DAMPER ADJUSTMENT MECHANISM

Abstract: A damper comprises a piston (1), including a port (5) for allowing fluid flow through the piston (1) from a first face (14) to a second face (7). A valve shim (6) mounted on the second face (7) of the piston covers the port (5) and is deformable to allow the flow of damping fluid through the port (5). Means (4) are provided for clamping an area of the valve shim (6) against the second face (7) of the piston, the clamping means (4) being adjustable to increase or decrease the area of the valve shim (6) that is clamped. Preferably, the valve shim (6) is generally disc-shaped; the clamping means (4) clamping an area of the valve shim (6) within an effective radius and being adjustable to change the effective radius. By changing the clamped area of the valve shim (6), deformation of the shim is aided or hindered, thus adjusting the damping characteristics of fluid flow through the port (5). Only a moderate and uniform force is needed to change the clamping area, thus avoiding damage to the mechanism and allowing a linear and repeatable response from the adjustment mechanism.

Description: TITLE Damper adjustment mechanism DESCRIPTION Technical field The invention relates to the field of dampers, e.g. for use as shock absorbers in vehicles, and to means for adjusting the damping characteristics of the dampers. Background to the Invention It is well known for dampers, for example for use as motor vehicle shock absorbers, to use a fluid damping medium. A piston rod carries a piston, which slides in a cylinder of the fluid. When the piston moves, the fluid is forced through ports in the piston controlled by valve shims on the piston. Thus movement of the piston relative to the cylinder is resisted and forces applied along the piston rod are damped by the dissipation of energy as heat in the damping fluid. The piston may also include bypass means to allow a degree of unrestricted fluid flow through or around the piston. There are many types of mechanism in current use allowing for external adjustment of the damping characteristics of such a damper. These generally fall into two categories: (1) Bypass adjustment. This is a means of regulating either the flow of bypass fluid around or through the main acting piston assembly within a damper, or the flow of fluid displaced by the volume of the piston rod and traveling from the primary damper cylinder into a second cylinder or reservoir. This may be achieved through the use of taper-needle type valves, rotary valves or other variable bypass systems. Through the use of non-return valves each design may limit the bypass to function only in compression or in extension. In some designs, normally where there is no non-return valve feature, the bypass control effects forces in both compression and extension simultaneously. Other derivations have independent controls for compression and extension bypass adjustment. The bypass adjustment generally has greatest influence on the lower velocity damping characteristics (0 - 0.025m/s range) although in many systems there is also a diminishing effect continuing into the medium velocity range (0.025 - 0.5 m/s). (2) Preload adjustment. This is a means of regulating the clamping or preload force of the valve shim arrangement upon either the main acting piston assembly within the damper, or a secondary flow restriction device or piston located between the primary damper cylinder and the secondary damper cylinder or reservoir. This effectively regulates the force at which the valve deflects or opens to permit direct fluid flow through the primary piston ports or fluid transfer from primary to secondary or reservoir cylinder. The velocity of movement of the damper piston at which this digression point occurs is dependent upon the preload force and the degree of bypass through or around the piston or secondary flow restriction device. In the case of a zero-bypass arrangement the digression point velocity will be very low irrespective of the preload force. As more bypass flow is allowed, the digression point velocity is increased. As more preload is added the digression point force is also increased. The preload adjustment has an influence on a wide range of velocity characteristics up to the point at which the dynamic flow capacity of the piston or any other flow restriction is approached. Most externally adjustable dampers work on some or all of the above principles. The external controls can be configured in a variety of ways and may be manually or electronically activated. However, the known dampers — and particularly those using the technique of preload adjustment — tend to suffer from the following problems: (i) Variation in the torque required to operate the external control throughout the range of adjustment. Generally the torque required increases as the preload force rises. (ii) Non-linearity of adjustment increments. The change in damping characteristics is generally not proportional to the degree of external control adjustment. (iii) Poor repeatability. Due to wear sensitive back-lash in the threaded components often found within adjustable preload systems, it is quite common for a particular adjustment position to provide differing damping characteristics depending on whether the adjustment position has been reached by increasing or decreasing the preload. Furthermore, when maximum preload is attained the torque required to operate the control mechanism can damage the often slender components through which the torque is transmitted. Summary of the invention The invention provides a