AU6512790A - A seat suspension - Google Patents

Description

"A SEAT SUSPENSION" THIS INVENTION relates to an adjustable suspension for a seat. It is particulary suitable for application to seats in vehicles adapted to traverse rough terrain; such as mining vehicles, earthmoving vehicles a d farming vehicles; though the invention is not intended ■ffco be restricted to use in these vehicles.

BACKGROUND ART

It is well known to provide seats for rough terrain vehicles with a suspension assembly intended to isolate the vehicle's occupants from at least some of the vibrations and impacts experienced by the vehicle. In general, these suspension assemblies allow the seat to move independently of the vehicle, and comprise a spring or other resilient means within the suspension assembly to attenuate at least some of the vibrations and impacts referred to above, as well as a damping mechanism to control or stabilise the movement of the seat.

Existing seat suspensions are normally designed to comply with various design standards that are based on what is termed a "standard or average person". This "standard or average person" is a hypothetical person developed by a statistical analysis of the human population, and can be approximately defined as a "man of average height and build", and thus he could be expected to weigh 75-80 kg.

It is apparent that a seat suspension designed using this approach would not be particularly suited to all actual users. If the person using the seat were of less than average weight, the seat would appear too firm, and impacts would cause a lesser degree of movement across the seat suspension than would occur for the "average" person. As a result the accelerations applied to the person of less than average weight would be significantly higher. Furthermore, the natural frequency of the seat-person system will be higher for a less than average weight person, resulting in reduced attenuation of low frequency vibration from the cabin floor. Both of these factors will combine to produce a generally uncomfortable ride for the less than average weight person.

Similarly, for a person of heavier than average build, the seat would appear too soft, and impacts could cause excessive movement across the seat suspension and harsh "bottoming-out " of the mechanism. The resultant high accelerations applied to the person have the potential to cause back and other injuries . Many seat suspensions now incorporate a mechanism for changing the suspension characteristics to accommodate users of different weight. In most cases however, such a mechanism only alters the pre-load applied to the spring or other resilient member in the suspension assembly, and the suspension has the same "stiffness" and damping characteristics, regardless of the actual weight of the user. These stiffness and damping characteristics are chosen to provide the best attenuation of vibration and impacts for the "standard or average person" as described above.

British patent application no. 2,057,260 discloses a spring suspension for a vehicle seat which includes means for varying the vertical spring rate or "stiffness" of the suspension.

It has been found that in existing seat suspensions, there is normally a threshold level of vibration and/or impact required before the suspension mechanism begins to attenuate vibrations, etc. This threshold level arises principally due to frictional forces inherent in the design and construction of the seat suspension. For example, frictional forces may arise from the use of plain bearings or bushings in pivoting or rotating joints, e.g. between pivoting or rotating linkages used to control movement of the suspension. Seat suspensions normally comprise two pairs of scissors linkages (as shown in British patent application no. 2,057,260) or two pairs of parallel linkages (as shown in West German patent application no, 2,156,786). Consequently the seat and base are each connected to the linkages at four points. Due to manufacturing tolerances, the four points may not lie exactly in the same plane. As a result, the movement of the two pairs of linkages is not restricted to two respective parallel planes, and frictional forces arise from the movement of the linkages, particularly if plain bushes or bearings are used.

Frictional forces may also arise from the seals used to seal between the relatively movable components of a damping cylinder. Dampers are commonly arranged so that the movement across them is similar to the actual relative displacement across the seat suspension. As a result, virtually the full magnitude of the damper seal friction acts as a force threshold that must be exceeded before any relative movement occurs across the seat suspension. Known suspension mechanisms are therefore only effective in attenuating vibrations and impacts above a threshold level due to the inherent frictional forces in the design of known suspensions. Small vibrations and impacts are consequently absorbed by the seat occupant. Such vibrations and impacts, although relatively small, can have a significant detrimental effect on the health of the occupant over a period of time, and at best make the ride uncomfortable for the occupant. Known seat suspensions are normally designed to cope with large vibrations and impacts, but fail to attenuate the relatively small steady state vibrations and impacts caused by corrugations in terrain for example.

It is an object of the present invention to provide an improved suspension for a seat which overcomes or substantially alleviates at least some of the problems of the prior art described above. SUMMARY OF THE INVENTION According to one aspect, the invention provides an adjustable suspension for a seat comprising: a top frame adapted to mount the seat; a base frame attachable to a support; a plurality of linking members pivotally connected to both the top frame and the base frame so that the top frame is movable independently of the base frame while remaining substantially parallel thereto; and a suspension mechanism for controlling relative movement of the top and base frames, said suspension mechanism being connected to at least one linking member, further comprising means for varying the distance between the suspension mechanism and a- pivot axis of the linking member so as to vary the suspension characteristics.

