GB2288218A - Controlling damper characteristics - Google Patents

Abstract

A damper system includes a control head 35 which has a low rate channel 43 and a high rate channel 44 through the head 35. The high rate channel 44 is provided with a unidirectional valve 45. A valve member 46 is detent-controlled so as normally to close the high rate channel but to allow the fluid to flow through the low rate channel. The head 35 is disposed in a hydraulic chamber 31. For low rates of displacement of the hydraulic fluid, valve 46 remains stationary and the damping force increases with increasing rate of displacement. For very high rates of displacement, valve 46 is displaced against the restoring force of the detont 47 so as to open high rate channel 44. The control head 35 remains in a low resistance regime until the direction of displacement changes whereupon valve 45 closes to allow fluid pressure to restore valve 46 to its original position and thereby automatically to restore the high resistance regime. The damper is of particular application to bogie suspension systems for railway vehicles. <IMAGE>

Description

DAMPING SYSTEMS, PARTICULARLY FOR RAILWAY VEHICLES The present invention relates to damping systems and particularly to hydraulic damping systems having a damping force characteristic which provides for an increase in damping force with increase in input rate (which be exhibited by velocity or pressure) up to a transition point and a reduced damping force, preferably a very low damping force for higher input rates. The invention particularly concerns such a system in which hydraulic switching is employed to provide a transition between a regime wherein damping force increases with increase in input rate to a regime wherein the damper provides less resistance to fluid flow and is arranged automatically to return to the first regime, for example when the input rate reverses.

A particular context in which the present invention is useful is the damping of movement in suspension systems for railway vehicles.

There are two main phenomena which particularly the dampers for yaw and lateral movement of the bogie are required to counteract. A first is low frequency oscillation of the bogies. Such low frequency oscillation is usually called 'hunting'. The oscillation normally starts at speeds of 100-110 kilometres per hour and if it is not damped, the bogie is likely to become derailed. The main task of the damper is to attenuate this low frequency oscillation.

A second phenomenon is a large disturbance caused when, for example, the railway vehicle traverses crossing points or track switches. Such irregularities in the track impart high transient loads at a much higher displacement rate than is characteristic of the low frequency hunting previously described. It is known to provide a discontinuity in the damping force characteristic such that above a threshold, i.e. a particular input rate, the rate of increase of damping force is reduced so that it increases more slowly than for input rates immediately below the threshold. Even so the damping force produced is very high and such a high damping force is liable to cause structural damage in the damper or its support. A yaw damper is particularly susceptible to damage from this cause. Although railway vehicles are usually strengthened to accommodate high damping forces, extra and unnecessary weight is the penalty.

One purpose of the present invention is to provide a damping system with a damping characteristic that is less liable to the production of unnecessarily high damping forces in response to large transient disturbances.

Another purpose of the present invention is to provide an improved damping system for use in railway vehicles and in particular a damping system for yaw damping of railway bogies.

A further purpose of the present invention is to provide a damping system which can automatically switch from a harmonic damping regime to a transient damping regime in an improved and efficient manner.

A yet further purpose of the present invention is to provide a damping system wherein a damping force for rates of displacement above a transition point or threshold is lower than for rates of displacement below the threshold and wherein the damping system remains in a low resistance regime until the sense of the displacement reverses, whereupon the system reverts to a regime providing an increase of damping force with rate of displacement.

In a preferred form of the invention, a hydraulic damper comprises a chamber containing hydraulic fluid and at least one control head which is disposed in the chamber and partitioned the chamber into at least a first part and a second part between which parts the fluid may flow by way of at least one channel through the control head. As will be explained hereinafter, the control head may be either fixed or movable and in a preferred form of the invention there are two such control heads, of which one is movable by means of, for example, a connecting rod and the other is stationary. The or each such control head may have a first channel which provides a relatively slow or comparatively restricted flow of hydraulic fluid through the control head from the aforementioned first to the aforementioned second part of the chamber and a second channel which can provide a much greater flow or less restricted flow through the control head between the parts of the chamber. The second channel may have a unidirectional valve, such as a flap valve. A flow control valve is preferably arranged to block the second channel but allow fluid flow normally through the first channel so that for damping low velocity displacement there is a relatively constricted flow or slow flow through the first channel. The flow control may be responsive to high transient disturbance to allow fluid flow through the second channel, thereby reducing the resistance to displacement. The valve may be maintained in a normal position, allowing the fluid flow through the first channel by means of a detent and may be freed from control by the detent in response to the large transient disturbance. The purpose of the one-way valve is to block the fluid flow through the second, larger channel for the reverse direction of displacement so that the fluid pressure can return the valve to its normal position. Such an action returns the control head automatically to the regime wherein a high resistance to low rates of displacement is provided.

