GB1579912A - Articulated vehicles - Google Patents

Description

(54) IMPPROVEMENTS IN AND RELATING TO ARTICULATED VEHICLES (71) We, SOCIETE NOUVELLE DES ATELIERS DE VENISSIEUX, a French corporate body of Chemin du Genie, 69631 Venissieux, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : - The invention relates to improvements in articulated vehicles and particularly to a system for holding an articulated rail vehicle in position during travel round a curve. The invention has particular but not exclusive application to articulated trucks, possessing three axles, which are used for transporting motor cars.

The trucks used for example for transporting motor cars are fairly lightly loaded.

it has therefore been proposed to increase the length of each truck and provide a truck having three axles which is made up of two half-chassis, the adjacent coupled ends of which are connected to each other by an articulation member and rest on an axle held in position, by any means whatsoever, along the bisector of the angle between the longitudinal axes of the two-half chassis. A longer truck is thus obtained, the articulation of which allows it to take a curve. The opposite coupling ends of the two halfchassis are of course equipped with buffer members and members for coupling them to other trucks.

In general, the articulation between the two half-chassis is free, the articulated ends resting on a sub-frame by means of sliding supports which offer a low resistance to the variation in the angle between the longitudinal axes of the two half-chassis, so as to allow the truck to take a curve. The articulation system is provided in order to transmit pulling or braking stresses which are applied to the coupling ends of the half-chassis. The pulling stresses tend to widen the angle formed by the axes of the half-chassis and thus cause the articulation system to undergo a reaction which is directed towards the centre of the curve and is taken up by the rails. In this direction, this reaction does not have severe disadvantages.On the other hand, the buffering and braking stresses, which are in addition generally more fierce and which tend to narrow the angle formed by the axes of the halfchassis, cause the articulation system to undergo a centrifugal reaction which is taken up by the rails but which, in the case of particularly fierce braking, would run the risk of causing a derailment of the intermediate axle. Furthermore, these transverse reactions can, in the long term, bring about a lateral shifting of the rails.

In order to overcome these disadvantages, the Applicant Company has already developed various devices which have formed the subject of patents, and in particular of U.K. Patent No. 1,531,565.

In this Patent, a device for holding the half-chassis in position on a curve is described, which device comprises linking devices of variable length between the coupled ends of the two half-chassis, the linking devices being for example connecting rods or jacks placed on either side of the articulation system, and means for locking the variations in length of these connecting rods, which means is controlled by a member for detecting when the coupling ends of the articulated truck are approaching adjacent coupled trucks, when this approach movement exceeds the normal clearance, during travel, without braking, on a curve, of the buffering member on the inside of the curve. As a result, the angle formed by the axes of the two half-chassis is locked, but in certain cases this locking runs the risk of being too fierce.

According to the invention there is provided an articulated truck comprising at least two chassis parts, the or the respective adjacent coupled ends of which are connected to each other by an articulation member. the coupling ends of the truck being provided with coupling and buffer members, means for detecting approach movement between the coulpling ends of the truck and trucks when coupled thereto, elastic means for resisting variation in the angle formed by the longitudinal axes of the or the respective connected chassis parts when on a curve, and actuating means controlled by the approach movement detecting means for progressively bringing into play, proportionally to the approach movement, the elastic resistance means.

In a first embodiment of the invention, the elastic resistance means comprises, symmetrically on each side of the longitudinal axis of the truck and each of the coupled ends of the chassis parts, a flexible leaf spring which extends parallel to the longitudinal axis, and the actuating means comprises, in respect of each spring, a stop member for contact by an end of the spring.

In a second embodiment of the invention, the elastic resistance means comprise, in respect of the or the respective coupled ends of the chassis parts and on each side of the longitudinal axis of the truck, two intermediate buffers which abut each other and are mounted for axial sliding movement one on each coupled end respectively of the coupled chassis parts, and an elastic member for supporting each buffer on a stop plate, each stop plate being mounted for axial sliding movement on the respective chassis part, and the actuating means comprises means for controlling displacement of the stop plates.

