Seat Tilting Mechanism With Two Springs

Abstract

A seat tilting mechanism comprising a main body, a support member rotationally mounted to said main body, and a first extension coil spring mounted between said main body and said support member to provide resistance to rotation of said support member, in which said seat tilting mechanism further comprises a second extension coil spring and an engagement mechanism which engages said second extension coil spring for resistance to rotation of said support member when said first extension coil spring reaches a pre-determined point of extension.

Claims

1. A seat tilting mechanism comprising a main body, a support member rotationally mounted to said main body, and a first extension coil spring mounted between said main body and said support member to provide resistance to rotation of said support member, and in which said seat tilting mechanism further comprises a second extension coil spring and an engagement mechanism which engages said second extension coil spring for resistance to rotation of said support member when said first extension coil spring reaches a pre-determined point of extension.

2. A seat tilting mechanism as claimed in claim 1 in which said seat tilting mechanism comprises an extension resistance mechanism comprising a first end part, a second end part and said first extension coil spring and second extension coil spring mounted between said first end part and said second end part.

3. A seat tilting mechanism as claimed in claim 2 in which said second extension coil spring comprises an axially extending spacer member mounted to a first end thereof, in which said engagement mechanism comprises a first radially extending portion provided at a first end of said spacer member and a second radially extending portion provided on said first end part, and in which said second radially extending portion engages said first radially extending portion when said first extension coil spring reaches said pre-determined point of extension.

4. A seat tilting mechanism as claimed in claim 3 in which said second extension coil spring is co-axial with said first extension coil spring.

5. A seat tilting mechanism as claimed in claim 4 in which said second extension coil spring and is mounted inside said first extension coil spring.

6. A seat tilting mechanism as claimed in claim 5 in which said spacer member comprises a sleeve with an annular flange at said first end, in which said first end part comprises a ring mounted around said sleeve, which abuts against said flange when said first extension coil spring reaches said pre-determined point of extension.

7. A seat tilting mechanism as claimed in claim 6 in which said first end part comprises a rod arranged inside said sleeve for telescoping movement in relation thereto.

8. A seat tilting mechanism as claimed in claim 7 in which said first end part is mounted to said support member and said second end part is mounted to said main body.

9. A seat tilting mechanism as claimed in any of the preceding claims in which said support member comprises a seat support.

10. A seat tilting mechanism as claimed in any of claims 1 to 8 in which said support member comprises a backrest support.

11. A seat tilting mechanism as claimed in any of claims 2 to 8 in which said support member comprises a lever rotationally mounted at a midpoint thereof to said main body, and comprising a first portion rotationally mounted at an upper point of said lever to a seat support, and a second portion rotationally mounted at a lower point of said lever to a backrest support, in which said seat tilting mechanism comprises an articulation mechanism comprising a first arm, a first end of which is rotationally mounted to said main body and a second end of which is rotationally mounted to said seat support, said first arm defining with said first portion, said seat support and said main body a first quadrilateral hinge, in which said articulation mechanism comprises a second arm, a first end of which is rotationally mounted to said main body and a second end of which is rotationally mounted to said backrest support, said second arm defining with said second portion, said backrest support and said main body a second quadrilateral hinge, in which interior corner angles and lengths of each side of said first quadrilateral hinge, and interior corner angles and lengths of each side of said second quadrilateral hinge are such that a predetermined angular displacement ratio of said backrest support in relation to said seat support is provided, in which said extension resistance mechanism is mounted between said main body and said first portion to provide resistance to rotation of said lever, in which said first end part is rotationally mounted to said lever at a first mounting point located between said midpoint and said upper point, and in which said first end part is angularly displaced from a line extending between said midpoint and said upper point.

12. A seat tilting mechanism as claimed in claim 11 in which said first quadrilateral hinge is movable between a rest position in which said seat support is substantially horizontal, and a fully tilted position in which said seat support is angularly displaced from horizontal, in which said second end part is rotationally mounted to said main body at a second mounting point, in which said first mounting point is located such that a rotation angle between an axis of said extension resistance mechanism and a line extending between said second mounting point and said upper point decreases as the first quadrilateral hinge moves from said rest position towards said fully tilted position.

13. A seat tilting mechanism as claimed in claim 12 in which said first extension coil spring reaches said pre-determined point of extension before said rotation angle reaches zero.

