VELOCITY-CONTROLLED RAILWAY BUFFER

Abstract: The invention refers to a buffer for railway vehicles, principally based on a hydraulic shock absorber. It comprises an internal speed control valve which maintains that the buffer, practically independent of the force affecting it, will move inwardly at a dertermined velocity which is reduced with a uniform deceleration of such a level that the main part of the possible buffer stroke is always used. The initial velocity corresponds to the maximum impact speed allowed at shunting operations, plus a safety margin given by experience.

Description: VELOCITY-CONTROLLED RAILlilAV BUFFER Background of the invention Railway vehicles in most countries εhoω a similar basic design, which in¬ ter alia includes that both ends of the vehicles are provided ωith a coupling device in the middle, surrounded by tωo buffers. The main task of the coupling device is to transfer traction forces from one vehicle to another, whilst the buffers have to take care of compressive forces or impacts between the vehicles elastically in order to reduce impact loads on structure DΓ cargo to a harmless level. The heaviest demands upon the function of buffers come from shunt yard operations, when railway wagons often bump together at considerable speeds. The resistance to closure of the buffers, combined with their stroke length, must be sufficient to absorb the energy of normally occur¬ ring impacts, because if a buffer reaches the end of its stroke, the system becomes rigid, and the remaining impact energy might lead to da¬ mage. It can be mentioned that a coupling system different from the one descri¬ bed above, is used by several railways εround the world, e.g. at the iron ore railroad between Lulea and Narvik in Northern 5candinaviε. In this case, the vehicles are provided with a central draw gear in which a built- in shock absorber has the same task as the separate buffers mentioned above. The demands upon such draw gears or central couplings regarding shock absorption are, of course, the same as when separate buffers are used, and the term "buffer" in the following description is to be under¬ stood as "draw gear shock absorber" when applicable. Due to practical reasons, it is not possible to design buffers with extre¬ mely long stroke, and a considerable energy absorption capacity thus im¬ plies a considerable resistance to closure. If buffers are εre designed to take cere of impacts between heavy wagons at quite high speeds, it is difficult to avoid that such buffers cause a brutal deceleration to a light wagon running into a heavy one at a moderate speed. The heavy wagon is in this case almost immovable, and the deceleration of the light one is equal to the buffer force divided by the mass of the wagon. Present state of technology The kind of buffer predominant in Sweden as well as many other countries is the ring spring buffer. It is here used in two basic models. LJagons with four axles, as well as new two axle wagons, are usually equipped ωith the heavier model, designed for a potential stored energy of 32 kϋ, whilst a majority of two axle wagons are equipped with the weaker model having only about the half of this capacity. The ratio between resistance to closure and stroke length is almost the same in both cases, although the weaker one reaches its bottom after a shorter stroke, corresponding to a lower maximum resistance. Ring spring buffers are generally a poor compromise, being too weak to cope with heavy wagons bumping at speeds much higher than 2 m/s, although rigid enough to give light wagons a deceleration of up to 3 g (~30 m/s 2 ) already at a bumping speed of 1 ,5 m/s which is the maximum value allowed in 5weden. The Swedish State Railways have recently started testing a buffer with a hydraulic shock absorber, which gives a resistance approximately propor¬ tional to the square of the closure speed. The energy absorption thus ad¬ justs itself to the demand, and the stroke will always be sufficient. The highest impact speed such a buffer can take care of is only limited by the hydraulic pressure ωhich its cylinder unit can stand, and is at least twice as high as the maximum possible speed for ring spring buffers. In relation to light wagons, however, even this hydraulic buffer is quite brutal because the resistance is depending on the speed rather than the weight of the wagons involved. Its behaviour is not mathematically well- defined, specially because the velocity of the inward movement might very considerably due to different weight of the bumping wagons. Usually, dif¬ ferent models σf hydraulic buffers are preferred for 2-axle and -axle wagons respectively, with further complicates the image. Typical values of deceleration for bumping wagons, both of which are equ¬ ipped ωith such hydraulic buffers, still tend to be high. Summary of the invention The invention provides a buffer which takes advantage of a specially con¬ trolled hydraulic shock absorber. It is based on the experience that the maximum bumping speed allowed, in our typical case 1 ,5 m/s, is often ex¬ ceeded so that impact speeds in the order of 2,5 m/s are not completely rare. The buffer is made in such a way that the entire