GB2096270A - Shock absorber with automatically variable flow restrictor - Google Patents

Abstract

A hydraulic shock absorber which is particularly for use on container rail vehicles to retard the relative movement which occurs between the sliding carrier and the running frame when the vehicle runs up against another vehicle (e.g. when the vehicles are being coupled together), has at least one piston (17, 18) which is movable in a damper cylinder (14) and responds to the impacts to be damped by moving into the working chamber of the cylinder causing a discharge of hydraulic fluid into a receiver (36). A restoring element (e.g. gas in the receiver) exerts on the piston a restoring force which increases with the piston stroke. An inertial sensor 81 controls a restrictor 66 to vary the resistance to flow of fluid along the flow path between the working chamber and the receiver, to an optimum value for uniform pressure equalisation in the damper cylinder working chamber. The restrictor 66 is biased to a position of maximum resistance. When a set value of acceleration is exceeded, the flow cross-section is increased. The weight 81 may be replaced by a sensor responsive to the pressure in the working chamber. <IMAGE>

Description

SPECIFICATION Hydraulic shock absorber The invention relates to a hydraulic shock absorber more particularly, a hydraulic shock absorber of the kind having at least one piston which is movable in a damper cylinder and responds to the impacts to be damped by moving in the working chamber of the cylinder, the impact-induced relative movement between the piston and the cylinder causing a working medium to discharge from the latter through a restrictor of variable flow cross-section or variable flow resistance into another pressure chamber, a restoring element being provided which exerts on the piston a restoring force increasing with piston stroke.

Shock absorbers of this kind are known as "long stroke" shock absorbers and are used for container rail vehicles such that the load-bearing chassis or running frame of a container vehicle is borne resiliently in relation to its sliding carrier which is guided for longitudinal reciprocation on the running frame and which comprises the buffers and drawgear for coupling the vehicle with other rail vehicles. The function of long stroke shock absorbers or dampers is to retard, with a very constant and limited delayed acceleration, the relative movement which occurs between the sliding carrier and the running frame when the vehicle runs up against another vehicle, for instance, when being coupled to the vehicle in front, the aim being to prevent damage to impactsensitive loads.The damper has to be designed so that a delayed acceleration of 1.5 g (g = 9.81 m/s2) at an impact speed of 15 km/h is not exceeded.

In long stroke dampers of this kind an oil-filled working chamber in the damper cylinder is bounded by two pistons rigidly interconnected by a piston rod, one of the pistons being a separating piston which separates the working chamber from a pressure gas chamber. An elongated helical compression spring which extends around the outside of the damper and acts between the cylinder end flanges and the moving piston arrangement biases the same into its normal position in which the separating piston is disposed very near an annular restrictor of the working chamber, the hydraulic fluid flowing therethrough during the operating stroke of the piston arrangement from the working chamber bounded on one side by the outer piston into the working chamber part bounded on one side by the separating piston.The piston rod between the two pistons is conical, so that the annular spill-gap left between the piston rod and the restrictor edge closes gradually on the damping stroke and, therefore, increases the flow resistance, thus providing progressive damping.

A disadvantage of this kind of restrictor damping is that the restrictor cross-section can be optimal only for a particular (e.g. average) load weight of approximately 20 tonnes. Consequently, piston travel that would be useful for effective retardation has to be wasted for appreciably heavier loadings of e.g. 60 tonnes, with excessive delayed accelerations occurring at the end of the damping stroke, whereas at much lower loading weights the delayed accelerations are excessive at the start of the damping stroke.

It is therefore an object of the invention to enable better limitation of impact-induced accelerations to permissible maxima.

In accordance with the invention, we propose a hydraulic shock absorber having at least one piston which is movable in a damper cylinder and responds to the impacts to be damped by moving into the working chamber of the cylinder, the impact-induced relative movement between the piston and the cylinder causing a working medium to discharge from the latter into a hydraulic fluid receiver, restoring element which exerts on the piston a restoring force increasing with piston stroke, and control means comprising a restrictor operable to vary the flow cross-section or flow resistance along a flow path between the working chamber of the damper cylinder and the hydraulic flow receiver at a value linked with a predetermined amount of damping.

