GB2057260A

GB2057260A - Vehicle seat suspension

Info

Publication number: GB2057260A
Application number: GB8026052A
Authority: GB
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1979-08-14
Filing date: 1980-08-11
Publication date: 1981-04-01

Abstract

A spring suspension for a vehicle seat comprises a seat support frame 10 for supporting a seat, a base frame 11 to be secured to the vehicle's floor, a system of scissor-action pivotal linkages (12), 12', 15, 15' interconnecting the base frame and seat support frame to guide the vertical movement of the seat support frame, and coil springs 20 to bias the seat support frame upwardly, the coil springs acting between the base frame 11 and a linkage 12' of the linkage system. The vertical spring rate of the suspension is varied by connecting the springs at one end thereof to a lever 24' which is pivoted via a support 23' to the linkage 12' and movable by a screw mechanism 25 to vary the moment of the force of the springs 20 about the pivot axis at 13 of the linkage 12' and hence to vary the torque applied to linkage 12'. Preferably the distance (e) between the line of action of the springs and the axis 13 and the sprung mass m are related by the expression e<2> alpha m. (Fig. 1.) A vibration damper is connected between the mountings to which the ends of the springs are secured so that the vibration damping effect is varied as the moment of the spring force is varied. <IMAGE>

Description

SPECIFICATION Improvements in and relating to vehicle seat suspensions This invention relates to vehicle seat suspensions and in particularto a seat suspension of the type in which the downward movement of the seat is opposed by spring means which counterbalance the weight of the seat and the seat occupant, and assist in isolating the seat occupant from vibratory move menttransmitted to the vehicle cab from the road wheels of the vehicle.

Atypical form of spring suspension comprises a seat support frame, on which a vehicle seat can be mounted, a base frame adapted to be bolted to the floor of the vehicle, a system of pivotal linkages interconnecting the base frame and the seat frame to guide the upwards and downwards movement of the seat frame relative to the base frame, and a spring means, acting between parts of the suspension which move relatively to each other during upward and downward movement of the seat frame relative to the base frame, to bias the seat frame in an upward direction.

The system of pivotal linkages can comprise a single linkage or parallel spaced pair of linkages connected at the upper end thereof to the seat frame and at the lower end thereof to the base frame, together with means for guiding the movement of the seat. Said means for guiding the movement of the seat can comprise a second linkage or a pair of parallel-spaced linkages also connected at the upper end thereof to the seat part and at the lower end thereof to the base part, the first and second linkages being disposed in intersecting relationship and pivoted at their intersection to form a scissors type linkage. Normally the pivotal connections between the linkages and the seat and base frames at the front of the seat will be slidable or roller connections to provide the necessary degree of freedom of movement of the linkages.

Examples of such seat suspensions are found in the following patent specifications: U.S. Patent Specifications 3,109,621 and 3,774,973, British Patent Specification 106,079 and German Patent Specification 863,303.

It is desirable, in such a seat suspension, that the natural frequency of oscillation of the suspension be relatively low, for example one cycle per second, and also that the frequency of oscillation be relatively constant irrespective of the weight of the seat occupant or the height at which the seat occupant chooses to ride.

In a scissor action type of seat suspension, it is known to provide spring means in the form of coil springs which are connected between the pivotal connections of the linkages to the base frame of the seat. However, as the seat travelling downward during oscillation approaches the base frame, the acute angle between the linkages is reduced and the effective (i.e. vertical) spring rate of the suspension is reduced. This is a very undesirable feature in that end-stop contact can easily occur.

One aim of the present invention is to provide a seat suspension which overcomes this defect, and in which the natural frequency of oscillation is maintained substantially constant, using springs of substantially constant spring rate.

According to the present invention there is provided a seat suspension comprising a seat support frame, a base frame, a linkage having a fixed pivotal connection at one end to one said frame, a pivotal connection at the opposite end to the other said frame and means for maintaining said frames substantially parallel to one another during their relative upward and downward movement, spring means extending between a first support mounted on said linkage adjacent said fixed pivotal connection and a second support on said one frame spaced from said fixed pivotal connection, and means operable to vary the torque applied about the fixed pivotal connection by moving the line of action of the spring means towards or away from said fixed pivotal connection.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a vertical section through a seat suspension according to the invention; Figure 2 is a plan view of part of the seat suspension of Figure 1; Figure 3 is a rear view of part of the seat suspension of Figure 1, and Figure 4 is a diagrammatic representation ofthe seat suspension.