The invention will now be described with reference to these two embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows an embodiment of an articulated truck on a straight stretch, Figure la showing travel under pull and Figure ib showing travel under restraint; Figure 2 shows the truck of Figure 1 during travel on a curve, Figure 2a showing travel under pull and Figure 2b showing travel under restraint; Figure 3 is a detailed view of part of the truck of Figure 1; Figure 4 shows another embodiment of an articulated truck on a straight stretch, under pull above the longitudinal axis and under restraint below the said axis; Figure 5 shows the truck of Figure 4 during travel on a curve under pull; Figure 6 shows the truck of Figure 4 during travel on a curve under restraint; and Figure 7 is a detailed view of part of the truck of Figure 4.

In Figures 1 to 3, a truck is shown comprising two chassis parts or half-chassis 1 and 2 of which the longitudinal axes 10 and 20 are in alignment and pass through the centre 3 of an articulation member (not shown) which connects the adjacent ends 11 and 21 of the half-chassis. The opposite ends 12 and 22 of the half-chassis are equipped with coupling members (not shown) and with buffers 4.

The truck rests on three axles 51, 52 and 53 of which two axles 51 and 52 are positioned close to each chassis coupling end 12, 22, and the third intermediate axle 53 is mounted on a sub-frame 5 on which the two chassis coupled ends 11 and 21 rest by means of sliding supports. This conventional arrangement is not shown in detail.

On each side of the sub-frame 5 there are mounted two pairs of leaf-springs 61 and 62, the springs of each pair being arranged one on each side of the articulation member. In the rest position, the springs are parallel to the longitudinal axis of the subframe 5, which axis is in alignment with the axes of the two half-chassis when the truck is on a straight stretch and forms the external bisector of the angle formed by the axes of the two half-chassis during travel on a curve, the sub-frame being equipped with conventional means for holding the axle in position along the internal bisector of this angle. The leaf-springs extend under the two ends 11 and 21 of the two half-chassis 1 and 2 and each end of a leaf-spring, in the position shown in Figure la, is close to a shoe extending between the spring and the longitudinal axis.

Each end 11 or 21 of a half-chassis is thus equipped with two shoes 7 arranged symmetrically on either side of the longitudinal axis of the half-chassis, close to the ends of the two leaf-springs 61, 62. In addition, each shoe 7 is mounted so as to slide along a vertical support surface 70 which is parallel to the longitudinal axis of the half-chassis, as shown in more detail in Figure 3. On the side facing the spring, each shoe 7 is equipped with a support face 71 which is inclined relative to the sliding surface 70. Each shoe 7 thus has the shape of a wedge which, by sliding longitudinally along the surface 70, is progressively interposed between the support surface and the leaf-spring and progressively moves the end of the spring outwardly of the chassis.

As shown in Figure 3, the end of each spring may advantageously be equipped with a wheel 65 resting on the inclined face 71.

In one embodiment, the longitudinal displacement of the shoe is proportional to the displacement of the normal approach movement, on a curve, of the corresponding coupling ends.

For this purpose, each chassis end 12, 22 is equipped with a member for detecting approach of the adjacent truck.

As described in the above referred to Patent Application, a device for detecting approach of an adjacent truck consists of a rod 8 associated with a respective buffer 4 and mounted to slide in a direction which is substantially parallel to the longitudinal axis and close to the axis of the buffer, in such a way that the end of the rod 8 comes into contact with the rear surface of the plate of the buffer after a certain displacement of the latter. The distance between the end of the rod and the plate of the buffer 4 is such that the rod is not displaced when the adjacent trucks draw closer to each other in a normal way during travel under pull on a curve.On the other hand, when the approach movement of the coupling ends of adjacent trucks exceeds the normal movement on a curve, for example in the case of braking, the plate of the buffer 4 presses against the rod 8 and causes the latter to slide inside the half-chassis. This thrust is transmitted to the shoe 7, positioned on the same half-chassis and on the same side of the longitudinal axis, by a stress transmission system which, in the example shown, is an articulated system which consists of a lever 81 mounted to pivot about a vertical axis and of which one arm is pivotally coupled to the end of the rod 8, the other arm being pivotally coupled to a traction rod 82 arranged along the side of the half-chassis, the other end of this rod 82 being pivotally coupled to a lever 83 which is mounted so as to pivot about a vertical axis, the other arm of the lever 93 being pivotally coupled to the end of a rod 84 which controls sliding movement of the shoe.

