft may be provided with a plurality of equidistantly spaced circumferential recesses.

Preferably, the recesses are arranged in pairs on opposite sides of the shaft. For example a total of eight recesses may be provided, spaced 45 degrees apart around the circumference of the shaft.

The pulley wheel may be engaged by an endless draw cord which, for example, may be used to raise or lower a blind. In such an arrangement, a first guide wheel may be provided for guiding the cord onto the pulley wheel and a second guide wheel may be provided for guiding the cord off the pulley wheel.

The pulley wheel preferably has a circumferential groove which receives the draw cord, the groove hiving tapered side walls and a central cylindrical drive portion. The drive portion is preferably provided with a gripping surface that will neither allow the cord to slip appreciably nor damage the surface of the cord significantly. The high friction surface may be provided by surface formations, a high friction coating or a high friction resilient sleeve which is fitted onto the drive portion.

Preferably the draw cord is led two full turns around the drive portion of the pulley wheel to provide good traction.

Preferably a plurality of pulley wheels are provided on a common shaft. The shaft may be turned by hand or may be driven by a motor. Suitable circuitry may be provided to control the operation of the motor.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a pulley mounted on a drive shaft; Figure 2 is a cross-section on line AA through the pulley and drive shaft of Figure 1; Figure 3 is a plan view of the drive shaft of Figure 1; and Figure 4 is a cross-section on line BB through the drive shaft of Figure 3.

Referring to Figure 1, a pulley or pulley wheel 2 is supported on a drive shaft 4 which is driven by an electric motor (not shown). The pulley 2 is a sliding fit on the drive shaft 4 and is releasably connected to it by means of a pair of spring biased engagement members 6 mounted in an enlarged flange 8 of the pulley 2.

A side wall 10 of the enlarged flange 8 tapers to a cylindrical drive portion 12 and is spaced apart from a tapered flange 14. The surface of the cylindrical drive portion 12 is provided with an aluminium oxide 80 grit coating.

The enlarged flange 8, cylindrical drive portion 12 and flange 14 together define a groove, having a substantially V-shaped cross-section, in which an endless draw cord (not shown) is wound.

The draw cord is guided to and from the pulley 2 by means of guide wheels 16, 18 rotatably mounted on an axle 20 which is fixed parallel to and spaced from the drive shaft 4. The shaft 4 and axle 20 are mounted on a common support 21. The shaft 4 is mounted with its axis of rotation a distance a from the support 21 and the axle 20 is mounted with its axis of rotation a distance ss from the support 21. The axle 20 is displaced from the shaft 4 by a distance 6.

Referring to Figure 2, each engagement member 6 comprises a ball bearing 22 located in a respective bore 24 which extends radially through the enlarged flange 8 of the pulley 2. Each ball 22 is biased radially inwardly towards the drive shaft 4 by means of a spring 26 housed in a respective bore 24. The radially outer ends of the bores 24 are threaded and receive correspondingly threaded plugs 28. In the illustrated embodiment, there are two engagement members 6 situated on opposite sides of the drive shaft 4, but any number of engagement members 6 are contemplated.

Referring to Figures 3 and 4, the drive shaft 4 comprises a hollow cylindrical drum having a register slot 30 at one end, which is engaged by a key located in the output shaft of the motor (not shown).

Six equidistantly spaced rings of openings are provided along the mid-portion of the drive shaft 4.

Each ring of openings comprises eight openings 34, which are equidistantly spaced around the circumference of the drive shaft 4. The openings 34 in each ring receive the balls 22 of a respective pulley 2. Thusw the drive shaft shown in Figure 3 is intended to support six pulleys 2.

In operation, each pulley 2 carries an endless draw cord (not shown) which is attached to the back side of a blind lath and is led on to one side of the pulley 2 by means of the guide pulley 18. The cord is then taken two full turns around the drive portion 12 of the pulley 2 and is led off the other side of the pulley 2 by the guide wheel 16. It is then led to the front of the blind lath, thus completing the endless cord.

Each pulley 2 on the drive shaft 4 is connected to a different blind by means of an endless cord. For the purpose of explanation, we will consider only two of the pulleys and assume that the first of these pulleys is connected to a blind of shorter drop length than the second pulley.

