DESCRIPTION

TECHNICAL FIELD

The present invention relates to a spring tensioning device for a roller blind. BACKGROUND ART A roller blind is often configured to operate with the blind fabric deploying in a vertical plane, e.g. adjacent a conventional window in a wall of a building. The blind fabric is held taut as it deploys simply by a weighted hem bar. However, there are situations where the use of a weighted hem bar alone is not sufficient or appropriate, for example if the blind fabric is to be deployed in a non-vertical plane or if the blind fabric is to be deployed upwards rather than downwards. In such situations, spring tensioning devices are used to apply tension to the blind fabric in the desired direction of blind fabric deployment.

Figure 1 shows schematically a pair of spring tensioning devices (10) used with a roller blind (12) configured such that its blind fabric (14) deploys horizontally. The spring tensioning devices (10) are positioned opposite the roller blind (12) at the end of guide channels (16) for guiding lateral sides of the blind fabric (14) as it deploys. Each spring tensioning device (10) comprises a length of cord (18), one end of which is attached to the blind fabric and the other end of which is wound on a spool (20) rotatably mounted in a housing (22). Rotation of the spool (20) in the housing (22) is subject to a resilient bias (24) such that the cord (16) between the housing (22) and the blind fabric (14) is maintained under tension, i.e. exerting a pull on the blind fabric (14) towards the housing (22). The resilient bias (24) comprises what is known in the art as a constant tension spring or a Conston™ spring.

Figure 2 is a cutaway schematic illustration of the spring tensioning device (10), showing the cord (18), spool (20) and constant tension spring (24). The latter comprises a first rotatable drum (30), a second rotatable drum (32), and an elongate spring strip (34) initially coiled around the second rotatable drum (32), but with one end attached to the first rotatable drum (30). The reel (20) is coaxially mounted on the first rotatable drum (30) such that pulling cord (18) from the housing (22) causes both the reel (20) and the first rotatable drum (30) to rotate in a first direction (e.g. anti-clockwise). Rotation of the first rotatable drum (30) in the first direction pulls the spring strip (34) from the second rotatable drum (32), causing the latter to rotate in a second direction opposite to the first direction (e.g. clockwise), and on to the first rotatable drum (30). Such uncoiling from one drum and coiling onto the other drum requires a near-constant force over the full range of possible drum rotations. This means that the constant tension spring (24) will exert a near-constant force on the cord (18) and the blind fabric ( 14) regardless of their positions relative to the spring tensioning devices (11). The present applicant has appreciated that there are number of disadvantages with conventional spring tensioning devices. First, despite its name, the constant tension spring (24) produces a bias which tends to increase slightly as the spring strip (34) uncoils from the second rotatable drum (32), i.e. as the length of cord (18) unwound from spool (20) increases. This has the unfortunate effect that the constant tension spring is able to exert a greater resilient bias when the cord is fully extended than when it is fully retracted. In addition, the effective diameter of the spool will increase as more cord is wound on to the spool, which is important since tension in the cord between the housing and the blind fabric will be inversely proportional to effective spool diameter. These two effects combined mean that the spring tensioning device generates a maximum force on the blind fabric when fully retracted (cord fully unwound from spool), and a minimum force on the blind fabric when fully deployed (cord fully wound on spool).

In actual fact, however, it would be better if the spring tensioning device exerted maximum force on the blind fabric when fully deployed in order to counter a resistance load which increases as the blind fabric deploys, caused for example by increasing friction between the blind fabric (14) and the side channels (16). In addition, it would be better if the spring tensioning device exerted minimum force on the blind fabric when fully retracted to avoid putting unnecessary stress on the roller blind that might cause creasing of the blind fabric on its roller. Furthermore, the conventional spring tensioning devices are considered to be noisy in operation due to repeated "twang" sounds which are audible as the blind fabric is deployed. The "twang" sounds are caused by the cord winding unevenly on its spool, repeatedly forming a ridge and then slipping suddenly into an adjacent furrow. The present invention has been made to address and even obviate at least one of the aforementioned problems with known cable tensioning devices for roller blinds.

