Trailers and caravans are today in most cases provided with mechanical brakes of pusher type. There are several pro¬ blems connected with this type of brakes. Firstly, it is almost impossible to back the trailer in a uphill slope, since the mechanical brake in this position is not applied against the wheels of the trailer. Secondly, there is no braking effect on the trailer when an equipage comprising a traction vehicle and a trailer shall be backed in a downhill slope, since the mecha¬ nical brake of pusher type is not applied against the wheels of the trailer in this position. Thirdly, the mechanical brake of pusher type can easily get rusted up whereby the braking func- tion is away.

This invention relates to remove the problems with this known technique and offer a hydraulic braking device which gives full braking effect on a trailer when that it needed and which makes backing operations in a uphill slope of an equipage comprising a traction vehicle and a trailer or the like con¬ nected to this vehicle possible. Moreover, this new invention makes full braking effect on the trailer during backing opera¬ tions in a downhill slope of an equipage comprising a traction vehicle and a trailer/caravan possible. However, a hydraulic braking device is previously known by German Offenlegungsschrift 21 27 717. This known braking device comprises a separate hydraulic braking system for brak¬ ing the trailer if it moves in one direction in relation to the traction vehicle and another separate hydraulic braking system for braking the trailer if this one moves in the other direc¬ tion in relation to the traction vehicle. Each of these two hydraulic systems comprises a special means for influencing a hydraulic cylinder which in its turn is connected to the wheel

couple of the trailer via a separate conduit. This means that the known braking device becomes very expensive to manufacture and very sensitive from a functional point of view.

This invention relates to offer a hydraulic braking de- vice which is simplier regarding the construction and which becomes inexpensive to manufacture and gets a more reliable function than the known one. This has been made possible by a hydraulic braking device of the kind mentioned by way of in¬ troduction which has the features appearing from the claims. A preferred embodiment of the invention which is shown in the accompanying drawing will be described more closely below. The drawing shows a plan view of the device.

Referrring to this drawing is shown there a braking de¬ vice provided with a base 1, in which the parts of the braking device are arranged. The base is stationarily arranged on the frame of the trailer and preferably in its fore part. The trailer comprises a draw bar 2 which is centrally positioned on the base 1 and is displaceably arranged in relation to this one by means of bearings 3, 4. These bearings comprise bearing sleeves 3, 4, the bearing sleeve 3 being that one which is positioned at the very front of the draw bar 2, i.e. closest to the traction vehicle. Around the draw bar 2 there is arranged a dog means 5 comprising a sleeve-formed means 6, in relation to which the draw bar is turnable. Thus, the draw bar is turnable in relation to the chassis of the trailer. In order to prevent an axial movement of the sleeve-formed means 6 there is a stop means 7, 8 stationarily arranged on the draw bar 2 on each side of the sleeve 6. The stop means in front of the sleeve-formed means 6, seen in the drawing direction, comprises another sleeve-formed means 7, which is stationarily arranged on the draw bar, while the stop means behind the means 6 comprises a locking ring 8 which is also stationarily arranged on the draw bar. Of course, it is possible to place the means 7 behind the sleeve 6 and the locking ring 8 in front of the sleeve 6. According to this embodiment there is a demand that the sleeve-formed means 6 shall not be able to be turned in rela¬ tion to the base. Thus, in order to prevent such a turning the sleeve-formed means 6 has two longitudinal flanges, not shown,

which are arranged on the lower part of the means 6 along the jacket surface of the means and on each side of a central vertical plane through the means 6. Each of these flanges rests on a longitudinal guide, which neither are shown in the drawing and which have a length exceeding the length of the mentioned flanges. These guides are fastened in the base. On the means 6 there is a plate 9 fastened which projects essentially radially in relation to the draw bar 2 so that the outer portion of the plate extends a bit outside the means 6. Thus, the longitudinal direction of the plate 9 is essentially perpendicular to the longitudinal direction of the draw bar. On the plate 9 there is a pin 10 detachably fastened. This pin projects essentially vertically from the plate 9 and is movable in the longitudinal direction of the plate 9. The pin is intended to cooperate with an oblong plate 11, the first end portion of which is pivotably fastened to a hinge 12 which in its turn is stationarily fastened in the base 1. The plate 11 in neutral position has an extension in the longitudinal direction which is essentially perpendicular to the longitudinal direction of the draw bar 2 and has a centrally arranged slot 13 which extends in the longitudinal direction of the plate from its other end portion to its first end portion. The slot 13 has a length correspond¬ ing to about 2/3 of the length of the plate 11 and is intended to cooperate with the pin 10 fastened on the plate 9. Due to the fact that the sleeve-formed means 6 is ar¬ ranged in relation to the base 1 in that way mentioned above the position of the plate 9 and the position of its pin means 10 in relation to the plate 11 can remain essentially intact during use even if the draw bar should be turned in relation to the chassis of the trailer, which could be the case if the one rear wheel of the trailer moves over a large stone.

