It is known to provide a roller blind comprising a roller on which the blind material is wound, supported at one end on a support bush which is rotatably supported on a support body. Where the blind is operated by a drive cord a drive mechanism is included to enable the cord to drive the bush to rotate the roller. It is desirable to include a degree of friction to control rotation of the roller so that a user has to apply a certain level of force on the drive cord to move the blind, and the blind will stop rapidly when the user stops pulling the cord so that the blind can be positioned accurately and easily. In some known blinds friction is provided by a clutch mechanism, such as a coil spring wound round a shaft which is loosened by pulling the cord to allow movement of the blind but which tightens on release of the cord to prevent further movement of the blind under its own weight. It can be a problem with this type of blind that if a user pulls on the blind itself rather than the cord, either the blind cannot easily be unwound or the clutch mechanism can become worn or damaged.

The present invention therefore provides a drive mechanism for a roller blind comprising a body which includes a support shaft, and a drive coupling member rotatably mounted on the support shaft and including drive means to enable it to be rotated by means of a cord, wherein the drive coupling member and body have mutually engaging friction surfaces thereon and the mechanism further comprises biasing means arranged to urge the friction surfaces together in a direction axial to the mechanism thereby providing frictional resistance to rotation of the drive coupling member.

This engagement can provide a blind in which the functional resistance to movement is substantially the same whether the cord is used or the blind itself if pulled.

Preferably the frictional resistance is sufficient to prevent unrolling of the blind under its own weight even when it is almost completely unwound.

Preferably the mechanism further comprises a drive bush rotatably mounted on the support shaft, the drive bush being drivingly engaged with the drive coupling member so as to be rotatable thereby and being arranged to support a roller for rotation therewith. In this case the biasing means is preferably arranged to act between the drive bush and the drive coupling member.

Preferably the drive bush comprises a tubular drive portion and a drive bush bearing portion extending radially inwards from the drive portion, and the biasing means acts against the drive bush bearing portion. The bearing portion preferably provides a bearing supporting the drive bush on the support shaft.

Preferably the drive coupling member includes a tubular portion and a drive coupling bearing portion extending radially inwards from the tubular portion, and the biasing means acts on the drive coupling bearing portion.

Preferably the drive coupling member comprises a drive wheel portion having the drive means thereon and having an end face which forms one of the friction surfaces. In this case the tubular portion preferably extends from the opposite side of the drive wheel portion to the end face, and is preferably of smaller diameter than the drive wheel portion. This allows

the end face to be annular and of substantially larger outer diameter than that of either of the tubular portions.

The biasing means can conveniently comprise a compression spring, preferably a helical spring, and preferably extends around the drive shaft.

The biasing means is preferably contained substantially within the tubular portion of the drive coupling member, which in turn is preferably supported within the tubular portion of the drive bush so as to provide support in the radial direction between the two, whilst allowing relative sliding in the axial direction.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure la is a top view of a roller blind assembly including a drive mechanism according to a first embodiment of the invention; Figure 1b is a front view of the roller blind assembly of Figure la; Figure le is a section through the roller blind assembly of Figures la and lb ; Figure 2 is a cut away perspective view of a drive mechanism of the assembly of Figure 1; Figure 3 is an exploded view of the drive mechanism of Figure 2; and

Figure 4 is a perspective view of the drive mechanism of Figures 2 and 3.

Referring to Figures la, lb and lc, a blind assembly 10 comprises a sheet of fabric 12 rolled onto a roller 14 which is supported in a horizontal orientation between a support bracket 16 and a dummy end 17 at one end, and a drive mechanism 18 at the other end which is supported on a further bracket 19. The support bracket 16 and dummy end 17 are arranged to support the roller 14 so that it can rotate relative to the support bracket 16. The drive mechanism 18 also supports the roller 14 such that it can rotate, and includes a drive mechanism operated by means of a cord 20 in the form of a ball chain to enable the blind to be raised and lowered.