As previously mentioned, there may be two control heads, one of which is constituted by a piston head movable in the chamber. The other head may be static and be arranged to partition the second part of the chamber from a third part, which may be connected to a pressure accumulator having a pneumatic fluid between a closed end and a movable piston and hydraulic fluid between the piston and the first chamber.

Various other features and objects of the invention will be apparent from the following detailed description, by way of example, of one embodiment of the invention.

In the accompanying drawings: Figure 1 is a schematic representation of a railway vehicle; Figure 2 illustrates a typical damping characteristic of damping force versus rate of displacement; Figure 3 illustrates a damping characteristic of a preferred damping system according to the invention; Figure 4 is a sectional view of one embodiment of a damper according to the invention; Figure 5 shows flow patterns for one direction of displacement in the damper according to Figure 4; Figure 6 shows a flow pattern for a reverse displacement in the damper according to Figure 4; Figure 7 illustrates the operation of the damper shown in Figure 4 in response to a high pressure transient for one direction of displacement; and Figure 8 illustrates the operation of the damper of Figure 4 for a high transient in a reverse or rebound direction.

Figure 1 illustrates schematically a railway vehicle. The vehicle 1 comprises a superstructure 2, two bogies 3 and 4 and respective wheels 5 and 6 associated with the bogies 3 and 4. Normally there is a multiplicity of wheels for each bogie. The wheels exemplified by wheels 5 and 6 run on a schematically illustrated track 7.

A primary suspension 8 links the wheel 5 and the bogie 3 and for the purpose of explanation is represented by a spring 9 and a damper 10. Likewise, a primary suspension 11 between wheel 6 and bogie 4 is shown as comprising a spring 12 and a damper 13.

A secondary suspension for the bogie 3 connects this bogie to the main superstructure 2 of the vehicle. This suspension is shown as comprising a spring 15, a vertical damper 16 and a yaw damper 17, the yaw damper being supported by a frame 18 relative to the superstructure 2.

The secondary suspension for bogie 4 is shown at 19 and is constituted by a spring 20 and a vertical damper 21.

A tertiary suspension for the vehicle is shown at 22 and is usually a buffer system. In Figure 1 this is shown as constituted by spring 23 and damper 24 between the superstructure 2 and an adjacent vehicle shown by the hatched line 25.

The main movements of the vehicle are a longitudinal movement, usually in either direction, as shown by arrow A, a pitching movement about the centre of gravity 26 and shown by arrow B, and a yaw movement (in a plane normal to the paper). Merely for convenience, the lateral dampers are not shown in Figure 1. However, the positioning of the lateral dampers and the yaw dampers is not crucial to the present invention, which is primarily concerned with a damper system that may be used for yaw and lateral damping.

As indicated previously, there are two phenomena which the yaw and lateral dampers need to counteract. The phenomenon known as 'hunting', exhibited by low frequency movements of the bogie, generally occurs at speeds in excess of 100 kilometres per hour, depending on the construction of the vehicle and the condition of the railway track. The oscillation or harmonic motion can be started by a small discontinuity or disturbance of the track or imperfections in the wheels. If the low velocity oscillation is left undamped, then the bogie is likely to become derailed. The task of yaw and lateral dampers are to prevent the increase of the low frequency oscillation to an amplitude rendering the bogie unstable.

The second phenomenon is due to disturbance caused by the passage of the vehicle over substantial discontinuities in the track, such as cross-overs and track switches. These discontinuities impart high transient loads characterised by a higher rate of displacement than is typical of low frequency hunting. Ordinary dampers, which provide for an increase in damping force for increase in rate of displacement, are liable to produce very high damping forces in response to large transient disturbances and such high forces are liable to cause structural damage in the damper or its supports.