14. A seat tilting mechanism as claimed in claim 13 in which said first mounting point is located such that said rotation angle is zero prior to said first quadrilateral hinge achieving said fully tilted position.

15. A seat tilting mechanism as claimed in claim 14 in which said interior corner angles and said lengths of each side of said first quadrilateral hinge, and said interior corner angles and said lengths of each side of said second quadrilateral hinge are such that said predetermined angular displacement ratio of said backrest support in relation to said seat support is greater than one to one and less than two to one.

16. A seat tilting mechanism as claimed in claim 15 in which said interior corner angles and said lengths of each side of said first quadrilateral hinge, and said interior corner angles and said lengths of each side of said second quadrilateral hinge are such that said predetermined angular displacement ratio of said backrest support in relation to said seat support is substantially 1.9 to one.

17. A seat tilting mechanism substantially as described herein and as shown in the accompanying drawings.

Description

[0037] Two embodiments of the present invention will now be described by way of example, and with reference to the accompanying drawings in which:

[0038] FIG. 1 is a side view of a first seat tilting mechanism according to the present invention in a rest position,

[0039] FIG. 2 is a side view of the seat tilting mechanism as shown in FIG. 1 in a mid-tilt position;

[0040] FIG. 3 is a side view of the seat tilting mechanism as shown in FIG. 1 in a fully tilted position; and,

[0041] FIG. 4 is a side view of a second seat tilting mechanism according to the present invention.

[0042] Referring to FIG. 1, a seat tilting mechanism 1 comprises a main body 2, a support member in the form of lever 5 rotationally mounted to the main body 2, and a first extension coil spring 26 mounted between the main body 2 and the support member 5 to provide resistance to rotation of the support member 5. The seat tilting mechanism 1 further comprises a second extension coil spring 50 and an engagement mechanism, generally designated 51, which engages the second extension coil spring 50 for resistance to rotation of the support member 5 when the first extension coil spring 26 reaches a pre-determined point of extension.

[0043] The seat tilting mechanism 1 incorporates the articulation mechanism invention of the Applicant's co-pending patent application, and the following description includes a detailed explanation of the workings of that mechanism, including how the first coil spring 26 and second coil spring 50 are incorporated within it. A second embodiment of the invention which does not feature the complex articulation mechanism of the Applicant's co-pending patent application is shown in FIG. 4 to illustrate how the present invention can also be used in a more traditional seat tilting mechanism.

[0044] Referring to FIG. 1, the seat tilting mechanism 1 comprises a seat support 3, a backrest support 4 and an articulation mechanism. The articulation mechanism comprises the lever 5 rotationally mounted at a midpoint 6 by annular hinge 7 to the main body 2, and comprising a first portion 8 rotationally mounted at an upper point 9 by annular hinge 10 to the seat support 3, and a second portion 11 rotationally mounted at a lower point 12 by annular hinge 13 to the backrest support 4.

[0045] The articulation mechanism further comprises a first arm 14, a first end 15 of which is rotationally mounted to the main body 2 by annular hinge 16, and a second end 17 of which is rotationally mounted to said seat support 3 by annular hinge 18. The first arm 14 defines with the first portion 8, the seat support 3 and the main body 2 a first quadrilateral hinge, indicated by hashed box 19. The articulation mechanism comprises a second arm 20, a first end 21 of which is rotationally mounted to the main body 2 by annular hinge 22, and a second end 23 of which is rotationally mounted to the backrest support 4 by annular hinge 24. The second arm 20 defines with the second portion 11, the backrest support 4 and the main body 2 a second quadrilateral hinge, indicated by hashed box 25.

[0046] As described further below, interior corner angles and lengths of each side of the first quadrilateral hinge 19, and interior corner angles and lengths of each side of the second quadrilateral hinge 25 are such that a predetermined angular displacement ratio of the backrest support 4 in relation to the seat support 3 is provided.

[0047] An extension resistance mechanism comprising first coil spring 26 and second coil spring 50, is mounted between the main body 2 and the first portion 8 to provide resistance to rotation of the lever 5. The components of the extension resistance mechanism are shown in the Figures in cross-section for ease of explanation. The extension resistance mechanism also comprises a first end part 27 and a second end part 30, between which the first coil spring 26 and the second coil spring 50 are mounted. The first end part 27 is rotationally mounted to the lever 5 at a first mounting point 28 by annular hinge 29, which first mounting point 28 is located between the midpoint 6 and the upper point 9. As is clear from FIG. 1, the first mounting point 28 is angularly displaced from a line (the rightmost side of hashed box 19) extending between the midpoint 6 and the upper point 9. The second end part 30 is rotationally mounted to the main body 2 at a second mounting point 31 by annular hinge 32.

