The invention relates to a towing gear for use between a motor car and a trailer and of the type having a longitudinally variable pull rod in the form of a telescopic unit comprising a closed cylindrical housing and a piston displaceable therein and provided with a piston rod which in a tight manner extends through the one end of the cylindrical housing, the cylindrical housing and the piston rod being mechanic¬ ally connected with a respective one of the motor car and the trailer, the length of the telescopic unit being adapted to be controlled by a pressurized working fluid in dependence on the mutual orientation between the motor car and the trailer.

Such towing gears are used in connection with a lorry with trailer in respect of obtaining the maximum load-carrying volume. This is obtained by means of the longitudinally variable pull rod serving to minimize the distance between the motor car and the trailer when driving generally straight ahead, because it will be possible in this way to increase the length of e.g. the trailer by up to about 1 m without the risk that the total length of the lorry and the trailer in such a situation exceeds the maximum allowable one.

This is particularly advantageous in respect of the carrying of bulky goods, such as batches of mineral wool, flowers and various forms of pallet goods, because in such cases the space and not the weight determines the load-carrying capacity of the lorry.

Further advantages of minimizing the distance between the lorry and the trailer consist in a not inconsiderable saving of fuel (up to about 15%) due to reduced turbulence and thus improved air flowing con¬ ditions between the lorry and the trailer.

By using a longitudinally variable pull rod it is possible to reduce the distance between the lorry and the trailer to about 20 to 25 cm. It will, however, be understood that said distance can only be maintained when driving straight ahead or driving through curves at a pretty large radius of curvature, since larger differences as to angular orientation (in a horoizontal and/or vertical plane) between the lorry and the trailer imply that they will come into collision. This is unallowable as it, beyond the immediate risk of damage to the equipments, involves the far more dangerous risk of loosing the total steering ability, because the lorry and the trailer due to the mutual abutment are prevented from "bending" enough at the transition between the lorry and the trailer and thus cannot "follow" the steering front wheels, but will continue in the same manner as if the front wheels were locked. With a view to continuously ensure the necessary distance between the motor car and trailer various forms of mechnically and/or electrically detecting means are used in practice which are disposed in the interspace between the rear wall of the body of the motor car and the front wall of the body of the trailer. When said means detect a distance that is shorter than a predetermined minimum distance between the lorry and the trailer, the longitudinally variable pull rod is activated so as to re-establish the correct clearance between the lorry and the trailer. When the actuation exterted by the detecting means ceases - as a sign that the lorry and the trailer are again oriented approximately in parallel - the length of the pull rod diminishes again.

A towing gear marketed by the company "C0NTAR" and of the type mentioned above makes use of a double telescopic cylinder consisting of two coaxial working cylinders driven by pressurized air and oil, respect-

ively. Both cylinders are mounted non-invertedly, meaning an orientation increasing the length of the pull rod in unison with pushing one or both piston rods out of the associated cylinder(s). The structure is rather complicated and dif¬ ficult as regards tightness, meaning that this solution is further comparatively expensive. Due to various facts it is, moreover, inexpedient to work with two different pressurized fluids in one and the same telescopic unit.

Eventually, the non-inverted orientation of the telescopic unit implies that the pull force prevailing at any time between lorry and trailer is to be trans¬ ferred through the volume of working medium (oil) available between the end of the working cylinder through which the piston rod extends and the associated piston. The consequence of this structure is, on one hand, that a high pressure is constantly exerted on the sealing means around the passage of the piston rod into the cylinder and, on the other hand, in case of failure a structure of mechanically minor flexural rigidity is concerned, in which the pull rod is locked in its "long" state, i.e. the piston rod being fully pushed out. The first mentioned circumstance involves a risk of leaks that entails possible functional irregulari¬ ties, while the second circumstance implies that the telescopic unit must be made stronger than actually necessary in respect of its primary function as a longitudinally variable unit. A second drawback of said prior towing gear is that the telescopic unit must further be dimensioned in consideration of the fact that under all working conditions it is merely this unit which transfers the stresses between the motor car and the trailer.