It is a simple matter to limit the accelerations occurring during the damping stroke and also to achieve far better utilisation of the permissible maxima of such accelerations and, therefore, to achieve correspondingly improved utilisation of the design damping stroke by a control facility which, in accordance with this invention, is disposed in an arrangement outside the working chamber of the damper cylinder, can be devised by simple means known per se in the hydraulic control art and which responds e.g. to the impactinduced decelerations or accelerations and/or to the pressure produced in the working chamber by adjusting the restrictor cross-section to the optimum value for uniform pressure equalisation in the damper cylinder working chamber.

Preferably, the restrictor is made responsive to an inertial sensor which may be of very simple construction.

In one embodiment, the sensor comprises a weight which is movable in response to accelerations thereon, movements of the weight being transmitted to a movable restrictor member which is spring biased toward a normal position in which the restrictor creases maximum resistance to flow. When the shock absorber is used for container vehicles, the control means ensures, within programmable safe load minima and maxima and independently of the safe load, the observation of maxima of effective accelerations.

Linearisation of the relationship between displacement of the restricting member i.e. the value of acceleration detected and the effective flow cross-section of the restrictor can be achieved quite simply by appropriate construction of the restricting member.

Preferably, the restoring element which controls the post-impact return of the working piston system of the damper to its normal position comprises at least one cylinder having a separating piston separating the hydraulic fluid receiver from a gas chamber in which the pressure is sufficient to restore the damper piston. The restoring force results from the pressure acting on a separating piston acted upon by the pressure in a high-pressure gas chamber and by the pressure which increases with the working stroke of the damper piston. Consequently, the need for heavy and costly helical compression springs conventionally used as restoring elements in the known long stroke dampers is obviated.

In a preferred embodiment, the restrictor comprises a restricting member mounted in a casing bore for movement along a longitudinal axis of the casing and symmetrical about a transverse centre-plane extending perpendicular to the said longitudinal axis, the restricting member being formed on either side of the said plane with annular grooves which, in a neutral central position of the restricting member, communicate by way of narrow gaps with an inlet duct leading to the damper cylinder working chamber.The advantage of such a construction and the fact that it is arranged in the neutral position specifically for the impact-free or acceleration-free state, is that even in cases in which opposite impact directions are possible, only a single restrictor facility or control facility is needed for appropriate damping control in both directions in order to achieve the required limitation of acceleration.

Advantageously, in such cases, the damper cylinder working chamber is bounded at each end by a piston the axially projecting rods of which engage when in their normal position abutments disposed opposite on another and forming part of a siider which is movable lengthwise on a frame on which impacts to be damped can act in two opposite directions, substantially in the direction of the longitudinal axis of the pistons; and the inertial sensor is disposed in a subunit rigidly secured to the frame. Also, the inertial sensor, the restrictor, the body barrel of the damper cylinder and the restoring element are combined in a fixed relation to one another to form the subunit rigidly connected to the frame.

A shock absorber having these features is particularly suitable for use as a long stroke shock absorber for container vehicles.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a cross-section of hydraulic shock absorber or damper having purely accelerationlinked control of damping, and for use as a long stroke damper for rail vehicles; and Figure 2 is a cross-section of a restrictor which is used in association with the damper of Figure 1, the restricting cross-section of which is controlled in dependence upon acceleration.

The hydraulic shock absorber or damper 10 shown in Figure 1, is designed as a long stroke impact damper for a rail vehicle such as a container vehicle. This comprises a running frame or chassis which carries the load and which is indicated in Figure 1 merely by blocks 11, 12 and, guided thereon for reciprocation lengthwise of the vehicle. A sliding carrier or slider 13 comprises drawgear (not shown) for connection to other rail vehicles and conventional short-stroke buffers which, for instance, during coupling together of two rail vehicles, damp the impact between the respective carriers 13 of the two vehicles.