[0048] As will be appreciated from the Figures, the second coil spring 50 is co-axial with the first coil spring 26, and it is mounted inside the first coil spring 26.

[0049] The second coil spring 50 comprises an axially extending spacer member mounted to a first end 52 thereof, in the form of sleeve 53. This has a first radially extending portion in the form of an annular flange 54 at a first end 55 thereof. The first end part 27 has a corresponding second radially extending portion in the form of ring 56, which is mounted around the sleeve 53. With this arrangement the movement apart of the first end part 27 and the second end part 30 when the first coil spring 26 extends brings the ring 56 towards, and then into engagement with, the flange 54. The ring 56 abuts against the flange 54 when the first coil spring 26 reaches the pre-determined point of extension, as explained further below.

[0050] In addition, the first end part 27 comprises a rod 57 (visible in FIG. 2) which is arranged inside the sleeve 53 for telescoping movement in relation thereto. This provides the engagement mechanism 50 with greater stability in use.

[0051] The seat tilting mechanism 1 is adapted for use in an office chair, and provides a user-adjustable tilting movement thereto by virtue of being able to move between a rest position as shown in FIG. 1, in which the seat support 3 and the backrest support 4 are substantially horizontal, and a fully tilted position as shown in FIG. 3 in which the seat support 3 and the backrest support 4 are angularly displaced from horizontal. The main body 2 is adapted to rest atop an upright stanchion (not shown), the seat support 3 is adapted to support a seat (not shown) arranged generally horizontally above it, and the backrest support 4 is adapted to support a backrest (not shown) arranged generally vertically above it. The various components of the seat tilting mechanism 1 are constructed from metal. The various annular hinges are of a known construction, and allow the various components to freely rotate in relation to one another in either direction.

[0052] The seat tilting mechanism 1 has a number of features designed to provide control over the way it can move between the rest position to the fully tilted position, and in particular to increase the resistance to tilting the greater the level of tilt. This is relevant in the context of seat tilting mechanism 1 because the force which moves it is provided by the user themselves, and it increases the further the seat tilting mechanism 1 is tilted, because the user's weight is increasingly brought to bear on the mechanism.

[0053] Initially, if the user places all their weight on the seat support 3, the seat tilting mechanism 1 will assume the rest position shown in FIG. 1. It is biased into this position because the upper point 9 is forward of the midpoint 6, and therefore weight applied to the upper point 9 serves to force it in a clockwise direction about the midpoint 6, thereby closing the first quadrilateral hinge 19. However, once the user starts to lean back and places some of their weight on the backrest support 4, a rotational force is applied to the lower point 12 to move it in an anti-clockwise direction about the midpoint 6. This force acts to open the first quadrilateral hinge 19 against the combined tensile force of the first coil spring 26 and the rest of the user's weight bearing down on the upper point 9. The rotational force applied to the lower point 12 must reach a break out point at which it exceeds the combined tensile force of the first coil spring 26 and the rest of the user's weight bearing down on the upper point 9, before the first quadrilateral hinge 19 will start to open. It will be appreciated that this will vary depending on the weight of the user.

[0054] What is relevant is that as the first quadrilateral hinge 19 opens up and moves from the rest position shown in FIG. 1 towards the fully tilted position shown in FIG. 3, the rotational force applied to the lower point 12 exponentially increases because the user is being rotated backwards, so their weight gradually shifts from acting on the seat support 3 to acting on the backrest support 4. The user can obviously compensate for this by adjusting their body position as the seat tilting mechanism 1 moves, for example by leaning forward slightly when the seat tilting mechanism 1 reaches a desired angle. However, the extension resistance mechanism made up of the first coil spring 26 and the second coil spring 50 functionally compensates for this because the force required to extend it increases with the length of extension. This is because a) the force required to extend the first coil spring 26 and the second coil spring 50 increases the more they extend by virtue of the fact they are extension coil springs, b) the second coil spring 50 is introduced part way through the tilting action to increase the level of resistance offered, and c) an axis 33 of both the first coil spring 26 and second coil spring 50 is initially angularly displaced from a line 34 extending between the second mounting point 31 and the upper point 9, which is effectively the direction of extension.

