Background of the Invention Stairlifts having a positive drive system, such as a rack and pinion drive system, are the most common. One example of such a stairlift is described in International Patent Application WO 96/20125 but there are numerous others including some of our own manufacture. Whilst positive drive stairlifts are well-known and have been sold and used for a number of years, the manufacture of such lifts involves considerable precision to ensure silent, reliable operation of the rack and pinion system. This, in turn, means that this form of lift is relatively expensive to manufacture, install and maintain.

Attempts have been made to substitute a friction drive system for the rack and pinion. Examples of friction drive stairlifts are described in European Patent 0 525,141 and in United States Patents 5,908,087 & 6,053,286. Friction drive stairlifts are generally smoother and quieter, can incorporate tighter inside bends, and allow the stairlift rail to be provided in modular form, with greater ease, as matching rail pitch is no longer an issue.

One of the problems with known friction drive stairlifts is that the degree of frictional engagement between the drive roller (s) and the rail is substantially constant and must be designed for the worst case scenario-a particularly heavy occupant. As a consequence, costly and often complex arrangements are provided to ensure adequate frictional engagement is provided at all times.

In the case of European Patent 0 525,141 and US Patent 6,053,286 referred to above, the stairlift rail is formed by upper and lower tubes and the stairlifts described address the need for friction by applying frictional drive to both upper and lower rails. In the case of the stairlift shown in US Patent 5,908,087 the frictional drive is applied to one rail only but by two friction drive units connected in tandem.

Obviously the incorporation of two drive units, of itself, adds expense and further expense can be generated ensured the two drive units operate in sync.

It is an object of this invention to provide a stairlift which addresses the above problems; or which will at least provide a useful choice.

Summary of the Invention Accordingly, in one aspect, the present invention provides a stairlift including a stairlift rail; a carriage displaceable along said rail; a seat mounted on said carriage for displacement therewith; a drive system including a frictional drive wheel engaging said rail to drive said carriage along said rail, said stairlift being characterised in that the contact pressure between said drive wheel and said rail varies with the weight acting on said seat.

Preferably said stairlift further includes weight transfer means operable to transfer weight applied to said seat into contact pressure between said drive wheel and said rail.

Preferably said drive roller acts in combination with at least one clamping roller such that said drive and clamping rollers are positioned on opposite sides of said rail, said drive and clamping rollers being displaceable toward and away from one another so as to vary the force on said rail clamped there between, said weight transfer means being operable to displace said drive and clamping rollers toward one another in response to a weight being applied to said seat.

Preferably said seat is displaceable with respect to said carriage said weight

transfer means being operable to transfer a displacement of said seat with respect to said carriage, under the influence of a weight being applied to said seat, into a displacement of said drive and clamping rollers toward one another.

Preferably said weight transfer means includes one, and more preferably two, Bowden cables. Alternatively, said weight transfer means includes a hydraulic or pneumatic system to transfer displacement of said seat with respect to said carriage into a displacement of said drive and clamping rollers toward one another.

Preferably said system includes two clamping rollers acting in combination with one drive roller. Said clamping rollers are preferably arranged to either side of, and equi-distant from, a vertical line passing through the axis of said drive roller when said carriage is located on a horizontal section of rail.

In a second aspect the invention provides a skate for a friction drive stairlift, said skate including a friction drive roller constructed and arranged to engage a first surface part of a stairlift rail; at least two clamping rollers constructed and arranged to engage an opposed surface part of said rail such that said rail is located between said drive roller and said clamping rollers, said skate being characterised in that the spacing between said drive roller and said clamping rollers, in a direction perpendicular to the tangent between said drive roller and said rail, is variable.

Preferably said skate further includes displacement means operable to vary said spacing.

Preferably said skate includes a lever bracket with respect to which said drive roller is rotatably mounted; and a clamping roller bracket on which said clamping rollers are rotatably mounted, said clamping roller bracket being connected to, but being slidably displaceable with respect to, said lever

bracket.

In a third aspect the invention provides a stairlift including the skate as set forth above.