The device 10 permits additional relative movement of the running frame 11, 12, relatively to the carrier 1 3 and provides within the utilizable stroke of this operative movement, by means of a maximum permissible delay value, very uniform retardation in order to reduce considerably the risk of damage to the load being carried by the vehicle.

The main operative element of the device 10 is a damper cylinder 14 which, in co-operation with two working pistons or plungers 17, 18 movable along central longitudinal axis 1 6 of cylinder 14, bounds a hydraulic fluid filled working chamber 19 of the device 10. The two pistons 1 7, 1 8 which are guided for axial movement in registering bores 21,22 in the opposite end walls of the cylinder 14, are sealed in their respective bores 21,22 within the cylinder barrel 26 by ring seals 23, 24.

Portions 27, 28 of the plungers 17, 18 respectively extending out of the cylinder barrel 16 at opposite ends thereof, have at their free ends a flange 29, 31 respectively which, with the pistons 1 7, 1 8 in their normal position indicated in solid lines, bear on abutments 32, 33 respectively of the carrier 13, the abutments 32,33 being transverse to the longitudinal axis 1 6 of the cylinder 14.

The cylinder 14, two parallel connected pressure accumulators 34, 36 and a damping or acceleration control facility 37 whose construction and operation will be described in greater detail hereinafter are incorporated in a block-like assembly unit 38 which, as shown in Figure 1, bears by way of guide rods 39, 41, extending through recesses 42,43 in the flanges 29,31 on the blocks 11, 12 of the running frame, whereby the unit 38 is restrained against axial movement relative thereto.

The two pressure accumulators 34, 36 are divided cylinders the respective pistons of which 44, 46 separate a gas chamber 47, 48 containing gas at a high pressure from an inlet hydraulic fluid chamber 49, 51; the inlet spigots or the like of the chambers 49, 51 are interconnected and linked by way of a flow path 52 of variable flow resistance, with the working chamber of cylinder 14. Also, the two accumulators 34, 36 are connected via a check valve 55 to working chamber 19 of damper cylinder 14, it being possible for the fluid to discharge more rapidly from the chambers 49, 57 into the working chamber 1 9 by way of the valve 55 than by way of the restrictor 61 , 66 when the latter is in its normal position.

The construction and operation of the control facility 37, is as follows: Secured to the unit 38 which moves with the running frame 11, 12 is a restrictor casing 61 (Figure 2) having a central bore 62 the longitudinal axis of which 63 extends parallel to the vehicle longitudinal axis 1 6 and, therefore, to the directions in which the vehicle is likely to experience impacts. Disposed in bore 62 for axial reciprocation is a restricting member 66 which is symmetrical with respect to its transverse centreplane 64, the latter extending, in Figure 2, at rightangles to the longitudinal axis 63.Member 66 resembles a spool, with two ports 68, 69 on either side of a central piston flange 67 and with outer piston flanges 71, 72. The casing 61 has a central annular duct 73 which communicates with chamber 19 of cylinder 14 in all positions of the member 66. By way of parts of the flow route 52 which are not shown, annular ducts 74, 76 disposed on either side of the central annular duct 73 and associated with the ports 68, 69 respectively communicate with the inlet chambers 49, 51 of the accumulators 34,35. Opposite flanks of the outer annular ducts 74,76 of the casing 61 and of the ports 68, 69 respectively have the conical shape visible in Figure 2, to which reference should be made for these details, and merge into one another by way of a smooth curve.

The axial internal width of the central duct 73 of casing 61 is slightly larger than the width, measured in the same direction, of the central flange 67 of member 66; consequently, with the member 66 in its normal position shown in Figure 2, the central duct 73 communicates by way of narrow annular gaps 77, 78 with the two ducts 74,76 of the casing 61.

The member 66 is movable in response to an inertial sensor. More particularly, the member 66 is rigidly coupled by way of a ball joint 79 for movement with an inertia weight 81 also adapted to reciprocate along the longitudinal axis 63.