The one end of an oblong arm 14, the length of which is only about 1/4 of the length of the plate 11 is fastened to the first end portion of the plate 11, that is the end portion which is pivotably fastened to the hinge 12. This means that when the plate 11 is pivoted round the hinge 12 to the left in the drawing, the arm 14 will be pivoted to the fight round the hinge 12 and vice versa. The other end of the arm 14 has a

wheel means 15 which when the arm 14 is pivoted to the left in the drawing is intended to influence the outer end of a first piston rod 16 arranged to a not shown piston which in its turn is displacebly arranged in a first hydraulic cylinder 17 that is stationarily fastened in the base 1 on one side of the draw bar 2. When the arm 14 is pivoted to the right in the drawing, the wheel means 15 will influence the outer end of a second piston rod 18 arranged to a not shown piston which is dis- placeably arranged in a second hydraulic cylinder 19 that is stationarily fastened in the base 1 on the same side of the draw bar as the first hydraulic cylinder.

By moving the pin 10 on the plate 9 the length L of the lever arm, i.e. the distance between the centre of the pin 10 and the centre of the hinge 12, can be changed. This means that it is possible to change the size of the force by which the wheel means 15 is forced against the piston rod 16, 18 of respective hydraulic cylinder 17, 19. Thus, a long lever arm L gives a larger press-ing force on the wheel means 15 than a short lever arm (a long lever arm L gives a larger braking force than a short lever arm) . Due to that fact the braking device according to the invention can be used on trailers for different total weights.

Inside of each hydraulic cylinder 17, 19 there is ar¬ ranged a not shown spring which is intended to rest against the inner portion of the piston means. This means that when the base 1 moves to the left in the drawing in relation to the draw bar 2, the plate 11 provided with the slot is pivoted round the hinge 12 to the right in the drawing, the arm 14 being pivoted round the hinge 12 to the left in the drawing and is according- ly applied against the outer end of the piston rod 16. Due to that fact the piston in the hydraulic cylinder 17 against the action of the mentioned spring will be forced further into the hydraulic cylinder so that a hydraulic medium is forced out of the cylinder via a conduit 20. This fact relates to braking the traction vehicle when driving forwards. However, if the base 1 moves to the right in the drawing in relation to the draw bar 2, the arm 14 is also pivoted to the right and is applied against the other piston rod 18. Due to that fact the plunger

of the second hydraulic cylinder 19 is forced further into the same against the action of the mentioned spring, whereby the hydraulic medium is forced out of the cylinder 19 via a conduit 21. This fact relates when braking the traction vehicle during backing operations in a downhill slope. Both conduits 20, 21 go to an electric-controlled hydraulic valve 22 which is suitably in connection with the gear box of the traction vehicle via a contact of the reverse light. From the electric-controlled hydraulic valve 22 there is a conduit 23 for the hydraulic medium, which preferably consists of oil, extending to the wheels of the trailer 1.

The device according to the invention is so constructed that if a gear is put in in the traction device for driving forwards, the hydraulic medium is intended to be conducted from the hydraulic cylinder 17 via the electric-controlled valve 22 to the brakes of the trailer. However, if the reverse gear is put in, the electric-controlled hydraulic valve 22 is turned over so that the connection is opened for the hydraulic medium between the hydraulic cylinder 19 and the brakes of the trailer. Thanks to the arrangement with the electric-controlled hydraulic valve only one conduit for the hydraulic medium needs to be drawn to the wheels of the trailer.