[0055] In the context of the present invention the most relevant of these three factors is the function of the second coil spring 50, and the engagement mechanism 51 which employs the second coil spring 50 at the pre-determined point of extension of the first coil spring 26. In particular, from the rest position shown in FIG. 1 to the mid-tilt position shown in FIG. 2, the resistance to tilt is provided only by the first coil spring 26. As such, the resistance is at a lower level. This means that moving the seat tilting mechanism 1 from the rest position shown in FIG. 1 to the mid-tilt position shown in FIG. 2 is easier for the user to perform. In particular the necessary rotational force which has to be applied to the lower point 12 to reach the break out point at which it exceeds the combined tensile force of the first coil spring 26 and the rest of the user's weight bearing down on the upper point 9 is lower than it would be if a larger single coil spring were used instead of first coil spring 26. This makes the seat tilting mechanism 1 more comfortable to use.

[0056] However, once the first coil spring 26 has reached the pre-determined point of extension, as shown in FIG. 2, the ring 56 abuts against the flange 54 and the first end part 27 engages the second coil spring 50. Therefore, from this mid-tilt position to the fully tilted position shown in FIG. 3, the resistance to tilt is provided by both the first coil spring 26 and the second coil spring 50. This means that moving the seat tilting mechanism 1 from the position shown in FIG. 2 to that shown in FIG. 3 is harder than moving it from the position shown in FIG. 1 to that shown in FIG. 2. This is advantageous because as explained above the rotational force applied to the lower point 12 exponentially increases the further the user leans back, because their weight continues to shift from acting on the seat support 3 to acting on the backrest support 4. The extra resistance provided by the second coil spring 50 helps to prevent, or at least mitigate against, an uncontrollable increase in the rate of tilting rotation after the mid-tilt position shown in FIG. 2.

[0057] With regard to the axial position of the first coil spring 26 and second coil spring 50, in the rest position shown in FIG. 1 the axis 33 of the first coil spring 26 and second coil spring 50 is angularly displaced from the line 34. However, as will be appreciated from the Figures, the first mounting point 28 is located such that the rotation angle between the axis 33 and the line 34 decreases as the first quadrilateral hinge 19 moves from the rest position towards the fully tilted position. This is because the mounting point 28 has a different planetary movement path about the midpoint 6 to that of the upper point 9. The result achieved is that the increasing resistance provided by the first coil spring 26, and then also the second coil spring 50, as they extend has an increasing impact on the first quadrilateral hinge 19 as it moves from the rest position as shown in FIG. 1 to the fully tilted position shown in FIG. 3. This is because the axis 33 of the first and second coil springs 26 and 50 is brought closer and closer to the actual direction of extension. As such the resistance provided by the first coil spring 26, and then also the second coil spring 50, increases as the first quadrilateral hinge 19 moves towards the fully tilted position to a greater extent that it would if axis 33 of the first and second coil springs 26 and 50 was always aligned in the direction of extension.

[0058] The combination of the introduction of the second coil spring 50 part way through the tilting process and the angular movement of the first coil spring 26 and the second coil spring 50 towards the direction of extension through the tilting process means that to the user the sensation is that substantially the same amount of force is required from them to tilt the seat whatever its angle of tilt. In particular, it is not harder to initiate the tilting action than to continue with it, and it does not become uncomfortably easier to tilt the seat the more tilted it becomes. These are undesirable features of known seat tilting mechanisms. It will be appreciated that the beneficial effects of the present arrangement will vary depending on the weight of the user, but the intention is to generate a feeling of control and comfort for most average sized users.

[0059] As will be appreciated from FIG. 3, the first mounting point 28 is located such that the axis 33 of the first coil spring 26 aligns with the line 34 prior to the first quadrilateral hinge 19 achieving the fully tilted position. As such, in FIG. 3 the axis 33 is now angularly displaced from opposite side of the line 34. What this means is that the above described increase in resistance provided as a result of the axis 33 of the first and second coil springs 26 and 50 approaching and then meeting the line 34 is beneficially located in a region of tilt more likely to be used by a user, which is prior to the fully tilted position. This makes this increase in resistance more useful.