Typically, the suspension mechanism comprises a coil spring or other resilient element, and a shock absorber or other damping member. The suspension mechanism applies stabilising forces to one end of the linking member(s) in reaction to the forces applied to the other end of the linking member(s) by the weight of the operator and vibrations and impacts.

As is common to existing designs, the seat suspension is preferably adjusted to be at mid stroke, i.e. to have approximately equal upward and downward relative movement available, when the person is seated in a normal manner. In the suspension mechanism, this requires that the moment Mgl about the appropriate pivot axis be balanced by the opposing moment Fx_β where M is the supported static mass of the seat and person, g is the acceleration due to gravity, 1 is the resolved component of the linking member length in the horizontal plane, F is the spring preload and x_β is the minimum distance from the pivot axis to the spring centreline (see Fig. 1).

The seat suspension of this invention incorporates means for varying the suspension characteristics, which in the preferred embodiment, is a single adjustment mechanism that not only provides the usual preload adjustment for operators of different weight, but also simultaneously increases or decreases the suspension stiffness and damping as the supported mass M of the seat and person likewise increases or decreases. The extent of this variation with supported mass is predetermined and can be controlled independently for the suspension stiffness and damping by adjusting the spring and damper characteristics and the geometry of the adjustment mechanism. As described below, in the preferred embodiment both the suspension stiffness and damping are varied proportionally to the supported mass M, though other combinations of variability can be used.

The stiffness of the seat suspension is proportional to Kx_β ² where K is the spring constant and x is the minimum distance from the pivot axis to the spring centreline. The present invention provides means for varying this distance x_β, and hence enables the stiffness of the seat suspension to be adjusted. As the supported mass M is increased or decreased, the distance x_β is changed to achieve a controlled and desired increase or decrease in suspension stiffness. The spring preload F is simultaneously altered as required to maintain the static moment balance about the pivot axis at the seat's mean position (as described above). For example, in the preferred embodiment a 40% increase in supported mass M dictates an 18.3% increase in x_β to provide a 40% stiffness increase. The spring preload F is simultaneously increased by 18.3% to maintain the static moment balance at the mean position.

The damping of the seat suspension is proportional to ex ² where c is the damping constant and _Λ is the minimum distance between the pivot axis and the damper centreline (see Fig. 1). As most dampers produce a damping constant which is velocity dependant the damping constant also changes as x changes, since the same relative velocity across the seat suspension causes a different relative velocity across the damper as the ratio of x_d to 1 changes.

By selection of dampers giving different damping constant-versus-velocity characteristics, and by controlling the change in x_d for operators of different weight, the desired characteristic of suspension damping-versus-supported mass can be achieved. For example, in the illustrated embodiment, the suspension damping has been increased proportionally to the supported mass. The damper has been attached at a larger radius from the pivot axis than the spring and therefore the same change in x_Λ and x_β produces a lower percentage change in the x² product for the damper. This is offset by choice of a damper whereby the damping constant increases as the relative velocity increases. These two factors combine to produce an effective suspension damping proportional to supported mass. The leng •'th of the effective radius arm x <___. is preferably small compared to the length of the linking member, so that there is minimum displacement of the point of connection of the suspension mechanism to the linking member caused by relative movement of the top and base frames. In this manner, the moment about the pivot axis resulting from the product of the damper friction and radius arm x_& is minimised, thereby reducing the frictional threshold in the suspension and making the suspension more responsive to low level vibrations. Preferably, a damper is used with a characteristic of increasing damping constant σ with relative velocity across the unit. For a fixed supported mass and seat adjustment, progressively higher impact amplitudes will cause progressively higher relative velocities across the damper and hence increased forces to retard opening and closure of the suspension. This damping characteristic will therefore generate the desired effect that mild impacts cause generous suspension movements and hence low forces and accelerations applied to the person, whilst maintaining a high level of impact required to approach "bottoming out" of the seat suspension. This damper characteristic will also be advantageous for maximising the attenuation of low amplitude vibrations, due to the presence of a minimised degree of damping at such low amplitude of relative displacement and velocity. The pivotal mountings of the linking members to the top and base frames preferably comprise rolling element bearings, such as ball bearings or roller bearings, to further minimise friction in the seat suspension.