In order that the device according to the invention shall function satisfactorily during use when travelling forwards, the draw bar ought to be able to be blocked in a certain posi¬ tion. For this purpose there is a pawl 24 arranged. This pawl 24 comprises an essentially right-angled formed means, which is pivotably journalled on a pin 25 which is stationarily fastened in the base 1 and projects essentially vertically from the same. The one part 26 of the right-angled formed means 26 ex¬ tends essentially parallel with the longitudinal direction of the draw bar 2 and is provided with a hook being into engage¬ ment with a corresponding projection 27 of the sleeve-formed means 6. The other part 28 of the right-angled formed means 24 extends essentially perpendicularly in relation to the longitu¬ dinal direction of the draw bar 2. The outer portion of the part 28 is pivotably fastened to the front end of a bar 29 being an integral part of a solenoid valve 30. The bar 29 ex-

tending essentially parallel with the draw bar 2 is displace¬ able in its longitudinal direction by means of electric cur¬ rent. The solenoid is constructed in that way that when the reverse gear is put in in the traction vehicle the bar 29 is pulled towards the valve house 31 via an impuls from the con¬ tact of the reverse light. The operation of the solenoid 30 and the hydraulic valve 22 is suitably made via a relay. Further¬ more, the solenoid 30 is provided with a return spring 32 which is intended to bring the bar 29 back to its original position when the reverse gear is put out and which is so arranged that during a falling off of the current, for instance during a brake in the connection from the contact of the reverse light, the solenoid takes such a position that a full braking effect exists when driving the equipage forwards. When the pawl 24 is into engagement with the projection

27 of the sleeve-formed means 6, as is the case when driving forwards, the base 1 is prevented to move to the right in the drawing in relation to the draw bar 2. When braking the hydrau¬ lic cylinder 17 will be influenced by the means 15 of the arm 14. When the pawl 25 is out of engagement with the projection 27, as is the case during backing operations, the base 1 will be able to move to the right in the drawing in relation to the draw bar 2. This means that the arm 14 and the wheel means 15 will be able to influence the hydraulic cylinder 19, which in this case has a direct connection with the brakes of the trailer. This happens if the traction car brakes after a backing operation in a downhill slope. However, if the equipage comprising a traction car and a trailer should make a backing operation in a uphill slope, the arm 14 and the means 15 will go into engagement with the piston bar 16 of the hydraulic cylinder 17. However, no hydraulic oil will be forced out from the hydraylic cylinder 17 to the brakes of the trailer, since these ones has a contact only with the hydraulic cylinder 19 during a backing operation. This means that partly there is a full braking effect on the trailer during braking in a downhill slope, partly there is no resistance from the brakes of the trailer during a backing operation in a uphill slope as is the case when using conventional brakes of pusher type.

As has mentioned in the application the pawl 24 is into engagement with the sleeve-formed means 6 fastened on the draw bar 2. The pulling force from the draw bar to the base 1 and by that to the chassis of the trailer will accordingly be trans- mitted via the pawl 24. This construction solution gives a more reliable function of the trailer device, since the pulling force from the draw bar to the chassis of the trailer otherwise should be transmitted via the hydraulic cylinder 19, where the piston in this position is essentially immovable, since the hydraulic conduit 2 via the electric valve 22 is blocked.

As appears from the drawing there is a container 33 which when needed is intended to deliver a hydraulic liquid to the hydraulic cylinders 17, 19 via conduits 34, 35. As also appears from the drawing the device according to the invention can be provided with a pressure accumulator 36 intended to moderate the pressure in the hydraulic conduit 20 which conducts a hydraulic liquid from the hydraulic cylinder 17, which is intended to be activated for braking when travelling forwards, to the brakes of the trailer. Especially in this conduit 20 the pressure otherwise could be very high.

In order to reduce the variations of the braking force and the vibrations during the course of the braking of the trailer there is a vibration damper 37 arranged inside the draw bar 2. The fore part of the vibration damper is fastened in the fore part of the draw bar 2 and the rear part of the damper 37 is fastened in the rear part of the base 1.

The whole construction in the base 1 is suitably enclosed by a dust proof and water proof casing, made of plastic mate¬ rial or metal. At the front portion 38 of the base 1 a protec- tive means can be arranged which prevents dirty particles to come into the bearing sleeve 3 of the draw bar.

The new invention can be applied on trailers independent¬ ly if they are provided with disc brakes or drum brakes. Furthermore, it is possible to build in a mechanical parking brake in the system.

The invention is, of course, not limited to the mentioned embodiment but can be modified within the scope of the follow¬ ing claims. Thus, for instance the means which is intended to

influence the two hydraulic cylinders can be fastened on the draw bar, the means performing an essentially linear movement, which is essentially parallel with the linear movement of the draw bar. Furthermore, the mentioned means may instead of performing a circular-like movement as according to the de¬ scribed embodiment be so arranged that it performs an essen¬ tially linear movement which is essentially parallel with the linear movement of the draw bar.