Many variations in the way the present invention may be performed will present themselves to those skilled in the art. The description which follows is intended as an illustration only and the absence of description of particular alternatives or variants should in no way be applied to limit the scope of the invention. Such description of specific elements which follows should also be interpreted as including equivalents whether existing now or in the future.

The scope of the invention should be defined solely by the appended claims.

Brief Description of the Drawings One form of stairlift incorporating the various aspects of the invention will now be described with reference to the accompanying drawings in which: Figure 1: shows a general front elevation of a stairlift assembly incorporating the invention; Figure 2: shows an enlarged rear view of the stairlift carriage shown in Figure 1, on a level section of rail; Figure 3: shows a side elevation of that which is shown in Figure 2; Figure 4: shows a view similar to Figure 2, but with the carriage positioned on a sloping section of rail; Figures SA : show details of alternative weight transfer arrangements &5B incorporated in a stairlift according to the invention; Figure 6: shows a similar view to Figure 2 but with the carriage positioned on a negative transition bend in the rail;

Figure 7: shows a side elevation of that which is shown in Figure 6; Figure 8: shows a view similar to Figure 6, but with the carriage positioned on a positive transition bend in the rail; Figure 9: shows a side elevation of that which is shown in Figure 8; Figure 10: shows a schematic plan view of the skate roller positioned on a straight horizontal section of rail; Figure 11: shows a similar view to Figure 10 but with the skate negotiating an outside bend (in a horizontal plane); and Figure 12: shows a view similar to Figure 10 but with the skate negotiating an inside bend.

Detailed View of Working Embodiment Referring firstly to Figure 1, a stairlift 10 is depicted including a carriage 11 mounted on a rail 12. In the conventional manner, the carriage provides for a chair (not shown) to be mounted on interface 13 thereon in a manner which will become clearer from the description which follows.

As can be seen the rail 12 comprises a top tube 14 and a bottom tube 15, the tubes being held in fixed relationship by rail brackets 16 (Figure 3). The brackets preferably serve as mounting points for posts or uprights 17 upon which the rail is mounted on a stairway. In a vertical plane, the rail 12 includes a negative transition bend 18 and a positive transition bend 19.

Typically the rail will also include inside and outside bends in a horizontal plane and, whilst these are not shown in Figure 1, they are shown in Figures 11 and 12.

In accordance with one aspect of this invention the stairlift is driven by frictional engagement between a friction drive roller and one of the tubes 14

or 15, the frictional engagement between the drive roller varying according to weight applied to the seat or chair mounted on the carriage i. e. weight applied vertically to interface 13. When an occupant is in place on the chair, the frictional engagement between the drive wheel and the rail increases; and reduces when the occupant alights from the stairlift.

Referring now to Figures 2 and 3, the carriage 11 includes an upright chassis defined by side plates 20, rear surface 21 and front surface 22. Enclosed within the carriage is a motor and gearbox, indicated in dotted outlines by reference numerals 23 and 24 respectively. The output of the gearbox projects through the rear 21 of the carriage and has a drive wheel 25 mounted thereon.

The surface of drive wheel is preferably formed with a high friction surface.

This surface may, for example, be formed from polyurethane with a shore hardness of 85 to 90.

The drive roller 25 forms part of a skate 26 (Figures 5A, 5B), the skate further including clamping rollers and roller mounts which are described in greater detail below.

The skate chassis is formed, in part, by a lever bracket 28, the lever bracket 28 being mounted over the gearbox output, between the carriage rear 21 and the drive roller 25. The bracket 28 is a sliding fit over the gearbox output so as not to rotate therewith but the bracket, and thus the entire skate 26, pivots about the same axis as the gearbox output and the drive roller 25. Slidably mounted on the lower side of the lever bracket 28 is a clamping roller bracket 30. As is best seen in Figures 5A & 5B, a pair of pillars 31 extend upwardly from the clamping roller bracket 30 and slidingly engage in vertical bores (not shown) formed in the lever bracket 28. This allows the clamping roller bracket 30 to pivot with the lever bracket 28 about the skate pivot axis, yet slide with respect to the lever bracket in the directions shown by arrow 32 in Figure 5B.