damper comprising: a piston, including a port for allowing fluid flow through the piston from a first face of the piston to a second face of the piston; a generally disc-shaped valve shim mounted on the second face of the piston, the valve shim at least partially covering the port and deformable to allow the flow of fluid through the port; and means for clamping an area of the valve shim against the second face of the piston, the clamping means being adjustable to increase or decrease the area of the valve shim that is clamped. CHARACTERIZED IN THAT the clamping means comprises a plurality of sliding blocks in contact with a surface of the valve shim, the sliding blocks being mounted for movement parallel to the surface of the valve shim. By changing the clamped area of the valve shim, deformation of the shim is aided or hindered, thus adjusting the damping characteristics of fluid flow through the port. Only a moderate and uniform force is needed to change the clamping area, thus overcoming the problems described in relation to the prior art. In a preferred embodiment of the invention, the sliding blocks are mounted for generally radial movement with respect to an axis of the piston in order to increase or decrease an effective radius within which the valve shim is clamped. A locator may be mounted for rotational movement relative to the piston; wherein the radial position of each sliding block is determined by engagement between an engagement means on the block and a spiral means on the locator. An adjustment shaft is preferably attached to or integral with the locator, the adjustment shaft extending to a location external to the damper through a hollow piston rod on which the piston is mounted. The aforementioned preferred arrangement is a convenient way of converting a rotational movement, applied externally of the damper, to radial movement of the clamping means parallel to the surface of the valve shim. The spiral location means allows the damping characteristic to be precisely adjusted. In a further preferred embodiment of the damper, the second surface of the piston is slightly concave; preferably in the form of a shallow cone. This allows a fixed preload by the clamping means without affecting the advantages provided by the dynamic preload adjustment of the invention. The adjustable damping mechanism of the invention may be used for damping in the compression direction of the damper, in the extension direction or in both directions. It may work in combination with a low velocity fluid bypass through the piston. The damping mechanism may be employed in a main piston or in a secondary piston for restricting the flow of displaced fluid into a secondary cylinder or reservoir. The drawings Fig. 1 shows a cross section along the axis of a damper adjustment mechanism in accordance with the invention. Figs. 2 and 3 show exploded isometric views of the most important components of the mechanism of Fig. 1. The main acting piston 1 is mounted on piston rod 2, with nut 3 exerting a static preload force on extension valve shims 6 and compression valve shims 13. The preload faces 7,14 of the piston are slightly concave, each having the form of a shallow cone. The degree of preload is determined by the angles of the preload faces 7 and 14, by the number and thickness of the valve shims 6,13 and by the diameter of compression control shims 15. A brake washer 16 prevents contact between the compression valve shims 13 and the damper end-closure assembly (not illustrated) when the damper is fully extended. It may also be used to influence the dynamic characteristics of the compression valve stack 13,15. The piston 1 in this embodiment has six compression ports 12 and six extension ports 5, arranged in pairs at 120° intervals. The nut 3 has three slideways 21 radially machined to support three sliding blocks 4 at 120° intervals. The slideways are machined to the same angle as the extension preload face 7 so that a constant clearance is maintained between the blocks 4 and the extension preload face 7 irrespective of their position along their radial axes. Each sliding block 4 has a raised section 19 on its upper surface that engages with a groove 20 machined in the form of a spiral in the underside of sliding block locator 8. As the locator 8 is turned the three blocks 4 move along their respective radial axes. An adjustment shaft 9 is attached at one end to the locator 8 by fixing 10 and at the other end to an external adjustment control mechanism. The illustrated embodiment shows a single spiral groove 20 in the sliding block locator 8. This allows precise adjustment of the damper because two or more full turns of the adjustment shaft 9 may be accommodated as the sliding blocks 4 traverse their full radial extent. However, the raised portions 19 must then be formed at different radial locations on the three blocks 4 to ensure that the blocks adopt the same radial positions when they are simultaneously in engagement with different portions of the spiral groove 20. An alternative embodiment could include three equal spiral grooves spaced at 120° intervals, in which were located three identical sliding blocks, but the alternative embodiment would lack precision of adjustment. In the minimum adjustment position of the device, the three sliding blocks 4 are fully inward, effectively forming a constant radius extension control shim similar to the compression control shim 15. During initial extension movement of the piston, fluid flows through a bypass valve 11 and on through a non-return mechanism (not shown) built