Movement of the member 66 in either of its two possible directions of movement, indicated by arrows 82 and 83, occurs against the restoring force of a preloaded helical compression spring 84 acting in each of the two directions 82, 83 and which, while no force is acting on the system, maintains the member 66 in the normal position shown.

It is a simple matter, by selection of appropriate dimensions of the weight 81 and of the spring 84 and the pre-loading thereof, to set a desired value of acceleration (e.g. 1.5 g); when the set value is exceeded the control facility 37 increases the restriction cross-section so that the pressure in chamber 1 9 of cylinder 14 can decrease relatively rapidly and an acceleration acting on the running frame 11, 12 decreases.

The control facility 37 described above operates in a typical case as follows: It will be assumed that the vehicle is moving to the left, as indicated by arrow 53, at a speed of about 1 5 km/h and the left-hand end 54 of carrier 1 3 runs into a stationary obstacle such as a buffer stop. Consequently, while the vehicle running frame, represented by the blocks 11, 12 continues to move relatively to the carrier 1 3 in the direction indicated by the arrow 53, the plunger 1 7 which is on the left in Figure 1 enters the damper cylinder 14.The hydraulic fluid displaced therefrom by the entering plunger or piston 1 7 discharges via the flow path 52 into the chambers 49, 51 of the accumulators 34, 36, the pistons 44, 46 thereof moving to the right against the increasing pressure in the gas chambers 47, 48. The pressure increase in the cylinder 14, resulting from plunger 17 entering chamber 1 9 of cylinder 14, causes the running frame 11, 12 to be braked increasingly, because of its rigid connection by way of the rods 39, 41 to the unit 38. The delayed deceleration increases continuously because the increasing pressure in the cylinder 14 makes the axial bearing relationship between the running frame 11, 12 and the carrier 13 increasingly "stiffer".

While the restricting member 66 remains in its normal position shown in Figure 2, the flow resistance along flow path 52 is at its maximum; consequently, when the vehicle strikes an obstacle, for example, when moving in the direction of the arrow 53 of Figure 1 , the pressure in chamber 1 9 of cylinder 14 increases fairly rapidly at first and the running frame 11, 12 experiences an acceleration or is moved relatively to the carrier 1 3. Once the limit of acceleration as determined by the pre-loading of the spring 84 in association with the inert weight 91 has been reached, any further increase in the acceleration a causes the weight 81 to move to the right in Figure 2 in relation to the casing 61, whereafter, because of the effective accleration a from the equations:: m a = CF-s(a) = Fdyn (1) s(a) = ma/C, (2) The equilibrium position of the member 66 alters; in equations (1) and (2) m denotes the value of the inert weight 81, cm denotes the spring constant of the restoring spring 84 and s(a) denotes deflection of the member 66 from its normal position in Figure 2 in dependence upon the effective acceleration a.

The characteristic radian frequency v of the control facility 37 is given in essence by the equation:

Consequently, it is a simple matter, involving an appropriate choice of the size of the weight 81 and of the spring rate CF of the spring 84, to set the characterising radian frequency of the facility 37 to a desired value; the frequency should be approximately from 10 to 100 times greater than the natural frequency of the mass system experiencing the impact accelerations.

Since, as will be immediately apparent from Figure 2, movement of the member 66 from its normal position causes a reduction in the flow resistance of flow route 72, which extends from the central annular duct 73 via one or the other annular ducts 74 or 76 to the pressure accumulators 34 and 36, there can now be a relative decrease in the pressure in chamber 1 9 of cylinder 14; consequently the acceleration a acting on the running frame 11, 12 decreases.