possible stroke is utilized for speeds in this order, no matter whether the involved wagons be light or heavy. This will dramatically reduce the top value of the im¬ pact deceleration in most critical cases which else - with conventional buffers - cause the majority of cargo damages. The buffer is shaped as a hydraulic capsule which fits in a conventional buffer casing. The invented buffer is dimensioned on basis of the fact that the speed limit of ,5 m/s in the reality is quite often exceeded. It is not realistic to reduce this speed limit because slow wagons tend to stop unintentionally, and as we cannot provide every track of all marshalling yards with an automatic re- terder system, we have to accept some oversteps now and then. As a reason¬ able compromise, the design is based on the postulation that impacts at speeds up to 2 , m/s are to be absorbed with a very low deceleration, whilst the buffers at heavier impacts mainly have to avcid reaching the bottom. These requirements are, of course, only valid provided that both wagons are equipped with these ideal buffers, as the invented buffer can¬ not influence the properties of other buffers. The deceleration at im¬ pacts between wagons with different kinds of buffers can, however, approx¬ imately be calculated as the mean value of the downright alternatives ωith similar buffers. In one arrangement, the control valve is made as a flow-limiting valve possessing means to restrict the flow of hydraulic liquid in the mentioned way. In a preferred arrangement, the mentioned flow-restricting means are mεdE to allow a pre-detεrmined maximum flow, which defines the maxirum impact speed allowed. In another arrangement, means are arranged to, under considerable pres¬ sure drop, shunt the flow beside the flow-limiting valve if the velocity of the inward movement of the buffer exceeds the velocity defined by said valve. These latter means are conveniently arranged in such a way that the rela¬ tion between the shunted flow and the pressure drop depends on how long the buffer has been moved inwardly. In a convenient arrangement, the flow restricting means are arranged to define an allowed velocity of the buffer movement, which, velocity regar¬ ded as a function of the buffer travel creates a horizontal parabola, through which the movement is uniformly decelerated. In yet another convenient arrangement, the valve system comprising con¬ trol valve and flow restricting means is arranged in order not to be¬ come initially activated at impact speeds below the allowed maximum, the buffer movement thus being initially retarded only by negligible hydrau¬ lic losses, although as soon as the movement reaches the allowed buffer velocity in relation to the travel, it is forced to follow this by the valve and flow restricting means defined relation. In a practical model, the arrangement is such that the flow control valve comprises a sleeve ωith an internal orifice plate, said sleeve being εlid- ably mounted in the plunger unit and affected by a recoil spring, whereas the orifice plate has a communicating connection with an outlet aperture for t;he hydraulic liquid, the area of said outlet aperture being defined by the position of the sleeve, and that a metering pin is mounted coaxiεl- ly ωith the orifice in such a way that said orifice combined ωith the cross sectional area of the pin determine the course of the movement. Conveniently, a reservoir chamber for the hydraulic liquid is surrounding the plunger/cylinder device, and a one-way valve is mounted between the outlet from the sleeve and the reservoir chamber. In one arrangement, the above-mentioned means for shunting the flow of hydraulic liquid comprises a thinned cylinder wall which is expandable when exposed to high pressure. If two vehicles, both equipped ωith buffers according to the invention, collide at the maximum speed mentioned above, the deceleration will be equally gentle independent of whether the vehicles are heavy or light. If the impact speed should be higher than intended, the pressure of the hydraulic liquid is prevented from increasing immensely ωith the help of a leakage slot, the area of ωhich depends on the pressure, by letting out the liquid flow ωhich the flow-limiting valve refuses to release. The de¬ pendence of the slot area on the pressure is progressively reduced during the inωard movement of the buffer. This way, the function of the buffer at abnormal high impact speeds is changed to absorbing corresponding impact energy almost like previously known hydraulic buffers, although, at moderate speeds, it gives a prede¬ termined low deceleration irrespective of the weight of the involved wagons. As practically the entire buffer stroke is always utilized, it is clear that most of the advantage of the buffer will be profitable even if the other wagon should have e.g. ring spring buffers. The fact that almost every bump causes the same buffer travel is used for a condition indication which is unique for the invented buffer. A mecha¬ nical wiper is mounted in such a ωay that a clean trace along the outside of the buffer casing indicates the length of the buffer's normal travel. Any malfunction ωill cause a different stroke length