As has been mentioned, in Figure la the truck is on a straight stretch and under pull.

In this case, the plates of the buffers are moved slightly away from the ends of the rods 8 and the shoes 7 are also moved away from the springs so that the ends of the springs are free. Resilient means such as a spring can move the shoes away from the springs when there is no pressure on the buffers. When, on a straight stretch, the adjacent trucks approach each other in a normal way, for example following a slight striking of the buffers, the rods 8 remain in position by reason of the gaps present between the ends of the rods and the plates of the corresponding buffers.

When the train brakes, a compressive stress is applied to the four buffers 4 positioned at the coupling ends 12, 22 of the truck, the latter being on a straight stretch as shown in Figure ib. In this case, the four shoes 7 are longitudinally displaced, towards the articulation member, by a distance corresponding to the amount by which the buffers are compressed, and move the ends of the springs 61 and 62 outwards.

The springs are subjected to four equal stresses which oppose each other in pairs, and the resultant stress exerted on the subframe 5 is therefore zero. The bending of the springs only adds its damping effect to that exerted by the buffers.

When the train travels on a curve and is under pull, as shown in Figure 2a, the buffers positioned towards the outside of the curve move away from the buffers of the adjacent trucks, whilst the buffers positioned towards the inside of the curve approach and are compressed over a distance equal to the normal clearance which corresponds, as has been stated, to the gap between the end of the rod 8 and the plate of the buffer. In this way, as long as the approach movement is normal, the buffer plates on the inside of the curve may come into contact with the corresponding rods but do not displace the rods. The corresponding shoes are therefore not displaced.

When the train is on a curve and is being braked, as shown in Figure 2b, the buffers 4 positioned towards the inside of the curve undergo a compressive stress and thus displace the corresponding rod 8. By means of the transfer mechansm 81, 82, 83, this displacement of the rods is converted into a proportional displacement of the corresponding shoes 7. The two buffers positioned at the two ends 12 and 22 of the two half-chassis, towards the inside of the curve, are subjected to equal compressive stresses and thus cause equal displacements of the corresponding shoes 7. The shoes positioned towards the outside of the curve do not of course move, and in addition the ends of the corresponding springs 62 move away from the support faces of the shoes.On the other hand, the springs 61 towards the inside of the curve are bent and as a result a stress directed towards the centre of the curve is exerted on the subframe 5, thus producing the desired counterregion opposing the centrifugal stress being exerted on the articulation system.

It will be noted that the bending of the leaf springs, and in consequence the extent of the centripetal stress of the counterreaction, depend both on the compressive stress applied to the buffers and on the radius of the curve. In fact, even if the braking stress is low and only brings about a slight displacement of the inside shoes on the inside of the curve, the bending of the leaf springs will be greater the smaller the radius of the curve.

In this same way, if a train of trucks undergoes a high compressive stress when on a straight stretch, the shoes are displaced as shown in Figure lb and increase the rigidity of the articulation system in proportion to the compressive stress. When the truck travels on a curve, the leaf springs towards the inside of the curve may already be bent and thus give rise to a greater reaction against the narrowing of the angle formed by the axes of the half-chassis.

The invention is of course not intended to be limited to the details of the embodiment which has just been described and which could be the subject of numerous variants, using, in particular, equivalent means.

Thus, for example, control of the sliding movement of each shoe by the corresponding buffer could be effected hydraulically.

In such a case each device for detecting the approach movement may comprise a jack, the rod of which is positioned behind the plate of the respective buffer and the chamber of which is connected to the chamber of a jack for controlling the sliding movement of the shoe. Any displacement of the jack following approach movement causes an equivalent displacement of the jack for controlling the shoe.