DISCLOSURE OF INVENTION In accordance with one aspect of the present invention, there is provided a spring tensioning device for applying tension to a blind fabric deployed from a roller blind, comprising: a body; a drum rotatably mounted on the body for rotation about a rotation axis; a cord having a leading part for attachment to deployed blind fabric, and a trailing part wound around the drum and configured to unwind therefrom as the drum rotates in a first direction around the rotation axis; and a constant tension spring mounted on the body and configured to apply a near-constant resilient bias to the drum as it rotates in the first direction, urging the drum to rotate in a second direction opposite to the first direction; characterised in that the constant tension spring is mounted alongside the drum, spaced in a radial direction from the rotation axis of the drum. With a conventional spring tensioning device, the spool and the constant tension spring are mounted end-to-end on the body, i.e. with the constant tension spring positioned at one axial end of the spool. In practice, this means that the spool can only take up part of the available height of the spring tensioning device. In contrast, with the present invention where the constant tension spring are radially offset rather than axially offset, the drum may span the full height of the spring tensioning device, allowing more room for consecutive windings of the cord on the drum to lay side by side rather than one on top of another. In turn, this should help to reduce or even obviate the "twang" sounds as the cord winds on to the drum. The drum may have a radially outermost periphery which is large enough to accommodate the full extent of the cord to be wound on the drum in a single layer, with each successive winding axially spaced from preceding windings (i.e. without any of the windings stacked on top of each other).

The device may further comprise a guide to help lay consecutive windings of the cord on the drum side by side as the cord winds on to the drum. The guide may comprise a groove on the radially outermost surface of the drum. The groove may be helical, and may have a pitch which urges consecutive windings of the cord on the drum to fit snuggly together. The helical groove may extend from a first axial end of the drum towards a second axial end of the drum. Alternatively or additionally, the guide may comprise a channel (for example, formed at least in part by a pulley wheel) on an opposite side of the constant tension spring to the drum, the channel being configured to steer the cord past the constant tension spring and towards the drum when being wound thereon. The channel may be spaced from the drum such that the maximum angle at the channel swept out by the cord when wound from one axial end of the drum to the other is less than about 30°, and may even be less than about 20°, for example it may be about 15°. By keeping the maximum angle below about 30°, consecutive windings are less likely to be urged together in a way which encourages a later winding to climb on top of an earlier winding.

In one embodiment of the present invention, the drum may have a frusto-conical profile, with the trailing part of the cord being configured to wind on the drum in a direction of decreasing drum diameter as the drum rotates in the second direction. For example, the trailing part of the cord may initially be wound on the drum in a first region before being wound on the drum in a second region which has a small diameter than the first region. A first axial end of the drum may have a first diameter, and an opposite axial end of the drum may have a second diameter, with the first diameter being larger than the second diameter by a factor of more than 1.2. For example, the first diameter may be larger than the second diameter by a factor of between 1.5 and 3.0, such as a factor of 2. In this way, tension in the leading part of the cord may increase as the trailing part of the cord winds on the drum towards a region of minimum diameter. This is because such tension is inversely proportional to the diameter of the latest winding on the drum. Advantageously, such an increase in tension in the leading part of the cord would correspond in use to when the blind fabric of the roller blind is fully deployed, which is exactly when it needs to be greatest in order to counter a resistance load which increases as the blind fabric deploys. The constant tension spring may comprise first and second rotatable drums and an elongate spring strip initially coiled around the second rotatable drum, with one end of the elongate spring strip being attached to the first rotatable drum such that the elongate spring strip will uncoil from the second rotatable drum and coil on to the first rotatable drum if one of the rotatable drums is rotated relative to the other. The constant tension spring may be configured to apply a near- constant resilient bias to the drum with the trailing part of the cord wound therearound via a gear mechanism. The gear mechanism may comprise a first gear or cogwheel coaxially attached to one of the first and second rotatable drums of the constant tension spring, and a second gear or cogwheel coaxially attached to the drum with the trailing part of the cord wound therearound. The first and second gears may mesh directly provided the trailing part of the cord unwinds from its drum as the elongate spring strip unwinds from the second rotatable drum and winds on to the first rotatable drum.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a schematic use of spring tensioning devices used with a roller blind; Figure 2 is cutaway schematic illustration of a prior art spring tensioning device;

Figure 3 is a cutaway perspective view from front, one side and above of a spring tensioning device in accordance with one embodiment of the present invention;

Figure 4 is cutaway inverted back view of key components of the spring tensioning device of Figure 3;

Figure 5 is a cutaway perspective view from front and above of the spring tensioning device of Figure 3; and

Figure 6 is a cutaway perspective view from front and below of the spring tensioning device of Figure 3

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

Figures 3-6 illustrate a spring tensioning device 100, embodying the present invention, for applying tension to blind fabric deployed from a roller blind. The spring tensioning device 100 comprises a housing defining a body 102, a drum 104 rotatably mounted on the body 102 for rotation about a rotation axis 106, a cord 108 and a constant tension spring 110. The cord 108 has a leading part 112 (shown schematically in phantom lines) for attachment to blind fabric deployed from the roller blind, and a trailing part 114 which in an initial configuration is wound 5 around the drum 104. The trailing part 114 is configured to unwind from the drum 104 when in use as the drum 104 rotates in a first direction 120 about the rotation axis 106. The constant tension spring 110 is mounted on the body 102 and is configured to apply a near-constant resilient bias to the drum 104 when the drum rotates in the first direction 120, urging the drum 104 to rotate in a second direction, opposite to the first direction 120, about the rotation axis 10 106. The constant tension spring 110 is mounted alongside the drum 104, spaced in a radial direction "R" from the rotation axis 106 of the drum 104.