As can be seen in Figure 2, the clamping roller bracket 30 serves as a mount for bottom skate rollers 34a and 34b, each of the rollers 34a, 34b being

rotatably mounted in U-shaped brackets 35, which brackets 35 are, themselves, mounted for rotation about vertical axes which pass through the centres of retaining bolts 36.

It will thus be appreciated that as the clamping roller bracket 30 moves with respect to the lever bracket 28, the spacing between the drive roller 25 on the one hand, and the clamping rollers 34a, 34b on the other hand, varies, this spacing being measured along a line perpendicular to the tangent between the drive roller and the rail. This, in turn, varies the clamping action of the rollers on rail tube 14 and thereby varies the frictional engagement between the drive roller 25 and the rail tube 14.

The combination of the drive roller, and two clamping rollers mounted equidistant from the drive roller is both vertical and horizontal planes, has particular advantages. Not only does this arrangement permit the skate to negotiate all types of vertical and horizontal bend, but the arrangement also provides inherent stability to the carriage by countering against twist and fore/aft rocking.

The skate further includes pressure varying means to vary the roller spacing, and thus the frictional engagement, as above described.

In the form shown in Figures 2 and 3, a pair of levers 38a and 38b are mounted on opposite sides of the lever bracket 28, the inner ends being pivoted at 39 to extensions 40 provided on the front of bracket 28. Pivoted at an intermediate point on each lever 38 is a link 42 which attaches the lever to the clamping roller bracket 30. Attached to the outer end of each lever 38 is one end of a Bowden cable 44. As can be seen from Figure 5A, one Bowden cable 44 can be attached directly to each of the levers 38a, 38b. Alternatively, as shown in Figure 5B, and in Figure 2, the two Bowden cables 44 can be attached, near centrally, to a balance bar 45 extending between the outer ends of the levers 38a and 38b. The latter arrangement is easier to adjust and is more effective in maintaining friction drive should one of the cables 44 break.

In the form shown, the outer casings of the Bowden cables are retained in slots (not shown) provided in shroud 46 extending outwardly from the upper edge of bracket 28, the shroud also covering, and preventing interference with, the drive roller 25.

The ends of the Bowden cables 44 not attached to levers 38a, 38b are attached to chair interface 13. In this case, the outer casings of the cables 44 are retained on the upper edge 47 of the carriage chassis, the inner cables passing over pulleys 48 before being fixed to a vertical part of the inner surface of the interface 13. As is perhaps best seen in Figure 4, the interface ends of the cables 44 preferably attach to cross piece 50 which, in turn, is pivotally fixed to the interface 13 at 51.

It will be appreciated that the combination of the Bowden cables and levers provides for the generation of considerable mechanical advantage. In the particular embodiment described, a mechanical advantage of six times the passenger weight is generated. This can be varied by changing the attachment points of the Bowden cables 44 to the levers 38a, 38b.

The interface 13, in the form shown, has a substantially channel shaped main section, the sides 54 of the channel closely overlying the sides 20 of the carriage. The upper part of the interface is formed into mounting flange 56 to allow a seat or chair to be fixed to the interface, while the lower part of the interface mounts a footrest 58.

As can be seen in Figure 3, the interface sides 54 include vertical slots 60 which engage over pegs 61 extending out from the side walls 20 of the carriage. The engagement of the pegs 61 in slots 60 not only retains the interface 13 on the carriage 11, but also controls and limits vertical sliding movement of the interface with respect to the carriage.

Springs 62 may be provided, acting between the interface and the carriage, to ensure a degree of clamping by the skate on rail 14 is maintained when there is no occupant on the stairlift. Whilst the springs 62 are shown as coil springs, they could also be provided in the form of gas

springs.

It will be appreciated that a stairlift of the type described must also include some form of levelling arrangement to ensure the mounting flange 56 of the interface remains substantially level regardless of the position of the carriage 11 on the rail 12. Unlike many prior art forms of stairlift in which levelling is effected by varying the spacing between the rail tubes, the form of stairlift described herein is based on a rail having rail tubes which are equi-spaced along the entire length of the rail. In this stairlift, the carriage 11 incorporates a sensor to sense out-of-level displacement of the carriage chassis, and combines this with an actuator which effects rotation between the skate and the carriage chassis, to maintain the chassis in such a position that flange 56 of interface 13 is always maintained level.