In addition, the displacement of the shoes may be controlled in accordance with the rigidity of the leaf springs, so as to obtain a desired counter-reaction. The displacement of the shoes is in fact proportional to the displacement of the detecting members. The ratio of proportionality can be adjusted very simply by altering the lengths of the arms of the levers 81 and 83, or the sections of the jacks in the case where an hydraulic device is used.

In the embodiment which has just been described, the device which provides elastic resistance to the variation in the angle between the two half-chassis is mounted on the intermediate bogie and requires leaf springs to be fitted to the latter, which method could be thought clumsy. However, the same principle can be applied by means of another device which does not bring the sub-frame into play and which is shown in Figures 4 to 7.

In Figure 4, a truck is shown on a straight stretch, under pull above the longitudinal axis and under restraint below this axis. The two half-chassis 1 and 2 are equipped, as in the preceding embodiment and as in the above referred to Patent Application, with devices for detecting the approach of the trucks, which devices comprise bars 8, each of which is associated with a buffer 4. Each bar is mounted so as to slide in a direction which is substantially parallel to the buffer and its end is positioned at a certain distance behind the plate of the buffer when the truck is under pull, whereas when it is under restraint, the plate of the buffer 4 presses against the bar 8 and controls sliding movement of the latter.The bar 8 is furthermore associated with a system for transmitting the thrust stresses which the bar receives from the buffer 4, which system may be either a mechanical system consisting, as shown in Figure 4, of levers 81, 83 connected by a traction rod 82 or, as was envisaged in the above referred to Patent Application, of a hydraulic system, both systems being of equal value.

The two adjacent ends 11 and 21 of the two half-chassis 1 and 2 are equipped with two pairs of intermediate buffers 9 which rest against each other and are positioned on either side of the articulation member 3.

Intermediate buffers on each side of the articulation member have been used for holding the two adjacent ends of an articulated truck against each other, but hitherto this has been done for the sole purpose of damping, so as to avoid rocking movement, particularly during travel on a straight stretch. In the present embodiment, the intermediate buffers are mounted in a different way and with a completely different purpose.

As shown very schematically in Figures 4, 5 and 6, each intermediate buffer comprises a rod 9, which is equipped at its end with a plate for abutting the adjacent buffer and is mounted so as to slide on the corresponding half-chassis, parallel to the longitudinal axis of the latter. The rod 9 acts against a stop plate 91 through an elastic member 92 which is intended to progressively offer resistance to the forcing in of the buffer 9, and which is placed between the stop plate 91 and a plate 95 positioned on the end of the rod 9.

The bar 8 for detecting approach of the trucks controls the progressive bringing into operation of the buffers 9. For this purpose, the stop plate 91 is mounted to slide parallel to the longitudinal axis of the chassis and the thrust stress exerted by the plate 4 on the bar 8 is transmitted to the stop plate to control the displacement of the latter.

In the simplest embodiment, which is shown in Figure 4, the top plate 91 forms the bottom of a cylinder in which the intermediate buffer 9 slides. This cylinder is itself mounted so as to slide on the chassis 9 and the bottom plate 91 is connected by a rod 93 to the lever 83, which is itself connected to the lever 81 by the traction rod 82.

In the improved embodiment shown schematically in Figures 5 and 6 and in detail in Figure 7, the intermediate buffer 9 is mounted so as to slide inside a fixed casing 90 in which the stop plate 91 also slides. The latter, as mentioned above, is fixed to a rod 93 which is actuated by the bar 8 by means of the system of levers.

This arrangement provides two stages of elasticity. The elastic member 92 between the buffer rod 9 and the stop plate 91 constitutes a firm stage and can be formed, for example, of a stack of Belleville rings or of biconical springs which are interposed between the stop plate 91 and the plate 95 positioned at the end of the rod 9. The stop plate 91 itself acts against the bottom 901 of the casing 90 through an elastic member 94 which constitutes a flexible stage and can be formed, for example, from a stack of rings made of an elastomer, as shown in Figure 7.