The constant tension spring 110 is well known in the art and comprises first and second rotatable drums 130, 132 and an elongate spring strip 134 initially coiled around the second rotatable

15 drum 132, with one end attached to the first rotatable drum 130. Rotation of the second drum 134 which causes the elongate spring strip 134 to uncoil from the second rotatable drum 132, will simultaneously induce counter rotation of the first rotatable drum 130 as the elongate spring strip 134 coils thereon and provide a near-constant resilient bias seeking to return the elongate spring strip 134 to the second rotatable drum 132. A cogwheel 136 is coaxially mounted on the 0 second rotatable drum 132, and meshes with another cogwheel 138 coaxially mounted to the drum 104. In this way, rotation of drum 104 in the first direction 120 causes a corresponding rotation of the second rotatable drum 134, generating the near-constant resilient bias urging the drum 104 to rotate back in the second direction. 5 The drum 104 has a frusto-conical profile, with the trailing part 114 of the cord 108 being configured to wind on to the radially outermost, frusto-conical surface of the drum 104 in a direction of decreasing drum diameter. In other words, the trailing part 114 of the cord 108 is attached to the first axial end 140 of the drum 104 where the diameter of the drum 104 is greatest so that initial windings will have a corresponding diameter. Subsequent windings around the0 drum 104 towards the opposite axial end 142 of the drum 104 where the diameter of the drum 104 is smallest will have a smaller diameter than the initial windings

The spring tensioning device 100 further comprises guide means 150 to help lay consecutive windings of the cord 108 side by side on the drum 104, rather than one on top of the other. The guide means 150 includes a channel 152 formed in a pulley wheel 154 which enables the leading part 112 of the cord 108 to be at an angle (e.g. 90°) to the trailing part 114 of the cord 108. The pulley wheel 154 is mounted on a bracket 156 which is reversibly mountable on the body 102 5 to allow both right and left handed variants of the device 100. The channel 152 is positioned on an opposite side of the constant tension spring 110 to the drum 104 in order to ensure there is a sufficient distance between the channel 152 and the drum 104 to prevent the cord sweeping out too large an angle "Θ" as it winds on the drum 104 from a fully unwound configuration to a fully wound configuration. The angle "Θ" (shown in Figure 6 with one extreme position of the cord 10 108 winding on the drum 104 indicated in phantom lines) should be less than 30°, for example about 15°. The guide means 150 also includes a roller 158 to steer the cord 108 from the channel 152 past the constant tension spring and towards the drum 104.

The guide means 150 also includes a helical groove 160 on the frusto-conical surface of the drum 15 104. The helical groove 160 has a width and depth to at least partially receive the cord 108 when laid therealong, and has a pitch which urges consecutive windings of the cord 108 on the drum 104 to fit snuggly together without piling one on top of the other. The helical groove 160 starts at the first axial end 140 of the drum 104 where its diameter is greatest, and extends all the way to the opposite axial end 142 of the drum 104 where its diameter is smallest. In this way, 0 the full extent of the cord 104 to be wound on the drum 104 may be accommodated in a single layer.

The spring tensioning device 100 may be used in exactly the same way as the conventional spring tensioning device 10 of Figure 1, with the leading part 112 of the cord 108 attached to 5 the blind fabric 14 deployed from roller blind 12. However, compared with the conventional spring tensioning device 10, the spring tensioning device 100 will deliver an improved pull force on the blind fabric 14, even if both devices employ identical constant tension springs. In particular, when the blind fabric 14 is fully retracted (i.e. cord 108 is fully extended), the spring tensioning device 100 will exert its lowest pull force because the bias from the constant tension0 spring 110 will be exerted on the cord 108 via windings at the first axial end 140 of the drum 104 where its diameter is greatest. Furthermore, when the blind fabric 14 is fully deployed (i.e. cord 108 is fully wound on drum 104), the spring tensioning device 100 will exert its highest pull force because the bias from the constant tension spring 110 will be exerted on the cord 108 via windings at the opposite axial end 142 of the drum 104 where its diameter is smallest. In addition, compared with the conventional spring tensioning device 10, the spring tensioning device 100 will operate more quietly because each successive winding of the cord 108 on the drum 104 will be positioned next to the immediately preceding winding rather than on top thereof.