As seen in the drawing, the seat suspension comprises a seat support frame 10, a base frame 11, a first rigid link 12 (see Figs. 2 & 3) having a fixed pivotal connection 13 at its lower forward end to the base frame 11 and a rolling pivotal connection 14 at its rearward upper end to the seat support frame 10.

In orderto maintain the seatsupportframe 10 parallel to the base frame 11 during rise and fall of the seat, a second rigid link 15 is provided having a fixed pivotal connection 16 at its upper forward end to the seat support frame 10, a rolling pivotal connection 17 at its lower rearward end to the base frame 11, and a fixed pivotal connection at the intersection 18 of the two links 12, 15. For stability, two further similar links 12', 15' are provided each parallel spaced in the horizontal direction (i.e. in a direction perpen dicularto the plane of Figure 1 ) from links 12, 15 respectively, as shown in Figures 2 and 3 and connected respectively to the seat and base frames as in the case of links 12 and 15. As seen in Figures 2 and 3, links 12, 12' are rigidly interconnected by a bar 19.

The base frame 11 is designed to be bolted to the floor of a vehicle cab, and the seat support frame 10 is designed to support a vehicle seat thereon. In order to counterbalance the weight of the seat and its occupant, spring means are provided in the form of a plurality of parallel spaced coil springs 20, extending between a first support bar 21 connected to linkages 12, 12' and a second support bar 22 connected to the base frame 11.

The connection between the first support bar 21 and the linkages 12, 12' comprises corresponding support arms 23, 23' extending from links 12, 12' perpendicular thereto, and levers 24, 24' pivoted to the support arms 23,23' respectively Lever 24' is a bellcrank lever which is movable pivotally by a screw and nut connection 25, 26 journalled in a support 27 fixed to the bar 19, the nut 26 being carried by the bellcrank lever 24' and the screw 25 being tethered in the support but rotatable by a handle (not shown).

By rotation of the bell crank lever 24' it will be seen that the first support bar 21 is moved towards the fixed pivotal connection 13 of links 12, 12' to reduce the torque applied by the coil spring 20, to links 12, 12' about the fixed pivotal connection 13, or away from the links 12,12' to increase the torque.

Figure 4 of the drawings is a diagrammatic representation of a seat suspension in which spring means 20 apply torque about a fixed pivotal connection 13.

It is known that the natural frequency of a sprung seat suspension is given by the expression

where k = vertical spring rate and m = mass on the suspension so that for constant natural frequency kam (1) If the spring means acts on the suspension through a leverage, such as 23, in the case described above, k=c2k5 where ks = actual spring rate (constant in this case) and c = leverage ratio so that k a c2 (2) and c = e (fbeing constant for the mid-ride position of the seat, being a function of geometry only, and e being the distance between the fixed pivotal connection and the line of action of the spring). The midride position is the position midway between upand down-stops.

Combining equations (1) and (2) cαm i.e. (e/f) 22 αm so e αm...................................... (3) Now for the spring means to support mass m at the mid-ride position, Spring force F = cm i.e.Fa" (4) e Let the extension of the spring means from the free length be d so that F a d (5) Combining equations (4) and (5) gives d αm and hence e deam (6) Combining equations (3) and (6) de a e2 sothatdae (7) From this, the conclusion can be drawn that, for constant natural frequency 1) the spring extension atthe mid-ride position should be proportional to the effective radius of action of the spring means about the fixed pivotal connection, and 2) the square of the radius of action should be proportional to the sprung mass.

The first of these criteria is closely approximated to by the construction described above, and if desired can be exactly met by providing a cam track or the like for constraining the movement of the first support bar to a path which meets the first of these criteria.

The second ofthese criteria can be met by selecting or adjusting the magnitude of e relative to the sprung mass. This can be achieved by appropriate design of the abovementioned cam track.