ωhich is easily ob¬ served at an early stage. Particularly for hydraulic buffers, whose mecha¬ nism is completely unacceεsible for active-service inspection, and for which a service interval of 5 - 1D years is desirable for economic reasons a simple check method for early detection of faulty shock-absorbing func¬ tion is very useful. Drawings Fig. 1 shoωs a basic arrangement of a buffer according to the invention. Fig. 2 shows an arrangement where the shock absorber according to the in¬ vention is made as a complete capsule to be mounted in a conventi¬ onal buffer casing, Fig. 3 shσws the forced floω-restricting action of the floω-control valve in terms of closing velocity of a buffer in relation to the stroke and Fig. 4 shoωs the closing velocity as a function of the time fσr a floω characteristic according to Fig. 3. Description of preferred embodiments Fig. 1 is a longitudinal sectional vieω of a buffer according to the in¬ vention. The buffer casing 10 forms together ωith the slidable jacket 11 and the buffer head 12 a fluid-tight case, most of ωhich is filled ωith hydraulic oil. A recoil spring 13 normally keeps the buffer in the exten¬ ded initial position. In the middle there is a hollow plunger 14 partially inserted in a cylin¬ der 15. These are completely filled with hydraulic oil. klhen the buffer is compressed, a certain amount of the oil inside the cylinder 15 and the plunger 14 has to be displaced, which is mainly done through the annulεr outlet channel 7. To arrive here, the oil has to pass the orifice plate 1B in a slidable sleeve 16 ωhich is kept in its shown neutral position by a tightened spring 2D. If the movement tends to get faster than intended, the pressure drop through the orifice 1B becomes big enough to overcome the spring force, and move the sleeve 16. The sleeve will thus partially close the outlet channel 17, thereby reducing the floω tD a level ωhich corresponds to balance between the spring force against the sleeve 16, and the pressure drop through the orifice 1B. The tension of the spring is chosen so as to make this balance floω correspond to the alloωed buf¬ fer closure velocity at the beginning of the stroke. The metering pin 19 ωill reduce the area of the orifice 1B as the stroke proceeds, thus reducing the flow required to achieve the pressure drop which balances the spring. The geometry is chosen so as to bring about the desired deceleration. At the end of the stroke, the pin 19 has a cross-section corresponding to the orifice 1B which makes it almost com¬ pletely choked. When the braking is finished, the recoil spring 13 returns the buffer, at which oil is sucked back to the cylinder 15 mainly through the bottom aperture of the outlet channel 17. The described arrangement should theoretically cause absurd oil pressures if two wagons should impact at a higher speed than permitted by the flow control valves of the buffers. Therefore, the cylinder 15 has to be provi¬ ded with some kind of safety valve. In order to obtain a characteristic suitable for the buffer function, its pressure drop should depend on the degree of over-speed, and also on how long the stroke has proceeded. In the preferred arrangement shown, the cylinder 15 is made with a princi¬ pally constant bore diameter, but somewhat varying outside diameter, thus making it thicker near the end wall. The plunger 14 has no sealing rings but forms a short sliding fit in the cylinder. An increased oil pressure will expand the cylinder and thus increase the leakage slot. Near the end of the stroke, the cylinder becomes more rigid, and here the leakage cau¬ sed by a given pressure will be considerably lower. In the figure, a wiper 21 is indicated on the buffer jacket 11. If the buffer function is correct, it scrapes a clean trace from the shown posi¬ tion to a point a couple Df centimetres from the flange of the casing. If the trace becomes apparently shorter or longer, the buffer is out of order and requires service. In the embodiment according to Fig. 2 a floω control valve is provided, comprising a spring-loaded sleeve 22 ωhich initially accepts an oil floω corresponding to 1 ,2 m/s (i.e. half the bump speed 2,4 m/s). Should the velocity tend to grow higher, the pressure drop along the sleeve 22 will overcome the spring force. The sleeve will then move until its rear end 23 chokes the radial outlet, thus maintaining the correct flow to balance the pressure drop against the spring force. The radial outlet channel leads through boreholes 24, 25 to an annular chamber 27 inside the cylinder 26. The chamber 27 is communicating with a hydraulic reservoir chamber 29 through a hole provided with a one-way valve 30. An over-pressure is kept in the reservoir chamber 29, a part of ωhich 31 being gas-filled. The more the buffer plunger 32 is moved inωardly, the more the area of the sleeve's orifice is reduced by a metering pin 33, and the alloωed velocity decreases. The metering pin 33 is shaped in such a ωay that the alloωed velocity as a function of the stroke forms a horizontal parabola, as shoωn in Fig. 3. The speed as a function of the time thus forms a straight line, i.e. the deceleration is constant, and ωith adequate di¬ mensions e.g. always = 0,6 g, ωhich is satisfactory low. The deceleration pattern appears from Fig. 4. If the impact speed is lower than 2,4 m/s (1 ,2 m/s per buffer) only a slight deceleration takes place initially, due to hydraulic losses and the like, until the plunger velocity (dotted line in Figs 3 and 4) hits the control curve (continuous line), at which moment the controlled decelera¬ tion starts. The fact that the deceleration of the buffer movement is al¬ ways limited to e.g. 0,6 g means in the ωorst impact case (a light ωagon hitting a heavy, immovable one) that the deceleration of the light ωagon cannot exceed 1,2 g (= 0,6 g per buffer). Higher deceleration cannot theoretically occur unless the impact speed exceeds 2,4 m/s. If it does, the flow control valve tries to close the outlet completely, but the end rim of the sleeve has such a shape that the valve in such case starts acting as a safety valve. The buffer then gets a characteris¬ tic similar to that of the earlier mentioned conventional hydraulic buf¬ fers, i.e. it absorbs the impact without exceeding the normal stroke, causing a deceleration rather equivalent to ring spring buffers. Fig. 2 thus shoωs the fundamental design of the complete hydraulic buf¬ fer capsule. The chamber 29 between the cylinder tube and the outer casing forms an oil reservoir , and ensures the proper function even if some de¬ cilitre of oil should leak out over the years. The reservoir 29 is half- filled ωith nitrogen to a pressure σf about 50 bar ωhich gives the per¬ manent recoil force the buffer must maintain. The connection between the cylinder and the reservoir is situated at the bottom and is provided ωith a Dne-ωay choking valve 30. The purpose is to slow down the return movement to prevent the ωagons from bouncing apart after the impact, and also to avoid that gas bubbles ωhich might have been flushed out during the quick damping movement be sucked back. This makes the cylinder self-degassing.

Claims: CLAIM5 1. Buffer, particularly intended for trackbound vehicles, comprising a hydraulic plunger/cylinder unit ωhich upon compression of the buffer gives a resistance, c h a r a c t e r i z e d b y a flσω control valve being arranged in the plunger/cylinder unit, and the valve being made such that it during an impact defines an allowed flow of hydraulic medium in such a ωay that the corresponding velocity of the buffer movement fol¬ lows the predetermined pattern. 2. Buffer according to claim 1, c h a r a c t e r i z e d b y the valve having means tσ restrict the hydraulic flow in the mentioned way. 3. Buffer according to claim 2, c h a r a c t e r i z e d b y said means being arranged to alloω a predetermined maximum floω, ωhich defines the maximum permissible impact speed. 4. Buffer according to claim 3, c h a r a c t e r i z e d b y means being arranged to shunt hydraulic medium beside the flow control valve, under considerable pressure drop, if the velocity of the buffer movement exceeds the limit defined by the valve. 5. Buffer according to claim 4, c h a r a c t e r i z e d b y the means to shunt the flow being made such that the relation between the shunted flow and the.pressure drop depends on the position of the buffer along the stroke. 6. Buffer according to claim 5, c h a r a c t e r i z e d b y the flow restricting means being made to achieve a curve of allowed buffer velocity, ωhich as a function of the buffer movement forms a part of a horizontal parabola, by ωhich the buffer velocity becomes uniformly de¬ celerated. 7. Buffer according to claim 6, c h a r a c t e r i z e d b y the valve arrangement comprising flow control valve and flow restricting means being designed in order not to be initially activated at impact speeds lower than the allowed maximum, whereby the compression of the buffer ωill initially only be resisted by negligible hydraulic forces, until the movement reaches the alloωed buffer velocity as a function of the movement, at which moment it changes to follow this velocity function defined by the valve and the flow restricting means. B. Buffer according to claim 7, c h a r a c t e r i z e d b y the flow control valve comprising a spring-loaded sleeve provided with an orifice plate, said sleeve being mounted slidably in the plunger and with its orifice in communicating connection with an outlet passage for the hydraulic medium, the area of which passage being determined by the position of the sleeve, and a metering pin arranged coaxially with the orifice plate whereby the area of the orifice together with the cross- section of the pin determine the inward movement of the buffer. 9. Buffer according to claim B, c h a r a c t e r i z e d b y a re¬ servoir for the hydraulic medium surrounding the plunger/cylinder unit, and a one-way valve being inserted between the outlet passage of the sleeve and the reservoir. 10. Buffer according to any of claims 4-9, c h a r a c t e r i z e d b y said means for shunting the hydraulic medium comprising a thinned cylinder wall part, expandable at higher pressure.

Inventors: DANIELI STEN HENRIK, DANIELI, STEN, HENRIK
Applicants: DANIELI STEN HENRIK, DANIELI, STEN, HENRIK
Application year: 1988
Main classification: B61G11
IPC codes: B61G11