[0060] It will also be appreciated from FIG. 2, that the pre-determined point of extension of the first coil spring 26 occurs just before the axis 33 of the first and second coil spring 26 and 50 aligns with the line 34. What this means is that the second coil spring 50 is effective in a region of tilt more likely to be used by a user, which extends from the mid-tilt position shown in FIG. 2 to the point the axis 33 of the first and second coil spring 26 and 50 aligns with the line 34. Again, this is the region of tilt most likely to be used by a user.

[0061] The first quadrilateral hinge 19 comprises four corners, namely midpoint 6, upper point 9, a centre 35 of the annular hinge 16, and a centre 36 of annular hinge 18, and it moves by virtue of the annular hinges 7, 10, 16 and 18 at those corners. The lengths of each side of the first quadrilateral hinge 19 are clear from the Figures, and the interior corner angles of the first quadrilateral hinge 19 when the seat tilting mechanism 1 is in the rest position are shown in FIG. 1. These interior corner angles change as the first quadrilateral hinge 19 moves, and FIG. 3 shows them when the seat tilting mechanism 1 is in the fully tilted position.

[0062] The important characteristics of the first quadrilateral hinge 19 are firstly the greater distance between centres 35 and 36, than between midpoint 6 and upper point 9. This means that as the first quadrilateral hinge 19 moves, a front 37 of the seat support 3 rises in relation to a rear 38 thereof, and hence the seat support 3 rotates. Secondly, throughout the entire movement range of the first quadrilateral hinge 19 the centre 36 is forward of the centre 35, which means that as the first quadrilateral hinge 19 moves from the rest position shown in FIG. 1 to the fully tilted position shown in FIG. 3, the front 37 of the seat support 3 continuously rises as the centre 36 follows its planetary path about the centre 35. Thirdly, the planetary movement path the upper point 9 follows about the midpoint as the first quadrilateral hinge 19 moves from the rest position shown in FIG. 1 to the fully tilted position shown in FIG. 3 sees it initially rise, but then fall, as it passes over centre. This means that the rear 38 of the seat support 3 initially rises slightly, but then it falls. These different movements of the centre 36 and the upper point 9, mean that the rate at which the seat support 3 rotates increases the further the first quadrilateral hinge 19 moves from the rest position to the fully tilted position. This makes for a more comfortable seating experience for the user because the angle of tilt can be more precisely controlled at first.

[0063] It will also be appreciated that as the first quadrilateral hinge 19 moves from the rest position to the fully tilted position the seat support 3 moves rearwardly. This ensures that the seat and backrest parts (not shown) supported by the seat tilting mechanism 1 do not move laterally apart from one another as the user tilts the backrest back. Once again, this makes for a more comfortable seating experience.

[0064] The second quadrilateral hinge 25 comprises four corners, namely midpoint 6, lower point 12, a centre 39 of the annular hinge 22, and a centre 40 of the annular hinge 24, and it moves by virtue of the annular hinges 7, 13, 22 and 24 at those corners. The lengths of each side of the second quadrilateral hinge 25 are clear from the Figures, and the interior corner angles of the second quadrilateral hinge 25 when the seat tilting mechanism 1 is in the rest position are shown in FIG. 1. These interior corner angles change as the second quadrilateral hinge 25 moves, and FIG. 3 shows them when the seat tilting mechanism 1 is in the fully tilted position.

[0065] The second quadrilateral hinge 25 shares similar characteristics to the first quadrilateral hinge 19. It is inverted relative to the first quadrilateral hinge 19, but the spatial relationship between centres 40 and 39 is like that between centres 36 and 35, in that centre 40 is always rearward of centre 39, so it continuously falls during movement of the second quadrilateral hinge 25 from the rest position to the fully tilted position. Further, the spatial relationship between the midpoint 6 and the lower point 12 is like that between the midpoint 6 and the upper point 9, in that the lower point 12 initially falls as the second quadrilateral hinge 25 begins its movement from the rest position to the fully tilted position, but it then rises as its planetary movement path about the midpoint 6 passes over centre. As such, a front 41 of the backrest support 4 initially falls slightly, but then it rises. Once again, these different movements of the centre 40 and the lower point 12, mean that the rate at which the backrest support 4 rotates increases the further the second quadrilateral hinge 25 moves from the rest position to the fully tilted position. This works in conjunction with the similar rotation rate characteristic of the seat support 3 to make for a more comfortable seating experience for the user.