Preferably, three linking members are used in conjunction with self aligning bearings on all linking members. This reduces the possibility of resistance to movement of the suspension seat due to overconstraint caused by inherent manufacturing tolerances, since a plane can always be drawn through the three pivotal attachment points at the ends of the three linking members. Furthermore, the linking members are arranged in parallel configuration, rather than the more conventional crossed or scissors arrangement. By arranging the pivot axis of the relevant linking member to be closer to the end to which the suspension mechanism is connected, there is less movement of that end and hence less friction as described above. In the preferred embodiment, the construction of the means for varying the suspension characteristics comprises a plate member to which the suspension mechanism is connected, the plate member being pivotally connected to the linking member by a parallelogram linkage. The plate member can be moved relative to the linking member, e.g. by means of a worm drive, to vary its spacing from the pivot axis of the linking member, thereby changing the effective radius arms

(x and x ) of the suspension mechanism. The worm drive can be driven by a lever at the front of the seat suspension so that it can be readily altered by the operator to suit.

In order that the invention may be more fully understood and put^{^~}±ntc---_p-ractice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side elevation of the seat suspension of the preferred embodiment;

Fig. 2 is a front elevation of the seat suspension of Fig. 1;

Fig. 3 is a part sectional side elevation along A-A of Fig. 2;

Fig. 4 is a part sectional side elevation along B-B of Fig. 2; Fig. 5 is a part sectional side elevation along C-C of Fig. 2; Fig. 6 is a side elevation corresponding to Fig. 5 with different suspension adjustment; and

Fig. 7 is a cross sectional front elevation of a pivotal mounting of a linking member of the seat suspension of Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in Figs. 1 and 2, the seat suspension of the preferred embodiment, illustrated generally at 10, comprises a top frame 11 adapted for attachment to a seat 12, a bottom frame 13 adapted to be mounted to the base of the driver's cabin of a vehicle, and a number of linking elements 14 pivotally connected to the top and base frames to allow relative movement between the top and base frames while maintaining the frames in substantially parallel configuration. In other words, the linking elements permit only relative translation, or non-rotational movement, between the frames. In the illustrated embodiment, three parallel linking members or link arms 14A, 14B, 14C are provided. Preferably, the pivotal mountings of the linking arms 14A, 14B, 14C to the top and bottom frames 11, 13 include spherical bearings or self-aligning rolling element bearings 30, for example as shown in Fig. 7. The use of such bearings reduces the inherent friction of the suspension linkages thereby making the suspension effective for a lower threshold of vibration amplitude. The use of three linking members and self-aligning or spherical bearings enables free movement to be retained despite tolerances in the positions of the pivotal attachments to the top and base frames, and errors in the length and parallelism of the linking members themselves, since it is always possible to draw a plan through three points and the position of this plan can freely change when rotations about all axes are allowed at each of these three points. The first linking member 14A is pivotally connected at its ends to brackets connected respectively to the top frame 11 and base frame 13 as shown. The two remaining linking members 14B, 14C are pivotally connected at their top ends to the top frame 11 on a common pivot axis. The linking members 14B, 14C are also pivotally mounted intermediate their ends to the base frame 13 on a common pivot axis 15.

The bottom part of each linking element 14B, 14C (as shown in Fig. 1) comprises a respective angled section 14B', 14C, the free ends of the angled sections being joined to a common cross bar 16 so that linking members 14B, 14C are constrained to pivot in unison about pivot axis 15.

A suspension mechanism, comprising a coil spring 20 and a damper or shock absorber 21, is connected between the base frame 13 and the cross bar 16 (which in turn is connected to both linking members 14B, 14C) . The spring 20 is placed under a predetermined tension or pre-load.

It will be apparent to those skilled in the art that the mechanism described above enables the top frame 11 and attached seat 12 to move relative to base frame 13, while retaining the two frames substantially parallel. Furthermore, the coil spring 20 will act to attenuate relative movement of the top and base frames towards and away from each other, while the shock absorber 21 will dampen relative movement of the frames toward and away from each other so that the combined spring and damping unit provides a stabilising suspension for the seat.

The seat suspension 10 includes a mechanism for varying the distances x_β (the spacing between pivot axis 15 and the line of action of spring 20) and X_Λ (the spacing between pivot-axis 15 and the line of action of damper 21) so as to vary the stiffness and damping characteristics of the suspension. Such mechanism is illustrated in more detail in Figs 3 to 6 , and comprises a plate or bar 22 having an aperture or eye 23 thereon to which coil spring 20 is hooked. The damper 21 is also pivotally connected to the plate 22. The plate 22 is connected to the cross bar 16 by means of a parallelogram linkage comprising at least two pairs of parallel arms 24A, 24B.