The towing gear according to the invention dif¬ fers from prior towing gears in that the telescopic

unit is arranged so that pull forces between the lorry and the trailer are transferred through the volume of working fluid encapsulated between the closed end of the cylindrical housing and the piston. This provides for obtaining, one one hand, that the gland around the piston rod passage during normal working by and large is relieved of pressure and, on the other hand, that the pressure on the piston sealing means proper between the piston and the inner side of the cylinder, other things being equal, decreases due to the larger pressure area.

Such an arrangement may in practice be obtained in that the piston rod is connected with the actual lorry through a connecting rod juxtaposed along the telescopic unit and extending apporoximately in parallel thereto, the free end of the piston rod being connected with the adjacent end of the connecting rod and the opposite end thereof being connected with the actual one of the lorry or the trailer. This "inverted" mounting causes the piston rod to be now pushed fully into the cylinder in the com¬ pletely extended condition of the pull rod with the positive consequence that the stresses on the gland are reduced. To a high degree said effect supported by the presence of the parallel connecting rod which to a large extent will assume the bending moments and lateral forces which also previously had to be assumed by the telescopic unit proper.

The invention will now be explained in detail with reference to the somewhat schematical drawings, in which

Fig. 1 is a partially sectional side elevation of an embodiment of the towing gear according to the invention, and Fig. 2 is a plan view of the towing gear illus¬ trated in Fig. 1.

As it will appear from the drawings, the towing gear according to the invention is in a first embodi¬ ment arranged about a hydraulic cylinder 2 with a cylindrical housing 3 and a piston rod 4. Cylinder 3 is over an angle bar 5 secured beneath a tri¬ angular frame 6 which is further secured via a trans¬ verse profile bar 11, in a manner not shown, to a com¬ mon swivel rim on a trailer. By means of a transverse pin 7 and a mounting 8 piston rod 4 is secured to the end of a connecting rod 9 extending parallel to the hydraulic cylinder 2 and which is displaceable in the central profile bar 10 of the frame 6. Said con¬ necting rod 9 is at the opposite end provided with an eye 12 by which said rod is adapted to be connected with a motor car.

The illustrated towing gear operates as follows: When driving normally straight ahead and in that respect at the desired minimum distance between the motor car and the trailer, the towing gear assumes its shortest possible condition obtained by pushing piston rod 4 completely out of cylindrical housing 3. This is effected by flowing e.g. pressurized oil to the relevant side of the piston within cylinder 2 in a manner not described in detail. This short condition substantially appears from Fig. 1. Pull forces are transferred from the motor car to the trailer through connecting rod 9, mounting 8 and pin 7 to piston rod 4, actuated in a direction into cylindrical housing 3. As previously stated, this is advantageous because the power transferring oil volume positioned at the closed end of the cylindrical housing actually does not actuate the gland at the opposite end of said housing.

If it is necessary to increase the distance between the lorry and the trailer, e.g. with respect to driving rounds corners, piston rod 4 is pulled

farther into cylindrical housing 3. This is effected as stated above by means of a suitable supply of pressurized oil. It will appear that the piston rod in the fully extended condition of the towing gear is pushed completely into the cylindrical housing, meaning as stated that the glands at the open end of the cylindrical housing as well as in the piston of the cylinder are only subjected to minimum stresses, thereby obtaining a longer lifetime of said glands. In the illustrated embodiment the working cylinder is on the whole relieved to a maximum as rela¬ tes "odd" streases of any kind. This is a result of the presence of the relatively long guide tube 10 in con¬ nection with the very rigid triangular frame 6 and connecting rod 9.

Irrespective of the fact that the towing gear is only illustrated with one working cylinder there is no hindrance to provide the gear with for instance two cylinders coupled in parallel side by side and also other configurations are likely. The central idea of the invention is the "inverted" mounting of the actual working cylinder(s) and the presence of at least one connecting rod arranged and extending parallel thereto.

The working cylinder is advantageously driven by means of pressurized oil which may either be discharged from the existing hydraulic system of the motor car or be generated by a separate system, but said working cylinder may as well be driven by means of e.g. pressurized gas. The working cylinder may also be designed in such a manner that the longitudinally reducing movement is effected by means of for example pressurized oil, while the force requiring longitudi¬ nally increasing movement that is considerably smaller under normal circumstances is effected by meams of one or more independent gas springs and/or purely mechanical spring means.

It should finally be mentioned that it is com¬ paratively simple to rebuild existing towing gears so as to utilize the advantages obtained according to the present invention.