Levelling of the chassis could be undertaken by the method and apparatus described in our European Patent 0 738 232, but we have also found that an electrolytic tilt sensor (not shown) provides satisfactory results. The output of the tilt sensor is linked to linear actuator 65 which serves to effect rotation between the skate and the carriage chassis.

Because of the relative bulk of linear actuator 65, we have devised a mounting arrangement which permits the actuator 65 to be mounted in a compact fashion yet still perform its intended function. Referring to Figures 2 to 4, it can be seen that the gearbox section 66 of the actuator is accommodated in the upper part of the carriage chassis, the actuator being mounted so that the output shaft 67 thereof extends down one side of the chassis. The free end of output shaft 67 is pivotally fixed to rocker arm 68 which is pivoted to the lower mid point of the carriage chassis. A connecting rod 70 attaches the opposite side of the rocker to the lever bracket 28. Thus, as the output shaft 67 of the actuator extends and retracts, the rocker is displaced about its pivot and the skate is rotated with respect to the carriage chassis. This is particularly evident from Figure 4. This mounting arrangement therefore allows the bulky linear actuator 65 to be accommodated in a compact manner whilst still allowing the full scope of the actuator movement to be utilized.

It will be seen that the stairlift as herein described obviates the need for a conventional cluster arrangement acting on bottom rail tube 15. In place of the cluster, the stairlift as herein described includes a simple support roller 74 which rotates about is vertical axis and reacts the forces tending to rotate the carriage in a clockwise sense as shown in Figure 3. Whilst one support roller only is shown in Figures 1 to 9 two parallel support rollers could be provided, as is shown in Figures 10 to 12.

Turning now to Figures 6 and 7, when the carriage encounters negative transition bend 18 in the rail, the horizontal spacing between the drive roller 25 and the clamping rollers 34a, 34b naturally reduces. This reduction is effected automatically by occupant weight and/or the springs 62 and, as can be seen in Figure 7, the levers 38a, 38b pivot upwardly and the slots 60 displace downwardly over pegs 61.

Turning now to Figures 8 and 9, when the carriage encounters positive transition bend 19 in the rail, the horizontal spacing between drive roller 25 and the clamping rollers 34a, 34b is naturally forced to increase. This increase is effected against the influence of occupant weight and/or the springs 62 and, as can be seen from Figure 9, the levers 38a, 38b pivot downwards and the slots 60 displaced upwardly over pegs 61.

Referring now to Figures 10 to 12, it will be noted that, in a horizontal plane, the relative positions of the drive roller 25, and clamping rollers 34a, 34b change. Unless allowance is made for this, the chair can tilt as the carriage negotiates inside and outside bends.

Figure 10 illustrates the roller positions when the carriage is positioned on a straight section of rail, such as is also shown in Figures 2 and 3. When the carriage negotiates an outside bend, as shown in Figure 11, the clamping rollers swivel outwardly in brackets 35 to accommodate the rail curvature and, in so doing, effectively project the drive roller outwardly in the direction of arrow 76. Since the form of the drive roller would normally constrain the generated tending to rotate the carriage about the rail. The present invention addresses this problem by providing the drive roller with an elongated shank

80 extending from the rounded conical end section 82 as can be clearly seen in Figure 9. Thus the contact point between the roller 25 and the rail can"float" over the axial length of the roller as the carriage negotiates inside and outside bends.

It will be noted from Figure 12 that, when the carriage negotiates inside bends, the rollers 34a and 34b swivel in the opposite sense to that shown in Figure 11 and effectively pull the drive roller 25 back in the direction of arrow 78.

It will thus be appreciated that the present invention provides an effective from of friction drive stairlift which enables high levels of frictional torque to be generated and applied though a single drive wheel. Further, the combination of a single top drive wheel in combination with two clamping rollers acting on the opposite sides of the rail tube, provides near perfect geometry for the carriage to negotiate bends in both vertical and horizontal planes.