There is also provided at least one bell crank lever or angled arm 25 having one end pivotally connected to the plate 22, and its other end pivotally connected to a sliding or follower nut 26 mounted on threaded member 27 in a worm drive arrangement. As the threaded member 27 is rotated, the follower nut 26 will move longitudinally thereon. The threaded member 27 has a socket head 28 adapted to receive a socket handle 29 as shown in Fig. 2.

By rotating the threaded member 27, the follower nut 26 will move longitudinally as shown in Fig. 6, and such movement is translated to the plate 22 via the angled arm 25. As the plate 22 is connected to the cross bar 16 by the parallel link arms 24A, 24B, it is constrained to move in an arcuate path. Movement of the plate 22 along this arcuate path will vary the lengths x_β and x_Λ, and hence the stiffness and damping characteristics of the suspension as described earlier. Thus the stiffness and damping characteristics of the seat suspension can be easily adjusted to suit the particular occupant of the seat.

The drawings illustrate the seat suspension at full height (no load) . At mid-stroke, cross bar 16 and components mounted thereon will be rotated from the orientation shown in the drawings. Typically, at mid-stroke, the angle θ (Fig. 4) is 25 to 30 degrees.

The seat suspension is operative to attenuate even small vibrations due to the very low friction threshold obtained by using three link arms and self-aligning bearings. The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention as defined in the following claims. For example, although Figs. 3 to 6 illustrate a particular mechanism for varying the distance between the suspension mechanism and the pivot axis, other mechanically equivalent mechanisms can be used.

Claims (14)

CLAIMS :

1. A seat suspension comprising: an upper frame adapted to mount a seat; a lower frame attachable to a support; and a linkage assembly connected between the upper and lower frames to permit relative vertical movement between the frames while maintaining the frames substantially parallel during their relative movement; characterised in that the linkage assembly comprises three link members each being pivotally connected at opposite ends to the upper and lower frames respectively.

2. A seat suspension as claimed in claim 1, wherein the pivotal connection of each of the link members to the upper and lower frames comprises a self-aligning bearing.

3. A seat suspension as claimed in claim 1 wherein the three link members are parallel link arms .

4. An adjustable seat suspension comprising: a top frame adapted to mount a seat; a base frame attachable to a support; a plurality of linking members each being pivotally connected to both the top frame and the base frame to permit relative parallel motion of the frames in the vertical plane; and

^' a suspension mechanism for controlling relative motion of the top and base frames, the suspension mechanism being connected to at least one linking member; characterised in that the seat suspension further comprises adjustment means for varying the distance between the suspension mechanism and a pivot axis of the linking member to which it is connected to thereby vary the suspension characteristics of the seat suspension.

5. A seat suspension as claimed in claim 4 wherein the suspension mechanism comprises a spring member and the adjustment means is ^' operable to vary the distance between the pivot axis and the line of action of the spring member.

6. A seat suspension as claimed in claim 5, wherein the suspension mechanism also comprises a damping member and the adjustment means is also operable to vary the distance between the pivot axis and the line of action of the damping member.

7. A seat suspension as claimed in claim 6, wherein the damping member is located at a greater distance from the pivot axis than the spring member.

8. A seat suspension as claimed in claim 6, wherein the damping constant of the damping member increases with relative velocity between the ends of the damping member.

9. A seat suspension as claimed in claim 6, wherein the distance between the damping member and the pivot axis is small relative to the length of the linking member to which the damping member is connected.

10. A seat suspension as claimed in claim 4, wherein the adjustment means comprises a transverse member to which the suspension mechanism is connected, a parallelogram linkage connected between the transverse member and the linking member, and a worm drive arrangement for moving the transverse member relative to the linking member to thereby vary the distance between the pivot axis of the linking member and the suspension mechanism.

11. A seat suspension as claimed in claim 10, wherein the worm drive arrangement includes a threaded member having a socket at one end thereof adjacent the front of the seat to enable the suspension to be adjusted manually by a user.

12. A seat suspension as claimed in claim 4, wherein there are three linking members.

13. A seat suspension as claimed in claim 12, wherein the linking members are in parallel configuration.

14. A seat suspension as claimed in claim 12, wherein the pivotal connection of each linking member to the top and base frames includes a self-aligning rolling element bearing.