Figure 2 illustrates a typical damping characteristic, damping force F against rate of displacement V. The normal or 'harmonic' regime of the damping system is illustrated by that part of the characteristic curve C adjacent the origin between two threshold or transition rates indicated as +Vt and -Vt. In this regime the damping force increases, usually monotonically but not necessary linearly, with rate of displacement. It is known to provide damping systems which exhibit a discontinuity in the damping characteristic at a threshold Vt such that for rates of displacement in excess of the threshold the damping force does not increase sharply (as shown by the dotted line 27a) but continues to increase at a lower rate than for rates of displacement just below the threshold. This regime of the damping system is known as the 'transient region' and is illustrated by the portion 28 of the damping characteristic X. As maybe seen, the portion 28 of the characteristic exhibits a monotonic increase of damping force with increased rate of displacement V.

However, even in systems having a damping characteristic such as is shown in Figure 2, the damping forces associated with high rates of displacement are still excessively high.

The characteristic shown in Figure 2 is anti-symmetrical about the origin and the portion of the characteristic for negative or reverse rates of displacement need not be described since it generally corresponds, apart from the change in sense, to that which is to the right of the origin.

One damping characteristic that may be achieved by a damping system according to the present invention is shown in Figure 3. This Figure again shows damping force F against rate of displacement V. The characteristic curve Y has a portion 27 which may typically resemble the portion 27 of the characteristic X in Figure 2. However, for increasing velocity, or rate of displacement, when the threshold Vt is reached, the characteristic shows a rapid reduction in the damping force to a low level, which may be predetermined, shown by the portion 30 of the characterstic Y. By means of, preferably, hydraulic switching to be described in relation to a specific embodiment, the damping system now operates in a low resistance regime, illustrated by the portion 30, which is typically a substantially linear curve extending through the origin and having a slope very substantially less than the gradient of the damping characteristic in the first regime, as illustrated by the portion 27.

Furthermore, as will be explained in relation to a specific embodiment, the damping system is preferably arranged to return automatically to the regime determined by the portion 27 of the characteristic, providing a much steeper increase of force with increasing rate of displacement. Such an automatic transition from the low resistance regime for high transient disturbances to the regime for low velocity disturbances is preferably achieved in response to a reversal of the rate of displacement. It is usual to call one direction of displacement as 'bump' and the other as 'rebound' and so the present invention may provide a low damping resistance to high velocity bump displacement but return to a high resistance regime for a rebound movement, and preferably vice versa.

The means for providing the distinct regimes described with reference to Figure 3 in a practical form of the invention is a control head which is disposed within a chamber and is adapted to switch automatically in response to high transient pressures and reversal of hydraulic pressure within the chamber. As indicated previously, in a preferred embodiment of the invention there are two such control heads disposed in series in a common chamber, a first of the control heads being a movable piston which may be connected by some connecting member such as a connecting rod to one portion of the vehicle and the cylinder or a support thereof being connected to another portion of the vehicle so as to provide damping of the relative displacement of these two portions of the vehicle. However, other embodiments such as comprising a single control head which may be fixed or movable in a hydraulic chamber are intended to be within the broad scope of the invention.

As shown in Figure 4, a damper system comprises a chamber 31 which contains hydraulic fluid such as oil O. The chamber has end closures 32 and 33, the closure 33 accommodating a connecting rod 34 which is attached to a movable control head 35 slidable in the chamber 31. This control head partitions the chamber 31 into parts C1 and C2 between which hydraulic fluid can flow by way of channels in the control head 35, in a manner to be explained hereinafter.

In this embodiment of the invention there is a second control head, which might be movable but in the embodiment shown in Figure 4 is a fixed head 36. As will be explained, the construction of the head 36 may resemble that of the head 35. The static head 36 partitions the chamber between the parts C2 and C3, one part to each side of the head 36.

One of the parts into which the chamber is partitioned by the heads, in this example the part C3, is connected by way of, for example, a connecting line 37 to a pressure accumulator 38. Such a pressure accumulator may have a hydraulic chamber and a pneumatic chamber separated by a free piston. In this particular example, the hydraulic portion of the pressure accumulator is shown at 39 and is a portion of variable volume occupied by hydraulic fluid. As more hydraulic fluid is displaced from the chamber 31 the volume of the hydraulic part of the pressure accumulator increases, moving a free piston 40 so as to compress the volume of a pneumatic portion 41 of the chamber, this portion being filled with pneumatic fluid such as air.