In the preferred embodiment of the invention, a linear viscous damper 30 is connected to act between the seat support frame and the base frame, the damper being connected at its opposite ends to the bars 21 and 22. In particular the damper is a cylinder and piston damper of conventional construction.

By this arrangement, a lightweight occupant of the seat, with the bar 21 nearest to the pivot 19, will experience a lower effective damping ratio than a heavyweight seat occupant who will have the bar 21 furthest away from the pivot 19. In other words, the damping will be in definite relationship with the effective spring rate. If the damping were constant, such as would be the case if the damper were mounted independently without its effective ratio being adjustable, the lightweight driver would tend to get an overdamped ride and therefore not benefit from the isolation potential ofthe suspension as much as he otherwise would, and the heavyweight driver would tend to get an underdamped ride, with the seat being uncomfortably "lively" and a risk of hitting the top or bottom travel stops when subjected to sudden jolts.

Although reference has been made herein to the vibration damper being connected at its opposite ends to the opposite ends of the spring means, it will be understood that this will include both direct and indirect connection such that each extension and contraction of the spring means results in a corresponding extension and contraction ofthe vibration damper.

Claims

1. A seat suspension comprising a seat support frame, a base frame, a linkage having a fixed pivotal connection at one end to one said frame, a pivotal connection at the opposite end to the other said frame and means for maintaining said frames substantially parallel to one another during their relative upward and downward movement, spring means extending between a first support mounted on said linkage adjacent said fixed pivotal connection and a second support on said frame spaced from said fixed pivotal connection, and means operable to vary the torque applied about the fixed pivotal connection by moving the line of action of the spring means towards or away from said fixed pivotal connection.

2. A seat suspension according to claim 1, wherein said linkage comprises a link having a fixed pivotal connection at one end thereof, a lever arm connected to said link adjacent its fixed pivotal con nection, means coupling one end of said spring i means to said lever arm and means for moving said one end of the spring means towards and away from the pivot axis of said fixed pivotal connection.

3. A seat suspension according to claim 2, wherein said coupling means comprise a bell crank lever pivoted to said lever, said one end of the spring means being connected to one end of the bell crank lever, and said means for moving the end of the spring means comprise a screw-action mechanism acting on the opposite end of the bell crank lever.

4. A seat suspension according to any preceding claim, comprising a second link intersecting said first link and having a pivotal connection thereto at the intersection; the opposite ends of the first and sec ond links having pivotal connections to the seat sup port frame and the base frame respectively, some of the said connections being fixed pivotal connections and others being slidable pivotal connections suffi cientto give freedom of movement.

5. A seat suspension according to any preceding claim, including a vibration damper connected at its opposite ends to the said ends of the spring means.

6. A seat suspension comprising a seat support frame, a base frame, a linkage mounting said seat sup port frame for upward and downward movement relative to said base frame, and spring means having connections at its opposite ends to parts of said seat suspension which move relatively to each other dur ing upward and downward movement of the seat support frame, the spring means acting to bias the seat support frame upwardly away from the base frame to counterbalance the weight of a seat and its occupant acting downwardly on the seat support frame, and having a mechanism operable to move one of said connections in a sense to vary the verti cal spring rate of the spring means, and a vibration damperconnected at its opposite ends to the oppo site ends of the spring means so that the damping effect of the damper varies with variation of the spring rate of the spring means.

7. A seat suspension according to claim 6, wherein said mechanism comprises a lever arm pivotally mounted on a part of the linkage and con nected to one end of said spring means, the lever arm being movable pivotally by a screw-action device, the mechanism being operable to apply tor que about a pivot of said linkage, the end of the spring means connected to said lever arm being movable towards or away from said pivot by said mechanism to vary the torque applied by the spring means about said pivot.

8. A seat suspension according to any preceding claim, wherein said spring means comprise one or more coil springs acting in tension.

9. A seat suspension according to claim 8, wherein said mechanism is operable to maintain the distance between the line of action of the spring means and said fixed pivotal connection substantially proportional to the extension of the spring means.

10. A seat suspension substantially as hereinbefore described with reference to the accompanying drawings.