into locator 8. Additional direct bypass around the piston 1 is prevented by a piston seal 17, which in turn is energized by an O-ring 18 to engage the cylinder wall. When the damping force that this bypass flow provides exceeds the extension preload force, the extension valve shims 6 deflect outside the radius of the effective control shim formed by the blocks 4, permitting fluid flow through the extension ports 5. As the locator 8 is adjusted, the three blocks 4 move outwards along their slideways 21 in nut 3 up to a maximum outward position 4 A (shown in Fig. 1). This movement increases the effective control radius and therefore the extension damping forces for a given fluid flow rate or piston velocity. Because the sliding blocks 4 move parallel to the surface of the valve shims 6, the adjustment shaft 9 can be rotated with uniform torque and there is accordingly a purely dynamic preload adjustment. With this constant torque arrangement there is very little backlash in the system and minimal wear problems thus establishing good repeatability properties. Furthermore, linearity of the adjustment increments can be readily provided as the effective digression point forces are proportional to the control mechanism movement. There are two more noteworthy features of this embodiment. Firstly, because the sliding blocks 4 engage the compression valve shims 6 only over three arcs and not over a complete circle, the effective preload force is not truly angularly uniform. The adjustment range of the device can therefore be varied as required by angularly positioning the tightened nut 3 relative to the extension flow ports 5 in piston 1 to have a greater or lesser effect on the extension valve shims 6. The adjustment range is greater if the sliding blocks 4 share the same radial axes as the extension ports 5 than if the nut 3 is positioned so that the blocks 4 share the radial axes of the compression ports 12. Secondly, by designing the external control mechanism to have a smaller movement range than available within the spiral system of the sliding block locator 8, it is possible to dynamically balance the adjustment characteristics of pairs or groups of dampers externally after assembly. It will be evident how the damper adjustment mechanism of the illustrated embodiment could be reconfigured to adjust the damping force in the compression direction; or by providing a second, concentric adjustment shaft to permit independent adjustment of both compressive and extensive damping forces.

Claims: CLAIMS 1. A damper comprising: a piston (1), including a port (5) for allowing fluid flow through the piston (1) from a first face (14) of the piston (1) to a second face (7) of the piston (1); a generally disc-shaped valve shim (6) mounted on the second face (7) of the piston (1), the valve shim (6) at least partially covering the port (5) and deformable to allow the flow of fluid through the port (5); and means for clamping an area of the valve shim (6) against the second face (7) of the piston (1), the clamping means being adjustable to increase or decrease the area of the valve shim (6) that is clamped; CHARACTERIZED IN THAT the clamping means comprises a plurality of sliding blocks (4) in contact with a surface of the valve shim (6), the sliding blocks (4) being mounted for movement parallel to the surface of the valve shim (6). 2. A damper according to claim 1, wherein the sliding blocks (4) are mounted for generally radial movement with respect to an axis of the piston (1) in order to increase or decrease an effective radius within which the valve shim (6) is clamped. 3. A damper according to claim 2, further comprising a locator (8) mounted for rotational movement relative to the piston (1); wherein the radial position of each sliding block (4) is determined by engagement between an engagement means (19) on the block and a spiral means (20) on the locator (8). 4. A damper according to claim 3, wherein the engagement means (19) of all the sliding blocks (4) are engaged with a single spiral means (20) on the locator (8). 5. A damper according to claim 3 or claim 4, wherein the spiral means on the locator (8) is a spiral groove (20) and the engagement means is a protrusion (19) on the block (4) for location in the spiral groove (20). 6. A damper according to any of claims 3 to 5, further comprising an adjustment shaft (9) attached to or integral with the locator (8), the adjustment shaft (9) extending to a location external to the damper through a hollow piston rod on which the piston (1) is mounted. 7. A damper according to any of claims 1 to 6, wherein the second surface (7) of the piston (1) is slightly concave. 8. A damper according to claim 7, wherein the second surface (7) of the piston (1) has the form of a shallow, concave cone. 9. A damper according to any preceding claim, further comprising one or more ports (12) for allowing fluid flow through the piston (1) from the second face (7) of the piston (1) to the first face (14) of the piston (1). 10. A damper according to any preceding claim, further including fluid bypass means (11) through the piston (1).

Inventors: ANDERSON RICHARD FRANKLIN, FRASER DUNCAN RICHARD, NEWBOULD ANDREW PAUL, ANDERSON, RICHARD, FRANKLIN, FRASER, DUNCAN, RICHARD, NEWBOULD, ANDREW, PAUL
Applicants: ANDERSON RICHARD FRANKLIN, FRASER DUNCAN RICHARD, NEWBOULD ANDREW PAUL, PILOT PRECISION DAMPERS LIMITE, ANDERSON, RICHARD, FRANKLIN, FRASER, DUNCAN, RICHARD, NEWBOULD, ANDREW, PAUL, PILOT PRECISION DAMPERS LIMITED
Application year: 1998
Main classification: F16F9
IPC codes: F16F9