To raise the blinds, the motor is turned in a first direction. Engagement of the balls 22 in respective pairs of openings 34 causes the pulleys 2 to rotate with the drive shaft 4 and hence to drive the endless cords which move the blind laths. As the drop length of the first blind is shorter than that of the second blind, the first blind will reach the end of its travel before the second blind. At this point the draw cord attached to the first blind will become stationary, thereby stalling the first pulley 2. As the drive shaft 4 continues to be driven by the motor, almost instantaneously the torque applied by the motor will overcome the tension in the springs 26 behind the balls 22. The balls 22 will then be driven back into the bores 24, allowing the first pulley 2 to remain stationary whilst the drive shaft 4 turns.

This process is repeated every time the balls 22 align with a respective pair of openings 34, until the second blind has reached the end of its travel. At this point the motor will be stopped, leaving both the first and second blinds fully raised under tension from their respective draw cords.

At the instant when the motor is stopped, the balls 22 in the first pulley 2 may be engaged in respective openings 34, so that the pulley 2 is held stationary with the drive shaft 4. However, if the balls 22 are not engaged, the weight of the first blind may cause the pulley 2 to counter rotate relative to the drive shaft 4 until the balls 22 engage in a pair of openings 34. Thus, the first blind may be able to slip back when the motor stops. The maximum amount of slip back which can occur is determined by the spacing of the openings 34 around the drive shaft 4. The closer the spacing, the less slip back which can occur.

When the direction of the motor is reversed to lower the blinds, the initial rotation of the drive shaft 4 will align a pair of openings 34 in the drive shaft 4 with the balls 22 in the first pulley 2, if they are not already aligned. Hence the first pulley 2 will again engage with the drive shaft 4 and will turn with the second pulley to lower the blinds. In the same manner as for raising the blinds, the first pulley will slip when it reaches the fully retracted position, until the longer blind is also fully retracted.

As will be appreciate, the engagement members 6 operate in the manner of an overload clutch, because when resistance is applied to the pulley, such that the torque applied by the motor overcomes the force applied by the springs 26, the drive shaft 4 is able to rotate relative to the pulley 2. In this way, if the blinds trap an object or become tangled or otherwise jammed, the motorised blind assembly will not be damaged.

Thus, not only does this arrangement enable blinds of varying drop length to be raised and lowered by a single motor using pulleys of the same diameter, but the general reliability and service life of the motorised blind is increased.

The torque at which each pulley 2 slips relative to the shaft 4 is adjustable by screwing the plugs 28 into or out of the threaded ends of the bores 24.

Slots or other suitable formations may be provided on the outwardly facing ends of the plugs 28, for engagement by a screw driver or similar adjusting device.

In a motorised blind arrangement comprising blinds of varying length, the pulley which operates the longest blind may be locked to the drive tube, by greatly increasing the spring pressure on the balls 22 or by replacing the engagement members by grub screws.

As will be appreciated, the torque at which the respective pulleys slip can be adjusted differently to accommodate blinds of different weights or to account for specific situations such as a particular blind which may become easily obstructed, such as over a window.

Any number of pulleys may be driven in the manner illustrated above, as long as the total load on the motor does not exceed its rated capacity.

Alternatively, the drive shaft 4 may be turned by hand or by any means whatsoever. It is also to be understood that the pulley 2 could be the drive member and the shaft 4 the driven member.

In an embodiment not illustrated, the engagement member may be provided in the shaft and corresponding recesses or openings provided in the pulley. The illustrated embodiment would however be more convenient, since it allows easy adjustment of the spring pressure applied to the balls 22.

In a particular embodiment of the invention which has been found to operate well, the drive shaft comprises a hollow roller of 41.28 mm external diameter and 38 mm internal diameter. The overall length of the roller is 465 mm and is provided with six rings of holes 34. The first ring of holes is spaced 163.75 mm from the free end of the drive shaft and the rings of holes are spaced 33.5 mm apart, centre to centre. Each ring comprises eight holes of 4.5 mm diameter, spaced equidistantly around the circumference of the roller and defines a track around the tube (on which a respective pulley 2 runs. If there are six pulleys 2 there will be forty-eight symmetrically placed holes.

These may be created by passing a drill right through the tube twenty-four times.