As a result, whilst retaining the necessary ability of the stop plate 91 to be displaced longitudinally, the latter can also be urged towards the adjacent end of the half-chassis by the flexible member 94 so as to ensure, on a straight stretch, that the intermediate buffers rest against each other even when the trucks are under pull.

When the truck is on a straight stretch under pull (Figure 4), and the buffers 9 are resting against each other, if the truck brakes and the end buffers 4 are subjected to a compressive stress, the bars 8 against which the buffers 4 press urge the stop plate 91 against the intermediate buffer and, since the intermediate buffers are already resting against each other, the displacement of the two half-chassis 1 and 2 towards each other is damped, in the case of a normal compressive stress, by the flexible stages 94.

Likewise, when the truck is travelling under pull on a curve, as shown in Figure 5, whereas the intermediate buffers move away from each other on the outside of the curve, the distance between the two half-chassis 1 and 2 shortens on the inside of the curve, and in consequence the stop plates 91 are each subjected to thrust in the direction of the arrows. This thrust manifests itself in a displacement of the bars 8 towards the outside, each bar draw ing closer to the plate of the corresponding buffers 4. However, it has already been mentioned that the detection member 8 is adjusted so as only to detect a displacement of the buffer which is greater than the normal displacement. This is why, when under pull, the bars 8 can move freely and therefore do not offer any resistance to the displacements of the stop plates 91.Consequently, the shortening of the distance between the two half-trucks during travel under pull on a curve merely causes the first, flexible, stage 94 of the intermediate buffers to be compressed, whilst giving rise only to a very minor reaction of the truck 5 on the rails, in the centripetal direction.

On the other hand, when the truck under goes a compressive stress which is due, for example, to braking, during travel on a curve, as shown in Figure 6, the buffers 4 on the inside of the curve are compressed by a distance greater than the normal clearance and first engage the bars 8 and thcn possibly exert a thrust on the latter. As soon as the stop plates 91 are locked by engagement of the bars 8 with the buffers 4 and even more so when they are subjected to a thrust stress transmitted by the bars 8, the second, firm, stage 92 of the intermediate buffers is compressed. As a result, a considerable reaction is caused on each buffer 9, which opposes the narrowing of the angle formed by the axes 10 and 20 of the two half-chassis.This reaction on the buffers 9 in fact gives rise to a cdnsiderable centripetal resultant which is inversely proportional to the radius of the curve and opposes the centrifugal reaction of the sub-frame on the rails.

Thus, by means of this buffer contact between the two half-chassis it as possible elastically to oppose any inopportune variation in the angle formed, on a curve, by the two half-chassis when they undergo considerable restraint. The embodiment which has just been described makes it possible to avoid positioning elastic devices, which might be clumsy, on the sub-frame.

In addition, the use of an elastic resisting means possessing two stages of flexibility makes it possible to ensure constantly that the intermediate buffers rest against each other under a low stress when the truck is on a straight stretch and, when the truck is on a curve, that the buffers on the inside of the curve rest against each other. Impacts on the buffers are thus avoided in the case of fierce braking, since the flexible resistance stage is always brought into play before the firm stage.

The invention is of course not limited to the embodiments which have just been described and which could not only form the subjects of variants but could also be replaced by equivalent systems. Likewise, the arragement of the system of rods between the detection member and the elastic resistance means could be altered and, as has already been mentioned, could be replaced by any other equivalent means such as a hydraulic system.

There is thus provided devices for holding the chassis parts in position on a curve, which devices make it possible to provide the articulated trucks with a certain amount of rigidity without, however, causing the unit formed by the chassis parts to be totally rigid.