Referring to Figures 1c and 2 to 4, the drive mechanism comprises a main body 22 which includes a housing portion 24 which forms part of a housing 25 and has an end wall 26 and a support shaft 28 extending from the end wall through the centre of the housing 25. The support shaft 28 has a first bearing portion 28a nearest the end wall 26 and extending through the housing 25, a central portion 28b which is slightly narrower than the first bearing portion 28a, a second bearing portion 28c which is narrower than the central portion 28b, and a retaining portion 28d at its free end furthest from the end wall 26.

The drive mechanism further comprises a drive coupling 30 extending around, and rotatably supported on, the support shaft 28. The drive coupling 30 includes a tubular portion 32 and a drive wheel 34, of larger diameter than the tubular portion 30, which is located within the housing 25. The drive wheel 34 has an annular end face 36 which rests against the inner surface 38 of the end wall 26 of the housing 24, and has a number of teeth 40 formed on its radially outer surface 42 each of

which has a slot 41 extending in the circumferential direction arranged to engage with the cord 20 so that the drive coupling 30 can be rotated by means of the cord 20. The inner diameter of the drive coupling 30 is stepped at 31, being narrower towards the end at which the drive wheel 34 is located thereby forming a bearing 35 which is supported on the first bearing portion 28a of the support shaft 28. The wider inner diameter extends over most of the length of the tubular portion 32 and is aligned with the central part 28b of the support shaft 32 such that a chamber 44 of annular cross section is defined between them. The end 46 of the tubular portion 32 is approximately aligned with the step between the central portion 28b and the second bearing portion 28c of the support shaft 28.

The mechanism further comprises a drive end or drive bush 50 which comprises a main tubular portion 52, and an end wall 54 at one end of the tubular portion 52 which forms one side of the housing 25, and a bearing portion 56 which projects radially inwards from the tubular portion 52 and is supported on the second bearing portion 28c of the drive shaft 28.

The tubular portion 52 of the drive bush 50 extends around the tubular portion 32 of the drive coupling 30 and has a number of longitudinal ribs 53 along its outer surface which engage with the inside of the roller 14.

The drive bush 50 has a drive rib (not shown) projecting radially inwards from its tubular portion 52 which engages in a slot 54 formed in the drive coupling 30 so that drive bush 50 and drive coupling 30 are drivingly interengaged. The slot 54 is wider than the drive rib allowing a degree of free travel between the drive bush 50 and the drive coupling 30. The drive bush 50 and coupling member 30 are a close sliding fit with each other so that there is substantially no radial movement between them, but they can slide axially relative to each other. They are therefore supported in the radial direction on the first and second bearing surfaces 28a, 28c of the support shaft 28.

A starlock washer 60 is located in a groove 62 in the retaining portion 28d of the drive shaft 28 and acts against the bearing portion 56 of the drive bush 50 to locate the drive bush 50 axially and prevent it from sliding off the support shaft 28. A helical compression spring 64 is located in the chamber 44 and is held in compression having one end acting on the bearing portion 56 of the drive bush 50 and the other end acting on the step 31 at the end of the bearing portion 35 of the drive coupling 30. This urges the end face 36 of the drive wheel 34 against the inner surface 38 of the end wall 26. These two surfaces therefore act as friction surfaces which slide against each other to provide frictional resistance to rotation of the drive coupling 30, and hence also the drive bush 50 and the roller 14, relative to the body 22. Because the friction surfaces 36,38 are of a relatively large area they can provide a frictional resistance to rotation that is substantially constant over the range ctf relative rotational positions of the two surfaces 36,38. Also their relatively large size means that wear of the friction surfaces 36,38 is slow, and will not significantly alter the amount of frictional resistance they provide.

It will be appreciated that the spring 64 also urges the bearing portion 56 of the drive bush 50 against the starlock washer 60. The produces further frictional resistance to rotation of the drive bush 50 relative to the body 22. However in order to ensure that the frictional resistance is predominantly provided by the friction surfaces 36,38 the friction between the starlock washer 60 and bearing portion 56 is kept to a minimum. This can be by coating either the bearing portion 56 of the washer 60 with a low friction coating. Alternatively a further support for the spring 64 could be provided on the drive shaft 28 inside the bearing portion 56 of the drive bush 50, for example a split ring located in a groove in the support shaft 28.