In a preferred form, suitable for lifting a roman blind, the pulleys 2 are formed from aluminium. The outside diameter of the enlarged flange 8 of each pulley 2 is 75.0 mm and the inside diameter of the opening through the pulley 2 is 41.7 mm. As mentioned above, the circumferential groove in the pulley around which the cord is wound is substantially V-shaped in crsss-section and has a maximum width of 15 mm and a minimum width of 5 mm. The overall width of the pulley 2 is 33 mm. The bores 24 which receive the engagement members 6 are tapped with an M12 thread and the diameter of the balls 22 is 8 mm. The spring pressure applied to the balls is between 25 and 50 Newtons.

The guide wheels 16, 18 rotate on an axis which is displaced form the axis of the shaft 4 by 70.75 mm and the dimensions indicated in Figure 2 are as follows: a = 50.0 mm ss =12.5 mm a = 60.0 mm

Claims

1. A pulley arrangement comprising a pulley wheel carried on a shaft which extends through an axial opening in the pulley wheel, the pulley arrangement comprising an engagement element slidably received in a bore which extends radially through the pulley wheel from a wall of the opening, the engagement element being resiliently biased radially inwardly towards the shaft, a recess being formed in a side wall of the shaft in which the engagement element engages to transmit drive from the shaft to the pulley wheel or from the pulley wheel to the shaft, the engagement element being disengaged from the recess when the applied drive torque overcomes the biasing force on the engagement element.

2. A pulley arrangement as claimed in claim 1, in which the engagement element comprises a ball.

3. A pulley arrangement as claimed in claim 1 or claim 2, in which the engagement element is biased by a compression spring situated in the bore.

4. A pulley arrangement as claimed in claim 3, in which the spring is situated between the engagement element and a movable plug.

5. A pulley arrangement as claimed in claim 4, in which the movable plug is provided with an external thread and is retained on a corresponding thread in the bore.

6. A pulley arrangement as claimed in claim 5, in which the biasing force applied by the engagement element is adjusted by screwing the plug into or out of the bore.

7. A pulley arrangement as claimed in any one of claims 4 to 6, in which the bore passes completely through the pulley wheel and the end of the plug remote from the engagement element is adapted to cooperate with a tool which is used to rotate the plug.

8. A pulley arrangement as claimed in any one of the preceding claims, in which a plurality of radial bores are provided at equidistant spacing around the pulley wheel, each bore housing a respective engagement element.

9. A pulley arrangement as claimed in any one of the preceding claims, in which the shaft is provided with a plurality of equidistantly spaced circumferential recesses.

10. A pulley arrangement as claimed in claim 9 in which the recesses comprise holes through the side wall of the shaft.

11. A pulley arrangement as claimed in claim 9 or 10, in which the engagement elements are arranged in opposing pairs on opposite sides of the pulley wheel, so that the biasing forces applied through the engagement elements to the shaft are equal and opposite, and so that the axis of rotation of the pulley wheel is aligned with the longitudinal axis of the shaft.

12. A pulley arrangement as claimed in any one of the preceding claims, in which the pulley wheel is engaged by an endless draw cord, and is provided with a first guide wheel for guiding the cord on to the pulley wheel and a second guide wheel for guiding the cord off the pulley wheel.

13. A pulley arrangement as claimed in claim 12, in which the pulley wheel has a circumferential groove which receives the draw cord, the groove having tapering side walls and a substantially cylindrical drive portion.

14. A pulley arrangement as claimed in claim 13, in which the drive portion is provided with a high friction surface.

15. A pulley arrangement as claimed in claim 14, in which the drive portion is provided with surface formations.

16. A pulley arrangement as claimed in claim 14 or 15, in which the drive portion is provided with an aluminium oxide 80 grit coating.

17. A pulley arrangement as claimed in any one of claims 12 to 16, in which the draw cord is led two full turns around the drive portion of the pulley wheel.

18. A pulley arrangement as claimed in any one of the preceding claims, in which a plurality of pulley wheels are provided on a common shaft.

19. A pulley arrangement as claimed in any one of the preceding claims in which the shaft is drive by an electric motor.

20. A pulley arrangement, substantially as described herein, with reference to, and as shown in the accompanying drawings.