WHAT WE CLAIM IS:- 1. An articulated truck comprising at least two chassis parts, the or the respective adjacent coupled ends of which are connected to each other by an articulation member, the coupling ends of the truck being provided with coupling and buffer members, means for detecting approach movement between the coupling ends of the truck and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. the buffer rod 9 and the stop plate 91 constitutes a firm stage and can be formed, for example, of a stack of Belleville rings or of biconical springs which are interposed between the stop plate 91 and the plate 95 positioned at the end of the rod 9. The stop plate 91 itself acts against the bottom 901 of the casing 90 through an elastic member 94 which constitutes a flexible stage and can be formed, for example, from a stack of rings made of an elastomer, as shown in Figure 7. As a result, whilst retaining the necessary ability of the stop plate 91 to be displaced longitudinally, the latter can also be urged towards the adjacent end of the half-chassis by the flexible member 94 so as to ensure, on a straight stretch, that the intermediate buffers rest against each other even when the trucks are under pull. When the truck is on a straight stretch under pull (Figure 4), and the buffers 9 are resting against each other, if the truck brakes and the end buffers 4 are subjected to a compressive stress, the bars 8 against which the buffers 4 press urge the stop plate 91 against the intermediate buffer and, since the intermediate buffers are already resting against each other, the displacement of the two half-chassis 1 and 2 towards each other is damped, in the case of a normal compressive stress, by the flexible stages 94. Likewise, when the truck is travelling under pull on a curve, as shown in Figure 5, whereas the intermediate buffers move away from each other on the outside of the curve, the distance between the two half-chassis 1 and 2 shortens on the inside of the curve, and in consequence the stop plates 91 are each subjected to thrust in the direction of the arrows. This thrust manifests itself in a displacement of the bars 8 towards the outside, each bar draw ing closer to the plate of the corresponding buffers 4. However, it has already been mentioned that the detection member 8 is adjusted so as only to detect a displacement of the buffer which is greater than the normal displacement. This is why, when under pull, the bars 8 can move freely and therefore do not offer any resistance to the displacements of the stop plates 91.Consequently, the shortening of the distance between the two half-trucks during travel under pull on a curve merely causes the first, flexible, stage 94 of the intermediate buffers to be compressed, whilst giving rise only to a very minor reaction of the truck 5 on the rails, in the centripetal direction. On the other hand, when the truck under goes a compressive stress which is due, for example, to braking, during travel on a curve, as shown in Figure 6, the buffers 4 on the inside of the curve are compressed by a distance greater than the normal clearance and first engage the bars 8 and thcn possibly exert a thrust on the latter. As soon as the stop plates 91 are locked by engagement of the bars 8 with the buffers 4 and even more so when they are subjected to a thrust stress transmitted by the bars 8, the second, firm, stage 92 of the intermediate buffers is compressed. As a result, a considerable reaction is caused on each buffer 9, which opposes the narrowing of the angle formed by the axes 10 and 20 of the two half-chassis.This reaction on the buffers 9 in fact gives rise to a cdnsiderable centripetal resultant which is inversely proportional to the radius of the curve and opposes the centrifugal reaction of the sub-frame on the rails. Thus, by means of this buffer contact between the two half-chassis it as possible elastically to oppose any inopportune variation in the angle formed, on a curve, by the two half-chassis when they undergo considerable restraint. The embodiment which has just been described makes it possible to avoid positioning elastic devices, which might be clumsy, on the sub-frame. In addition, the use of an elastic resisting means possessing two stages of flexibility makes it possible to ensure constantly that the intermediate buffers rest against each other under a low stress when the truck is on a straight stretch and, when the truck is on a curve, that the buffers on the inside of the curve rest against each other. Impacts on the buffers are thus avoided in the case of fierce braking, since the flexible resistance stage is always brought into play before the firm stage. The invention is of course not limited to the embodiments which have just been described and which could not only form the subjects of variants but could also be replaced by equivalent systems. Likewise, the arragement of the system of rods between the detection member and the elastic resistance means could be altered and, as has already been mentioned, could be replaced by any other equivalent means such as a hydraulic system. There is thus provided devices for holding the chassis parts in position on a curve, which devices make it possible to provide the articulated trucks with a certain amount of rigidity without, however, causing the unit formed by the chassis parts to be totally rigid. WHAT WE CLAIM IS:-

1. An articulated truck comprising at least two chassis parts, the or the respective adjacent coupled ends of which are connected to each other by an articulation member, the coupling ends of the truck being provided with coupling and buffer members, means for detecting approach movement between the coupling ends of the truck and

trucks when coupled thereto, elastic means for resisting variation in the angle formed by the longitudinal axes of the or the respective connected chassis parts when on a curve and actuating means controlled by the approach movement detecting means for progressively bringing into play, proportionally to the approach movement, the elastic resistance means.