There is a corresponding decrease in the dynamic force Fdvn acting on the weight 81 and the member 66 takes up a new equilibrium position which, in accordance with equation (2), corresponds to a reduced movement s(a) (where a - is the reduced acceleration), from its normal position and therefore to a relatively higher flow resistance in the flow path 52.Consequently, during the remainder of the operating stroke of whichever plunger 1 7 or 1 8 is entering the cylinder 14, the member 66 takes up an equilibrium position corresponding to a definite value aD which can be programmed by the design of the weight 81 and of the restoring spring 84 and of the geometry of the restricting member 66 and of the casing 61, with the result that the accelerations arising from an impact on the running frame are limited in the required manner to permissible maxima and full use is made of the opportunity to achieve effective and gentle braking of the running frame within the available travel determined by the design stroke of the damper.

In the particular construction of hydraulic impact damper 10 described above, the flow resistance along the flow path 52 is adjusted in dependence upon the acceleration which acts on the running frame 11, 12 and which is detected by means of the inertia weight 81. Appropriate control or adjustment can of course be based upon the working pressure in the cylinder 14; in which case, it is convenient to have a programmed set value which takes account of the state of loading.

Claims (11)

1. A hydraulic shock absorber having at least one piston which is movable in a damper cylinder and responds to the impacts to be damped by moving into the working chamber of the cylinder, the impact-induced relative movement between the piston and the cylinder causing a working medium to discharge from the latter into a hydraulic fluid receiver, restoring element which exerts on the piston a restoring force increasing with piston stroke, and control means comprising a restrictor operable to vary the flow cross-section or flow resistance along a flow path between the working chamber of the damper cylinder and the hydraulic fluid receiver at a value linked with a predetermined amount of damping.

2. A shock absorber according to claim 1 wherein the restrictor is operable to vary the flow cross-section or flow resistance in the flow path, by an inertial sensor responsive to acceleration acting thereon, the restrictor being biased toward a normal position corresponding to maximum restriction or flow resistance.

3. A shock absorber according to claim 2 wherein the inertial sensor comprises a weight which is movable in response to acceleration acting thereon movements of the weight being transmitted to a movable restricting member which is spring biased toward the said normal position.

4. A shock absorber according to claim 3, wherein the variation of the cross-section of the flow path is directly proportional to the movement of the restricting member.

5. A shock absorber according to any of the previous claims, wherein the restoring element acting on the working piston of the damper cylinder having a separating piston separating the hydraulic fluid receiver from a gas chamber in which the pressure is sufficient to restore the damper piston.

6. A shock absorber according to any one of the preceding claims, wherein the restrictor comprises a restricting member mounted in a casing bore for movement along a longitudinal axis of the casing and symmetrical about a transverse centre-plane extending perpendicular to the said longitudinal axis, the restricting member being formed on either side of the said plane with annular grooves which, in a neutral central position of the restricting member, communicate by way of narrow gaps with an inlet duct leading to the damper cylinder working chamber.

7. A shock absorber according to claim 5 or 6, wherein the damper cylinder working chamber is bounded at each end by a piston, the axially projecting rods of which engage when in their normal position abutments disposed opposite one another and forming part of a slider which is movable lengthwise on a frame on which impacts to be damped can act in two opposite directions, substantially in the direction of the longitudinal axis of the pistons and the inertial sensor is disposed in a subunit rigidly secured to the frame.

8. A shock absorber according to claim 7 wherein the inertial sensor, the restrictor, the barrel of the damper cylinder and the restoring element are combined in a fixed relation to one another to form the subunit rigidly connected to the frame.

9. A shock absorber according to claim 8, for a container vehicle and wherein the frame, with which the damper cylinder and the inertial sensor are rigidly coupled for movement, the loadcarrying chassis and the slider movable lengthwise relatively thereto is the slider having short stroke buffers.

10. A shock absorber according to claim 9, for a container vehicle for loads between 4 and 60 metric tonnes, wherein the design piston stroke of the damper cylinder is approximately 60 cm, the restoring facility is preloaded to a minimum restoring force of approximately 3 metric tonnes, the inertia weight weighs about 5 kg and the restoring spring is preloaded to a restoring force of 75N.

11. A hydraulic shock absorber constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.