[0066] It will also be appreciated that as the second quadrilateral hinge 25 moves from the rest position to the fully tilted position the backrest support 4 moves forward. This also serves to ensure that the seat and backrest parts (not shown) supported by the seat tilting mechanism 1 do not move laterally apart from one another as the user tilts the backrest back.

[0067] However, the most important characteristic of the second quadrilateral hinge 25 is that it is configured to rotate the backrest support 4 more than the first quadrilateral hinge 13 is configured to rotate the seat support 3. The angular displacement ratio between the backrest support 4 and the seat support 3 is 1.9 to one. This is achieved because centre 39 is closer to midpoint 6 than it is to centre 40, which results in a greater angular change in the shape of the second quadrilateral hinge 25 than is achieved by the first quadrilateral hinge 25, where centre 35 is closer to centre 36 than it is to midpoint 6.

[0068] As is clear from the Figures, the first portion 8 of the lever 5 is L shaped, with the first mounting point 28 located at the corner of the L shape. This is a compact and efficient arrangement.

[0069] The seat tilting mechanism 1 operates as follows. When no user is seated on the office chair incorporating the seat tilting mechanism 1 it adopts the rest position shown in FIG. 1. This is due to the force of the first coil spring 26 acting to rotate the first portion 8 of the lever 5 about the midpoint 6. The first coil spring 26 comprises a sufficient tensile force to lift the backrest (not shown) back up into its generally vertical position by levering the lower point 12 clockwise about the midpoint 6.

[0070] When a user sits on the seat (not shown) mounted to the seat support 3, their weight acts to maintain the seat tilting mechanism 1 in the rest position, because the upper point 9 is forward of the midpoint 6, and therefore weight applied to the upper point 9 serves to force it in a clockwise direction about the midpoint 6, thereby closing the first quadrilateral hinge 19.

[0071] The user can lean back against the backrest (not shown) mounted to the backrest support 4 without the seat tilting mechanism 1 moving from the rest position as shown in FIG. 1, provided the force applied by the user is less than a break out point at which it exceeds the combined forces of the first coil spring 26 and the rest of the user's weight bearing down on the upper point 9.

[0072] To force the office chair into a more relaxed seating position the user simply leans back against the backrest. Once the weight applied to the backrest exceeds the break out point, the first and second quadrilateral hinges 19 and 25 will start to move from the rest position to the fully tilted position. The rate at which this movement occurs will depend on the amount of force applied by the user, and they will be able to increase or decrease the rate by adjusting their body position accordingly. However, the force required to initiate the tilting action, and to move the seat tilting mechanism from the rest position to the mid-tilt position shown in FIG. 2 is small because the user only needs to act against the strength of the first coil spring 26. If a single coil spring were used in place of the first and second coil springs 26 and 50, it would have to be equal in strength to the combined resistive strength of the first and second coil springs 26 and 50 towards the end of the tilting process. As such, it would exert a greater resistance to tilting from the rest position to the mid-tilt position, making it harder for the user to tilt the seat initially. In fact, the use of the dual spring arrangement of the invention allows for an easy and comfortable initial tilting action.

[0073] As the angular displacement ratio between the seat support 3 and the backrest support 4 is 1.9 to one, the backrest will rotate more than the seat, creating a more relaxed relationship between backrest and seat the more the seat tilting mechanism 1 moves towards the fully tilted position.

[0074] Further, due to the relative positions of the upper point 9 and the lower point 12 about the midpoint 6, and in particular the fact that they initially travel upwards and downwards respectively, before travelling downwards and upwards respectively, the rate at which the seat and backrest rotate increases the further the seat tilting mechanism 1 moves towards the fully tilted position. This makes for a more comfortable seating experience.

[0075] In addition, once the seat tilting mechanism 1 reaches the mid-tilt position as shown in FIG. 2, the resistance to tilt increases to counteract the increase in the driving force applied by the user as they shift more weight onto the backrest support 4. This extra resistance provided by the second coil spring 50 helps to prevent, or at least mitigate against, an uncontrollable increase in the rate of tilting rotation after the mid-tilt position shown in FIG. 2. Once again, this makes for a more comfortable seating experience.