2. A truck according to claim 1, wherein the elastic resistance means comprises, symmetrically on each side of the longitudinal axis of the truck and on each of the coupled ends of the chassis parts, a flexible leaf spring which extends parallel to the longitudinal axis, and the actuating means comprises, in respect of each spring, a stop member for contact by an end of the spring.

3. A truck according to claim 2, wherein the flexible leaf springs in respect of the or the respective coupled ends of the chassis parts are mounted on a sub-frame on which the or the respective coupled ends of the chassis parts rest, the spring being arranged symmetrically relative to, and parallel to the longitudinal axis of, the sub-frame, the stop members being mounted on the corresponding chassis part.

4. A truck according to either claim 2 or claim 3, wherein each stop member is mounted on the respective chassis part for sliding movement parallel to the corresponding elastic leaf spring and the actuating means comprise means for controlling sliding movement of the stop member between a first position spaced from the leaf spring and a second position in contact with the leaf spring, proportionally to the approach movement.

5. A truck according to any of claims 2 to 4, wherein each stop member comprises a wedge for progressively moving the end of the leaf spring transversely by displacement from its first position to its second position.

6. A truck according to claim 5 comprising two chassis parts, wherein the means for detecting approach movement comprise a pair of members at each coupling end of the truck, each pair of members being arranged symmetrically one on each side of the longitudinal axis thereof, and each support wedge is mounted to slide parallel to the longitudinal axis of the corresponding chassis part and is connected to one of said members for detecting the approach movement, which one member is positioned on the same chassis part and on the same side of the longitudinal axis, by a stress transmission system which determines a displacement of the wedge proportional to the movement detected.

7. A truck according to claim 6, wherein each member for detecting the approach movement comprises a rod, associated with a respective one of the buffers, which is slidable in accordance with the compression of the buffer, and each stress transmission system comprises two levers, one of which is pivotted to the detecting rod and the other to a thrust rod connected to the corresponding stop member, the two levers being connected by a connecting rod which extends along the corresponding side of the respective chassis part.

8. A truck according to claim 1, wherein the elastic resistance means comprise, in respect of the or the respective coupled ends of the chassis parts and on each side of the longitudinal axis of the truck, two intermediate buffers which abut each other and are mounted for axial sliding movement one on each coupled end respectively of the coupled chassis parts, and an elastic member for supporting each buffer on a stop plate, each stop plate being mounted for axial sliding movement on the respective chassis part, and the actuating means comprises means for controlling displacement of the stop plates.

9. A truck according to claim 8, wherein each intermediate buffer comprises a first stage and a second stage possessing different flexibilities, the first stage possessing relatively higher flexibility, for biassing the stop plate in a direction to maintain the intermediate buffers in contact under low stress, and a second stage possessing relatively lower flexibility, comprising the elastic member for preventing the end of the couple chassis parts from approaching.

10. A truck according to claim 9, wherein each intermediate buffer and its stop plate mounted for sliding movement inside a cylinder fixed to the respective chassis part, the first stage being positioned between the stop plate and the bottom of the cylinder on that side facing the intermediate buffer, and the second stage being positioned on the other side of the stop plate between the stop plate and a support plate provided at the end of the rod of the intermediate buffer.

11. A truck according to any of claims 8 to 10, wherein the approach movement detecting means comprise a rod associated with each coupling end buffer, each rod being axially slidable in accordance with the extent of compression of the associated buffer, the thrust stress on the rod controlling, by means of a transmission system, displacement of a respective stop plate proportional to the displacement of the rod.

12. A truck substantially as herein des cribed with reference to Figures 1 to 3 of the accompanying drawings.

13. A truck substantially as herein described with reference to Figures 4 to 7 of the accompanying drawings.