Other forms of pressure accumulator, employing springs instead of pneumatic fluid, are known in the art.

The control head 35 in this example has a central bore 42 and two channels 43 and 44. These channels may be constituted by a series or multiplicity of channels but for the sake of convenience will be described as single channels.

The channel 43 extends from one side of the control head to the other but includes the right hand part of the central bore 42 so that the channel can be blocked as hereinafter described. The channel 44 includes the left hand end of the central bore 42 and extends to the right hand face of the head 35. The channel 44 is controlled by a unidirectional valve 45 which in this embodiment of the invention allows fluid flow through the channel 44 from the chamber part C1 to the chamber part C2 (rightwards in the drawing) but not in the reverse direction.

The first channel 43 may include a constriction but in any event allows a much slower or comparatively restricted flow of hydraulic fluid than the second channel (or series of channels) 44, which provides very little resistance to fluid flow.

Within the bore 42 is a valve 46, preferably constituted by a spool valve, which is maintained normally in a condition that blocks the rapid flow, through channel 44 and therefore only allows a low rate of fluid flow by way of the channel 43. The valve, which might in other embodiments be constituted by a set of valve means, is preferably maintained in the normal condition by a detent, in this embodiment comprising a detent ball 46 engaging a reduced waist 48 of the valve. The ball 47 is urged against the waist 48 by a spring 49 in a blind bore 50 extending in a transverse direction relative to the bore 42.

As noted previously, the static control head resembles the movable control head and will not therefore be described in detail. For convenience in the later description, the control head 36 is shown as having a relatively restricted channel 43a corresponding to channel 43 in the head 35, a low resistance channel 44a corresponding to the low resistance channel 44 of control head 35, valve 45a controlling channel 44a in a manner similar to the valve 45 controlling channel 44 in control head 35, and valve 46a corresponding to the valve 46 of head 35.

The remaining Figures, Figure 5 to 8, illustrate four main phases or condition of operation, namely normal or harmonic damping in a forward or bump direction, normal damping in a rebound direction, high transient damping for the forward or bump direction and high transient damping for the reverse direction or rebound direction respectively.

As is shown in Figure 5, for the bump direction, wherein the piston 35 is moving to the right, hydraulic fluid flows from the part C2 of the chamber 31 to the part C1, by way of the low rate channel 43. The particular shape of the portion 27 of curve Y is not important and the low rate channel may include various devices (not shown) which may control the precise configuration of the curve. In any case, it is intended that the damping force should increase with increase in rate of displacement of the head 35.

As may further be seen from Figure 5, for low rates of displacement the valve 46 is held in its home position by the detent, the left hand part of the valve blocking high rate channel 44. Similarly, as the volume of portion C2 decreases, there is also hydraulic fluid flow through low rate channel 43 in static head 36, valve 46a being held in its home position by the respective detent and blocking the high rate channel 44a in the control head 36. Hydraulic fluid flows from part C3 of chamber 31 to the pressure accumulator 38.

In a similar, but converse, manner, if the piston rod and the valve head 35 are moving in the leftward direction, fluid flows through the low rate channel 43 from chamber part C1 into chamber part C2 and fluid flows from the pressure accumulator through low rate channel 43 in control head 36. In this mode, the pressure accumulator makes up the fluid requirements of the chamber 31 whereas it takes the excess fluid for the situation shown in Figure 5.

In this embodiment, whichever direction may be the movement of the connecting rod and the movable control head 35, half the displaced fluid passes through the head 35 by way of channel 43 and half the displaced fluid flows through head 36 by way of channel 43a in the reverse direction.

Figure 7 illustrates the effect of a high transient input for the bump direction wherein the connecting rod and the control head are moving rightwards. The rise in pressure in part C2 of chamber 31 forces the valve 46a of head 36 away from its home position so that this valve blocks the low rate channel 43a in head 36 and opens the high rate channel 44a, the valve 45a being automatically opened by the fluid pressure to allow rapid fluid flow, with low resistance, through control head 36 to the chamber part C3 and the accumulator 38.

Once the valve has moved sufficiently to free itself from the action of the detent, there is no restoring force from the detent after the transient input abates and so the bypass or high rate channel 44a remains open.

The damping system is then operating on the region 30 of the characteristic curve Y in Figure 3, there being very little resistance to the movement of the head 35 and therefore very little transmission of any damping force to the supports of the damper. This minimizes the danger of structural damage to the damper and the supports.