[0076] In addition, due to the location of the first mounting point 28, the resistance to tilting also increases the further the seat tilting mechanism 1 moves towards the fully tilted position by virtue of the axis 33 of the first and second coil springs 26 and 50 moving closer to the direction of extension. This also compensates for the exponential increase in weight a user places on the backrest the further they tilt it back. Furthermore, this beneficial increase in resistance is applied in a middle region of tilt most used, which is prior to the fully tilted position.

[0077] If the user leans forward, and transfers their weight back again from the backrest to the seat, the seat tilting mechanism 1 will move back towards the rest position as shown in FIG. 1. If the user gets up from the office chair, the seat tilting mechanism 1 will return to the rest position as described above.

[0078] Of note is the fact that the contraction force applied decreases as the angle of tilt decreases, because the second coil spring 50 no longer acts on the lever 5 after the mid-tilt position, and because the axis 33 of the first and second coil springs 26 and 50 rotates away from the direction of contraction. This means that when the seat tilting mechanism 1 returns to the rest position this is done in a controlled and comfortable way. In particular, the back of the seat does not flip back up into an upright position in an unpleasant manner.

[0079] The seat tilting mechanism 1 shown in the figures can be used in isolation with an office chair or the like, but two can also be provided, one on either side of an office chair, to provide greater stability.

[0080] The present invention can be altered without departing from the scope of claim 1. In particular, as shown in FIG. 4, the dual spring arrangement can be used in a simple seat tilting mechanism which does not feature any interrelationship between the seat and the backrest. In FIG. 4, seat tilting mechanism 100 comprises a main body 101, a support member in the form of lever 102 rotationally mounted to the main body 101, and a first extension coil spring 103 mounted between the main body 101 and the support member 102 to provide resistance to rotation of the support member 102. The seat tilting mechanism 100 further comprises a second extension coil spring 104 and an engagement mechanism, generally designated 105, which engages the second extension coil spring 104 for resistance to rotation of the support member 102 when the first extension coil spring 103 reaches a pre-determined point of extension. This is the position which is illustrated in FIG. 4.

[0081] The resistance to extension mechanism which comprises the first and second coil springs 103 and 104 is identical to that shown in seat tilting mechanism 1 described above. However, the lever 102 is a simplex rotational component which is mounted at a midpoint 106 by annular hinge 107 to the main body 101, and comprises a first portion 108 which is rotationally connected at an upper point 109 to first end part 110 by an annular hinge 111, and a second portion 112 rotationally connected at a lower point 113 by annular hinge 114 to backrest support 115.

[0082] In use a rotational force applied by a user to the backrest support 115 rotates the lever 102 about the midpoint 106. This rotational motion is resisted initially by just the first coil spring 103, and then once the first coil spring 103 reaches the pre-determined point of extension as shown in FIG. 4, and ring 116 engages annular flange 117, the rotational motion is resisted by both the first coil spring 103 and the second coil spring 104. This additional resistance to extension counteracts the increase in the driving force applied by the user as they shift more of their weight onto the backrest support 115 the further back they lean. The extra resistance provided by the second coil spring 104 helps to prevent, or at least mitigate against, an uncontrollable increase in the rate of tilting rotation after the mid-tilt position. This makes for a more comfortable seating experience.

[0083] In another alternative embodiment (not shown) the second coil spring is mounted independently of the first, and the engagement mechanism involves a physical part of the first coil spring or the support member physically engaging the second coil spring.

[0084] In another alternative embodiment (not shown) the first and second coil springs are mounted in parallel with one another, rather than co-axially.

[0085] In another alternative embodiment (not shown) the whole extension resistance mechanism is mounted the opposite way up to in seat tilting mechanism 1, with the first end part mounted to the main body, and the second end part mounted to the lever.

[0086] In another alternative embodiment (not shown), instead of using a ring and a flange to provide the engagement mechanism, the spacer member is provided with a radially extending pin, and the first end part comprises a sleeve with an axially extending slot in which the pin is disposed. In the rest position the pin is located at a first end of the slot, and at the pre-determined point of extension the pin is located at a second end of the slot, and the first end part and the spacer member are linked for extension of the second coil spring.

[0087] Therefore, the present invention provides a resistance to rotation which beneficially increases at the pre-determined point of extension. As outlined above, this can help to provide a more user-friendly seating experience, because the seat can be easy to tilt at first, but is controlled as the weight of the user is shifted more and more onto the backrest.