It may be seen from Figure 7 that if the direction of movement of the head 35 and the connecting rod reverses, so that the head 35 is moving to the left, there is fluid flow into the chamber part C2 from the part C1, by way of channel 43 and also fluid flow from chamber part C3, and the accumulator 38, into chamber part C2. Such reverse fluid flow closes valve 45a and the accumulated pressure acting on the right hand end of valve 46a restores this valve to its home position under control of the associated detent. This action therefore restores the damper system to the condition shown in Figures 5 and 6 such that the damper system is now operating in the normal harmonic region 27 of the characteristic curve Y.

Similarly, if there is a high transient moving the control head and the piston leftwards, as shown in Figure 8, the valve 46 is moved to the right by virtue of the increase in pressure in chamber part C1. The detent loses control of the valve, which blocks the low rate channel 43 and opens the high rate, low resistance, channel 44 so that there is low resistance to fluid flow from chamber part C1 into chamber part C2. There is some but in this mode an insignificant flow of fluid from the accumulator 38 through low rate channel 43a in head 36.

As may be seen by consideration of Figure 8, when the direction of movement of the head 35 and rod 34 and accordingly the sense of displacement of fluid in the chamber reverses from the direction shown in Figure 8, so that volume of chamber part C1 now increases at the expense of volume of chamber part C2, the fluid pressure is such as to close valve 45 and to force valve 46 leftwards to bring it back under control of the detent and also to open low rate channel 43. The closure of valve 45 and the restoration of the valve 46 to its home position thus re-establish the first or harmonic regime of the damper system.

The present invention is not confined to use in railway bogies but may be employed in circumstances where, for example, a folded damping characteristic of the kind generally shown in Figure 3 is useful. Moreover, systems employing only one control head, either static or movable, may be useful for systems wherein a folded characteristic is desired only for rates of displacement in one sense rather than bidirectionally as provided by the specific embodiment.

Claims (10)

1. A damper system comprising a chamber (31) containing hydraulic fluid, a control head (35 or 36) disposed within the chamber and partitioning the chamber into first and second parts, the control head allowing hydraulic fluid flow between the said parts and including a low rate channel (43,43a) and a high rate channel (44,44a) valve means (46,46a) having a first condition blocking said high rate channel and allowing fluid flow through said low rate channel and a second condition which allows fluid flow through said high rate channel, means (47) for resisting change of said condition of said valve means for rates of displacement below a threshold in a first direction, said means allowing change of said valve means from said first condition to said second condition in response to rates of displacement above a threshold and means (45,45a) responsive to reversal of the sense of displacement to close said high rate channel and cause restoration of said valve means to said first condition.

2. A damper system according to claim 1, wherein said control head (35) is movable in said chamber.

3. A damper system according to claim 1, wherein said control head (36) is static in said chamber.

4. A damper system according to claim 1, and comprising a first control head (35) which is movable in said chamber, a second control head (36) in said chamber, and a connecting member (34) coupled to said first control head.

5. A damper system according to any foregoing claim, wherein said chamber (31) is connected to a accumulator (38) for taking up and providing hydraulic fluid displaced from and required by said chamber.

6. A damper system according to claim 1, wherein said valve means comprises a valve member (46) which has a home position blocking said high rate channel and allowing flow through said low rate channel and said means for resisting (47) comprises a detent for said valve member.

7. A damper system according to claim 6 wherein said valve means is responsive to fluid pressure to move from said first condition to a second condition out of control by said detent.

8. A damper system according to claim 1 wherein said means (45,45a) responsive to reversal of said displacement comprises a unidirectional valve for said high rate channel.

9. A damper system according to claim 9 wherein said unidirectional valve is adapted to close on reversal of said fluid pressure whereby the valve means (46) changes from the second condition to said first condition.

10. A damper system employing hydraulic fluid damping and comprising a control head including a first low rate channel providing a damping force that increases with increasing rate of displacement of hydraulic fluid, high rate channel that provides a substantially smaller damping force than said low rate channel and means responsive to pressure in said fluid for allowing fluid flow through the high rate channel in response to fluid displacement in a particular sense and above a threshold rate and to permit fluid flow through that channel until the sense of fluid displacement reverses.